Merge latest master into subagent policy inheritance

Retarget the feature branch to the current master tip without rewriting its existing review history. Keeping this as a dedicated merge checkpoint makes the later simplification diff attributable to the stacked child rather than mixing base movement with design changes. Resolve the identified-message API drift in the feature tests by constructing complete user messages, reading the nested tool-result message shape, and adapting the prompt-submit listener signature. Preserve both sides of the user-approval conflict: master’s createUserMessage wrapper and the feature’s inherited-policy attribution. Regenerate the Cordis and persistence catalogs, re-record the session README pair, and refresh the affected ACP/headless fixtures so derived artifacts describe the merged source rather than either parent in isolation. Validated with the focused policy/session/persistence/query suites (430 tests), focused ACP/headless snapshots (3 tests), build, doc-sync (25 gates), lint, hygiene, and git diff checks.
2026-07-28 21:11:20 +08:00
parent 2f93614efb cad0615261
commit afa38c4b2f
425 changed files with 7530 additions and 3109 deletions
--- a/packages/core/session/README.i18n.yaml
+++ b/packages/core/session/README.i18n.yaml
@@ -2,5 +2,5 @@
 # side as of the last confirmed-consistent state. Both languages carry equal authority;
 # after editing either side, bring the other along and re-record with:
 #   pnpm run verify-translation-pairing --write packages/core/session/README.md
-README.md: 6c5ec11bdfb575050b26813ed03395b3cc5e0923
-README.zh.md: 41d937119ef92f2658f305f52c5a34eb4cefdc43
+README.md: 5168a08d293b71a60f23ed65c434ad6d33a7cdfe
+README.zh.md: 236ee2a66efc5742765c062c20a4eadde34dffdb
--- a/packages/core/session/README.md
+++ b/packages/core/session/README.md
@@ -38,7 +38,7 @@ The store pairs announced creation with disposal, publishes post-commit append n
 Plain class (not a Cordis Service). Create via `ctx.sessions.create()`.

 - `session.append(type, data, opts?)` snapshots and freezes durable data and surface metadata, validates marker shape, provenance, complete replacement coverage, and content-only single-result `tool/result` rewrites, commits synchronously, then notifies observers with independent failure containment. Reentrant attached-session appends reject, and runtime checks cover widened unions and loaded logs.
- `session.deriveMessages()` incrementally projects each new surface entry once and returns a fresh array over shared frozen messages. Assistant projections preserve provider/model provenance and adapter-private replay state. A surface rewrite rebuilds the projection; there is no raw-log fallback.
+- `session.deriveMessages()` incrementally projects each new surface entry once and returns a fresh array over the complete identified, frozen messages stored by those entries. Assistant messages preserve provider/model provenance and adapter-private replay state in their model source. A surface rewrite rebuilds the projection; there is no raw-log fallback.
 - `session.deriveEventMessage(event)` is the canonical per-event projection used by reconstruction and request checks.
 - `session.surface` exposes the readonly `SessionSurface` view owned by the session's single incremental surface manager; `replaceGeneration` changes on every committed rewrite.
 - `session.events` is a cached frozen snapshot invalidated by append; accepted events remain deeply frozen.
@@ -65,9 +65,9 @@ Providers stream token-sized deltas, so a raw log stores hundreds of `assistant/

 `request/header` records a full canonical snapshot of the non-history request envelope with reason `initial`, `resume`, or `change`. `foldRequestHeader()` selects the latest snapshot; legacy delta events and the removed `fallback` reason are rejected. See the [reconstructable-requests Agent Note](../../../.agents/notes/implemented/architecture/2026-07-05-reconstructable-requests.md).

-A `user/message` renders its `content` verbatim as a user-role message whether it is a direct human prompt, a synthetic injection, or an admitted goal round; its typed `source` is the only channel that tells them apart and carries any domain-specific durable facts. Turn execution remains enclosed by `turn/start` and `turn/end`, while an idle injection may append and flush a `user/message` between turns without running the model.
+A `user/message` stores the complete `UserMessage` directly, including the identity created before routing or prompt admission. It renders its `content` verbatim whether it is a direct human prompt, a synthetic injection, or an admitted goal round; its typed `source` is the only channel that tells them apart and carries any domain-specific durable facts. `assistant/message`, `tool/result`, and `steering/message` likewise store complete message values. Turn execution remains enclosed by `turn/start` and `turn/end`, while an idle injection may append and flush a `user/message` between turns without running the model.

-`tool/result` persists the model-facing content, optional internal failure identity, and optional presentation metadata. A tool's successful canonical `value` and human-readable canonical failure message remain execution-local; rendered error content is the replay-authoritative message. This preserves the existing event shape and does not change `SESSION_FORMAT_VERSION`.
+`tool/result` persists one identified user-role tool-result message, optional internal failure identity, and optional presentation metadata. A tool's successful canonical `value` and human-readable canonical failure message remain execution-local; rendered error content is the replay-authoritative message.

 ### Session event vocabulary (`types.ts`)

@@ -100,7 +100,7 @@ Every `SessionEvent` carries two optional top-level fields (structural metadata)

 #### What the model sees

-The model receives projections of `user/message`, `assistant/message`, `tool/result`, and `steering/message` surface entries verbatim: each is a user- or assistant-role message carrying its content blocks unchanged. A prompt envelope changes only human presentation; its prefix context and request delimiter are already present in the event content. Tool calls live inside assistant messages. Chunks, boundaries, usage, hook records, todo records, and other log-only events add no message.
+The model receives the complete messages from `user/message`, `assistant/message`, `tool/result`, and `steering/message` surface entries verbatim. Their identities, roles, sources, and content blocks are the same values established at creation; projections do not mint identities. A prompt envelope changes only human presentation; its prefix context and request delimiter are already present in the event content. Tool calls live inside assistant messages. Chunks, boundaries, usage, hook records, todo records, and other log-only events add no message.

 #### Token effect

--- a/packages/core/session/README.zh.md
+++ b/packages/core/session/README.zh.md
@@ -38,7 +38,7 @@
 普通类（不是 Cordis 服务）。通过 `ctx.sessions.create()` 创建。

 - `session.append(type, data, opts?)` 会为持久数据和 surface 元数据制作快照并冻结它们，校验标记形态、溯源信息、替换覆盖完整性，以及仅修改内容的单个 `tool/result` 重写，随后同步提交，再在彼此独立的失败收容下通知观察者。对已附加会话的重入追加会被拒绝，运行时检查也覆盖扩宽后的联合类型和已加载日志。
- `session.deriveMessages()` 对每个新的 surface 条目只做一次增量投影，并返回一个新数组，数组元素引用共享的冻结消息。assistant 投影保留提供方／模型溯源信息及适配器私有回放状态。surface 重写会重建投影；不存在原始日志回退。
+- `session.deriveMessages()` 对每个新的 surface 条目只做一次增量投影，并返回一个新数组，其中包含这些条目存储的完整、带标识且冻结的消息。assistant 消息会在其模型来源中保留提供方／模型溯源信息及适配器私有回放状态。surface 重写会重建投影；不存在原始日志回退。
 - `session.deriveEventMessage(event)` 是重建和请求检查使用的规范逐事件投影。
 - `session.surface` 暴露只读 `SessionSurface` 视图，由会话唯一的增量 surface 管理器所有；每次提交重写，`replaceGeneration` 都会变化。
 - `session.events` 是按追加失效的缓存冻结快照；已接受事件保持深度冻结。
@@ -65,9 +65,9 @@

 `request/header` 记录非历史请求封装的完整规范快照，其原因为 `initial`、`resume` 或 `change`。`foldRequestHeader()` 选择最新快照；旧版增量事件和已移除的 `fallback` 原因会被拒绝。详见[可重建请求 Agent Note](../../../.agents/notes/implemented/architecture/2026-07-05-reconstructable-requests.md)。

-`user/message` 会将其 `content` 原样呈现为 user-role 消息，无论它是直接人类提示词、合成注入，还是已准入的 Goal Round；带类型的 `source` 是区分三者的唯一通道，并携带各领域专有的持久事实。轮次执行仍由 `turn/start` 与 `turn/end` 包围，而空闲注入可以在轮次之间追加并刷新一条 `user/message`，无需运行模型。
+`user/message` 会直接存储完整的 `UserMessage`，其中包括路由或提示词准入前创建的标识。无论它是直接人类提示词、合成注入，还是已准入的 Goal Round，都会原样呈现其 `content`；带类型的 `source` 是区分三者的唯一通道，并携带各领域专有的持久事实。`assistant/message`、`tool/result` 和 steering（中途引导）对应的 `steering/message` 也会存储完整的消息值。轮次执行仍由 `turn/start` 与 `turn/end` 包围，而空闲注入可以在轮次之间追加并刷新一条 `user/message`，无需运行模型。

-`tool/result` 持久保存面向模型的内容、可选内部失败标识和可选呈现元数据。工具成功时的规范 `value` 和便于人类阅读的规范失败消息只存在于执行本地；渲染后的错误内容是回放权威消息。这样会保留现有事件形态，且不改变 `SESSION_FORMAT_VERSION`。
+`tool/result` 持久保存一条带标识、user-role 的工具结果消息，以及可选内部失败标识和可选呈现元数据。工具成功时的规范 `value` 和便于人类阅读的规范失败消息只存在于执行本地；渲染后的错误内容是回放权威消息。

 ### 会话事件词汇（`types.ts`）

@@ -100,7 +100,7 @@

 #### 模型看到的内容

-模型会原样接收 `user/message`、`assistant/message`、`tool/result` 和 `steering/message` surface 条目的投影：每个投影都是一条 user-role 或 assistant-role 消息，其内容块保持不变。提示词封装只改变面向人的呈现；其前缀上下文和请求分隔符已经位于事件内容中。工具调用包含在 assistant 消息内。分片、边界、用量、hook 记录、todo 记录以及其他仅日志事件不会添加消息。
+模型会原样接收 `user/message`、`assistant/message`、`tool/result` 和 `steering/message` surface 条目中的完整消息。其标识、角色、来源和内容块都与创建时确定的值相同；投影不会生成标识。提示词封装只改变面向人的呈现；其前缀上下文和请求分隔符已经位于事件内容中。工具调用包含在 assistant 消息内。分片、边界、用量、hook 记录、todo 记录以及其他仅日志事件不会添加消息。

 #### Token 影响

--- a/packages/core/session/src/index.ts
+++ b/packages/core/session/src/index.ts
@@ -20,6 +20,7 @@ import type { SessionSurface } from './surface.ts'
 import { foldRequestHeader } from './request-header.ts'

 export * from './types.ts'
+export type { AssistantMessage, ToolResultMessage, UserMessage } from '@deepseek-ai/dsh-llm'
 export { isJsonValue, snapshotJsonValue } from './json.ts'
 export type { JsonValue } from './json.ts'
 export { interruptedTurnClosers, TOOL_NOT_STARTED, TOOL_OUTCOME_UNKNOWN } from './repair.ts'
@@ -153,6 +154,33 @@ function snapshotSessionHeader(id: SessionId, source?: SessionHeader): SessionHe
  return deepFreeze(record as unknown as SessionHeader)
 }

+/**
+ * Detach one event while preserving deep immutability for its identified message.
+ * @param event - event imported across a query or persistence boundary.
+ * @returns a detached event snapshot with a validated, deeply frozen message.
+ */
+export function snapshotSessionEvent<T extends SessionEvent>(event: T): T {
+  const snapshot = structuredClone(event)
+  assertMessageEventShape(
+    snapshot,
+    `session event at seq ${snapshot.seq}`,
+  )
+  switch (snapshot.type) {
+    case 'user/message':
+      deepFreeze(snapshot.data)
+      break
+    case 'assistant/message':
+    case 'tool/result':
+    case 'steering/message':
+      deepFreeze(snapshot.data.message)
+      break
+    default:
+      // SessionEventMap is merge-extensible; plugin-owned events carry no core message.
+      break
+  }
+  return snapshot
+}
+
 /** Validate the fixed event envelope after one-pass JSON materialization. */
 function assertSessionEventEnvelope(value: Record<string, unknown>, index: number): asserts value is SessionEvent {
  const event = value
@@ -173,13 +201,14 @@ function assertSessionEventEnvelope(value: Record<string, unknown>, index: numbe
  assertCurrentTurnEndShape(event, index)
 }

-/** Reject pre-provider request headers and assistant messages at the seed/load boundary. */
+/** Reject obsolete request headers and malformed messages at the seed/load boundary. */
 function assertCurrentLlmShape(event: Record<string, unknown>, index: number): void {
  const data = event['data']
-  if (typeof data !== 'object' || data === null) return
-  const record = data as Record<string, unknown>
+  const record = typeof data === 'object' && data !== null
+    ? data as Record<string, unknown>
+    : undefined
  if (event['type'] === 'request/header') {
-    const header = record['header']
+    const header = record?.['header']
    const config = typeof header === 'object' && header !== null ? (header as Record<string, unknown>)['config'] : undefined
    if (!hasProviderModel(config)) throw new Error(`seed request/header at index ${index} lacks provider/model`)
    const reasoningEffort = (config as Record<string, unknown>)['reasoningEffort']
@@ -188,8 +217,63 @@ function assertCurrentLlmShape(event: Record<string, unknown>, index: number): v
      throw new Error(`seed request/header at index ${index} has an invalid reasoningEffort`)
    }
  }
-  if (event['type'] === 'assistant/message' && !hasProviderModel(record['provenance'])) {
-    throw new Error(`seed assistant/message at index ${index} lacks provider/model provenance`)
+  const type = event['type']
+  if (type !== 'user/message' && type !== 'assistant/message'
+    && type !== 'tool/result' && type !== 'steering/message') return
+  assertMessageEventShape(event, `seed ${type} at index ${index}`)
+}
+
+/** Validate only the event-specific invariants needed to safely replay a message. */
+function assertMessageEventShape(event: Record<string, unknown>, subject: string): void {
+  const type = event['type']
+  if (type !== 'user/message' && type !== 'assistant/message'
+    && type !== 'tool/result' && type !== 'steering/message') return
+  const data = event['data']
+  const record = typeof data === 'object' && data !== null
+    ? data as Record<string, unknown>
+    : undefined
+  const message = type === 'user/message' ? record : record?.['message']
+  if (typeof message !== 'object' || message === null
+    || typeof (message as Record<string, unknown>)['id'] !== 'string'
+    || (message as Record<string, unknown>)['id'] === '') {
+    throw new Error(`${subject} lacks an identified message`)
+  }
+  const messageRecord = message as Record<string, unknown>
+  const expectedRole = type === 'assistant/message' ? 'assistant' : 'user'
+  if (messageRecord['role'] !== expectedRole) {
+    throw new Error(`${subject} message must have role "${expectedRole}"`)
+  }
+  const source = messageRecord['source']
+  if (typeof source !== 'object' || source === null
+    || typeof (source as Record<string, unknown>)['kind'] !== 'string'
+    || (source as Record<string, unknown>)['kind'] === '') {
+    throw new Error(`${subject} message has invalid source`)
+  }
+  if (!Array.isArray(messageRecord['content'])) {
+    throw new Error(`${subject} message has invalid content`)
+  }
+  const sourceRecord = source as Record<string, unknown>
+  if (type === 'assistant/message') {
+    if (sourceRecord['kind'] !== 'model' || !hasProviderModel(sourceRecord)) {
+      throw new Error(`${subject} message must have model source`)
+    }
+    return
+  }
+  if (type !== 'tool/result') return
+  if (sourceRecord['kind'] !== 'tool'
+    || typeof sourceRecord['callId'] !== 'string'
+    || sourceRecord['callId'] === '') {
+    throw new Error(`${subject} message must have tool source`)
+  }
+  const content = messageRecord['content'] as unknown[]
+  const block = content[0]
+  if (content.length !== 1 || typeof block !== 'object' || block === null
+    || (block as Record<string, unknown>)['type'] !== 'tool-result'
+    || !Array.isArray((block as Record<string, unknown>)['content'])) {
+    throw new Error(`${subject} message must contain one tool-result block`)
+  }
+  if ((block as Record<string, unknown>)['toolCallId'] !== sourceRecord['callId']) {
+    throw new Error(`${subject} message has mismatched tool call ids`)
  }
 }

@@ -525,7 +609,7 @@ export class Session {
      // A surface node is one of the five message-producing types, but an
      // empty-content assistant/message (a max-tokens step that hosts only
      // usage) derives to null and must not enter the transcript.
-      if (msg) this.derived.push(deepFreeze(msg))
+      if (msg) this.derived.push(msg)
    }
    this.derivedNodes = nodes.length
    return [...this.derived]
@@ -538,10 +622,9 @@ export class Session {
   * The per-node pure function {@link deriveMessages} folds over the surface;
   * an external reconstructor (or the dev invariant) folds the same function
   * over a log prefix's surface to rebuild the exact messages any request was
-   * built from (the reconstructability Agent Note). The returned message wrapper is
-   * fresh; its content reuses the logged event's already deep-frozen durable
-   * data, so changing the wrapper cannot rewrite the log and changing content
-   * throws.
+   * built from (the reconstructability Agent Note). The returned message is
+   * the already frozen message nested in the event wrapper and shared by
+   * delivery, durable history, and model requests.
   * @param event - the event to project.
   * @returns the derived message, or null when the event produces none.
   */
@@ -553,30 +636,28 @@ export class Session {
    switch (event.type) {
      // Ordinary prompts, injected context, and mid-turn steering project
      // identically in user role: the event's model-facing content stays
-      // verbatim. The message's `source` and steering's `turn` are log-only. Do NOT
+      // verbatim. Steering's `turn` is log-only. Do NOT
      // re-add per-type framing (e.g. `<context>`/`<steering>`) here: framing is
      // caller-owned — a producer bakes it into `content`, as workspace-context
      // does with `<system-reminder>` — or, if reintroduced, must be driven by
      // the event `meta` map and a dedicated renderer, keeping this projection a
      // verbatim pass-through. See the deferred design note in
      // ../../../../.agents/notes/implemented/simplification/2026-07-20-unwrap-injected-content-envelopes.md
-      case 'user/message':
+      case 'user/message': {
+        return event.data
+      }
      case 'steering/message': {
-        return { role: 'user', content: event.data.content }
+        return event.data.message
      }
      case 'assistant/message': {
        // Skip an empty-content assistant/message: it exists only to host a
        // max-tokens step's usage and must not inject a content-less assistant
        // turn into the provider transcript.
-        if (event.data.content.length === 0) return null
-        return { role: 'assistant', content: event.data.content, provenance: event.data.provenance }
+        if (event.data.message.content.length === 0) return null
+        return event.data.message
      }
      case 'tool/result': {
-        const { callId, content, isError } = event.data
-        return {
-          role: 'user',
-          content: [{ type: 'tool-result', toolCallId: callId, content, isError }],
-        }
+        return event.data.message
      }
      default:
        // A non-surface event (boundary, chunk, log-only record) projects to
--- a/packages/core/session/src/invariant.ts
+++ b/packages/core/session/src/invariant.ts
@@ -134,11 +134,12 @@ function validateEvent(
        break
      }
      requireOpenStep(trace, 'tool/result', event.data.turn, event.data.step, fail)
-      const syntheticNotStarted = event.data.isError && event.data.error?.code === TOOL_NOT_STARTED
-      if (!trace.pendingCalls.has(event.data.callId) && !syntheticNotStarted) {
-        fail(`tool/result for ${event.data.callId} with no prior tool/call in this step`)
+      const callId = event.data.message.source.callId
+      const syntheticNotStarted = event.data.message.content[0].isError === true && event.data.error?.code === TOOL_NOT_STARTED
+      if (!trace.pendingCalls.has(callId) && !syntheticNotStarted) {
+        fail(`tool/result for ${callId} with no prior tool/call in this step`)
      }
-      pendingCalls = { kind: 'delete', callId: event.data.callId }
+      pendingCalls = { kind: 'delete', callId }
      break
    }
    case 'user/message':
--- a/packages/core/session/src/repair.ts
+++ b/packages/core/session/src/repair.ts
@@ -5,7 +5,8 @@
 * @module @deepseek-ai/dsh-session/repair
 */

-import type { CallId } from '@deepseek-ai/dsh-llm'
+import { MessageId, freezeMessage, type CallId } from '@deepseek-ai/dsh-llm'
+import type { ToolResultMessage } from '@deepseek-ai/dsh-llm'
 import type { SessionEvent } from './types.ts'

 /** Recovery code for an assistant tool request that never reached a recorded call start. */
@@ -51,7 +52,7 @@ export function interruptedTurnClosers(events: readonly SessionEvent[]): Session
      case 'assistant/message':
        // The assistant message carries the tool-call blocks; each is pending
        // until a tool/result event with the same callId is logged.
-        for (const block of event.data.content) {
+        for (const block of event.data.message.content) {
          if (block.type === 'tool-call') pendingCalls.set(block.id, { step: event.data.step })
        }
        break
@@ -65,7 +66,7 @@ export function interruptedTurnClosers(events: readonly SessionEvent[]): Session
        }
        break
      case 'tool/result':
-        pendingCalls.delete(event.data.callId)
+        pendingCalls.delete(event.data.message.source.callId)
        break
      // Other event types do not move the turn/step boundary cursor.
      default:
@@ -89,6 +90,22 @@ export function interruptedTurnClosers(events: readonly SessionEvent[]): Session
  // and Map insertion order preserves their transcript order.
  for (const [callId, { step, callSeq }] of pendingCalls) {
    const started = callSeq !== undefined
+    const message: ToolResultMessage = freezeMessage({
+      id: MessageId(`interrupted-tool-result-${callId}-${seq}`),
+      role: 'user',
+      source: { kind: 'tool', callId },
+      content: [{
+        type: 'tool-result',
+        toolCallId: callId,
+        isError: true,
+        content: [{
+          type: 'text',
+          text: started
+            ? 'The tool call was interrupted after it was recorded, but no result was durably recorded. Its outcome is unknown. Decide whether to retry from the tool semantics: retry only if the operation is read-only or idempotent; if it may have side effects, first verify external state or ask the user. Do not retry blindly.'
+            : 'The tool call was interrupted before the Harness recorded it as started. Retry it if it is still needed.',
+        }],
+      }],
+    })
    closers.push({
      type: 'tool/result',
      seq: seq++,
@@ -96,14 +113,7 @@ export function interruptedTurnClosers(events: readonly SessionEvent[]): Session
      data: {
        turn: openTurn,
        step,
-        callId,
-        content: [{
-          type: 'text',
-          text: started
-            ? 'The tool call was interrupted after it was recorded, but no result was durably recorded. Its outcome is unknown. Decide whether to retry from the tool semantics: retry only if the operation is read-only or idempotent; if it may have side effects, first verify external state or ask the user. Do not retry blindly.'
-            : 'The tool call was interrupted before the Harness recorded it as started. Retry it if it is still needed.',
-        }],
-        isError: true,
+        message,
        error: started
          ? { name: 'ToolOutcomeUnknownError', code: TOOL_OUTCOME_UNKNOWN }
          : { name: 'ToolNotStartedError', code: TOOL_NOT_STARTED },
--- a/packages/core/session/src/surface.ts
+++ b/packages/core/session/src/surface.ts
@@ -224,8 +224,16 @@ function assertToolResultRewrite(
    }
    const originalRest = { ...original.data } as Record<string, unknown>
    const replacementRest = { ...event.data } as Record<string, unknown>
-    delete originalRest['content']
-    delete replacementRest['content']
+    const originalResult = original.data.message.content[0]
+    const replacementResult = event.data.message.content[0]
+    originalRest['message'] = {
+      ...original.data.message,
+      content: [{ ...originalResult, content: null }],
+    }
+    replacementRest['message'] = {
+      ...event.data.message,
+      content: [{ ...replacementResult, content: null }],
+    }
    if (!isDeepEqualJson(originalRest, replacementRest)) {
      throw new Error('tool/result surface replacement may change only content')
    }
--- a/packages/core/session/src/types.ts
+++ b/packages/core/session/src/types.ts
@@ -1,5 +1,16 @@
 import type { Branded } from '@deepseek-ai/dsh-brand'
-import type { AssistantProvenance, CallId, ContentBlock, LlmCallConfig, LlmFailure, MessageSource, StreamChunk, TokenUsage, ToolSchema } from '@deepseek-ai/dsh-llm'
+import type {
+  AssistantMessage,
+  CallId,
+  LlmCallConfig,
+  LlmFailure,
+  MessageSource,
+  StreamChunk,
+  TokenUsage,
+  ToolResultMessage,
+  ToolSchema,
+  UserMessage,
+} from '@deepseek-ai/dsh-llm'
 import type { JsonValue } from './json.ts'

 /** Identifies one session in the store (and its persistence artifacts). */
@@ -186,20 +197,6 @@ export interface EpochHeader {
 */
 export type RequestHeaderReason = 'initial' | 'resume' | 'change'

-/**
- * Shared payload for user, injected-context, and steering messages. A
- * direct human prompt, a synthetic `agent.inject()` context, and mid-turn
- * steering all project into the model transcript as verbatim user-role content;
- * they are told apart by `source` (a non-`user` kind marks injected context),
- * not by event type.
- */
-export interface UserMessageData {
-  /** Exact model-facing blocks. */
-  content: ContentBlock[]
-  /** Producer provenance. */
-  source: MessageSource
-}
-
 /**
 * The merge-extensible, append-only source of truth for an agent interaction.
 * Message history is derived from this log. Every event is lossless JSON and
@@ -230,7 +227,7 @@ export interface SessionEventMap {
   * project their `content` verbatim; `source` tells them apart. An idle
   * injection may append this event between turns without running the model.
   */
-  'user/message': UserMessageData
+  'user/message': UserMessage
  /** Raw stream chunk — token-level replay fidelity. */
  'assistant/chunk': { turn: number; step: number; chunk: StreamChunk }
  /**
@@ -239,7 +236,7 @@ export interface SessionEventMap {
   * the model output and its accounting travel together (there is no separate
   * usage record). `usage` is absent when the adapter reported none.
   */
-  'assistant/message': { turn: number; step: number; content: ContentBlock[]; provenance: AssistantProvenance; usage?: TokenUsage }
+  'assistant/message': { turn: number; step: number; message: AssistantMessage; usage?: TokenUsage }
  /**
   * The model requested one tool invocation: `name` with the raw `arguments`
   * JSON string exactly as the model produced it (unparsed). `callId` pairs the
@@ -260,14 +257,12 @@ export interface SessionEventMap {
  'tool/result': {
    turn: number
    step: number
-    callId: CallId
-    content: ContentBlock[]
-    isError: boolean
+    message: ToolResultMessage
    error?: { name: string; code: string }
    meta?: JsonValue
  }
  /** Steering content injected between steps of a running turn. */
-  'steering/message': UserMessageData & { turn: number }
+  'steering/message': { turn: number; message: UserMessage }
  /** Whole-list snapshot; latest write wins on replay. Log-only UI state; never derived history. */
  'todo/write': { todos: TodoItem[] }
  /**
--- a/packages/core/session/tests/derived-cache.spec.ts
+++ b/packages/core/session/tests/derived-cache.spec.ts
@@ -1,3 +1,4 @@
+import { createUserMessage, createMessage } from '@deepseek-ai/dsh-llm'
 /**
 * Derived-message cache contract against a scratch oracle: project new nodes
 * once, rebuild on surface replacements, return fresh arrays over shared
@@ -8,7 +9,9 @@ import { describe, expect, it } from 'vitest'
 import { Session, SessionId } from '@deepseek-ai/dsh-session'

 function userText(session: Session, text: string): void {
-  session.append('user/message', { content: [{ type: 'text', text }], source: { kind: 'user' } }, { surfaceOp: 'append' })
+  session.append('user/message', createUserMessage({
+    content: [{ type: 'text', text }], source: { kind: 'user' },
+  }), { surfaceOp: 'append' })
 }

 /** From-scratch oracle: replay the log into a fresh session and derive. */
@@ -23,9 +26,30 @@ describe('derived-message cache', () => {
    userText(session, 'one')
    expect(session.deriveMessages()).toEqual(scratch(session))
    userText(session, 'two')
-    session.append('assistant/message', { provenance: { provider: 'mock', model: 'mock' }, turn: 1, step: 1, content: [{ type: 'text', text: 'reply' }] }, { surfaceOp: 'append' })
+    session.append('assistant/message', {
+      turn: 1, step: 1,
+      message: createMessage({
+        role: 'assistant',
+        content: [{ type: 'text', text: 'reply' }],
+        source: {
+          kind: 'model',
+          ...{ provider: 'mock', model: 'mock' },
+        },
+      }),
+    }, { surfaceOp: 'append' })
    expect(session.deriveMessages()).toEqual(scratch(session))
-    session.append('assistant/message', { provenance: { provider: 'mock', model: 'mock' }, turn: 1, step: 2, content: [], usage: { inputTokens: 1, outputTokens: 0 } }, { surfaceOp: 'append' })
+    session.append('assistant/message', {
+      turn: 1, step: 2,
+      message: createMessage({
+        role: 'assistant',
+        content: [],
+        source: {
+          kind: 'model',
+          ...{ provider: 'mock', model: 'mock' },
+        },
+      }),
+      usage: { inputTokens: 1, outputTokens: 0 },
+    }, { surfaceOp: 'append' })
    expect(session.deriveMessages()).toEqual(scratch(session))
  })

@@ -38,9 +62,9 @@ describe('derived-message cache', () => {
    expect(beforeReplace).toHaveLength(2)

    const nodes = session.surface.nodes
-    session.append('user/message', {
+    session.append('user/message', createUserMessage({
      content: [{ type: 'text', text: 'summary' }], source: { kind: 'plugin', plugin: 'compact' },
-    }, { surfaceOp: { op: 'replace', start: nodes[0]!, end: nodes[1]! }, sourceEventSeqs: [nodes[0]!, nodes[1]!] })
+    }), { surfaceOp: { op: 'replace', start: nodes[0]!, end: nodes[1]! }, sourceEventSeqs: [nodes[0]!, nodes[1]!] })

    expect(session.deriveMessages()).toHaveLength(1)
    expect(session.deriveMessages()).toEqual(scratch(session))
@@ -67,7 +91,9 @@ describe('Session.deriveEventMessage — the per-event projection', () => {
  it('projects one appended event exactly as the full derivation projects its node', () => {
    const session = new Session(SessionId('per-event'))
    session.append('turn/start', { turn: 1, trigger: { kind: 'message', source: { kind: 'user' } } })
-    const event = session.append('user/message', { content: [{ type: 'text', text: 'hi' }], source: { kind: 'user' } }, { surfaceOp: 'append' })
+    const event = session.append('user/message', createUserMessage({
+      content: [{ type: 'text', text: 'hi' }], source: { kind: 'user' },
+    }), { surfaceOp: 'append' })
    // Full and per-event derivation share one projection.
    expect(session.deriveEventMessage(event)).toEqual(session.deriveMessages().at(-1))
  })
@@ -75,7 +101,9 @@ describe('Session.deriveEventMessage — the per-event projection', () => {
  it('reuses the logged event\'s already frozen content', () => {
    const session = new Session(SessionId('per-event-clone'))
    session.append('turn/start', { turn: 1, trigger: { kind: 'message', source: { kind: 'user' } } })
-    const event = session.append('user/message', { content: [{ type: 'text', text: 'orig' }], source: { kind: 'user' } }, { surfaceOp: 'append' })
+    const event = session.append('user/message', createUserMessage({
+      content: [{ type: 'text', text: 'orig' }], source: { kind: 'user' },
+    }), { surfaceOp: 'append' })
    const message = session.deriveEventMessage(event)!
    expect(message.content).toBe(event.data.content)
    expect(Object.isFrozen(message.content)).toBe(true)
@@ -89,7 +117,17 @@ describe('Session.deriveEventMessage — the per-event projection', () => {
    session.append('turn/start', { turn: 1, trigger: { kind: 'message', source: { kind: 'user' } } })
    const boundary = session.append('step/start', { turn: 1, step: 1 })
    expect(session.deriveEventMessage(boundary)).toBeNull()
-    const empty = session.append('assistant/message', { provenance: { provider: 'mock', model: 'mock' }, turn: 1, step: 1, content: [] }, { surfaceOp: 'append' })
+    const empty = session.append('assistant/message', {
+      turn: 1, step: 1,
+      message: createMessage({
+        role: 'assistant',
+        content: [],
+        source: {
+          kind: 'model',
+          ...{ provider: 'mock', model: 'mock' },
+        },
+      }),
+    }, { surfaceOp: 'append' })
    expect(session.deriveEventMessage(empty)).toBeNull()
  })
 })
--- a/packages/core/session/tests/fork.spec.ts
+++ b/packages/core/session/tests/fork.spec.ts
@@ -1,6 +1,6 @@
 import { describe, expect, it } from 'vitest'
 import { Context } from 'cordis'
-import { CallId } from '@deepseek-ai/dsh-llm'
+import { createUserMessage, CallId , createMessage } from '@deepseek-ai/dsh-llm'
 import SessionStore, { Session, SessionForkError, SessionId } from '@deepseek-ai/dsh-session'
 import type { SessionEvent, TurnEndReason } from '@deepseek-ai/dsh-session'

@@ -23,19 +23,19 @@ function appendClosedTurn(
  reason: TurnEndReason = { kind: 'completed' },
 ): void {
  session.append('turn/start', { turn, trigger: { kind: 'message', source: { kind: 'user' } } })
-  session.append('user/message', {
+  session.append('user/message', createUserMessage({
    content: [{ type: 'text', text }],
    source: { kind: 'user' },
-  }, { surfaceOp: 'append' })
+  }), { surfaceOp: 'append' })
  session.append('turn/end', { turn, reason })
 }

 function appendOpenTurn(session: Session, turn: number): void {
  session.append('turn/start', { turn, trigger: { kind: 'message', source: { kind: 'user' } } })
-  session.append('user/message', {
+  session.append('user/message', createUserMessage({
    content: [{ type: 'text', text: `open ${turn}` }],
    source: { kind: 'user' },
-  }, { surfaceOp: 'append' })
+  }), { surfaceOp: 'append' })
 }

 function firstUserMessage(events: readonly SessionEvent[]): SessionEvent<'user/message'> {
@@ -116,7 +116,12 @@ describe('SessionStore.fork', () => {

    expect(child.events).toEqual(source.events.slice(0, firstBoundary + 1))
    expect(child.header.seedLength).toBe(firstBoundary + 1)
-    expect(child.deriveMessages()).toEqual([{ role: 'user', content: [{ type: 'text', text: 'first' }] }])
+    expect(child.deriveMessages()).toEqual([{
+      id: expect.any(String) as unknown,
+      role: 'user',
+      content: [{ type: 'text', text: 'first' }],
+      source: { kind: 'user' },
+    }])
  })

  it('accepts every turn/end reason as an explicit fork boundary', async () => {
@@ -210,23 +215,42 @@ describe('SessionStore.fork', () => {
      }],
      ['user/message', (session) => {
        session.append('turn/start', { turn: 1, trigger: { kind: 'message', source: { kind: 'user' } } })
-        session.append('user/message', { content: [{ type: 'text', text: 'open' }], source: { kind: 'user' } }, { surfaceOp: 'append' })
+        session.append('user/message', createUserMessage({
+          content: [{ type: 'text', text: 'open' }], source: { kind: 'user' },
+        }), { surfaceOp: 'append' })
        return lastSeq(session)
      }],
      ['assistant/message', (session) => {
        session.append('turn/start', { turn: 1, trigger: { kind: 'message', source: { kind: 'user' } } })
        session.append('step/start', { turn: 1, step: 1 })
-        session.append('assistant/message', { provenance: { provider: 'mock', model: 'mock' }, turn: 1, step: 1, content: [{ type: 'text', text: 'partial' }] }, { surfaceOp: 'append' })
+        session.append('assistant/message', {
+          turn: 1, step: 1,
+          message: createMessage({
+            role: 'assistant',
+            content: [{ type: 'text', text: 'partial' }],
+            source: {
+              kind: 'model',
+              ...{ provider: 'mock', model: 'mock' },
+            },
+          }),
+        }, { surfaceOp: 'append' })
        return lastSeq(session)
      }],
      ['tool/call', (session) => {
        const callId = CallId('call-open')
        session.append('turn/start', { turn: 1, trigger: { kind: 'message', source: { kind: 'user' } } })
        session.append('step/start', { turn: 1, step: 1 })
-        session.append('assistant/message', { provenance: { provider: 'mock', model: 'mock' },
+        session.append('assistant/message', {
          turn: 1,
          step: 1,
-          content: [{ type: 'tool-call', id: callId, name: 'bash', arguments: '{}' }],
+          message: createMessage({
+            role: 'assistant',
+            content: [{ type: 'tool-call', id: callId, name: 'bash', arguments: '{}' }],
+            source: {
+              kind: 'model',
+              ...{ provider: 'mock', model: 'mock' },
+            },
+          }),
        }, { surfaceOp: 'append' })
        session.append('tool/call', { turn: 1, step: 1, callId, name: 'bash', arguments: '{}' })
        return lastSeq(session)
--- a/packages/core/session/tests/invariant.spec.ts
+++ b/packages/core/session/tests/invariant.spec.ts
@@ -1,7 +1,7 @@
 import { describe, expect, it } from 'vitest'
 import { Context } from 'cordis'
 import { createScope, scopeTarget } from '@deepseek-ai/dsh-scope'
-import { CallId } from '@deepseek-ai/dsh-llm'
+import { createUserMessage, CallId, createMessage, createToolResultMessage, freezeMessage } from '@deepseek-ai/dsh-llm'
 import SessionStore, { SessionId, TOOL_NOT_STARTED } from '@deepseek-ai/dsh-session'
 import * as SessionInvariant from '@deepseek-ai/dsh-session/invariant'
 import InvariantService, { InvariantError } from '@deepseek-ai/dsh-invariants'
@@ -36,17 +36,32 @@ describe('session-log invariants', () => {
    const session = ctx.sessions.create()
    expect(() => {
      session.append('turn/start', { turn: 1, trigger: { kind: 'message', source: { kind: 'user' } } })
-      session.append('user/message', { content: [{ type: 'text', text: 'hi' }], source: { kind: 'user' } }, { surfaceOp: 'append' })
+      session.append('user/message', createUserMessage({
+        content: [{ type: 'text', text: 'hi' }], source: { kind: 'user' },
+      }), { surfaceOp: 'append' })
      session.append('step/start', { turn: 1, step: 1 })
      session.append('assistant/chunk', { turn: 1, step: 1, chunk: { type: 'text-delta', index: 0, text: 'h' } })
      session.append('assistant/message', {
-        provenance: { provider: 'mock', model: 'mock' },
        turn: 1,
        step: 1,
-        content: [{ type: 'tool-call', id: CallId('c1'), name: 'echo', arguments: '{}' }],
+        message: createMessage({
+          role: 'assistant',
+          content: [{ type: 'tool-call', id: CallId('c1'), name: 'echo', arguments: '{}' }],
+          source: {
+            kind: 'model',
+            ...{ provider: 'mock', model: 'mock' },
+          },
+        }),
      }, { surfaceOp: 'append' })
      session.append('tool/call', { turn: 1, step: 1, callId: CallId('c1'), name: 'echo', arguments: '{}' })
-      session.append('tool/result', { turn: 1, step: 1, callId: CallId('c1'), content: [], isError: false }, { surfaceOp: 'append' })
+      session.append('tool/result', {
+        turn: 1, step: 1,
+        message: createToolResultMessage({
+          callId: CallId('c1'),
+          content: [],
+          isError: false,
+        }),
+      }, { surfaceOp: 'append' })
      session.append('step/end', { turn: 1, step: 1 })
      session.append('turn/end', { turn: 1, reason: { kind: 'completed' } })
    }).not.toThrow()
@@ -118,14 +133,16 @@ describe('session-log invariants', () => {
      .toThrow(/expected turn 2, got 3/)

    const outside = (await setup()).ctx.sessions.create()
-    expect(() => outside.append('user/message', {
+    expect(() => outside.append('user/message', createUserMessage({
      content: [{ type: 'text', text: 'idle context' }],
      source: { kind: 'plugin', plugin: 'test' },
-    }, { surfaceOp: 'append' })).not.toThrow()
+    }), { surfaceOp: 'append' })).not.toThrow()
    expect(() => outside.append('steering/message', {
      turn: 1,
-      content: [{ type: 'text', text: 'go' }],
-      source: { kind: 'user' },
+      message: createUserMessage({
+        content: [{ type: 'text', text: 'go' }],
+        source: { kind: 'user' },
+      }),
    }, { surfaceOp: 'append' })).toThrow(/outside any open turn/)
    // The owning plugin decides whether a merge-extensible event is log-only.
    const appendUnknown = outside.append.bind(outside) as (type: string, data: unknown) => unknown
@@ -149,10 +166,16 @@ describe('session-log invariants', () => {
      .toThrow(/while step 1 is still open/)
    expect(() => nested.append('step/end', { turn: 1, step: 2 })).toThrow(/open is turn 1\/step 1/)
    expect(() => nested.append('assistant/message', {
-      provenance: { provider: 'mock', model: 'mock' },
      turn: 1,
      step: 2,
-      content: [],
+      message: createMessage({
+        role: 'assistant',
+        content: [],
+        source: {
+          kind: 'model',
+          ...{ provider: 'mock', model: 'mock' },
+        },
+      }),
    }, { surfaceOp: 'append' })).toThrow(/open is turn 1\/step 1/)

    const skipped = (await setup()).ctx.sessions.create()
@@ -178,9 +201,11 @@ describe('session-log invariants', () => {
    expect(() => tool.append('tool/result', {
      turn: 1,
      step: 1,
-      callId: CallId('ghost'),
-      content: [],
-      isError: false,
+      message: createToolResultMessage({
+        callId: CallId('ghost'),
+        content: [],
+        isError: false,
+      }),
    }, { surfaceOp: 'append' })).toThrow(/no prior tool\/call/)
  })

@@ -191,9 +216,11 @@ describe('session-log invariants', () => {
    expect(() => session.append('tool/result', {
      turn: 1,
      step: 1,
-      callId: CallId('closed'),
-      content: [],
-      isError: false,
+      message: createToolResultMessage({
+        callId: CallId('closed'),
+        content: [],
+        isError: false,
+      }),
    }, { surfaceOp: 'append' })).toThrow(/open is turn 1\/step null/)
  })

@@ -212,9 +239,11 @@ describe('session-log invariants', () => {
    const original = session.append('tool/result', {
      turn: 1,
      step: 1,
-      callId: CallId('rewrite'),
-      content: [{ type: 'text', text: 'original' }],
-      isError: false,
+      message: createToolResultMessage({
+        callId: CallId('rewrite'),
+        content: [{ type: 'text', text: 'original' }],
+        isError: false,
+      }),
    }, { surfaceOp: 'append' })
    session.append('step/end', { turn: 1, step: 1 })
    session.append('turn/end', { turn: 1, reason: { kind: 'completed' } })
@@ -222,7 +251,13 @@ describe('session-log invariants', () => {
    session.append('turn/start', { turn: 2, trigger: { kind: 'message', source: { kind: 'user' } } })
    expect(() => session.append('tool/result', {
      ...original.data,
-      content: [{ type: 'text', text: 'pruned' }],
+      message: freezeMessage({
+        ...original.data.message,
+        content: [{
+          ...original.data.message.content[0],
+          content: [{ type: 'text', text: 'pruned' }],
+        }],
+      }),
    }, {
      surfaceOp: { op: 'replace', start: original.seq, end: original.seq },
      sourceEventSeqs: [original.seq],
@@ -244,16 +279,24 @@ describe('session-log invariants', () => {
    const original = session.append('tool/result', {
      turn: 1,
      step: 1,
-      callId: CallId('rewrite'),
-      content: [{ type: 'text', text: 'original' }],
-      isError: false,
+      message: createToolResultMessage({
+        callId: CallId('rewrite'),
+        content: [{ type: 'text', text: 'original' }],
+        isError: false,
+      }),
    }, { surfaceOp: 'append' })
    session.append('step/end', { turn: 1, step: 1 })
    session.append('turn/end', { turn: 1, reason: { kind: 'completed' } })

    expect(() => session.append('tool/result', {
      ...original.data,
-      content: [{ type: 'text', text: 'pruned' }],
+      message: freezeMessage({
+        ...original.data.message,
+        content: [{
+          ...original.data.message.content[0],
+          content: [{ type: 'text', text: 'pruned' }],
+        }],
+      }),
    }, {
      surfaceOp: { op: 'replace', start: original.seq, end: original.seq },
      sourceEventSeqs: [original.seq],
@@ -268,9 +311,11 @@ describe('session-log invariants', () => {
      repaired.append('tool/result', {
        turn: 1,
        step: 1,
-        callId: CallId('crashed'),
-        content: [],
-        isError: true,
+        message: createToolResultMessage({
+          callId: CallId('crashed'),
+          content: [],
+          isError: true,
+        }),
        error: { name: 'ToolNotStartedError', code: TOOL_NOT_STARTED },
      }, { surfaceOp: 'append' })
      repaired.append('step/end', { turn: 1, step: 1 })
@@ -298,9 +343,11 @@ describe('session-log invariants', () => {
    expect(() => session.append('tool/result', {
      turn: 1,
      step: 2,
-      callId: CallId('c1'),
-      content: [],
-      isError: false,
+      message: createToolResultMessage({
+        callId: CallId('c1'),
+        content: [],
+        isError: false,
+      }),
    }, { surfaceOp: 'append' })).toThrow(/no prior tool\/call in this step/)
  })

--- a/packages/core/session/tests/properties.spec.ts
+++ b/packages/core/session/tests/properties.spec.ts
@@ -9,7 +9,7 @@

 import { describe, expect, it } from 'vitest'
 import fc from 'fast-check'
-import { CallId } from '@deepseek-ai/dsh-llm'
+import { createUserMessage, CallId , createMessage, createToolResultMessage } from '@deepseek-ai/dsh-llm'
 import { Session, SessionId } from '@deepseek-ai/dsh-session'
 import type { SessionEventMap, SessionEventType, SurfaceIntent } from '@deepseek-ai/dsh-session'

@@ -27,11 +27,45 @@ const textContentArb = fc.array(
 // A message-producing event (these DO affect derived history). Each carries an
 // explicit `surfaceOp: 'append'` intent — the marker the real loop passes.
 const messageEventArb: fc.Arbitrary<Appendable> = fc.oneof(
-  textContentArb.map((content): Appendable => ({ type: 'user/message', data: { content, source: { kind: 'user' } }, intent: { surfaceOp: 'append' } })),
-  textContentArb.map((content): Appendable => ({ type: 'assistant/message', data: { turn: 1, step: 1, content, provenance: { provider: 'mock', model: 'mock' } }, intent: { surfaceOp: 'append' } })),
-  textContentArb.map((content): Appendable => ({ type: 'assistant/message', data: { turn: 1, step: 1, content, provenance: { provider: 'mock', model: 'mock' }, usage: { inputTokens: 1, outputTokens: 1 } }, intent: { surfaceOp: 'append' } })),
+  textContentArb.map((content): Appendable => ({ type: 'user/message', data: createUserMessage({
+    content, source: { kind: 'user' },
+  }), intent: { surfaceOp: 'append' } })),
+  textContentArb.map((content): Appendable => ({
+    type: 'assistant/message',
+    data: {
+      turn: 1,
+      step: 1,
+      message: createMessage({
+        role: 'assistant',
+        content,
+        source: { kind: 'model', provider: 'mock', model: 'mock' },
+      }),
+    },
+    intent: { surfaceOp: 'append' },
+  })),
+  textContentArb.map((content): Appendable => ({
+    type: 'assistant/message',
+    data: {
+      turn: 1,
+      step: 1,
+      message: createMessage({
+        role: 'assistant',
+        content,
+        source: { kind: 'model', provider: 'mock', model: 'mock' },
+      }),
+      usage: { inputTokens: 1, outputTokens: 1 },
+    },
+    intent: { surfaceOp: 'append' },
+  })),
  fc.record({ id: fc.string({ minLength: 1 }), content: textContentArb, isError: fc.boolean() })
-    .map((r): Appendable => ({ type: 'tool/result', data: { turn: 1, step: 1, callId: CallId(r.id), content: r.content, isError: r.isError }, intent: { surfaceOp: 'append' } })),
+    .map((r): Appendable => ({ type: 'tool/result', data: {
+      turn: 1, step: 1,
+      message: createToolResultMessage({
+        callId: CallId(r.id),
+        content: r.content,
+        isError: r.isError,
+      }),
+    }, intent: { surfaceOp: 'append' } })),
 )

 // A non-message event (trace/replay data — must NOT affect derived history).
--- a/packages/core/session/tests/repair.spec.ts
+++ b/packages/core/session/tests/repair.spec.ts
@@ -1,5 +1,5 @@
 import { describe, expect, it } from 'vitest'
-import { CallId } from '@deepseek-ai/dsh-llm'
+import { CallId , createMessage, createToolResultMessage } from '@deepseek-ai/dsh-llm'
 import { interruptedTurnClosers, TOOL_NOT_STARTED, TOOL_OUTCOME_UNKNOWN } from '../src/index.ts'
 import type { SessionEvent, SurfaceEvent } from '../src/index.ts'

@@ -51,10 +51,20 @@ describe('interruptedTurnClosers', () => {
    const events: SessionEvent[] = [
      userTurnStart(2, 0),
      { type: 'step/start', seq: 1, time: 1, data: { turn: 2, step: 1 } },
-      { type: 'assistant/message', seq: 2, time: 2, data: { turn: 2, step: 1, content: [
-        { type: 'text', text: 'calling a tool' },
-        { type: 'tool-call', id: CallId('call-1'), name: 'bash', arguments: '{}' },
-      ], provenance: { provider: 'mock', model: 'mock' } } },
+      { type: 'assistant/message', seq: 2, time: 2, data: {
+        turn: 2, step: 1,
+        message: createMessage({
+          role: 'assistant',
+          content: [
+            { type: 'text', text: 'calling a tool' },
+            { type: 'tool-call', id: CallId('call-1'), name: 'bash', arguments: '{}' },
+          ],
+          source: {
+            kind: 'model',
+            ...{ provider: 'mock', model: 'mock' },
+          },
+        }),
+      } },
    ]
    const closers = interruptedTurnClosers(events)
    // tool/result (for the orphaned call) → step/end → turn/end, contiguous seqs.
@@ -62,9 +72,15 @@ describe('interruptedTurnClosers', () => {
    expect(closers.map(e => e.seq)).toEqual([3, 4, 5])
    const result = closers[0]!
    expect(result.type === 'tool/result' && result.data).toMatchObject({
-      turn: 2, step: 1, callId: CallId('call-1'), isError: true, error: { code: TOOL_NOT_STARTED },
+      turn: 2,
+      step: 1,
+      message: {
+        source: { callId: CallId('call-1') },
+        content: [{ isError: true }],
+      },
+      error: { code: TOOL_NOT_STARTED },
    })
-    expect(result.type === 'tool/result' && result.data.content).toEqual([{
+    expect(result.type === 'tool/result' && result.data.message.content[0].content).toEqual([{
      type: 'text', text: 'The tool call was interrupted before the Harness recorded it as started. Retry it if it is still needed.',
    }])
  })
@@ -73,10 +89,27 @@ describe('interruptedTurnClosers', () => {
    const events: SessionEvent[] = [
      userTurnStart(2, 0),
      { type: 'step/start', seq: 1, time: 1, data: { turn: 2, step: 1 } },
-      { type: 'assistant/message', seq: 2, time: 2, data: { turn: 2, step: 1, content: [
-        { type: 'tool-call', id: CallId('call-1'), name: 'bash', arguments: '{}' },
-      ], provenance: { provider: 'mock', model: 'mock' } } },
-      { type: 'tool/result', seq: 3, time: 3, data: { turn: 2, step: 1, callId: CallId('call-1'), content: [{ type: 'text', text: 'ok' }], isError: false } },
+      { type: 'assistant/message', seq: 2, time: 2, data: {
+        turn: 2, step: 1,
+        message: createMessage({
+          role: 'assistant',
+          content: [
+            { type: 'tool-call', id: CallId('call-1'), name: 'bash', arguments: '{}' },
+          ],
+          source: {
+            kind: 'model',
+            ...{ provider: 'mock', model: 'mock' },
+          },
+        }),
+      } },
+      { type: 'tool/result', seq: 3, time: 3, data: {
+        turn: 2, step: 1,
+        message: createToolResultMessage({
+          callId: CallId('call-1'),
+          content: [{ type: 'text', text: 'ok' }],
+          isError: false,
+        }),
+      } },
    ]
    // The call is answered, so only the open step + turn need closing.
    const closers = interruptedTurnClosers(events)
@@ -87,9 +120,19 @@ describe('interruptedTurnClosers', () => {
    const events: SessionEvent[] = [
      userTurnStart(2, 0),
      { type: 'step/start', seq: 1, time: 1, data: { turn: 2, step: 1 } },
-      { type: 'assistant/message', seq: 2, time: 2, data: { turn: 2, step: 1, content: [
-        { type: 'tool-call', id: CallId('call-1'), name: 'bash', arguments: '{}' },
-      ], provenance: { provider: 'mock', model: 'mock' } } },
+      { type: 'assistant/message', seq: 2, time: 2, data: {
+        turn: 2, step: 1,
+        message: createMessage({
+          role: 'assistant',
+          content: [
+            { type: 'tool-call', id: CallId('call-1'), name: 'bash', arguments: '{}' },
+          ],
+          source: {
+            kind: 'model',
+            ...{ provider: 'mock', model: 'mock' },
+          },
+        }),
+      } },
      { type: 'step/end', seq: 3, time: 3, data: { turn: 2, step: 1 } },
    ]

@@ -104,48 +147,102 @@ describe('interruptedTurnClosers', () => {
    const events: SessionEvent[] = [
      userTurnStart(1, 0),
      { type: 'step/start', seq: 1, time: 1, data: { turn: 1, step: 1 } },
-      { type: 'assistant/message', seq: 2, time: 2, data: { turn: 1, step: 1, content: [
-        { type: 'tool-call', id: CallId('old-call'), name: 'bash', arguments: '{}' },
-      ], provenance: { provider: 'mock', model: 'mock' } } },
-      { type: 'tool/result', seq: 3, time: 3, data: { turn: 1, step: 1, callId: CallId('old-call'), content: [], isError: false } },
+      { type: 'assistant/message', seq: 2, time: 2, data: {
+        turn: 1, step: 1,
+        message: createMessage({
+          role: 'assistant',
+          content: [
+            { type: 'tool-call', id: CallId('old-call'), name: 'bash', arguments: '{}' },
+          ],
+          source: {
+            kind: 'model',
+            ...{ provider: 'mock', model: 'mock' },
+          },
+        }),
+      } },
+      { type: 'tool/result', seq: 3, time: 3, data: {
+        turn: 1, step: 1,
+        message: createToolResultMessage({
+          callId: CallId('old-call'),
+          content: [],
+          isError: false,
+        }),
+      } },
      { type: 'step/end', seq: 4, time: 4, data: { turn: 1, step: 1 } },
      { type: 'turn/end', seq: 5, time: 5, data: { turn: 1, reason: { kind: 'completed' } } },
      userTurnStart(2, 6),
      { type: 'step/start', seq: 7, time: 7, data: { turn: 2, step: 1 } },
-      { type: 'assistant/message', seq: 8, time: 8, data: { turn: 2, step: 1, content: [
-        { type: 'tool-call', id: CallId('new-call'), name: 'bash', arguments: '{}' },
-      ], provenance: { provider: 'mock', model: 'mock' } } },
+      { type: 'assistant/message', seq: 8, time: 8, data: {
+        turn: 2, step: 1,
+        message: createMessage({
+          role: 'assistant',
+          content: [
+            { type: 'tool-call', id: CallId('new-call'), name: 'bash', arguments: '{}' },
+          ],
+          source: {
+            kind: 'model',
+            ...{ provider: 'mock', model: 'mock' },
+          },
+        }),
+      } },
    ]
    const closers = interruptedTurnClosers(events)
    expect(closers.map(e => e.type)).toEqual(['tool/result', 'step/end', 'turn/end'])
    const result = closers[0]!
-    expect(result.type === 'tool/result' && result.data.callId).toBe('new-call')
+    expect(result.type === 'tool/result' && result.data.message.source.callId).toBe('new-call')
  })

  it('synthesizes a result for each of multiple unanswered calls, in log order', () => {
    const events: SessionEvent[] = [
      userTurnStart(1, 0),
      { type: 'step/start', seq: 1, time: 1, data: { turn: 1, step: 1 } },
-      { type: 'assistant/message', seq: 2, time: 2, data: { turn: 1, step: 1, content: [
-        { type: 'tool-call', id: CallId('call-a'), name: 'bash', arguments: '{}' },
-        { type: 'tool-call', id: CallId('call-b'), name: 'bash', arguments: '{}' },
-      ], provenance: { provider: 'mock', model: 'mock' } } },
+      { type: 'assistant/message', seq: 2, time: 2, data: {
+        turn: 1, step: 1,
+        message: createMessage({
+          role: 'assistant',
+          content: [
+            { type: 'tool-call', id: CallId('call-a'), name: 'bash', arguments: '{}' },
+            { type: 'tool-call', id: CallId('call-b'), name: 'bash', arguments: '{}' },
+          ],
+          source: {
+            kind: 'model',
+            ...{ provider: 'mock', model: 'mock' },
+          },
+        }),
+      } },
      // call-a got answered before the crash; call-b did not.
-      { type: 'tool/result', seq: 3, time: 3, data: { turn: 1, step: 1, callId: CallId('call-a'), content: [], isError: false } },
+      { type: 'tool/result', seq: 3, time: 3, data: {
+        turn: 1, step: 1,
+        message: createToolResultMessage({
+          callId: CallId('call-a'),
+          content: [],
+          isError: false,
+        }),
+      } },
    ]
    const closers = interruptedTurnClosers(events)
    expect(closers.map(e => e.type)).toEqual(['tool/result', 'step/end', 'turn/end'])
    const result = closers[0]!
-    expect(result.type === 'tool/result' && result.data.callId).toBe('call-b')
+    expect(result.type === 'tool/result' && result.data.message.source.callId).toBe('call-b')
  })

  it('synthesized tool/result carries surfaceOp and sourceEventSeqs when tool/call was logged', () => {
    const events: SessionEvent[] = [
      userTurnStart(1, 0),
      { type: 'step/start', seq: 1, time: 1, data: { turn: 1, step: 1 } },
-      { type: 'assistant/message', seq: 2, time: 2, data: { turn: 1, step: 1, content: [
-        { type: 'tool-call', id: CallId('call-1'), name: 'bash', arguments: '{}' },
-      ], provenance: { provider: 'mock', model: 'mock' } } },
+      { type: 'assistant/message', seq: 2, time: 2, data: {
+        turn: 1, step: 1,
+        message: createMessage({
+          role: 'assistant',
+          content: [
+            { type: 'tool-call', id: CallId('call-1'), name: 'bash', arguments: '{}' },
+          ],
+          source: {
+            kind: 'model',
+            ...{ provider: 'mock', model: 'mock' },
+          },
+        }),
+      } },
      { type: 'tool/call', seq: 3, time: 3, data: { turn: 1, step: 1, callId: CallId('call-1'), name: 'bash', arguments: '{}' } },
    ]
    const closers = interruptedTurnClosers(events)
@@ -156,11 +253,11 @@ describe('interruptedTurnClosers', () => {
    expect(result.type === 'tool/result' && result.data.error).toEqual({
      name: 'ToolOutcomeUnknownError', code: TOOL_OUTCOME_UNKNOWN,
    })
-    if (result.type !== 'tool/result' || result.data.content[0]?.type !== 'text') {
+    if (result.type !== 'tool/result' || result.data.message.content[0].content[0]?.type !== 'text') {
      throw new Error('expected a text tool result')
    }
-    expect(result.data.content[0].text).toContain('retry only if the operation is read-only or idempotent')
-    expect(result.data.content[0].text).toContain('first verify external state or ask the user')
+    expect(result.data.message.content[0].content[0].text).toContain('retry only if the operation is read-only or idempotent')
+    expect(result.data.message.content[0].content[0].text).toContain('first verify external state or ask the user')
  })

  it('handles tool/call without a matching assistant/message entry gracefully', () => {
--- a/packages/core/session/tests/request-header.spec.ts
+++ b/packages/core/session/tests/request-header.spec.ts
@@ -3,8 +3,8 @@
 import { describe, expect, it } from 'vitest'
 import { Session, SessionId, canonicalHeader, foldRequestHeader, headerEquals } from '@deepseek-ai/dsh-session'
 import type { EpochHeader, SessionEvent } from '@deepseek-ai/dsh-session'
+import { createUserMessage, ReasoningEffortId } from '@deepseek-ai/dsh-llm'
 import type { ToolSchema } from '@deepseek-ai/dsh-llm'
-import { ReasoningEffortId } from '@deepseek-ai/dsh-llm'

 const CONFIG = { provider: 'mock', model: 'm' }

@@ -55,7 +55,9 @@ describe('foldRequestHeader', () => {
    const session = new Session(SessionId('fold'))
    session.append('turn/start', { turn: 1, trigger: { kind: 'message', source: { kind: 'user' } } })
    session.append('request/header', { header: { config: CONFIG, system: 'first' }, reason: 'initial' })
-    session.append('user/message', { content: [{ type: 'text', text: 'hi' }], source: { kind: 'user' } }, { surfaceOp: 'append' })
+    session.append('user/message', createUserMessage({
+      content: [{ type: 'text', text: 'hi' }], source: { kind: 'user' },
+    }), { surfaceOp: 'append' })
    session.append('request/header', { header: { config: { provider: 'mock', model: 'other' }, tools: [] }, reason: 'change' })
    expect(foldRequestHeader(session.events)).toEqual({ config: { provider: 'mock', model: 'other' } })
  })
--- a/packages/core/session/tests/session.spec.ts
+++ b/packages/core/session/tests/session.spec.ts
@@ -1,12 +1,13 @@
 import { describe, expect, expectTypeOf, it, vi } from 'vitest'
 import { Context } from 'cordis'
-import { CallId, ReasoningEffortId } from '@deepseek-ai/dsh-llm'
+import { createUserMessage, CallId, createMessage, createToolResultMessage, MessageId, ReasoningEffortId } from '@deepseek-ai/dsh-llm'
 import SessionStore, {
  findLastMessageTurnEnd,
  SESSION_FORMAT_VERSION,
  Session,
  SessionEvent,
  SessionId,
+  snapshotSessionEvent,
 } from '@deepseek-ai/dsh-session'
 import type { CreateSessionOptions, SessionEventType, SessionHeader, SessionSurface, TodoItem } from '@deepseek-ai/dsh-session'

@@ -22,16 +23,32 @@ describe('Session', () => {
  it('derives message history from the event log', () => {
    const session = new Session(SessionId('s1'))
    session.append('turn/start', { turn: 1, trigger: { kind: 'message', source: { kind: 'user' } } })
-    session.append('user/message', { content: [{ type: 'text', text: 'hello' }], source: { kind: 'user' } }, { surfaceOp: 'append' })
+    session.append('user/message', createUserMessage({
+      content: [{ type: 'text', text: 'hello' }], source: { kind: 'user' },
+    }), { surfaceOp: 'append' })
    session.append('assistant/chunk', { turn: 1, step: 1, chunk: { type: 'text-delta', index: 0, text: 'hi' } })
-    session.append('assistant/message', { provenance: { provider: 'mock', model: 'mock' },
+    session.append('assistant/message', {
      turn: 1, step: 1,
-      content: [
-        { type: 'text', text: 'let me check' },
-        { type: 'tool-call', id: CallId('c1'), name: 'echo', arguments: '{}' },
-      ],
+      message: createMessage({
+        role: 'assistant',
+        content: [
+          { type: 'text', text: 'let me check' },
+          { type: 'tool-call', id: CallId('c1'), name: 'echo', arguments: '{}' },
+        ],
+        source: {
+          kind: 'model',
+          ...{ provider: 'mock', model: 'mock' },
+        },
+      }),
+    }, { surfaceOp: 'append' })
+    session.append('tool/result', {
+      turn: 1, step: 1,
+      message: createToolResultMessage({
+        callId: CallId('c1'),
+        content: [{ type: 'text', text: 'ok' }],
+        isError: false,
+      }),
    }, { surfaceOp: 'append' })
-    session.append('tool/result', { turn: 1, step: 1, callId: CallId('c1'), content: [{ type: 'text', text: 'ok' }], isError: false }, { surfaceOp: 'append' })
    session.append('turn/end', { turn: 1, reason: { kind: 'completed' } })

    const messages = session.deriveMessages()
@@ -61,10 +78,10 @@ describe('Session', () => {
      turn: 1,
      trigger: { kind: 'injection', source: { kind: 'plugin', plugin: 'before' } },
    })
-    session.append('user/message', {
+    session.append('user/message', createUserMessage({
      content: [{ type: 'text', text: 'before' }],
      source: { kind: 'plugin', plugin: 'before' },
-    }, { surfaceOp: 'append' })
+    }), { surfaceOp: 'append' })
    session.append('turn/end', { turn: 1, reason: { kind: 'completed' } })
    expect(findLastMessageTurnEnd(session.events)).toBeUndefined()

@@ -72,19 +89,19 @@ describe('Session', () => {
      turn: 2,
      trigger: { kind: 'message', source: { kind: 'user' } },
    })
-    session.append('user/message', {
+    session.append('user/message', createUserMessage({
      content: [{ type: 'text', text: 'bounded prompt' }],
      source: { kind: 'user' },
-    }, { surfaceOp: 'append' })
+    }), { surfaceOp: 'append' })
    const messageEnd = session.append('turn/end', { turn: 2, reason: { kind: 'max-tokens' } })
    session.append('turn/start', {
      turn: 3,
      trigger: { kind: 'injection', source: { kind: 'plugin', plugin: 'after' } },
    })
-    session.append('user/message', {
+    session.append('user/message', createUserMessage({
      content: [{ type: 'text', text: 'after' }],
      source: { kind: 'plugin', plugin: 'after' },
-    }, { surfaceOp: 'append' })
+    }), { surfaceOp: 'append' })
    session.append('turn/end', { turn: 3, reason: { kind: 'completed' } })

    expect(findLastMessageTurnEnd(session.events)).toBe(messageEnd)
@@ -118,14 +135,16 @@ describe('Session', () => {

  it('renders injected-context and steering messages as plain user content', () => {
    const session = new Session(SessionId('s2'))
-    session.append('user/message', {
+    session.append('user/message', createUserMessage({
      content: [{ type: 'text', text: 'file changed: a.ts' }],
      source: { kind: 'plugin', plugin: 'watcher' },
-    }, { surfaceOp: 'append' })
+    }), { surfaceOp: 'append' })
    session.append('steering/message', {
      turn: 1,
-      content: [{ type: 'text', text: 'focus on tests' }],
-      source: { kind: 'user' },
+      message: createUserMessage({
+        content: [{ type: 'text', text: 'focus on tests' }],
+        source: { kind: 'user' },
+      }),
    }, { surfaceOp: 'append' })

    const [contextMessage, steeringMessage] = session.deriveMessages()
@@ -135,17 +154,15 @@ describe('Session', () => {
    expect(steeringMessage!.content).toEqual([{ type: 'text', text: 'focus on tests' }])
  })

-  it('keeps context source durable in the event while hiding it from the projection', () => {
+  it('keeps the exact identified context message in durable history and projection', () => {
    const session = new Session(SessionId('s2-raw'))
-    session.append('user/message', {
+    const message = createUserMessage({
      content: [{ type: 'text', text: '<system-reminder>Additional instructions from: pkg/AGENTS.md</system-reminder>' }],
      source: { kind: 'plugin', plugin: 'workspace-context' },
-    }, { surfaceOp: 'append' })
+    })
+    session.append('user/message', message, { surfaceOp: 'append' })

-    expect(session.deriveMessages()).toEqual([{
-      role: 'user',
-      content: [{ type: 'text', text: '<system-reminder>Additional instructions from: pkg/AGENTS.md</system-reminder>' }],
-    }])
+    expect(session.deriveMessages()).toEqual([message])
    const event = session.events[0]
    expect(event?.type === 'user/message' && event.data.source).toEqual({ kind: 'plugin', plugin: 'workspace-context' })
  })
@@ -153,8 +170,20 @@ describe('Session', () => {
  it('replays identically from a seeded event log', () => {
    const original = new Session(SessionId('s3'))
    original.append('turn/start', { turn: 1, trigger: { kind: 'message', source: { kind: 'user' } } })
-    original.append('user/message', { content: [{ type: 'text', text: 'q' }], source: { kind: 'user' } }, { surfaceOp: 'append' })
-    original.append('assistant/message', { provenance: { provider: 'mock', model: 'mock' }, turn: 1, step: 1, content: [{ type: 'text', text: 'a' }] }, { surfaceOp: 'append' })
+    original.append('user/message', createUserMessage({
+      content: [{ type: 'text', text: 'q' }], source: { kind: 'user' },
+    }), { surfaceOp: 'append' })
+    original.append('assistant/message', {
+      turn: 1, step: 1,
+      message: createMessage({
+        role: 'assistant',
+        content: [{ type: 'text', text: 'a' }],
+        source: {
+          kind: 'model',
+          ...{ provider: 'mock', model: 'mock' },
+        },
+      }),
+    }, { surfaceOp: 'append' })
    original.append('turn/end', { turn: 1, reason: { kind: 'completed' } })

    const replayed = new Session(SessionId('s3-replay'), [...original.events])
@@ -176,7 +205,7 @@ describe('Session', () => {
      surfaceOp: 'append',
    } as unknown as SessionEvent
    expect(() => new Session(SessionId('old-assistant'), [assistantMessage]))
-      .toThrow('seed assistant/message at index 0 lacks provider/model provenance')
+      .toThrow('seed assistant/message at index 0 lacks an identified message')

    const malformedHeader = {
      type: 'request/header', seq: 0, time: 1,
@@ -192,6 +221,156 @@ describe('Session', () => {
      .toEqual([unrelatedPrimitiveData])
  })

+  it('rejects event-specific malformed message shapes on seed/load', () => {
+    const user = {
+      id: 'user',
+      role: 'user',
+      content: [{ type: 'text', text: 'content' }],
+      source: { kind: 'user' },
+    }
+    const assistant = {
+      id: 'assistant',
+      role: 'assistant',
+      content: [{ type: 'text', text: 'content' }],
+      source: { kind: 'model', provider: 'mock', model: 'mock' },
+    }
+    const tool = {
+      id: 'tool',
+      role: 'user',
+      content: [{
+        type: 'tool-result',
+        toolCallId: 'call',
+        content: [{ type: 'text', text: 'result' }],
+      }],
+      source: { kind: 'tool', callId: 'call' },
+    }
+    const invalid = [
+      {
+        name: 'message record',
+        event: {
+          type: 'user/message', seq: 0, time: 1, surfaceOp: 'append',
+          data: null,
+        },
+        message: 'lacks an identified message',
+      },
+      {
+        name: 'user role',
+        event: {
+          type: 'user/message', seq: 0, time: 1, surfaceOp: 'append',
+          data: { ...user, role: 'assistant' },
+        },
+        message: 'message must have role "user"',
+      },
+      {
+        name: 'source',
+        event: {
+          type: 'user/message', seq: 0, time: 1, surfaceOp: 'append',
+          data: { ...user, source: null },
+        },
+        message: 'message has invalid source',
+      },
+      {
+        name: 'assistant source',
+        event: {
+          type: 'assistant/message', seq: 0, time: 1, surfaceOp: 'append',
+          data: {
+            turn: 1,
+            step: 1,
+            message: { ...assistant, source: { kind: 'user' } },
+          },
+        },
+        message: 'message must have model source',
+      },
+      {
+        name: 'content block',
+        event: {
+          type: 'steering/message', seq: 0, time: 1, surfaceOp: 'append',
+          data: {
+            turn: 1,
+            message: { ...user, content: 'not-an-array' },
+          },
+        },
+        message: 'message has invalid content',
+      },
+      {
+        name: 'tool source',
+        event: {
+          type: 'tool/result', seq: 0, time: 1, surfaceOp: 'append',
+          data: {
+            turn: 1,
+            step: 1,
+            message: { ...tool, source: { kind: 'user' } },
+          },
+        },
+        message: 'message must have tool source',
+      },
+      {
+        name: 'tool tuple',
+        event: {
+          type: 'tool/result', seq: 0, time: 1, surfaceOp: 'append',
+          data: {
+            turn: 1,
+            step: 1,
+            message: { ...tool, content: [{ type: 'text', text: 'not a result' }] },
+          },
+        },
+        message: 'message must contain one tool-result block',
+      },
+      {
+        name: 'tool correlation',
+        event: {
+          type: 'tool/result', seq: 0, time: 1, surfaceOp: 'append',
+          data: {
+            turn: 1,
+            step: 1,
+            message: {
+              ...tool,
+              source: { kind: 'tool', callId: 'other-call' },
+            },
+          },
+        },
+        message: 'message has mismatched tool call ids',
+      },
+    ] as const
+
+    for (const { name, event, message } of invalid) {
+      expect(
+        () => new Session(SessionId(`invalid-${name}`), [event as unknown as SessionEvent]),
+        name,
+      ).toThrow(message)
+    }
+  })
+
+  it('snapshots message events without validating plugin-owned block details', () => {
+    const boundary = snapshotSessionEvent({
+      type: 'turn/start',
+      seq: 0,
+      time: 1,
+      data: { turn: 1, trigger: { kind: 'message', source: { kind: 'user' } } },
+    })
+    expect(boundary).toEqual({
+      type: 'turn/start',
+      seq: 0,
+      time: 1,
+      data: { turn: 1, trigger: { kind: 'message', source: { kind: 'user' } } },
+    })
+
+    const extended = snapshotSessionEvent({
+      type: 'user/message',
+      seq: 0,
+      time: 1,
+      surfaceOp: 'append',
+      data: {
+        id: 'extended-message',
+        role: 'user',
+        content: [{ type: 'plugin-block', value: 1 }],
+        source: { kind: 'plugin-source', value: 1 },
+      },
+    } as unknown as SessionEvent)
+    expect(extended.type === 'user/message' && extended.data.content)
+      .toEqual([{ type: 'plugin-block', value: 1 }])
+  })
+
  it('round-trips a non-empty reasoning effort and rejects invalid durable values', () => {
    const valid = {
      type: 'request/header',
@@ -223,10 +402,16 @@ describe('Session', () => {

  it('isolates the log from mutation through a derived message (append-only contract)', () => {
    const session = new Session(SessionId('s4'))
-    session.append('user/message', { content: [{ type: 'text', text: 'original' }], source: { kind: 'user' } }, { surfaceOp: 'append' })
+    session.append('user/message', createUserMessage({
+      content: [{ type: 'text', text: 'original' }], source: { kind: 'user' },
+    }), { surfaceOp: 'append' })
    session.append('tool/result', {
-      turn: 1, step: 1, callId: CallId('c1'),
-      content: [{ type: 'text', text: 'tool out' }], isError: false,
+      turn: 1, step: 1,
+      message: createToolResultMessage({
+        callId: CallId('c1'),
+        content: [{ type: 'text', text: 'tool out' }],
+        isError: false,
+      }),
    }, { surfaceOp: 'append' })
    const before = structuredClone(session.events)

@@ -282,7 +467,9 @@ describe('Session', () => {
    // A widened SessionEventType bypasses the overload's conditional requirement,
    // so the runtime guard must still reject the missing surface marker.
    const widenedType = 'user/message' as SessionEventType
-    expect(() => session.append(widenedType, { content: [{ type: 'text', text: 'hi' }], source: { kind: 'user' } }))
+    expect(() => session.append(widenedType, createUserMessage({
+      content: [{ type: 'text', text: 'hi' }], source: { kind: 'user' },
+    })))
      .toThrow(/surface-eligible and requires a surfaceOp marker/)
    // The rejected append never entered the log (only turn/start is present).
    expect(session.events).toHaveLength(1)
@@ -318,7 +505,9 @@ describe('Session', () => {
    // compile time; a raw seed must be rejected at runtime to match.
    const markerlessSeed = [
      { type: 'turn/start' as const, seq: 0, time: 1, data: { turn: 1, trigger: { kind: 'message' as const, source: { kind: 'user' as const } } } },
-      { type: 'user/message' as const, seq: 1, time: 2, data: { content: [{ type: 'text' as const, text: 'hi' }], source: { kind: 'user' as const } } },
+      { type: 'user/message' as const, seq: 1, time: 2, data: createUserMessage({
+        content: [{ type: 'text' as const, text: 'hi' }], source: { kind: 'user' as const },
+      }) },
      { type: 'turn/end' as const, seq: 2, time: 3, data: { turn: 1, reason: { kind: 'completed' as const } } },
    ] as SessionEvent[]
    expect(() => new Session(SessionId('seed-no-marker'), markerlessSeed)).toThrow(/requires a surfaceOp marker/)
@@ -327,7 +516,9 @@ describe('Session', () => {
  it('accepts a well-formed contiguous serializable seed', () => {
    const goodSeed = [
      { type: 'turn/start' as const, seq: 0, time: 1, data: { turn: 1, trigger: { kind: 'message' as const, source: { kind: 'user' as const } } } },
-      { type: 'user/message' as const, seq: 1, time: 2, data: { content: [{ type: 'text' as const, text: 'hi' }], source: { kind: 'user' as const } }, surfaceOp: 'append' as const },
+      { type: 'user/message' as const, seq: 1, time: 2, data: createUserMessage({
+        content: [{ type: 'text' as const, text: 'hi' }], source: { kind: 'user' as const },
+      }), surfaceOp: 'append' as const },
      { type: 'turn/end' as const, seq: 2, time: 3, data: { turn: 1, reason: { kind: 'completed' as const } } },
    ] as SessionEvent[]
    const session = new Session(SessionId('seed-ok'), goodSeed)
@@ -380,7 +571,9 @@ describe('Session', () => {
      type: 'user/message',
      seq: 0,
      time: 1,
-      data: { content: [{ type: 'text', text: 'hello' }], source: { kind: 'user' } },
+      data: createUserMessage({
+        content: [{ type: 'text', text: 'hello' }], source: { kind: 'user' },
+      }),
      surfaceOp: { op: 'replace', start: 1n, end: 2 },
    }] as unknown as SessionEvent[]

@@ -398,7 +591,9 @@ describe('Session', () => {
      type: 'user/message',
      seq: 0,
      time: 1,
-      data: { content: [{ type: 'text', text: 'hello' }], source: { kind: 'user' } },
+      data: createUserMessage({
+        content: [{ type: 'text', text: 'hello' }], source: { kind: 'user' },
+      }),
      surfaceOp: new ReplaceOp(),
    }] as unknown as SessionEvent[]

@@ -445,13 +640,17 @@ describe('Session', () => {
      type: 'user/message',
      seq: 0,
      time: 1,
-      data: { content: [{ type: 'text', text: 'source' }], source: { kind: 'user' } },
+      data: createUserMessage({
+        content: [{ type: 'text', text: 'source' }], source: { kind: 'user' },
+      }),
      surfaceOp: 'append',
    }, {
      type: 'user/message',
      seq: 1,
      time: 2,
-      data: { content: [{ type: 'text', text: 'hello' }], source: { kind: 'user' } },
+      data: createUserMessage({
+        content: [{ type: 'text', text: 'hello' }], source: { kind: 'user' },
+      }),
      surfaceOp,
      sourceEventSeqs: [0],
    }] as unknown as SessionEvent[]
@@ -477,13 +676,17 @@ describe('Session', () => {
      type: 'user/message',
      seq: 0,
      time: 1,
-      data: { content: [{ type: 'text', text: 'source' }], source: { kind: 'user' } },
+      data: createUserMessage({
+        content: [{ type: 'text', text: 'source' }], source: { kind: 'user' },
+      }),
      surfaceOp: 'append',
    }, {
      type: 'user/message',
      seq: 1,
      time: 2,
-      data: { content: [{ type: 'text', text: 'hello' }], source: { kind: 'user' } },
+      data: createUserMessage({
+        content: [{ type: 'text', text: 'hello' }], source: { kind: 'user' },
+      }),
      surfaceOp: { op: 'replace', start: 0, end: 0 },
      sourceEventSeqs: [0],
    }] as unknown as SessionEvent[]
@@ -499,7 +702,11 @@ describe('Session', () => {
  it('snapshots the seed: mutating the original after construction does not affect session.events', () => {
    const seed = [
      { type: 'turn/start' as const, seq: 0, time: 1, data: { turn: 1, trigger: { kind: 'message' as const, source: { kind: 'user' as const } } } },
-      { type: 'user/message' as const, seq: 1, time: 2, data: { content: [{ type: 'text' as const, text: 'original' }], source: { kind: 'user' as const } }, surfaceOp: 'append' as const },
+      { type: 'user/message' as const, seq: 1, time: 2, data: {
+        id: MessageId('seed-input'),
+        role: 'user' as const,
+        content: [{ type: 'text' as const, text: 'original' }], source: { kind: 'user' as const },
+      }, surfaceOp: 'append' as const },
      { type: 'turn/end' as const, seq: 2, time: 3, data: { turn: 1, reason: { kind: 'completed' as const } } },
    ] as SessionEvent[]
    const session = new Session(SessionId('seed-snapshot'), seed)
@@ -516,7 +723,12 @@ describe('Session', () => {

  it('snapshots append data: mutating the passed object after append does not affect session.events', () => {
    const session = new Session(SessionId('append-snapshot'))
-    const data = { content: [{ type: 'text' as const, text: 'original' }], source: { kind: 'user' as const } }
+    const data = {
+      id: MessageId('append-input'),
+      role: 'user' as const,
+      content: [{ type: 'text' as const, text: 'original' }],
+      source: { kind: 'user' as const },
+    }
    const event = session.append('user/message', data, { surfaceOp: 'append' })
    // Mutate the caller's object after append returns. A shared reference would
    // make session.events diverge from the value that passed validation.
@@ -552,7 +764,9 @@ describe('Session', () => {

    expect(() => session.append(
      'user/message',
-      { content: [{ type: 'text', text: 'hello' }], source: { kind: 'user' } },
+      createUserMessage({
+        content: [{ type: 'text', text: 'hello' }], source: { kind: 'user' },
+      }),
      { surfaceOp: { op: 'replace', start: 1n, end: 2 } } as never,
    )).toThrow(/non-JSON-serializable surface metadata/)
    expect(session.events).toEqual([])
@@ -568,7 +782,9 @@ describe('Session', () => {

    expect(() => session.append(
      'user/message',
-      { content: [{ type: 'text', text: 'hello' }], source: { kind: 'user' } },
+      createUserMessage({
+        content: [{ type: 'text', text: 'hello' }], source: { kind: 'user' },
+      }),
      { surfaceOp: new ReplaceOp() },
    )).toThrow(/non-JSON-serializable surface metadata/)
    expect(session.events).toEqual([])
@@ -578,7 +794,9 @@ describe('Session', () => {
    const session = new Session(SessionId('append-unstable-metadata'))
    const source = session.append(
      'user/message',
-      { content: [{ type: 'text', text: 'source' }], source: { kind: 'user' } },
+      createUserMessage({
+        content: [{ type: 'text', text: 'source' }], source: { kind: 'user' },
+      }),
      { surfaceOp: 'append' },
    )
    let reads = 0
@@ -592,7 +810,9 @@ describe('Session', () => {

    const event = session.append(
      'user/message',
-      { content: [{ type: 'text', text: 'hello' }], source: { kind: 'user' } },
+      createUserMessage({
+        content: [{ type: 'text', text: 'hello' }], source: { kind: 'user' },
+      }),
      { surfaceOp, sourceEventSeqs: [0] } as never,
    )

@@ -811,7 +1031,9 @@ describe('SessionStore', () => {
    // but cannot suppress the durable event feed.
    expect(Reflect.set(session, 'onAppend', undefined)).toBe(true)
    session.append('turn/start', { turn: 1, trigger: { kind: 'message', source: { kind: 'user' } } })
-    session.append('user/message', { content: [{ type: 'text', text: 'x' }], source: { kind: 'user' } }, { surfaceOp: 'append' })
+    session.append('user/message', createUserMessage({
+      content: [{ type: 'text', text: 'x' }], source: { kind: 'user' },
+    }), { surfaceOp: 'append' })
    expect(events).toHaveLength(2)
    expect(events[1]![0]).toBe(session)
    expect(events[1]![1].type).toBe('user/message')
@@ -827,7 +1049,9 @@ describe('SessionStore', () => {
    expect(() => ctx.sessions.create(SessionId('fixed'))).toThrow('already exists')

    a.append('turn/start', { turn: 1, trigger: { kind: 'message', source: { kind: 'user' } } })
-    a.append('user/message', { content: [{ type: 'text', text: 'q' }], source: { kind: 'user' } }, { surfaceOp: 'append' })
+    a.append('user/message', createUserMessage({
+      content: [{ type: 'text', text: 'q' }], source: { kind: 'user' },
+    }), { surfaceOp: 'append' })
    const forked = ctx.sessions.create(SessionId('fork'), { seed: [...a.events] })
    expect(forked.deriveMessages()).toEqual(a.deriveMessages())
  })
@@ -1047,7 +1271,9 @@ describe('SessionStore', () => {

    await fiber.dispose()
    expect(ctx.sessions.get(SessionId('scoped'))).toBeUndefined()
-    session.append('user/message', { content: [{ type: 'text', text: 'late' }], source: { kind: 'user' } }, { surfaceOp: 'append' })
+    session.append('user/message', createUserMessage({
+      content: [{ type: 'text', text: 'late' }], source: { kind: 'user' },
+    }), { surfaceOp: 'append' })
    expect(observed).toBe(0)
  })

@@ -1074,7 +1300,9 @@ describe('SessionStore', () => {
    const session = ctx.sessions.create(SessionId('fixed'))
    expect(ctx.sessions.get(SessionId('fixed'))).toBe(session)
    session.append('turn/start', { turn: 1, trigger: { kind: 'message', source: { kind: 'user' } } })
-    session.append('user/message', { content: [{ type: 'text', text: 'hi' }], source: { kind: 'user' } }, { surfaceOp: 'append' })
+    session.append('user/message', createUserMessage({
+      content: [{ type: 'text', text: 'hi' }], source: { kind: 'user' },
+    }), { surfaceOp: 'append' })
    expect(events.at(-1)?.type).toBe('user/message')
  })

@@ -1161,10 +1389,10 @@ describe('SessionStore', () => {
    const session = ctx.sessions.create(SessionId('surface-dispatch-veto'))
    session.append('turn/start', { turn: 1, trigger: { kind: 'message', source: { kind: 'user' } } })
    session.append('step/start', { turn: 1, step: 1 })
-    session.append('user/message', {
+    session.append('user/message', createUserMessage({
      content: [{ type: 'text', text: 'source' }],
      source: { kind: 'user' },
-    }, { surfaceOp: 'append' })
+    }), { surfaceOp: 'append' })
    const surface = session.surface
    let reject = true
    ctx.on('internal/dispatch', (_mode, name) => {
@@ -1175,10 +1403,16 @@ describe('SessionStore', () => {
    })

    expect(() => session.append('assistant/message', {
-      provenance: { provider: 'mock', model: 'mock' },
      turn: 1,
      step: 1,
-      content: [{ type: 'text', text: 'replacement' }],
+      message: createMessage({
+        role: 'assistant',
+        content: [{ type: 'text', text: 'replacement' }],
+        source: {
+          kind: 'model',
+          ...{ provider: 'mock', model: 'mock' },
+        },
+      }),
    }, {
      surfaceOp: { op: 'replace', start: 2, end: 2 },
      sourceEventSeqs: [2],
@@ -1188,10 +1422,10 @@ describe('SessionStore', () => {
    expect(surface.nodes).toEqual([2])
    expect(surface.replaceGeneration).toBe(0)

-    session.append('user/message', {
+    session.append('user/message', createUserMessage({
      content: [{ type: 'text', text: 'next' }],
      source: { kind: 'user' },
-    }, { surfaceOp: 'append' })
+    }), { surfaceOp: 'append' })
    expect(surface.nodes).toEqual([2, 3])
    expect(surface.replaceGeneration).toBe(0)
  })
@@ -1397,7 +1631,9 @@ describe('todo/write event', () => {

  it('is NOT a surface event: it produces no derived message and joins no surface node', () => {
    const session = new Session(SessionId('t3'))
-    session.append('user/message', { content: [{ type: 'text', text: 'q' }], source: { kind: 'user' } }, { surfaceOp: 'append' })
+    session.append('user/message', createUserMessage({
+      content: [{ type: 'text', text: 'q' }], source: { kind: 'user' },
+    }), { surfaceOp: 'append' })
    const before = session.deriveMessages().length
    session.append('todo/write', { todos: [{ content: 'a task', status: 'pending' }] })
    // The todo event must not add a message to the derived history…
--- a/packages/core/session/tests/surface.spec.ts
+++ b/packages/core/session/tests/surface.spec.ts
@@ -7,14 +7,33 @@ import {
  isSurfaceEligibleType,
  isSurfaceEvent,
 } from '@deepseek-ai/dsh-session'
-import { CallId } from '@deepseek-ai/dsh-llm'
+import {
+  createMessage,
+  createToolResultMessage,
+  createUserMessage,
+  freezeMessage,
+  CallId,
+  MessageId,
+} from '@deepseek-ai/dsh-llm'

 /** Build a minimal session with turn boundaries and a single user message. */
 function surfaceSession(): Session {
  const s = new Session(SessionId('ss'))
  s.append('turn/start', { turn: 1, trigger: { kind: 'message', source: { kind: 'user' } } })
-  s.append('user/message', { content: [{ type: 'text', text: 'hello' }], source: { kind: 'user' } }, { surfaceOp: 'append' })
-  s.append('assistant/message', { provenance: { provider: 'mock', model: 'mock' }, turn: 1, step: 1, content: [{ type: 'text', text: 'hi' }] }, { surfaceOp: 'append' })
+  s.append('user/message', createUserMessage({
+    content: [{ type: 'text', text: 'hello' }], source: { kind: 'user' },
+  }), { surfaceOp: 'append' })
+  s.append('assistant/message', {
+    turn: 1, step: 1,
+    message: createMessage({
+      role: 'assistant',
+      content: [{ type: 'text', text: 'hi' }],
+      source: {
+        kind: 'model',
+        ...{ provider: 'mock', model: 'mock' },
+      },
+    }),
+  }, { surfaceOp: 'append' })
  s.append('turn/end', { turn: 1, reason: { kind: 'completed' } })
  return s
 }
@@ -24,7 +43,9 @@ function provenanceEvent(seq: number, sourceEventSeqs: unknown): SessionEvent {
    type: 'user/message',
    seq,
    time: seq,
-    data: { content: [], source: { kind: 'user' } },
+    data: createUserMessage({
+      content: [], source: { kind: 'user' },
+    }),
    surfaceOp: 'append',
    ...sourceEventSeqs === undefined ? {} : { sourceEventSeqs },
  } as unknown as SessionEvent
@@ -43,9 +64,11 @@ function toolResultEvent(
    data: {
      turn: 1,
      step: 1,
-      callId: CallId(callId),
-      content: [{ type: 'text', text: `result ${seq}` }],
-      isError: false,
+      message: createToolResultMessage({
+        callId: CallId(callId),
+        content: [{ type: 'text', text: `result ${seq}` }],
+        isError: false,
+      }),
    },
    surfaceOp,
    ...sourceEventSeqs === undefined ? {} : { sourceEventSeqs },
@@ -82,10 +105,16 @@ describe('foldSurface provenance', () => {
      seq: 0,
      time: 0,
      data: {
-        provenance: { provider: 'mock', model: 'mock' },
        turn: 1,
        step: 1,
-        content: [],
+        message: createMessage({
+          role: 'assistant',
+          content: [],
+          source: {
+            kind: 'model',
+            ...{ provider: 'mock', model: 'mock' },
+          },
+        }),
      },
      surfaceOp: 'append',
      sourceEventSeqs: [],
@@ -142,10 +171,42 @@ describe('foldSurface tool-result rewrites', () => {
    expect(() => foldSurface(events)).toThrow(/may change only content/)
  })

+  it.each([
+    ['toolCallId', { toolCallId: CallId('changed') }],
+    ['isError', { isError: true }],
+  ] as const)('rejects a replacement that changes the result block %s', (_field, patch) => {
+    const original = toolResultEvent(0, 'original')
+    const data = original.data as Extract<SessionEvent, { type: 'tool/result' }>['data']
+    const result = data.message.content[0]
+    const replacement = {
+      ...original,
+      seq: 1,
+      time: 1,
+      data: {
+        ...data,
+        message: freezeMessage({
+          ...data.message,
+          content: [{ ...result, ...patch }] as [typeof result],
+        }),
+      },
+      surfaceOp: { op: 'replace', start: 0, end: 0 },
+      sourceEventSeqs: [0],
+    } as SessionEvent
+    expect(() => foldSurface([original, replacement])).toThrow(/may change only content/)
+  })
+
  it('compares array-valued rest fields structurally (meta arrays: equal accepted, drifted rejected)', () => {
    const withMeta = (seq: number, meta: unknown, surfaceOp: SurfaceEvent['surfaceOp'] = 'append', sourceEventSeqs?: number[]): SessionEvent => {
      const event = toolResultEvent(seq, 'c-meta', surfaceOp, sourceEventSeqs)
-      return { ...event, data: { ...(event.data as object), meta } } as SessionEvent
+      const data = event.data as Extract<SessionEvent, { type: 'tool/result' }>['data']
+      return {
+        ...event,
+        data: {
+          ...data,
+          message: freezeMessage({ ...data.message, id: MessageId('meta-message') }),
+          meta,
+        },
+      } as SessionEvent
    }
    // Structurally equal arrays (fresh references) pass the rest-field equality.
    expect(() => foldSurface([
@@ -178,10 +239,34 @@ describe('foldSurface tool-result rewrites', () => {
 describe('SurfaceManager', () => {
  it('shares ordered entries and nested replacement ranges with foldSurface', () => {
    const s = new Session(SessionId('shared-fold'))
-    s.append('user/message', { content: [{ type: 'text', text: 'a' }], source: { kind: 'user' } }, { surfaceOp: 'append' })
-    s.append('user/message', { content: [{ type: 'text', text: 'b' }], source: { kind: 'user' } }, { surfaceOp: 'append' })
-    s.append('assistant/message', { provenance: { provider: 'mock', model: 'mock' }, turn: 1, step: 1, content: [{ type: 'text', text: 'summary' }] }, { surfaceOp: { op: 'replace', start: 0, end: 0 }, sourceEventSeqs: [0] })
-    s.append('assistant/message', { provenance: { provider: 'mock', model: 'mock' }, turn: 1, step: 2, content: [{ type: 'text', text: 'summary 2' }] }, { surfaceOp: { op: 'replace', start: 2, end: 1 }, sourceEventSeqs: [2, 1] })
+    s.append('user/message', createUserMessage({
+      content: [{ type: 'text', text: 'a' }], source: { kind: 'user' },
+    }), { surfaceOp: 'append' })
+    s.append('user/message', createUserMessage({
+      content: [{ type: 'text', text: 'b' }], source: { kind: 'user' },
+    }), { surfaceOp: 'append' })
+    s.append('assistant/message', {
+      turn: 1, step: 1,
+      message: createMessage({
+        role: 'assistant',
+        content: [{ type: 'text', text: 'summary' }],
+        source: {
+          kind: 'model',
+          ...{ provider: 'mock', model: 'mock' },
+        },
+      }),
+    }, { surfaceOp: { op: 'replace', start: 0, end: 0 }, sourceEventSeqs: [0] })
+    s.append('assistant/message', {
+      turn: 1, step: 2,
+      message: createMessage({
+        role: 'assistant',
+        content: [{ type: 'text', text: 'summary 2' }],
+        source: {
+          kind: 'model',
+          ...{ provider: 'mock', model: 'mock' },
+        },
+      }),
+    }, { surfaceOp: { op: 'replace', start: 2, end: 1 }, sourceEventSeqs: [2, 1] })

    const folded = foldSurface(s.events)
    expect(folded.nodes).toEqual(s.surface.nodes)
@@ -198,8 +283,20 @@ describe('SurfaceManager', () => {

  it('does not retain fold-only replacement history in incremental state', () => {
    const s = new Session(SessionId('incremental-state'))
-    s.append('user/message', { content: [{ type: 'text', text: 'a' }], source: { kind: 'user' } }, { surfaceOp: 'append' })
-    s.append('assistant/message', { provenance: { provider: 'mock', model: 'mock' }, turn: 1, step: 1, content: [{ type: 'text', text: 'b' }] }, { surfaceOp: { op: 'replace', start: 0, end: 0 }, sourceEventSeqs: [0] })
+    s.append('user/message', createUserMessage({
+      content: [{ type: 'text', text: 'a' }], source: { kind: 'user' },
+    }), { surfaceOp: 'append' })
+    s.append('assistant/message', {
+      turn: 1, step: 1,
+      message: createMessage({
+        role: 'assistant',
+        content: [{ type: 'text', text: 'b' }],
+        source: {
+          kind: 'model',
+          ...{ provider: 'mock', model: 'mock' },
+        },
+      }),
+    }, { surfaceOp: { op: 'replace', start: 0, end: 0 }, sourceEventSeqs: [0] })

    expect(s.surface.nodes).toEqual([1])
    const manager = s.surface as unknown as { _state: object }
@@ -222,7 +319,9 @@ describe('SurfaceManager', () => {

  it('leaves incremental state unchanged when candidate validation fails', () => {
    const s = new Session(SessionId('atomic-validation'))
-    s.append('user/message', { content: [{ type: 'text', text: 'a' }], source: { kind: 'user' } }, { surfaceOp: 'append' })
+    s.append('user/message', createUserMessage({
+      content: [{ type: 'text', text: 'a' }], source: { kind: 'user' },
+    }), { surfaceOp: 'append' })
    const surface = s.surface
    const nodes = surface.nodes

@@ -231,7 +330,17 @@ describe('SurfaceManager', () => {

    expect(() => s.append(
      'assistant/message',
-      { provenance: { provider: 'mock', model: 'mock' }, turn: 1, step: 1, content: [{ type: 'text', text: 'invalid' }] },
+      {
+        turn: 1, step: 1,
+        message: createMessage({
+          role: 'assistant',
+          content: [{ type: 'text', text: 'invalid' }],
+          source: {
+            kind: 'model',
+            ...{ provider: 'mock', model: 'mock' },
+          },
+        }),
+      },
      { surfaceOp: { op: 'replace', start: 0, end: 0 } },
    )).toThrow(/missing 0/)

@@ -241,7 +350,9 @@ describe('SurfaceManager', () => {
    expect(surface.replaceGeneration).toBe(0)
    expect(surface.nodes).toEqual(foldSurface(s.events).nodes)

-    s.append('user/message', { content: [{ type: 'text', text: 'b' }], source: { kind: 'user' } }, { surfaceOp: 'append' })
+    s.append('user/message', createUserMessage({
+      content: [{ type: 'text', text: 'b' }], source: { kind: 'user' },
+    }), { surfaceOp: 'append' })
    expect(surface.nodes).toBe(nodes)
    expect(surface.nodes).toEqual([0, 1])
    expect(surface.replaceGeneration).toBe(0)
@@ -253,7 +364,9 @@ describe('SurfaceManager', () => {
      type: 'user/message',
      seq: 0,
      time: 1,
-      data: { content: [{ type: 'text', text: 'hidden' }], source: { kind: 'user' } },
+      data: createUserMessage({
+        content: [{ type: 'text', text: 'hidden' }], source: { kind: 'user' },
+      }),
    }

    expect(() => foldSurface([malformed]))
@@ -294,14 +407,28 @@ describe('SurfaceManager', () => {
  it('picks up new events incrementally (delta processing)', () => {
    const s = surfaceSession()
    expect(s.surface.nodes.length).toBe(2)
-    s.append('tool/result', { turn: 1, step: 1, callId: CallId('c1'), content: [{ type: 'text', text: 'ok' }], isError: false }, { surfaceOp: 'append' })
+    s.append('tool/result', {
+      turn: 1, step: 1,
+      message: createToolResultMessage({
+        callId: CallId('c1'),
+        content: [{ type: 'text', text: 'ok' }],
+        isError: false,
+      }),
+    }, { surfaceOp: 'append' })
    expect(s.surface.nodes.length).toBe(3)
    expect(s.surface.nodes[2]!).toBe(4) // seq 4: after turn/end at seq 3
  })

  it('replays identically from a seeded log with surface markers', () => {
    const original = surfaceSession()
-    original.append('tool/result', { turn: 1, step: 1, callId: CallId('c1'), content: [{ type: 'text', text: 'ok' }], isError: false }, { surfaceOp: 'append' })
+    original.append('tool/result', {
+      turn: 1, step: 1,
+      message: createToolResultMessage({
+        callId: CallId('c1'),
+        content: [{ type: 'text', text: 'ok' }],
+        isError: false,
+      }),
+    }, { surfaceOp: 'append' })
    const replayed = new Session(SessionId('replay'), [...original.events])
    expect(replayed.surface.nodes).toEqual([1, 2, 4])
    expect(replayed.deriveMessages()).toEqual(original.deriveMessages())
@@ -310,7 +437,17 @@ describe('SurfaceManager', () => {
  it('rebuild with replace operation splices out shadowed nodes', () => {
    const s = surfaceSession()
    s.append('assistant/message',
-      { provenance: { provider: 'mock', model: 'mock' }, turn: 2, step: 1, content: [{ type: 'text', text: 'summary' }] },
+      {
+        turn: 2, step: 1,
+        message: createMessage({
+          role: 'assistant',
+          content: [{ type: 'text', text: 'summary' }],
+          source: {
+            kind: 'model',
+            ...{ provider: 'mock', model: 'mock' },
+          },
+        }),
+      },
      { surfaceOp: { op: 'replace', start: 1, end: 2 }, sourceEventSeqs: [1, 2] },
    )
    expect(s.surface.nodes).toEqual([4])
@@ -318,12 +455,28 @@ describe('SurfaceManager', () => {

  it('replace with both ends at real nodes splices only the range', () => {
    const s = new Session(SessionId('range'))
-    s.append('user/message', { content: [{ type: 'text', text: 'a' }], source: { kind: 'user' } }, { surfaceOp: 'append' }) // seq 0
-    s.append('user/message', { content: [{ type: 'text', text: 'b' }], source: { kind: 'user' } }, { surfaceOp: 'append' }) // seq 1
-    s.append('user/message', { content: [{ type: 'text', text: 'c' }], source: { kind: 'user' } }, { surfaceOp: 'append' }) // seq 2
+    s.append('user/message', createUserMessage({
+      content: [{ type: 'text', text: 'a' }], source: { kind: 'user' },
+    }), { surfaceOp: 'append' }) // seq 0
+    s.append('user/message', createUserMessage({
+      content: [{ type: 'text', text: 'b' }], source: { kind: 'user' },
+    }), { surfaceOp: 'append' }) // seq 1
+    s.append('user/message', createUserMessage({
+      content: [{ type: 'text', text: 'c' }], source: { kind: 'user' },
+    }), { surfaceOp: 'append' }) // seq 2
    // Replace seq 0 through 1 inclusive: shadow a and b, keep c.
    s.append('assistant/message',
-      { provenance: { provider: 'mock', model: 'mock' }, turn: 1, step: 1, content: [{ type: 'text', text: 'summary' }] },
+      {
+        turn: 1, step: 1,
+        message: createMessage({
+          role: 'assistant',
+          content: [{ type: 'text', text: 'summary' }],
+          source: {
+            kind: 'model',
+            ...{ provider: 'mock', model: 'mock' },
+          },
+        }),
+      },
      { surfaceOp: { op: 'replace', start: 0, end: 1 }, sourceEventSeqs: [0, 1] },
    ) // seq 3
    expect(s.surface.nodes).toEqual([3, 2])
@@ -331,11 +484,25 @@ describe('SurfaceManager', () => {

  it('single-node replacement (start === end)', () => {
    const s = new Session(SessionId('single'))
-    s.append('user/message', { content: [{ type: 'text', text: 'a' }], source: { kind: 'user' } }, { surfaceOp: 'append' }) // seq 0
-    s.append('user/message', { content: [{ type: 'text', text: 'b' }], source: { kind: 'user' } }, { surfaceOp: 'append' }) // seq 1
+    s.append('user/message', createUserMessage({
+      content: [{ type: 'text', text: 'a' }], source: { kind: 'user' },
+    }), { surfaceOp: 'append' }) // seq 0
+    s.append('user/message', createUserMessage({
+      content: [{ type: 'text', text: 'b' }], source: { kind: 'user' },
+    }), { surfaceOp: 'append' }) // seq 1
    // Replace only seq 1 (single node).
    s.append('assistant/message',
-      { provenance: { provider: 'mock', model: 'mock' }, turn: 1, step: 1, content: [{ type: 'text', text: 'x' }] },
+      {
+        turn: 1, step: 1,
+        message: createMessage({
+          role: 'assistant',
+          content: [{ type: 'text', text: 'x' }],
+          source: {
+            kind: 'model',
+            ...{ provider: 'mock', model: 'mock' },
+          },
+        }),
+      },
      { surfaceOp: { op: 'replace', start: 1, end: 1 }, sourceEventSeqs: [1] },
    ) // seq 2
    expect(s.surface.nodes).toEqual([0, 2])
@@ -343,38 +510,88 @@ describe('SurfaceManager', () => {

  it('throws when replace start is not found', () => {
    const s = new Session(SessionId('bad-start'))
-    s.append('user/message', { content: [{ type: 'text', text: 'a' }], source: { kind: 'user' } }, { surfaceOp: 'append' }) // seq 0
+    s.append('user/message', createUserMessage({
+      content: [{ type: 'text', text: 'a' }], source: { kind: 'user' },
+    }), { surfaceOp: 'append' }) // seq 0
    expect(() => s.append('assistant/message',
-      { provenance: { provider: 'mock', model: 'mock' }, turn: 1, step: 1, content: [{ type: 'text', text: 'y' }] },
+      {
+        turn: 1, step: 1,
+        message: createMessage({
+          role: 'assistant',
+          content: [{ type: 'text', text: 'y' }],
+          source: {
+            kind: 'model',
+            ...{ provider: 'mock', model: 'mock' },
+          },
+        }),
+      },
      { surfaceOp: { op: 'replace', start: 5, end: 0 }, sourceEventSeqs: [0] },
    )).toThrow(/surface replace: start seq 5 not found/)
  })

  it('throws when replace end is not found', () => {
    const s = new Session(SessionId('bad-end'))
-    s.append('user/message', { content: [{ type: 'text', text: 'a' }], source: { kind: 'user' } }, { surfaceOp: 'append' }) // seq 0
+    s.append('user/message', createUserMessage({
+      content: [{ type: 'text', text: 'a' }], source: { kind: 'user' },
+    }), { surfaceOp: 'append' }) // seq 0
    expect(() => s.append('assistant/message',
-      { provenance: { provider: 'mock', model: 'mock' }, turn: 1, step: 1, content: [{ type: 'text', text: 'y' }] },
+      {
+        turn: 1, step: 1,
+        message: createMessage({
+          role: 'assistant',
+          content: [{ type: 'text', text: 'y' }],
+          source: {
+            kind: 'model',
+            ...{ provider: 'mock', model: 'mock' },
+          },
+        }),
+      },
      { surfaceOp: { op: 'replace', start: 0, end: 99 }, sourceEventSeqs: [0] },
    )).toThrow(/surface replace: end seq 99 not found/)
  })

  it('throws when start is after end', () => {
    const s = new Session(SessionId('reversed'))
-    s.append('user/message', { content: [{ type: 'text', text: 'a' }], source: { kind: 'user' } }, { surfaceOp: 'append' }) // seq 0
-    s.append('user/message', { content: [{ type: 'text', text: 'b' }], source: { kind: 'user' } }, { surfaceOp: 'append' }) // seq 1
+    s.append('user/message', createUserMessage({
+      content: [{ type: 'text', text: 'a' }], source: { kind: 'user' },
+    }), { surfaceOp: 'append' }) // seq 0
+    s.append('user/message', createUserMessage({
+      content: [{ type: 'text', text: 'b' }], source: { kind: 'user' },
+    }), { surfaceOp: 'append' }) // seq 1
    // start=1, end=0 would be reversed order.
    expect(() => s.append('assistant/message',
-      { provenance: { provider: 'mock', model: 'mock' }, turn: 1, step: 1, content: [{ type: 'text', text: 'y' }] },
+      {
+        turn: 1, step: 1,
+        message: createMessage({
+          role: 'assistant',
+          content: [{ type: 'text', text: 'y' }],
+          source: {
+            kind: 'model',
+            ...{ provider: 'mock', model: 'mock' },
+          },
+        }),
+      },
      { surfaceOp: { op: 'replace', start: 1, end: 0 }, sourceEventSeqs: [1, 0] },
    )).toThrow(/start seq 1.*after end seq 0/)
  })

  it('sourceEventSeqs is snapshot so caller mutation does not affect logged event', () => {
    const s = new Session(SessionId('immutable'))
-    s.append('user/message', { content: [{ type: 'text', text: 'source' }], source: { kind: 'user' } }, { surfaceOp: 'append' })
+    s.append('user/message', createUserMessage({
+      content: [{ type: 'text', text: 'source' }], source: { kind: 'user' },
+    }), { surfaceOp: 'append' })
    const sources = [0]
-    s.append('assistant/message', { provenance: { provider: 'mock', model: 'mock' }, turn: 1, step: 1, content: [{ type: 'text', text: 'h' }] }, { surfaceOp: 'append', sourceEventSeqs: sources })
+    s.append('assistant/message', {
+      turn: 1, step: 1,
+      message: createMessage({
+        role: 'assistant',
+        content: [{ type: 'text', text: 'h' }],
+        source: {
+          kind: 'model',
+          ...{ provider: 'mock', model: 'mock' },
+        },
+      }),
+    }, { surfaceOp: 'append', sourceEventSeqs: sources })
    // Mutate caller's array after append.
    sources.push(1)
    sources[0] = 99
@@ -384,12 +601,28 @@ describe('SurfaceManager', () => {

  it('replace starting at non-head position preserves surrounding order', () => {
    const s = new Session(SessionId('mid-replace'))
-    s.append('user/message', { content: [{ type: 'text', text: 'a' }], source: { kind: 'user' } }, { surfaceOp: 'append' }) // seq 0
-    s.append('user/message', { content: [{ type: 'text', text: 'b' }], source: { kind: 'user' } }, { surfaceOp: 'append' }) // seq 1
-    s.append('user/message', { content: [{ type: 'text', text: 'c' }], source: { kind: 'user' } }, { surfaceOp: 'append' }) // seq 2
+    s.append('user/message', createUserMessage({
+      content: [{ type: 'text', text: 'a' }], source: { kind: 'user' },
+    }), { surfaceOp: 'append' }) // seq 0
+    s.append('user/message', createUserMessage({
+      content: [{ type: 'text', text: 'b' }], source: { kind: 'user' },
+    }), { surfaceOp: 'append' }) // seq 1
+    s.append('user/message', createUserMessage({
+      content: [{ type: 'text', text: 'c' }], source: { kind: 'user' },
+    }), { surfaceOp: 'append' }) // seq 2
    // Replace the middle node (seq 1) only, keeping seq 0 and seq 2.
    s.append('assistant/message',
-      { provenance: { provider: 'mock', model: 'mock' }, turn: 1, step: 1, content: [{ type: 'text', text: 'x' }] },
+      {
+        turn: 1, step: 1,
+        message: createMessage({
+          role: 'assistant',
+          content: [{ type: 'text', text: 'x' }],
+          source: {
+            kind: 'model',
+            ...{ provider: 'mock', model: 'mock' },
+          },
+        }),
+      },
      { surfaceOp: { op: 'replace', start: 1, end: 1 }, sourceEventSeqs: [1] },
    ) // seq 3
    expect(s.surface.nodes).toEqual([0, 3, 2])
@@ -397,9 +630,21 @@ describe('SurfaceManager', () => {

  it('surfaceOp replace object is snapshot so caller mutation is isolated', () => {
    const s = new Session(SessionId('immutable-op'))
-    s.append('user/message', { content: [{ type: 'text', text: 'a' }], source: { kind: 'user' } }, { surfaceOp: 'append' })
+    s.append('user/message', createUserMessage({
+      content: [{ type: 'text', text: 'a' }], source: { kind: 'user' },
+    }), { surfaceOp: 'append' })
    const op = { op: 'replace' as const, start: 0, end: 0 }
-    s.append('assistant/message', { provenance: { provider: 'mock', model: 'mock' }, turn: 1, step: 1, content: [{ type: 'text', text: 's' }] }, { surfaceOp: op, sourceEventSeqs: [0] })
+    s.append('assistant/message', {
+      turn: 1, step: 1,
+      message: createMessage({
+        role: 'assistant',
+        content: [{ type: 'text', text: 's' }],
+        source: {
+          kind: 'model',
+          ...{ provider: 'mock', model: 'mock' },
+        },
+      }),
+    }, { surfaceOp: op, sourceEventSeqs: [0] })
    // Mutate caller's object after append.
    op.start = 99
    const logged = s.events[1]! as SurfaceEvent
@@ -423,8 +668,20 @@ describe('deriveMessages with surface', () => {
    s.append('turn/start', { turn: 1, trigger: { kind: 'message', source: { kind: 'user' } } })
    s.append('assistant/chunk', { turn: 1, step: 1, chunk: { type: 'text-delta', index: 0, text: 'h' } })
    s.append('assistant/chunk', { turn: 1, step: 1, chunk: { type: 'text-delta', index: 1, text: 'i' } })
-    s.append('user/message', { content: [{ type: 'text', text: 'hello' }], source: { kind: 'user' } }, { surfaceOp: 'append' })
-    s.append('assistant/message', { provenance: { provider: 'mock', model: 'mock' }, turn: 1, step: 1, content: [{ type: 'text', text: 'hi' }] }, { surfaceOp: 'append' })
+    s.append('user/message', createUserMessage({
+      content: [{ type: 'text', text: 'hello' }], source: { kind: 'user' },
+    }), { surfaceOp: 'append' })
+    s.append('assistant/message', {
+      turn: 1, step: 1,
+      message: createMessage({
+        role: 'assistant',
+        content: [{ type: 'text', text: 'hi' }],
+        source: {
+          kind: 'model',
+          ...{ provider: 'mock', model: 'mock' },
+        },
+      }),
+    }, { surfaceOp: 'append' })
    s.append('turn/end', { turn: 1, reason: { kind: 'completed' } })
    // Chunks and boundaries are NOT in the surface, so only 2 messages.
    expect(s.deriveMessages()).toHaveLength(2)
@@ -432,8 +689,20 @@ describe('deriveMessages with surface', () => {

  it('deriveMessages via surface respects replace (shadowed nodes are excluded)', () => {
    const s = new Session(SessionId('compacted'))
-    s.append('user/message', { content: [{ type: 'text', text: 'original' }], source: { kind: 'user' } }, { surfaceOp: 'append' })
-    s.append('assistant/message', { provenance: { provider: 'mock', model: 'mock' }, turn: 1, step: 1, content: [{ type: 'text', text: 'compacted' }] }, { surfaceOp: { op: 'replace', start: 0, end: 0 }, sourceEventSeqs: [0] })
+    s.append('user/message', createUserMessage({
+      content: [{ type: 'text', text: 'original' }], source: { kind: 'user' },
+    }), { surfaceOp: 'append' })
+    s.append('assistant/message', {
+      turn: 1, step: 1,
+      message: createMessage({
+        role: 'assistant',
+        content: [{ type: 'text', text: 'compacted' }],
+        source: {
+          kind: 'model',
+          ...{ provider: 'mock', model: 'mock' },
+        },
+      }),
+    }, { surfaceOp: { op: 'replace', start: 0, end: 0 }, sourceEventSeqs: [0] })
    // Only the compaction node is visible.
    const messages = s.deriveMessages()
    expect(messages).toHaveLength(1)
@@ -442,8 +711,16 @@ describe('deriveMessages with surface', () => {

  it('injected-context and steering/message appear on surface', () => {
    const s = new Session(SessionId('ctx'))
-    s.append('user/message', { content: [{ type: 'text', text: 'file changed' }], source: { kind: 'plugin', plugin: 'watcher' } }, { surfaceOp: 'append' })
-    s.append('steering/message', { turn: 1, content: [{ type: 'text', text: 'focus' }], source: { kind: 'user' } }, { surfaceOp: 'append' })
+    s.append('user/message', createUserMessage({
+      content: [{ type: 'text', text: 'file changed' }], source: { kind: 'plugin', plugin: 'watcher' },
+    }), { surfaceOp: 'append' })
+    s.append('steering/message', {
+      turn: 1,
+      message: createUserMessage({
+        content: [{ type: 'text', text: 'focus' }],
+        source: { kind: 'user' },
+      }),
+    }, { surfaceOp: 'append' })
    const messages = s.deriveMessages()
    expect(messages).toHaveLength(2)
    expect(messages[0]!.content).toEqual([{ type: 'text', text: 'file changed' }])
@@ -457,7 +734,17 @@ describe('Session.append surface opts', () => {
    s.append('turn/start', { turn: 1, trigger: { kind: 'message', source: { kind: 'user' } } })
    s.append('step/start', { turn: 1, step: 1 })
    const event = s.append('assistant/message',
-      { provenance: { provider: 'mock', model: 'mock' }, turn: 1, step: 1, content: [{ type: 'text', text: 'h' }] },
+      {
+        turn: 1, step: 1,
+        message: createMessage({
+          role: 'assistant',
+          content: [{ type: 'text', text: 'h' }],
+          source: {
+            kind: 'model',
+            ...{ provider: 'mock', model: 'mock' },
+          },
+        }),
+      },
      { surfaceOp: 'append', sourceEventSeqs: [0, 1] },
    )
    expect(event.sourceEventSeqs).toEqual([0, 1])
@@ -474,7 +761,17 @@ describe('Session.append surface opts', () => {
    const seed: SessionEvent[] = [
      { type: 'turn/start', seq: 0, time: 1, data: { turn: 1, trigger: { kind: 'message', source: { kind: 'user' } } } },
      { type: 'step/start', seq: 1, time: 2, data: { turn: 1, step: 1 } },
-      { type: 'assistant/message', seq: 2, time: 3, data: { turn: 1, step: 1, content: [], provenance: { provider: 'mock', model: 'mock' } }, surfaceOp: 'append' },
+      { type: 'assistant/message', seq: 2, time: 3, data: {
+        turn: 1, step: 1,
+        message: createMessage({
+          role: 'assistant',
+          content: [],
+          source: {
+            kind: 'model',
+            ...{ provider: 'mock', model: 'mock' },
+          },
+        }),
+      }, surfaceOp: 'append' },
      { type: 'step/end', seq: 3, time: 4, data: { turn: 1, step: 1 } },
      { type: 'turn/end', seq: 4, time: 5, data: { turn: 1, reason: { kind: 'completed' } } },
    ]
@@ -492,7 +789,17 @@ describe('Session.append surface opts', () => {

  it('surfaceOp primitives are not cloned (they are immutable)', () => {
    const s = new Session(SessionId('prim'))
-    const event = s.append('assistant/message', { provenance: { provider: 'mock', model: 'mock' }, turn: 1, step: 1, content: [] }, { surfaceOp: 'append' })
+    const event = s.append('assistant/message', {
+      turn: 1, step: 1,
+      message: createMessage({
+        role: 'assistant',
+        content: [],
+        source: {
+          kind: 'model',
+          ...{ provider: 'mock', model: 'mock' },
+        },
+      }),
+    }, { surfaceOp: 'append' })
    // The string 'append' is a primitive — identity-preserving is fine.
    expect(event.surfaceOp).toBe('append')
  })
@@ -503,7 +810,9 @@ describe('Session.append surface opts', () => {
    // SurfaceEvent — it would otherwise be silently dropped from the surface.
    const noMarker: SessionEvent = {
      type: 'user/message', seq: 0, time: 1,
-      data: { content: [{ type: 'text', text: 'hi' }], source: { kind: 'user' } },
+      data: createUserMessage({
+        content: [{ type: 'text', text: 'hi' }], source: { kind: 'user' },
+      }),
    }
    expect(isSurfaceEvent(noMarker)).toBe(false)
    // A non-surface type is rejected too (the type gate).
@@ -545,7 +854,9 @@ describe('surface type guards', () => {
      type: 'user/message',
      seq: 0,
      time: 0,
-      data: { content: [{ type: 'text', text: 'hi' }], source: { kind: 'user' } },
+      data: createUserMessage({
+        content: [{ type: 'text', text: 'hi' }], source: { kind: 'user' },
+      }),
    }
    expect(isSurfaceEligibleType(markerless.type)).toBe(true)
    expect(isSurfaceEvent(markerless)).toBe(false)
@@ -556,16 +867,20 @@ describe('SurfaceManager.replaceGeneration', () => {
  it('folds the pending log delta on access and counts replaces', () => {
    const s = new Session(SessionId('gen'))
    s.append('turn/start', { turn: 1, trigger: { kind: 'message', source: { kind: 'user' } } })
-    s.append('user/message', { content: [{ type: 'text', text: 'one' }], source: { kind: 'user' } }, { surfaceOp: 'append' })
-    s.append('user/message', { content: [{ type: 'text', text: 'two' }], source: { kind: 'user' } }, { surfaceOp: 'append' })
+    s.append('user/message', createUserMessage({
+      content: [{ type: 'text', text: 'one' }], source: { kind: 'user' },
+    }), { surfaceOp: 'append' })
+    s.append('user/message', createUserMessage({
+      content: [{ type: 'text', text: 'two' }], source: { kind: 'user' },
+    }), { surfaceOp: 'append' })
    // Read the generation FIRST — before nodes — so the getter itself folds
    // the pending delta rather than piggybacking on a nodes read.
    expect(s.surface.replaceGeneration).toBe(0)

    const nodes = s.surface.nodes
-    s.append('user/message', {
+    s.append('user/message', createUserMessage({
      content: [{ type: 'text', text: 'summary' }], source: { kind: 'plugin', plugin: 'compact' },
-    }, { surfaceOp: { op: 'replace', start: nodes[0]!, end: nodes[1]! }, sourceEventSeqs: [nodes[0]!, nodes[1]!] })
+    }), { surfaceOp: { op: 'replace', start: nodes[0]!, end: nodes[1]! }, sourceEventSeqs: [nodes[0]!, nodes[1]!] })
    expect(s.surface.replaceGeneration).toBe(1)
  })
 })