fix(web): record which preset a session actually runs

The creation header names the preset a session STARTED with and is frozen,
which is correct — it is a creation fact. Switching is legal only while a
session is blank, and that looked like enough: no history exists yet.

It is not, because the switch's effect outlives the blank window. The user
switches, then sends the first message; every turn from there runs under the
new composition while the header still names the old one. The session is
then locked around a misrecorded preset, and resume reads the header to
rebuild it — composing one preset's tools over a history another produced,
which is exactly the replay the blank-only lock exists to prevent, reached
by another route. A picker showed `standard` for a session running
`core-web`.

A switch is now an `agent-preset/selected` event appended after the swap
commits, and `resolveSessionPreset()` (last selection, else the header) is
what every reconstruction reads: the summary, resume, the conflict guard,
and the fork introduced one layer down.

packages/preset/agent-presets/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/preset/agent-presets/README.md
-README.md: 5e785b747209d4c0cedaebbd3a90ba1b46dcd1c6
-README.zh.md: 4c40d7b7bfabb83dba2859251ad189a2646170c6
+README.md: b60d89b6dcda97a7570680072195231885fd0d45
+README.zh.md: f2485663ada031a9e8fa5ce3a06327e6cbc1de10

packages/preset/agent-presets/README.md

@@ -21,6 +21,12 @@ Discovery is unmemoized: `list()` and `resolve()` re-read the roots on every cal
 
 The agent factory's `setup(agentCtx)` hook is the one supported call site. Only there is the composition installed while the agent is still unpublished, so a rejected mount rolls the whole creation back rather than leaving a half-composed session. The subtree is owned by `agentCtx`'s fiber, so it unwinds with the agent and the caller receives no disposer.
 
+### Which preset a session runs
+
+The creation header names the preset a session STARTED with; `resolveSessionPreset(session)` names the one it RUNS. They differ whenever a blank session switched, so every reconstruction path — the summary a picker reads, a resume, a fork — resolves rather than reading the header.
+
+The header stays frozen because it is a creation fact. A switch is an `agent-preset/selected` session event appended after the swap commits, which is what the model-visible ⟺ logged rule requires: the preset decides the tool schemas and prompt sections the model sees, so it has to be reconstructable from the log. Reading the header alone would rebuild a switched session under the composition it was created with, replaying history the new tool set cannot act on — the exact hazard the blank-only lock exists to prevent.
+
 ## Config
 
 | Field | Default | Meaning |

packages/preset/agent-presets/README.zh.md

@@ -21,6 +21,12 @@
 
 agent 工厂的 `setup(agentCtx)` 钩子是唯一受支持的调用点。只有在那里，组装是在 agent 尚未发布时装入的，因此挂载被拒绝会让整次创建回滚，而不会留下一个组装到一半的会话。子树归 `agentCtx` 的 fiber 所有，随 agent 一起卸载，调用方无需持有 disposer。
 
+### 会话实际运行的是哪个 preset
+
+创建头部记录的是会话**以什么开始**，`resolveSessionPreset(session)` 给出的才是它**实际运行的**。空白会话一旦切换过，两者就不同，因此所有重建路径——选择器读取的摘要、resume、fork——都走解析，而非直接读头部。
+
+头部保持冻结，因为它是创建期事实。切换以 `agent-preset/selected` 会话事件记录，在替换提交之后追加；这正是 model-visible ⟺ logged 规则的要求：preset 决定模型看到的工具 schema 与提示词段落，因此必须能从日志重建。只读头部会让切换过的会话按创建时的组装重建，从而重放新工具集无法执行的历史——这正是「仅空白可切」那道锁要防的危险。
+
 ## 配置
 
 | 字段 | 默认值 | 含义 |

packages/preset/agent-presets/package.json

@@ -30,6 +30,7 @@
     "@deepseek-ai/dsh-invariants": "^0.0.1",
     "@deepseek-ai/dsh-paths": "^0.0.1",
     "@deepseek-ai/dsh-scope": "^0.0.1",
+    "@deepseek-ai/dsh-session": "^0.0.1",
     "@deepseek-ai/dsh-settings": "^0.0.1",
     "cordis": "^4.0.0-rc.7"
   },

packages/preset/agent-presets/src/index.ts

@@ -37,6 +37,7 @@ export {
   inactiveRows, leakedServices, livePresetMounts, mountPreset, serviceForAgent,
   unmountPresetFor, type PresetMount,
 } from './mount.ts'
+export { resolveSessionPreset, type PresetBearingSession } from './session.ts'
 export { PresetMountError, UnknownPresetError } from './types.ts'
 export type { AgentPreset, Config, PresetRoot, PresetTrust } from './types.ts'
 

packages/preset/agent-presets/src/session.ts

@@ -0,0 +1,54 @@
+/**
+ * The session-log record of which preset a session actually runs.
+ *
+ * The creation header names the preset a session STARTED with, and it is
+ * deep-frozen because that is a creation fact. A session may still change
+ * preset while it is blank, and the effect of that change outlives the blank
+ * window: the first turn — and every turn after it — runs under the newly
+ * mounted composition. Recording the change is what keeps the log honest, and
+ * it is required outright by the repo's model-visible ⟺ logged rule, since the
+ * preset decides the tool schemas and prompt sections the model sees.
+ *
+ * Reconstruction reads {@link resolveSessionPreset}, never the header alone.
+ * @module @deepseek-ai/dsh-agent-presets/session
+ */
+
+import type { SessionEvent, SessionHeader } from '@deepseek-ai/dsh-session'
+
+declare module '@deepseek-ai/dsh-session' {
+  interface SessionEventMap {
+    /**
+     * The session's agent preset was chosen after creation, while the session
+     * was still blank. Log-only: it records the composition later turns ran
+     * under, so a resumed or forked session rebuilds the same one instead of
+     * the header's creation-time value.
+     */
+    'agent-preset/selected': { agentPreset: string }
+  }
+}
+
+/** The minimum a caller must supply to resolve a session's preset. */
+export interface PresetBearingSession {
+  /** The session's creation header. */
+  readonly header: SessionHeader
+  /** The session's event log, oldest first. */
+  readonly events: readonly SessionEvent[]
+}
+
+/**
+ * The preset a session actually runs, newest selection winning.
+ *
+ * The header supplies the creation-time value; every later selection is a
+ * logged event, so the last one is the answer. Reading the header alone
+ * rebuilds a switched session under the composition it was created with, not
+ * the one its history was produced under.
+ * @param session - the session's header and event log.
+ * @returns the preset id, or `undefined` when the deployment composes none.
+ */
+export function resolveSessionPreset(session: PresetBearingSession): string | undefined {
+  for (let index = session.events.length - 1; index >= 0; index -= 1) {
+    const event = session.events[index]
+    if (event?.type === 'agent-preset/selected') return event.data.agentPreset
+  }
+  return session.header.agentPreset
+}

packages/preset/agent-presets/tsconfig.json

@@ -21,6 +21,9 @@
     {
       "path": "../../core/scope"
     },
+    {
+      "path": "../../core/session"
+    },
     {
       "path": "../../settings/settings"
     },