refactor(agent): remove message metadata channel

packages/core/session/README.md

@@ -64,7 +64,7 @@ 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. `messagePrefix` remains separate from derived history. 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 (`user` source), a synthetic injection (`plugin`/`goal` source), or an admitted goal round — `source` is the only channel that tells them apart. It may attach JSON `meta` for replayable plugin state; metadata remains durable but is excluded from `deriveMessages()`. A `user/message` or `steering/message` with prompt-prefix context keeps the exact combined model bytes in `content` and stores a model-hidden `envelope` containing the direct `displayContent` and prefix context source/metadata descriptors. `displayPromptContent()` selects the human-facing prompt without changing derived history.
+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. A `user/message` or `steering/message` with prompt-prefix context keeps the exact combined model bytes in `content` and stores a model-hidden `envelope` containing the direct `displayContent` and prefix context sources. `displayPromptContent()` selects the human-facing prompt without changing derived history.
 
 `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`.
 

packages/core/session/src/types.ts

@@ -187,8 +187,6 @@ export type RequestHeaderReason = 'initial' | 'resume' | 'change'
 export interface PromptPrefixContext {
   /** Producer provenance retained for transcript presentation and inspection. */
   source: MessageSource
-  /** Opaque JSON state retained in the session event but hidden from the model. */
-  meta?: JsonValue
 }
 
 /**
@@ -209,7 +207,7 @@ export interface PromptMessageEnvelope {
  * 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. `meta` carries durable model-hidden producer state.
+ * not by event type.
  */
 export interface PromptMessageData {
   /** Exact model-facing blocks, including any baked prompt-prefix contexts. */
@@ -218,15 +216,6 @@ export interface PromptMessageData {
   source: MessageSource
   /** Present only when prompt-prefix contexts were baked into `content`. */
   envelope?: PromptMessageEnvelope
-  /**
-   * Opaque durable JSON state retained on the event but hidden from the model
-   * projection. It is the intended channel for a future framing directive (a
-   * producer declares the frame, a dedicated renderer applies it — see the
-   * deferred note in
-   * ../../../../.agents/notes/implemented/simplification/2026-07-20-unwrap-injected-content-envelopes.md),
-   * so the surface keeps projecting `content` verbatim rather than wrapping it.
-   */
-  meta?: JsonValue
 }
 
 /**

packages/core/session/tests/session.spec.ts

@@ -147,7 +147,7 @@ describe('Session', () => {
       source: { kind: 'user' },
       envelope: {
         displayContent: [{ type: 'text', text: 'question' }],
-        prefixContexts: [{ source: { kind: 'plugin', plugin: 'reference' }, meta: { kind: 'card' } }],
+        prefixContexts: [{ source: { kind: 'plugin', plugin: 'reference' } }],
       },
     }, { surfaceOp: 'append' })
 
@@ -165,17 +165,11 @@ describe('Session', () => {
       .toEqual(session.deriveMessages())
   })
 
-  it('keeps context meta durable in the event while hiding it from the projection', () => {
+  it('keeps context source durable in the event while hiding it from the projection', () => {
     const session = new Session(SessionId('s2-raw'))
-    const meta = {
-      kind: 'workspace-instructions',
-      version: 1,
-      changes: [{ action: 'set', scope: 'pkg', path: 'pkg/AGENTS.md', digest: 'abc123' }],
-    }
     session.append('user/message', {
       content: [{ type: 'text', text: '<system-reminder>Additional instructions from: pkg/AGENTS.md</system-reminder>' }],
       source: { kind: 'plugin', plugin: 'workspace-context' },
-      meta,
     }, { surfaceOp: 'append' })
 
     expect(session.deriveMessages()).toEqual([{
@@ -183,7 +177,7 @@ describe('Session', () => {
       content: [{ type: 'text', text: '<system-reminder>Additional instructions from: pkg/AGENTS.md</system-reminder>' }],
     }])
     const event = session.events[0]
-    expect(event?.type === 'user/message' && event.data.meta).toEqual(meta)
+    expect(event?.type === 'user/message' && event.data.source).toEqual({ kind: 'plugin', plugin: 'workspace-context' })
   })
 
   it('replays identically from a seeded event log', () => {