fix(web): bind prompts to plan selection

packages/host/apiproxy/README.md

@@ -10,7 +10,7 @@ The layering/protocol decisions are recorded in the [GUI layering and RPC protoc
 
 The mux stream projects the latest log-backed title as a validated `session/title` control frame after each attached-session subscription baseline and immediately after the corresponding live raw title event. This projection does not add titles to `session.list`; cold sessions remain metadata-only there until opening or resuming attaches their logs.
 
-Plan mode uses two unary methods instead of deriving current state from a history page: `session.planMode` returns the committed state plus any boundary-pending selection, and `session.setPlanMode` records a selection and returns the same authoritative shape. A present `pending` target must differ from `active`; a net-zero service cleanup intent projects as `{ active }`, and the wire schema rejects equal values. Both methods return `null` when the optional host service is absent; `null` is capability absence, while `{ active: false }` is a supported inactive session. Committed changes still arrive through the raw logged `plan/mode` session event.
+Plan mode uses two unary methods instead of deriving current state from a history page: `session.planMode` returns the committed state plus any boundary-pending selection, and `session.setPlanMode` records a selection and returns the same authoritative shape. A present `pending` target must differ from `active`; a net-zero service cleanup intent projects as `{ active }`, and the wire schema rejects equal values. Both methods return `null` when the optional host service is absent; `null` is capability absence, while `{ active: false }` is a supported inactive session. Committed changes still arrive through the raw logged `plan/mode` session event. `session.prompt` may carry a `planMode` target so the host records that selection immediately before accepting the prompt; it fails when the capability is absent and restores the prior target after a synchronous prompt rejection.
 
 ## Carrier layer (`/client` + root)
 

packages/host/apiproxy/src/api/sessions.schema.ts

@@ -92,6 +92,7 @@ export const sessionPromptRequestSchema = z.object({
   sessionId: sessionIdSchema,
   mode: z.union([z.literal('queue'), z.literal('steer')]),
   content: z.array(contentBlockSchema),
+  planMode: z.boolean().optional(),
 }) as unknown as z.ZodType<RequestPayload<'session.prompt'>>
 
 /** session.prompt response value. */

packages/host/apiproxy/src/api/sessions.ts

@@ -74,8 +74,17 @@ export interface SessionsApi {
   history(request: RpcRequest<{ sessionId: SessionId; beforeSeq?: number; maxMessages?: number }>):
   Promise<RpcResponse<{ events: HistoryEntry[]; hasMore: boolean }>>
 
-  /** Sends a message. content is core's ContentBlock[] verbatim; mode maps 1:1 — queue→send, steer→steer. */
-  prompt(request: RpcRequest<{ sessionId: SessionId; mode: 'queue' | 'steer'; content: ContentBlock[] }>):
+  /**
+   * Sends a message. `content` is core's ContentBlock[] verbatim and `mode`
+   * maps 1:1 — queue→send, steer→steer. An optional `planMode` target is
+   * admitted atomically with the prompt.
+   */
+  prompt(request: RpcRequest<{
+    sessionId: SessionId
+    mode: 'queue' | 'steer'
+    content: ContentBlock[]
+    planMode?: boolean
+  }>):
   Promise<RpcResponse<{ accepted: true }>>
 
   /** Stops: clears both FIFOs + aborts the current step (1:1 with agent.cancel). */

packages/host/apiproxy/tests/fetch-carrier.spec.ts

@@ -84,7 +84,12 @@ describe('unary round trip (handler ⇄ client, no network)', () => {
   it('covers create/prompt/cancel/plan/describe passthrough', async () => {
     const c = client()
     expect((await c.sessions.create({})).result.ok).toBe(true)
-    expect((await c.sessions.prompt({ sessionId: 's' as never, mode: 'queue', content: [{ type: 'text', text: 'x' }] })).result.ok).toBe(true)
+    expect((await c.sessions.prompt({
+      sessionId: 's' as never,
+      mode: 'queue',
+      content: [{ type: 'text', text: 'x' }],
+      planMode: true,
+    })).result.ok).toBe(true)
     expect((await c.sessions.cancel({ sessionId: 's' as never })).result.ok).toBe(true)
     expect((await c.sessions.planMode({ sessionId: 's' as never })).result).toEqual({ ok: true, value: null })
     expect((await c.sessions.setPlanMode({ sessionId: 's' as never, active: true })).result).toEqual({ ok: true, value: null })

packages/host/apiproxy/tests/rpc-schemas.spec.ts

@@ -101,9 +101,14 @@ describe('sessions domain schemas', () => {
     expect(sessionHistoryRequestSchema.parse({ sessionId: 's1', beforeSeq: 3, maxMessages: 5 }).beforeSeq).toBe(3)
     expect(() => sessionHistoryRequestSchema.parse({ sessionId: 's1', maxMessages: 0 })).toThrow()
     expect(sessionHistoryValueSchema.parse({ events: [], hasMore: false }).hasMore).toBe(false)
-    const prompt = sessionPromptRequestSchema.parse({ sessionId: 's1', mode: 'queue', content: [{ type: 'text', text: 'hi' }] })
-    expect(prompt.mode).toBe('queue')
+    const prompt = sessionPromptRequestSchema.parse({
+      sessionId: 's1', mode: 'queue', content: [{ type: 'text', text: 'hi' }], planMode: true,
+    })
+    expect(prompt).toMatchObject({ mode: 'queue', planMode: true })
     expect(() => sessionPromptRequestSchema.parse({ sessionId: 's1', mode: 'inject', content: [] })).toThrow()
+    expect(() => sessionPromptRequestSchema.parse({
+      sessionId: 's1', mode: 'queue', content: [], planMode: 'plan',
+    })).toThrow()
     expect(sessionPromptValueSchema.parse({ accepted: true }).accepted).toBe(true)
     expect(sessionCancelRequestSchema.parse({ sessionId: 's1' }).sessionId).toBe('s1')
     expect(sessionCancelValueSchema.parse({ accepted: true }).accepted).toBe(true)