fix(subagent): preserve follow-up provenance

packages/subagent/tool-subagent-control/README.md

@@ -2,7 +2,7 @@
 
 The globally named `send_message` tool: a thin adapter over `ctx.subagentControl.sendMessage()`. Provider-bound `@deepseek-ai/dsh-tool-subagent` instances register distinct delegation tools per transport; this separately loaded package registers the one shared control tool, so multiple delegation tools never register duplicate global controls.
 
-The tool performs no lifecycle routing. The control service decides between live delivery to the running activation's existing Task and a fresh Task that cold-resumes the durable child; the tool renders which route was taken and the relevant Task id. A control-service throw becomes an errored tool result stating the message was not delivered.
+The tool performs no lifecycle routing. It attributes every follow-up as `{ kind: 'coordinator', senderSessionId: parent.id }`; the control service preserves that source while deciding between live delivery to the running activation's existing Task and a fresh Task that cold-resumes the durable child. The tool renders which route was taken and the relevant Task id. A control-service throw becomes an errored tool result stating the message was not delivered.
 
 ## Model Experience
 

packages/subagent/tool-subagent-control/src/index.ts

@@ -67,7 +67,12 @@ export function apply(ctx: Context): void {
         throw new Error('send_message requires a calling agent (exec.agent was undefined)')
       }
       const message: ContentBlock[] = [{ type: 'text', text: args.message }]
-      const result = ctx.subagentControl.sendMessage(parent, SessionId(args.subagent_id), message)
+      const result = ctx.subagentControl.sendMessage(
+        parent,
+        SessionId(args.subagent_id),
+        message,
+        { kind: 'coordinator', senderSessionId: parent.id },
+      )
       return Promise.resolve(result)
     },
   }))

packages/subagent/tool-subagent-control/tests/tool-subagent-control.spec.ts

@@ -83,17 +83,27 @@ describe('dsh-tool-subagent-control', () => {
     expect(text(result)).toBe(`message started task subagent-2 continuing subagent ${started.childId}`)
     const collected = await callTool(ctx, 'task_output', { task_id: 'subagent-2', wait: true }, parent)
     expect(text(collected)).toBe('second answer\n[status: completed]')
+    const loaded = await ctx.sessionPersistence.load(started.childId)
+    const followUp = loaded.events.findLast(event =>
+      event.type === 'user/message',
+    )
+    expect(followUp?.type === 'user/message' && followUp.data.source).toEqual({
+      kind: 'coordinator',
+      senderSessionId: parent.id,
+    })
   })
 
   it('renders the steered route when the child is still running', async () => {
     // Script the child's single turn as two steps: the steer joins mid-turn.
     const { ctx, parent } = await setup([])
     let steered: string | undefined
+    let source: unknown
     // Reach past the tool into the control service to fake a running route
     // deterministically: the tool is a thin adapter, so its steered wording is
     // what this test pins.
-    ctx.subagentControl.sendMessage = (agent, _childId, message) => {
+    ctx.subagentControl.sendMessage = (agent, _childId, message, messageSource) => {
       steered = (message[0] as { text: string }).text
+      source = messageSource
       return { route: 'steered', taskId: ctx.tasks.list(agent)[0]?.id ?? ('subagent-9' as never) }
     }
     const result = await callTool(ctx, 'send_message', {
@@ -102,6 +112,7 @@ describe('dsh-tool-subagent-control', () => {
     }, parent)
     expect(result.isError).toBe(false)
     expect(steered).toBe('also consider Y')
+    expect(source).toEqual({ kind: 'coordinator', senderSessionId: parent.id })
     expect(text(result)).toBe('message delivered to running task subagent-9')
   })