fix(acp): surface subagent activity

packages/cli/src/acp/event.ts

@@ -1,4 +1,4 @@
-import type { AgentSideConnection, PromptResponse } from "@agentclientprotocol/sdk"
+import type { AgentSideConnection, PromptResponse, SessionUpdate } from "@agentclientprotocol/sdk"
 import type {
   EventSubscribeOutput,
   OpenCodeClient,
@@ -37,6 +37,34 @@ export type TurnStart =
   | { readonly type: "skill"; readonly id: string }
   | { readonly type: "compaction"; readonly id: string }
 
+export const ChildSessionUpdatesCapability = "opencode/child-session-updates"
+export const ChildSessionUpdateMethod = "opencode/session/child_update"
+
+type ChildSessionUpdateBase = {
+  readonly rootSessionId: string
+  readonly childSessionId: string
+  readonly parentSessionId: string
+  readonly depth: number
+  readonly title?: string
+}
+
+export type ChildSessionUpdate = ChildSessionUpdateBase &
+  (
+    | { readonly type: "update"; readonly update: SessionUpdate }
+    | {
+        readonly type: "status"
+        readonly status: "created" | "running" | "completed" | "failed" | "interrupted"
+        readonly error?: { readonly type: string; readonly message: string }
+      }
+  )
+
+type ChildSession = {
+  readonly id: string
+  readonly parentID: string
+  readonly depth: number
+  readonly title?: string
+}
+
 function emptyToolState(): ToolState {
   return { name: "tool", input: {}, metadata: {}, content: [] }
 }
@@ -50,8 +78,13 @@ export async function streamTurn(input: {
   readonly writeTextFile: boolean
   readonly submit: (signal: AbortSignal) => Promise<unknown>
   readonly control: TurnControl
+  readonly childSessionUpdate?: (update: ChildSessionUpdate) => Promise<void>
+  readonly connectionSignal?: AbortSignal
+  readonly sessionSignal?: AbortSignal
 }): Promise<PromptResponse> {
   const streamController = new AbortController()
+  const connectionAbort = () => streamController.abort()
+  input.connectionSignal?.addEventListener("abort", connectionAbort, { once: true })
   const stream = input.client.event.subscribe({ signal: streamController.signal })[Symbol.asyncIterator]()
   const connected = await stream.next()
   if (connected.done) throw new Error("event stream disconnected before prompt admission")
@@ -62,47 +95,113 @@ export async function streamTurn(input: {
   let finish: SessionMessageAssistant["finish"]
   let executionError: { readonly type: string; readonly message: string } | undefined
   const tools = new Map<string, ToolState>()
+  const children = new Map<string, ChildSession>()
+  const openChildren = new Set<string>()
+  let standard = true
+  let handedOff = false
 
-  const update = (value: Parameters<Connection["sessionUpdate"]>[0]["update"]) =>
-    input.connection.sessionUpdate({ sessionId: input.sessionID, update: value })
+  const notifyChild = async (
+    child: ChildSession,
+    value:
+      | { readonly type: "update"; readonly update: SessionUpdate }
+      | {
+          readonly type: "status"
+          readonly status: "created" | "running" | "completed" | "failed" | "interrupted"
+          readonly error?: { readonly type: string; readonly message: string }
+        },
+  ) => {
+    if (!input.childSessionUpdate) return
+    await input
+      .childSessionUpdate({
+        rootSessionId: input.sessionID,
+        childSessionId: child.id,
+        parentSessionId: child.parentID,
+        depth: child.depth,
+        ...(child.title ? { title: child.title } : {}),
+        ...value,
+      })
+      .catch(() => {})
+  }
 
-  const consume = async () => {
+  const updateSession = async (value: SessionUpdate, child?: ChildSession) => {
+    const projected = child ? projectChildUpdate(value, child) : value
+    if (standard && (!child || !input.childSessionUpdate)) {
+      await input.connection.sessionUpdate({ sessionId: input.sessionID, update: projected })
+    }
+    if (child) await notifyChild(child, { type: "update", update: projected })
+  }
+
+  const consume = async (mode: "turn" | "background") => {
     while (!streamController.signal.aborted) {
       const next = await stream.next()
       if (next.done) throw new Error("event stream disconnected during prompt execution")
       const event = next.value
-      if (event.type === "permission.asked" && event.data.sessionID === input.sessionID) {
-        const tool = event.data.source?.callID ? tools.get(event.data.source.callID) : undefined
+      if (event.type === "session.created") {
+        const parentID = event.data.info.parentID
+        if (!parentID) continue
+        const parent = parentID === input.sessionID ? undefined : children.get(parentID)
+        if ((mode === "turn" && parentID === input.sessionID) || parent) {
+          const child = {
+            id: event.data.sessionID,
+            parentID,
+            depth: parent ? parent.depth + 1 : 1,
+            title: event.data.info.title,
+          }
+          children.set(child.id, child)
+          openChildren.add(child.id)
+          await notifyChild(child, { type: "status", status: "created" })
+        }
+        continue
+      }
+
+      const eventSessionID = sessionID(event)
+      const child = eventSessionID ? children.get(eventSessionID) : undefined
+      const send = (update: SessionUpdate) => updateSession(update, child)
+      if (mode === "background" && !child) continue
+
+      if (event.type === "permission.asked" && (event.data.sessionID === input.sessionID || child)) {
+        const tool = event.data.source?.callID
+          ? tools.get(toolKey(event.data.sessionID, event.data.source.callID))
+          : undefined
         await replyPermission({
           client: input.client,
           connection: input.connection,
           event,
-          sessionID: input.sessionID,
+          sessionID: event.data.sessionID,
+          clientSessionID: input.sessionID,
           cwd: input.cwd,
           tool,
+          ...(child ? { toolCallPrefix: child.id, titlePrefix: child.title } : {}),
         })
         continue
       }
-      if (event.type === "form.created" && event.data.form.sessionID === input.sessionID) {
+      if (event.type === "form.created" && (event.data.form.sessionID === input.sessionID || child)) {
         await input.client.form
-          .cancel({ sessionID: input.sessionID, formID: event.data.form.id })
-          .catch(() => input.client.session.interrupt({ sessionID: input.sessionID }).catch(() => {}))
+          .cancel({ sessionID: event.data.form.sessionID, formID: event.data.form.id })
+          .catch(() => input.client.session.interrupt({ sessionID: event.data.form.sessionID }).catch(() => {}))
         continue
       }
-      if (!("sessionID" in event.data) || event.data.sessionID !== input.sessionID) continue
+      if (!eventSessionID || (eventSessionID !== input.sessionID && !child)) continue
       if (matchesStart(event, input.start)) {
         started = true
         continue
       }
       if (!started) continue
 
+      if (event.type === "session.execution.started") {
+        if (child) {
+          await notifyChild(child, { type: "status", status: "running" })
+        }
+        continue
+      }
+
       if (event.type === "session.step.started") {
-        assistantMessageID = event.data.assistantMessageID
+        if (!child) assistantMessageID = event.data.assistantMessageID
         continue
       }
       if (event.type === "session.text.delta") {
-        assistantMessageID = event.data.assistantMessageID
-        await update({
+        if (!child) assistantMessageID = event.data.assistantMessageID
+        await send({
           sessionUpdate: "agent_message_chunk",
           messageId: event.data.assistantMessageID,
           content: { type: "text", text: event.data.delta },
@@ -110,8 +209,8 @@ export async function streamTurn(input: {
         continue
       }
       if (event.type === "session.reasoning.delta") {
-        assistantMessageID = event.data.assistantMessageID
-        await update({
+        if (!child) assistantMessageID = event.data.assistantMessageID
+        await send({
           sessionUpdate: "agent_thought_chunk",
           messageId: event.data.assistantMessageID,
           content: { type: "text", text: event.data.delta },
@@ -119,9 +218,14 @@ export async function streamTurn(input: {
         continue
       }
       if (event.type === "session.tool.input.started") {
-        assistantMessageID = event.data.assistantMessageID
-        tools.set(event.data.callID, { name: event.data.name, input: {}, metadata: {}, content: [] })
-        await update({
+        if (!child) assistantMessageID = event.data.assistantMessageID
+        tools.set(toolKey(event.data.sessionID, event.data.callID), {
+          name: event.data.name,
+          input: {},
+          metadata: {},
+          content: [],
+        })
+        await send({
           sessionUpdate: "tool_call",
           ...pendingToolCall({
             toolCallId: event.data.callID,
@@ -133,11 +237,12 @@ export async function streamTurn(input: {
         continue
       }
       if (event.type === "session.tool.called") {
-        assistantMessageID = event.data.assistantMessageID
-        const current = tools.get(event.data.callID) ?? emptyToolState()
+        if (!child) assistantMessageID = event.data.assistantMessageID
+        const key = toolKey(event.data.sessionID, event.data.callID)
+        const current = tools.get(key) ?? emptyToolState()
         current.input = event.data.input
-        tools.set(event.data.callID, current)
-        await update({
+        tools.set(key, current)
+        await send({
           sessionUpdate: "tool_call_update",
           ...runningToolUpdate({
             toolCallId: event.data.callID,
@@ -149,10 +254,10 @@ export async function streamTurn(input: {
         continue
       }
       if (event.type === "session.tool.progress") {
-        const current = tools.get(event.data.callID)
+        const current = tools.get(toolKey(event.data.sessionID, event.data.callID))
         if (!current) continue
         current.metadata = event.data.metadata
-        await update({
+        await send({
           sessionUpdate: "tool_call_update",
           ...runningToolUpdate({
             toolCallId: event.data.callID,
@@ -164,8 +269,9 @@ export async function streamTurn(input: {
         continue
       }
       if (event.type === "session.tool.success") {
-        const current = tools.get(event.data.callID) ?? emptyToolState()
-        tools.delete(event.data.callID)
+        const key = toolKey(event.data.sessionID, event.data.callID)
+        const current = tools.get(key) ?? emptyToolState()
+        tools.delete(key)
         await syncEditedFiles({
           connection: input.connection,
           writeTextFile: input.writeTextFile,
@@ -175,7 +281,7 @@ export async function streamTurn(input: {
           toolInput: current.input,
           metadata: event.data.metadata ?? {},
         }).catch(() => {})
-        await update({
+        await send({
           sessionUpdate: "tool_call_update",
           ...completedToolUpdate({
             toolCallId: event.data.callID,
@@ -188,9 +294,10 @@ export async function streamTurn(input: {
         continue
       }
       if (event.type === "session.tool.failed") {
-        const current = tools.get(event.data.callID) ?? emptyToolState()
-        tools.delete(event.data.callID)
-        await update({
+        const key = toolKey(event.data.sessionID, event.data.callID)
+        const current = tools.get(key) ?? emptyToolState()
+        tools.delete(key)
+        await send({
           sessionUpdate: "tool_call_update",
           ...errorToolUpdate({
             toolCallId: event.data.callID,
@@ -205,13 +312,33 @@ export async function streamTurn(input: {
         continue
       }
       if (event.type === "session.step.ended") {
-        assistantMessageID = event.data.assistantMessageID
-        finish = event.data.finish
+        if (!child) {
+          assistantMessageID = event.data.assistantMessageID
+          finish = event.data.finish
+        }
+        continue
+      }
+      if (event.type === "session.execution.succeeded") {
+        if (!child) return "succeeded" as const
+        openChildren.delete(child.id)
+        await notifyChild(child, { type: "status", status: "completed" })
+        if (mode === "background" && openChildren.size === 0) return "succeeded" as const
+        continue
+      }
+      if (event.type === "session.execution.interrupted") {
+        if (!child) return "interrupted" as const
+        openChildren.delete(child.id)
+        await notifyChild(child, { type: "status", status: "interrupted" })
+        if (mode === "background" && openChildren.size === 0) return "interrupted" as const
         continue
       }
-      if (event.type === "session.execution.succeeded") return "succeeded" as const
-      if (event.type === "session.execution.interrupted") return "interrupted" as const
       if (event.type === "session.execution.failed") {
+        if (child) {
+          openChildren.delete(child.id)
+          await notifyChild(child, { type: "status", status: "failed", error: event.data.error })
+          if (mode === "background" && openChildren.size === 0) return "failed" as const
+          continue
+        }
         executionError = event.data.error
         return "failed" as const
       }
@@ -219,7 +346,13 @@ export async function streamTurn(input: {
     return "interrupted" as const
   }
 
-  const completed = consume()
+  const completed = consume("turn")
+  const closeStream = async () => {
+    streamController.abort()
+    input.connectionSignal?.removeEventListener("abort", connectionAbort)
+    input.sessionSignal?.removeEventListener("abort", connectionAbort)
+    await stream.return?.(undefined).catch(() => {})
+  }
   try {
     await input.submit(control.admission.signal).catch((error) => {
       if (!control.cancelled) throw error
@@ -233,6 +366,14 @@ export async function streamTurn(input: {
       }
     }
     const terminal = await completed
+    if (input.childSessionUpdate && openChildren.size > 0 && !input.sessionSignal?.aborted) {
+      standard = false
+      handedOff = true
+      input.sessionSignal?.addEventListener("abort", connectionAbort, { once: true })
+      void consume("background")
+        .catch(() => {})
+        .finally(closeStream)
+    }
     const assistant = assistantMessageID
       ? await input.client.session
           .message({ sessionID: input.sessionID, messageID: assistantMessageID })
@@ -250,11 +391,37 @@ export async function streamTurn(input: {
     await completed.catch(() => {})
     throw error
   } finally {
-    streamController.abort()
-    await stream.return?.(undefined).catch(() => {})
+    if (!handedOff) await closeStream()
   }
 }
 
+function sessionID(event: EventSubscribeOutput) {
+  if ("sessionID" in event.data && typeof event.data.sessionID === "string") return event.data.sessionID
+  if (event.type === "form.created") return event.data.form.sessionID
+  return undefined
+}
+
+function toolKey(sessionID: string, callID: string) {
+  return `${sessionID}:${callID}`
+}
+
+function projectChildUpdate(update: SessionUpdate, child: ChildSession) {
+  update._meta = {
+    ...update._meta,
+    "opencode/child-session": {
+      id: child.id,
+      parentID: child.parentID,
+      depth: child.depth,
+      ...(child.title ? { title: child.title } : {}),
+    },
+  }
+  if (update.sessionUpdate === "tool_call" || update.sessionUpdate === "tool_call_update") {
+    update.toolCallId = `${child.id}:${update.toolCallId}`
+    if (update.title && child.title) update.title = `${child.title}: ${update.title}`
+  }
+  return update
+}
+
 export async function replayMessages(
   connection: Pick<AgentSideConnection, "sessionUpdate">,
   sessionID: string,

packages/cli/src/acp/permission.ts

@@ -20,20 +20,30 @@ export async function replyPermission(input: {
   readonly connection: Connection
   readonly event: PermissionEvent
   readonly sessionID: string
+  readonly clientSessionID?: string
   readonly cwd: string
   readonly tool?: Tool
+  readonly toolCallPrefix?: string
+  readonly titlePrefix?: string
 }) {
   const toolName = input.tool?.name ?? input.event.data.action
   const toolInput = { ...input.event.data.metadata, ...input.tool?.input }
   const previews = await permissionPreviews(toolName, toolInput, input.cwd)
   const result = await input.connection
     .requestPermission({
-      sessionId: input.sessionID,
+      sessionId: input.clientSessionID ?? input.sessionID,
       toolCall: {
         ...pendingToolCall({
-          toolCallId: input.event.data.source?.callID ?? input.event.data.id,
+          toolCallId: [input.toolCallPrefix, input.event.data.source?.callID ?? input.event.data.id]
+            .filter((value) => value !== undefined)
+            .join(":"),
           toolName,
-          state: { input: toolInput, title: permissionTitle(toolName, toolInput, previews) },
+          state: {
+            input: toolInput,
+            title: [input.titlePrefix, permissionTitle(toolName, toolInput, previews)]
+              .filter((value) => value !== undefined)
+              .join(": "),
+          },
           cwd: input.cwd,
         }),
         locations: permissionLocations(toolName, toolInput, input.event.data.resources, input.cwd, previews),

packages/cli/src/acp/service.ts

@@ -43,13 +43,21 @@ import { OPENCODE_VERSION } from "../version"
 import { SessionMessage } from "@opencode-ai/schema/session-message"
 import { buildConfigOptions, parseModelSelection, type ConfigOptionProvider } from "./config-option"
 import { promptContentToParts } from "./content"
-import { replayMessages, streamTurn, type TurnControl, type TurnStart } from "./event"
+import {
+  ChildSessionUpdateMethod,
+  ChildSessionUpdatesCapability,
+  replayMessages,
+  streamTurn,
+  type ChildSessionUpdate,
+  type TurnControl,
+  type TurnStart,
+} from "./event"
 import { ACPError } from "./error"
 
 export const AuthMethodID = "opencode-login"
 
 type Connection = Pick<AgentSideConnection, "sessionUpdate" | "requestPermission"> &
-  Partial<Pick<AgentSideConnection, "writeTextFile">>
+  Partial<Pick<AgentSideConnection, "writeTextFile" | "extNotification" | "signal">>
 
 type Catalog = {
   readonly providers: ConfigOptionProvider[]
@@ -64,6 +72,7 @@ type Catalog = {
 type Attached = {
   readonly id: string
   readonly cwd: string
+  readonly abort: AbortController
   catalog: Catalog
   model: ModelRef
   modeID: string
@@ -100,7 +109,7 @@ export function make(input: { readonly client: OpenCodeClient; readonly connecti
   const catalogs = new Map<string, Promise<Catalog>>()
   const registeredMcp = new Map<string, Set<string>>()
   const active = new Map<string, TurnControl>()
-  const capabilities = { writeTextFile: false }
+  const capabilities = { writeTextFile: false, childSessionUpdates: false }
 
   const catalog = (cwd: string) => {
     const cached = catalogs.get(cwd)
@@ -121,9 +130,11 @@ export function make(input: { readonly client: OpenCodeClient; readonly connecti
 
   const attach = async (session: SessionInfo, cwd: string, mcpServers: readonly McpServer[]) => {
     const currentCatalog = await catalog(cwd)
+    sessions.get(session.id)?.abort.abort()
     const state: Attached = {
       id: session.id,
       cwd,
+      abort: new AbortController(),
       catalog: currentCatalog,
       model: session.model ?? currentCatalog.defaultModel,
       modeID: session.agent ?? currentCatalog.defaultModeID,
@@ -161,6 +172,7 @@ export function make(input: { readonly client: OpenCodeClient; readonly connecti
   return {
     initialize: async (params) => {
       capabilities.writeTextFile = params.clientCapabilities?.fs?.writeTextFile === true
+      capabilities.childSessionUpdates = params.clientCapabilities?._meta?.[ChildSessionUpdatesCapability] === true
       const authMethod: AuthMethod = {
         description: "Run `opencode auth login` in the terminal",
         name: "Login with opencode",
@@ -178,6 +190,7 @@ export function make(input: { readonly client: OpenCodeClient; readonly connecti
           mcpCapabilities: { http: true, sse: false },
           promptCapabilities: { embeddedContext: true, image: true },
           sessionCapabilities: { close: {}, delete: {}, fork: {}, list: {}, resume: {} },
+          _meta: { [ChildSessionUpdatesCapability]: true },
         },
         authMethods: [authMethod],
         agentInfo: { name: "OpenCode", version: OPENCODE_VERSION },
@@ -224,6 +237,7 @@ export function make(input: { readonly client: OpenCodeClient; readonly connecti
       await input.client.session.remove({ sessionID: params.sessionId }).catch((error) => {
         if (!isSessionNotFoundError(error)) throw error
       })
+      sessions.get(params.sessionId)?.abort.abort()
       sessions.delete(params.sessionId)
       registeredMcp.delete(params.sessionId)
       return {}
@@ -234,6 +248,7 @@ export function make(input: { readonly client: OpenCodeClient; readonly connecti
       return { configOptions: configOptions(state) }
     },
     closeSession: async (params) => {
+      sessions.get(params.sessionId)?.abort.abort()
       sessions.delete(params.sessionId)
       registeredMcp.delete(params.sessionId)
       const turn = active.get(params.sessionId)
@@ -296,6 +311,7 @@ export function make(input: { readonly client: OpenCodeClient; readonly connecti
       const messageID = SessionMessage.ID.create()
       const prepared = preparePrompt(state.catalog, params.prompt, messageID)
       const control: TurnControl = { cancelled: false, admission: new AbortController() }
+      const extNotification = input.connection.extNotification
       active.set(state.id, control)
       const response = await streamTurn({
         client: input.client,
@@ -305,7 +321,15 @@ export function make(input: { readonly client: OpenCodeClient; readonly connecti
         start: prepared.start,
         writeTextFile: capabilities.writeTextFile,
         control,
+        connectionSignal: input.connection.signal,
+        sessionSignal: state.abort.signal,
         submit: (signal) => submitPrompt(input.client, state, prepared, signal),
+        ...(capabilities.childSessionUpdates && extNotification
+          ? {
+              childSessionUpdate: (update: ChildSessionUpdate) =>
+                extNotification(ChildSessionUpdateMethod, update).then(() => {}),
+            }
+          : {}),
       }).finally(() => {
         if (active.get(state.id) === control) active.delete(state.id)
       })

packages/cli/test/acp/event-behavior.test.ts

@@ -2,7 +2,7 @@ import { describe, expect, test } from "bun:test"
 import type { AgentSideConnection } from "@agentclientprotocol/sdk"
 import type { SessionMessageInfo } from "@opencode-ai/client/promise"
 import { resolve } from "node:path"
-import { replayMessages, streamTurn, type TurnControl } from "../../src/acp/event"
+import { replayMessages, streamTurn, type ChildSessionUpdate, type TurnControl } from "../../src/acp/event"
 import { createSseFixture, durableEvent, ephemeralEvent, withTimeout } from "./sse-fixture"
 
 type SessionUpdateParams = Parameters<AgentSideConnection["sessionUpdate"]>[0]
@@ -191,6 +191,181 @@ describe("acp event behavior", () => {
     }
   })
 
+  test("projects foreground child session updates onto the parent turn", async () => {
+    const updates: SessionUpdateParams[] = []
+    const fixture = createSseFixture({
+      onPrompt({ id, send }) {
+        send(durableEvent("session.input.promoted", { sessionID: "ses_parent", inputID: id }))
+        send(
+          durableEvent("session.created", {
+            sessionID: "ses_child",
+            info: childSession("ses_child", "ses_parent", "Explore code"),
+          }),
+        )
+        send(durableEvent("session.execution.started", { sessionID: "ses_child" }))
+        send(
+          durableEvent("session.tool.input.started", {
+            sessionID: "ses_child",
+            assistantMessageID: "msg_child",
+            callID: "call_read",
+            name: "read",
+          }),
+        )
+        send(
+          durableEvent("session.tool.called", {
+            sessionID: "ses_child",
+            assistantMessageID: "msg_child",
+            callID: "call_read",
+            input: { path: "/workspace/src/index.ts" },
+            executed: false,
+          }),
+        )
+        send(
+          durableEvent("session.tool.success", {
+            sessionID: "ses_child",
+            assistantMessageID: "msg_child",
+            callID: "call_read",
+            metadata: {},
+            content: [{ type: "text", text: "source" }],
+            executed: true,
+          }),
+        )
+        send(durableEvent("session.execution.succeeded", { sessionID: "ses_child" }))
+        send(durableEvent("session.execution.succeeded", { sessionID: "ses_parent" }))
+      },
+    })
+
+    try {
+      const response = await turn({
+        fixture,
+        connection: recordingConnection(updates),
+        sessionID: "ses_parent",
+        inputID: "input_parent",
+      })
+
+      expect(updates.map((item) => [item.sessionId, item.update.sessionUpdate])).toEqual([
+        ["ses_parent", "tool_call"],
+        ["ses_parent", "tool_call_update"],
+        ["ses_parent", "tool_call_update"],
+      ])
+      expect(updates.map((item) => ("toolCallId" in item.update ? item.update.toolCallId : undefined))).toEqual([
+        "ses_child:call_read",
+        "ses_child:call_read",
+        "ses_child:call_read",
+      ])
+      expect(updates[0]?.update).toMatchObject({
+        title: "Explore code: read",
+        _meta: {
+          "opencode/child-session": {
+            id: "ses_child",
+            parentID: "ses_parent",
+            depth: 1,
+            title: "Explore code",
+          },
+        },
+      })
+      expect(response.stopReason).toBe("end_turn")
+    } finally {
+      await fixture.stop()
+    }
+  })
+
+  test("continues child extension updates after the parent turn ends", async () => {
+    const updates: SessionUpdateParams[] = []
+    const childUpdates: ChildSessionUpdate[] = []
+    const completed = Promise.withResolvers<void>()
+    const fixture = createSseFixture({
+      onPrompt({ id, send }) {
+        send(durableEvent("session.input.promoted", { sessionID: "ses_parent", inputID: id }))
+        send(
+          durableEvent("session.created", {
+            sessionID: "ses_background",
+            info: childSession("ses_background", "ses_parent", "Background research"),
+          }),
+        )
+        send(durableEvent("session.execution.succeeded", { sessionID: "ses_parent" }))
+      },
+    })
+
+    try {
+      const response = await turn({
+        fixture,
+        connection: recordingConnection(updates),
+        sessionID: "ses_parent",
+        inputID: "input_parent",
+        childSessionUpdate: async (update) => {
+          childUpdates.push(update)
+          if (update.type === "status" && update.status === "completed") completed.resolve()
+        },
+      })
+      expect(response.stopReason).toBe("end_turn")
+
+      fixture.send(
+        durableEvent("session.created", {
+          sessionID: "ses_future",
+          info: childSession("ses_future", "ses_parent", "Later turn child"),
+        }),
+      )
+      fixture.send(durableEvent("session.execution.started", { sessionID: "ses_future" }))
+      fixture.send(durableEvent("session.execution.started", { sessionID: "ses_background" }))
+      fixture.send(
+        durableEvent("session.tool.input.started", {
+          sessionID: "ses_background",
+          assistantMessageID: "msg_background",
+          callID: "call_shell",
+          name: "shell",
+        }),
+      )
+      fixture.send(
+        durableEvent("session.tool.called", {
+          sessionID: "ses_background",
+          assistantMessageID: "msg_background",
+          callID: "call_shell",
+          input: { command: "pwd" },
+          executed: false,
+        }),
+      )
+      fixture.send(
+        durableEvent("session.tool.success", {
+          sessionID: "ses_background",
+          assistantMessageID: "msg_background",
+          callID: "call_shell",
+          metadata: { exit: 0 },
+          content: [{ type: "text", text: "/workspace" }],
+          executed: true,
+        }),
+      )
+      fixture.send(durableEvent("session.execution.succeeded", { sessionID: "ses_background" }))
+      await withTimeout(completed.promise, "background child completion was not delivered")
+
+      expect(updates).toEqual([])
+      expect(
+        childUpdates.map((update) =>
+          update.type === "status" ? [update.type, update.status] : [update.type, update.update.sessionUpdate],
+        ),
+      ).toEqual([
+        ["status", "created"],
+        ["status", "running"],
+        ["update", "tool_call"],
+        ["update", "tool_call_update"],
+        ["update", "tool_call_update"],
+        ["status", "completed"],
+      ])
+      expect(childUpdates[2]).toMatchObject({
+        rootSessionId: "ses_parent",
+        childSessionId: "ses_background",
+        parentSessionId: "ses_parent",
+        depth: 1,
+        title: "Background research",
+        type: "update",
+        update: { toolCallId: "ses_background:call_shell" },
+      })
+      expect(childUpdates.some((update) => update.childSessionId === "ses_future")).toBe(false)
+    } finally {
+      await fixture.stop()
+    }
+  })
+
   test("streams tool pending, progress, success, and failure updates", async () => {
     const updates: SessionUpdateParams[] = []
     const fixture = createSseFixture({
@@ -556,6 +731,7 @@ function turn(input: {
   readonly connection: Connection
   readonly sessionID: string
   readonly inputID: string
+  readonly childSessionUpdate?: (update: ChildSessionUpdate) => Promise<void>
 }) {
   return streamTurn({
     client: input.fixture.client,
@@ -565,11 +741,25 @@ function turn(input: {
     start: { type: "input", id: input.inputID },
     writeTextFile: false,
     control: { cancelled: false, admission: new AbortController() },
+    childSessionUpdate: input.childSessionUpdate,
     submit: (signal) =>
       input.fixture.client.session.prompt({ sessionID: input.sessionID, id: input.inputID, text: "hello" }, { signal }),
   })
 }
 
+function childSession(id: string, parentID: string, title: string) {
+  return {
+    id,
+    slug: id,
+    projectID: "project",
+    directory: "/workspace",
+    parentID,
+    title,
+    version: "test",
+    time: { created: 1, updated: 1 },
+  }
+}
+
 function tokens() {
   return { input: 1, output: 1, reasoning: 0, cache: { read: 0, write: 0 } }
 }

packages/cli/test/acp/permission-behavior.test.ts

@@ -153,6 +153,68 @@ describe("acp permission behavior", () => {
     }
   })
 
+  test("routes foreground child permissions through the parent ACP session", async () => {
+    const permissionRequests: RequestPermissionRequest[] = []
+    const fixture = createSseFixture({
+      onPrompt({ id, send }) {
+        send(durableEvent("session.input.promoted", { sessionID: "ses_parent", inputID: id }))
+        send(
+          durableEvent("session.created", {
+            sessionID: "ses_child",
+            info: {
+              id: "ses_child",
+              slug: "ses_child",
+              projectID: "project",
+              directory: "/workspace",
+              parentID: "ses_parent",
+              title: "Review code",
+              version: "test",
+              time: { created: 1, updated: 1 },
+            },
+          }),
+        )
+        send(durableEvent("session.execution.started", { sessionID: "ses_child" }))
+        send(
+          permissionAsked("ses_child", "perm_child", {
+            action: "read",
+            metadata: { path: "/workspace/child.ts" },
+            source: { type: "tool", messageID: "msg_child", callID: "call_child" },
+          }),
+        )
+        send(durableEvent("session.execution.succeeded", { sessionID: "ses_child" }))
+        send(durableEvent("session.execution.succeeded", { sessionID: "ses_parent" }))
+      },
+    })
+    const connection = {
+      sessionUpdate: async () => {},
+      requestPermission: async (request) => {
+        permissionRequests.push(request)
+        return { outcome: { outcome: "selected", optionId: "once" } } as const
+      },
+    } satisfies Connection
+
+    try {
+      await startTurn(fixture, connection, "ses_parent", "input_parent")
+
+      expect(permissionRequests).toHaveLength(1)
+      expect(permissionRequests[0]).toMatchObject({
+        sessionId: "ses_parent",
+        toolCall: {
+          toolCallId: "ses_child:call_child",
+          title: "Review code: /workspace/child.ts",
+        },
+      })
+      expect(fixture.requests).toContainEqual(
+        expect.objectContaining({
+          method: "POST",
+          path: "/api/session/ses_child/permission/perm_child/reply",
+        }),
+      )
+    } finally {
+      await fixture.stop()
+    }
+  })
+
   test("previews edits during approval and syncs the completed file", async () => {
     const cwd = await fs.mkdtemp(path.join(os.tmpdir(), "opencode-acp-permission-"))
     const file = path.join(cwd, "file.ts")

packages/cli/test/acp/service.test.ts

@@ -2,6 +2,7 @@ import { describe, expect, test } from "bun:test"
 import type { AgentSideConnection } from "@agentclientprotocol/sdk"
 import { OpenCode } from "@opencode-ai/client/promise"
 import { ACPService } from "../../src/acp/service"
+import { ChildSessionUpdatesCapability } from "../../src/acp/event"
 
 describe("acp service", () => {
   test("creates a v2 session, registers mcp, and publishes commands", async () => {
@@ -39,11 +40,17 @@ describe("acp service", () => {
     })
 
     try {
+      const initialized = await service.initialize({
+        protocolVersion: 1,
+        clientCapabilities: { _meta: { [ChildSessionUpdatesCapability]: true } },
+        clientInfo: { name: "test", version: "1" },
+      })
       const result = await service.newSession({
         cwd: "/workspace",
         mcpServers: [{ name: "docs", command: "bun", args: ["docs.ts"], env: [{ name: "TOKEN", value: "x" }] }],
       })
       expect(result.sessionId).toBe("ses_acp")
+      expect(initialized.agentCapabilities?._meta).toEqual({ [ChildSessionUpdatesCapability]: true })
       expect(result.configOptions?.map((option) => option.id)).toEqual(["model", "effort", "mode"])
       expect(requests).toContainEqual({
         method: "PUT",