fix(task): handle running background resumes

packages/opencode/src/background/job.ts

@@ -0,0 +1,173 @@
+import { InstanceState } from "@/effect/instance-state"
+import { Identifier } from "@/id/id"
+import { Cause, Deferred, Effect, Fiber, Layer, Scope, Context } from "effect"
+
+export type Status = "running" | "completed" | "error" | "cancelled"
+
+export type Info = {
+  id: string
+  type: string
+  title?: string
+  status: Status
+  started_at: number
+  completed_at?: number
+  output?: string
+  error?: string
+  metadata?: Record<string, unknown>
+}
+
+type Active = {
+  info: Info
+  done: Deferred.Deferred<Info>
+  fiber?: Fiber.Fiber<void, unknown>
+}
+
+type State = {
+  jobs: Map<string, Active>
+  scope: Scope.Scope
+}
+
+export type StartInput = {
+  id?: string
+  type: string
+  title?: string
+  metadata?: Record<string, unknown>
+  run: Effect.Effect<string, unknown>
+}
+
+export type WaitInput = {
+  id: string
+  timeout?: number
+}
+
+export type WaitResult = {
+  info?: Info
+  timedOut: boolean
+}
+
+export interface Interface {
+  readonly list: () => Effect.Effect<Info[]>
+  readonly get: (id: string) => Effect.Effect<Info | undefined>
+  readonly start: (input: StartInput) => Effect.Effect<Info>
+  readonly wait: (input: WaitInput) => Effect.Effect<WaitResult>
+  readonly cancel: (id: string) => Effect.Effect<Info | undefined>
+}
+
+export class Service extends Context.Service<Service, Interface>()("@opencode/BackgroundJob") {}
+
+function snapshot(job: Active): Info {
+  return {
+    ...job.info,
+    ...(job.info.metadata ? { metadata: { ...job.info.metadata } } : {}),
+  }
+}
+
+function errorText(error: unknown) {
+  if (error instanceof Error) return error.message
+  return String(error)
+}
+
+export const layer = Layer.effect(
+  Service,
+  Effect.gen(function* () {
+    const state = yield* InstanceState.make<State>(
+      Effect.fn("BackgroundJob.state")(function* () {
+        return {
+          jobs: new Map(),
+          scope: yield* Scope.Scope,
+        }
+      }),
+    )
+
+    const finish = Effect.fn("BackgroundJob.finish")(function* (
+      job: Active,
+      status: Exclude<Status, "running">,
+      data?: { output?: string; error?: string },
+    ) {
+      if (job.info.status !== "running") return snapshot(job)
+      job.info.status = status
+      job.info.completed_at = Date.now()
+      if (data?.output !== undefined) job.info.output = data.output
+      if (data?.error !== undefined) job.info.error = data.error
+      job.fiber = undefined
+      const info = snapshot(job)
+      yield* Deferred.succeed(job.done, info).pipe(Effect.ignore)
+      return info
+    })
+
+    const list: Interface["list"] = Effect.fn("BackgroundJob.list")(function* () {
+      const s = yield* InstanceState.get(state)
+      return Array.from(s.jobs.values())
+        .map(snapshot)
+        .toSorted((a, b) => a.started_at - b.started_at)
+    })
+
+    const get: Interface["get"] = Effect.fn("BackgroundJob.get")(function* (id) {
+      const s = yield* InstanceState.get(state)
+      const job = s.jobs.get(id)
+      if (!job) return
+      return snapshot(job)
+    })
+
+    const start: Interface["start"] = Effect.fn("BackgroundJob.start")(function* (input) {
+      const s = yield* InstanceState.get(state)
+      const id = input.id ?? Identifier.ascending("job")
+      const existing = s.jobs.get(id)
+      if (existing?.info.status === "running") return snapshot(existing)
+
+      const job: Active = {
+        info: {
+          id,
+          type: input.type,
+          title: input.title,
+          status: "running",
+          started_at: Date.now(),
+          metadata: input.metadata,
+        },
+        done: yield* Deferred.make<Info>(),
+      }
+      s.jobs.set(id, job)
+      job.fiber = yield* input.run.pipe(
+        Effect.matchCauseEffect({
+          onSuccess: (output) => finish(job, "completed", { output }),
+          onFailure: (cause) =>
+            finish(job, Cause.hasInterruptsOnly(cause) ? "cancelled" : "error", {
+              error: errorText(Cause.squash(cause)),
+            }),
+        }),
+        Effect.asVoid,
+        Effect.forkIn(s.scope),
+      )
+      return snapshot(job)
+    })
+
+    const wait: Interface["wait"] = Effect.fn("BackgroundJob.wait")(function* (input) {
+      const s = yield* InstanceState.get(state)
+      const job = s.jobs.get(input.id)
+      if (!job) return { timedOut: false }
+      if (job.info.status !== "running") return { info: snapshot(job), timedOut: false }
+      if (!input.timeout) return { info: yield* Deferred.await(job.done), timedOut: false }
+      return yield* Effect.raceAll([
+        Deferred.await(job.done).pipe(Effect.map((info) => ({ info, timedOut: false }))),
+        Effect.sleep(input.timeout).pipe(Effect.as({ info: snapshot(job), timedOut: true })),
+      ])
+    })
+
+    const cancel: Interface["cancel"] = Effect.fn("BackgroundJob.cancel")(function* (id) {
+      const s = yield* InstanceState.get(state)
+      const job = s.jobs.get(id)
+      if (!job) return
+      if (job.info.status !== "running") return snapshot(job)
+      const fiber = job.fiber
+      const info = yield* finish(job, "cancelled")
+      if (fiber) yield* Fiber.interrupt(fiber).pipe(Effect.ignore)
+      return info
+    })
+
+    return Service.of({ list, get, start, wait, cancel })
+  }),
+)
+
+export const defaultLayer = layer
+
+export * as BackgroundJob from "./job"

packages/opencode/src/cli/cmd/tui/routes/session/index.tsx

@@ -1960,12 +1960,15 @@ function Task(props: ToolProps<typeof TaskTool>) {
   const { navigate } = useRoute()
   const sync = useSync()
 
-  onMount(() => {
-    if (props.metadata.sessionId && !sync.data.message[props.metadata.sessionId]?.length)
-      void sync.session.sync(props.metadata.sessionId)
+  createEffect(() => {
+    const sessionID = props.metadata.sessionId
+    if (!sessionID) return
+    if (sync.data.message[sessionID]?.length) return
+    void sync.session.sync(sessionID)
   })
 
-  const messages = createMemo(() => sync.data.message[props.metadata.sessionId ?? ""] ?? [])
+  const childSessionID = createMemo(() => props.metadata.sessionId)
+  const messages = createMemo(() => sync.data.message[childSessionID() ?? ""] ?? [])
 
   const tools = createMemo(() => {
     return messages().flatMap((msg) =>
@@ -1979,7 +1982,16 @@ function Task(props: ToolProps<typeof TaskTool>) {
     tools().findLast((x) => (x.state.status === "running" || x.state.status === "completed") && x.state.title),
   )
 
-  const isRunning = createMemo(() => props.part.state.status === "running")
+  const isBackground = createMemo(() => props.metadata.background === true)
+  const isBackgroundRunning = createMemo(() => {
+    const sessionID = childSessionID()
+    if (!isBackground() || !sessionID) return false
+    const status = sync.data.session_status[sessionID]?.type
+    if (status === "busy" || status === "retry") return true
+    if (status === "idle") return false
+    return !messages().some((x) => x.role === "assistant" && x.time.completed)
+  })
+  const isRunning = createMemo(() => props.part.state.status === "running" || isBackgroundRunning())
 
   const duration = createMemo(() => {
     const first = messages().find((x) => x.role === "user")?.time.created
@@ -1990,7 +2002,8 @@ function Task(props: ToolProps<typeof TaskTool>) {
 
   const content = createMemo(() => {
     if (!props.input.description) return ""
-    let content = [`${Locale.titlecase(props.input.subagent_type ?? "General")} Task — ${props.input.description}`]
+    const description = isBackground() ? `${props.input.description} (background)` : props.input.description
+    let content = [`${Locale.titlecase(props.input.subagent_type ?? "General")} Task — ${description}`]
 
     if (isRunning() && tools().length > 0) {
       // content[0] += ` · ${tools().length} toolcalls`
@@ -2001,7 +2014,7 @@ function Task(props: ToolProps<typeof TaskTool>) {
       } else content.push(`↳ ${tools().length} toolcalls`)
     }
 
-    if (props.part.state.status === "completed") {
+    if (!isRunning() && props.part.state.status === "completed") {
       content.push(`└ ${tools().length} toolcalls · ${Locale.duration(duration())}`)
     }
 
@@ -2016,8 +2029,9 @@ function Task(props: ToolProps<typeof TaskTool>) {
       pending="Delegating..."
       part={props.part}
       onClick={() => {
-        if (props.metadata.sessionId) {
-          navigate({ type: "session", sessionID: props.metadata.sessionId })
+        const sessionID = childSessionID()
+        if (sessionID) {
+          navigate({ type: "session", sessionID })
         }
       }}
     >

packages/opencode/src/effect/app-runtime.ts

@@ -50,6 +50,7 @@ import { SessionShare } from "@/share/session"
 import { SyncEvent } from "@/sync"
 import { Npm } from "@opencode-ai/core/npm"
 import { memoMap } from "@opencode-ai/core/effect/memo-map"
+import { BackgroundJob } from "@/background/job"
 
 export const AppLayer = Layer.mergeAll(
   Npm.defaultLayer,
@@ -75,6 +76,7 @@ export const AppLayer = Layer.mergeAll(
   Todo.defaultLayer,
   Session.defaultLayer,
   SessionStatus.defaultLayer,
+  BackgroundJob.defaultLayer,
   SessionRunState.defaultLayer,
   SessionProcessor.defaultLayer,
   SessionCompaction.defaultLayer,

packages/opencode/src/id/id.ts

@@ -2,6 +2,7 @@ import z from "zod"
 import { randomBytes } from "crypto"
 
 const prefixes = {
+  job: "job",
   event: "evt",
   session: "ses",
   message: "msg",

packages/opencode/src/session/prompt.ts

@@ -117,6 +117,7 @@ export const layer = Layer.effect(
         cancel: (sessionID: SessionID) => run.fork(cancel(sessionID)),
         resolvePromptParts: (template: string) => resolvePromptParts(template),
         prompt: (input: PromptInput) => prompt(input),
+        loop: (input: LoopInput) => loop(input),
       } satisfies TaskPromptOps
     })
 

packages/opencode/src/tool/registry.ts

@@ -7,6 +7,7 @@ import { GlobTool } from "./glob"
 import { GrepTool } from "./grep"
 import { ReadTool } from "./read"
 import { TaskTool } from "./task"
+import { TaskStatusTool } from "./task_status"
 import { TodoWriteTool } from "./todo"
 import { WebFetchTool } from "./webfetch"
 import { WriteTool } from "./write"
@@ -46,6 +47,8 @@ import { Bus } from "../bus"
 import { Agent } from "../agent/agent"
 import { Skill } from "../skill"
 import { Permission } from "@/permission"
+import { SessionStatus } from "@/session/status"
+import { BackgroundJob } from "@/background/job"
 
 const log = Log.create({ service: "tool.registry" })
 
@@ -78,11 +81,13 @@ export const layer: Layer.Layer<
   | Agent.Service
   | Skill.Service
   | Session.Service
+  | SessionStatus.Service
   | Provider.Service
   | LSP.Service
   | Instruction.Service
   | AppFileSystem.Service
   | Bus.Service
+  | BackgroundJob.Service
   | HttpClient.HttpClient
   | ChildProcessSpawner
   | Ripgrep.Service
@@ -113,6 +118,7 @@ export const layer: Layer.Layer<
     const greptool = yield* GrepTool
     const patchtool = yield* ApplyPatchTool
     const skilltool = yield* SkillTool
+    const taskstatus = yield* TaskStatusTool
     const agent = yield* Agent.Service
 
     const state = yield* InstanceState.make<State>(
@@ -193,6 +199,7 @@ export const layer: Layer.Layer<
           edit: Tool.init(edit),
           write: Tool.init(writetool),
           task: Tool.init(task),
+          taskstatus: Tool.init(taskstatus),
           fetch: Tool.init(webfetch),
           todo: Tool.init(todo),
           search: Tool.init(websearch),
@@ -215,6 +222,7 @@ export const layer: Layer.Layer<
             tool.edit,
             tool.write,
             tool.task,
+            ...(Flag.OPENCODE_EXPERIMENTAL ? [tool.taskstatus] : []),
             tool.fetch,
             tool.todo,
             tool.search,
@@ -331,11 +339,13 @@ export const defaultLayer = Layer.suspend(() =>
     Layer.provide(Skill.defaultLayer),
     Layer.provide(Agent.defaultLayer),
     Layer.provide(Session.defaultLayer),
+    Layer.provide(SessionStatus.defaultLayer),
     Layer.provide(Provider.defaultLayer),
     Layer.provide(LSP.defaultLayer),
     Layer.provide(Instruction.defaultLayer),
     Layer.provide(AppFileSystem.defaultLayer),
     Layer.provide(Bus.layer),
+    Layer.provide(BackgroundJob.defaultLayer),
     Layer.provide(FetchHttpClient.layer),
     Layer.provide(Format.defaultLayer),
     Layer.provide(CrossSpawnSpawner.defaultLayer),

packages/opencode/src/tool/task.ts

@@ -1,17 +1,23 @@
 import * as Tool from "./tool"
 import DESCRIPTION from "./task.txt"
+import { Bus } from "@/bus"
 import { Session } from "@/session/session"
 import { SessionID, MessageID } from "../session/schema"
 import { MessageV2 } from "../session/message-v2"
 import { Agent } from "../agent/agent"
 import type { SessionPrompt } from "../session/prompt"
+import { SessionStatus } from "@/session/status"
+import { TuiEvent } from "@/cli/cmd/tui/event"
+import { Cause, Effect, Option, Schema, Scope, Stream } from "effect"
 import { Config } from "@/config/config"
-import { Effect, Schema } from "effect"
+import { BackgroundJob } from "@/background/job"
+import { Flag } from "@opencode-ai/core/flag/flag"
 
 export interface TaskPromptOps {
   cancel(sessionID: SessionID): void
   resolvePromptParts(template: string): Effect.Effect<SessionPrompt.PromptInput["parts"]>
   prompt(input: SessionPrompt.PromptInput): Effect.Effect<MessageV2.WithParts>
+  loop(input: SessionPrompt.LoopInput): Effect.Effect<MessageV2.WithParts>
 }
 
 const id = "task"
@@ -20,24 +26,67 @@ export const Parameters = Schema.Struct({
   description: Schema.String.annotate({ description: "A short (3-5 words) description of the task" }),
   prompt: Schema.String.annotate({ description: "The task for the agent to perform" }),
   subagent_type: Schema.String.annotate({ description: "The type of specialized agent to use for this task" }),
-  task_id: Schema.optional(Schema.String).annotate({
+  task_id: Schema.optional(SessionID).annotate({
     description:
       "This should only be set if you mean to resume a previous task (you can pass a prior task_id and the task will continue the same subagent session as before instead of creating a fresh one)",
   }),
   command: Schema.optional(Schema.String).annotate({ description: "The command that triggered this task" }),
+  background: Schema.optional(Schema.Boolean).annotate({
+    description: "When true, launch the subagent in the background and return immediately",
+  }),
 })
 
+function output(sessionID: SessionID, text: string) {
+  return [
+    `task_id: ${sessionID} (for resuming to continue this task if needed)`,
+    "",
+    "<task_result>",
+    text,
+    "</task_result>",
+  ].join("\n")
+}
+
+function backgroundOutput(sessionID: SessionID) {
+  return [
+    `task_id: ${sessionID} (for polling this task with task_status)`,
+    "state: running",
+    "",
+    "<task_result>",
+    "Background task started. Continue your current work and call task_status when you need the result.",
+    "</task_result>",
+  ].join("\n")
+}
+
+function backgroundMessage(input: { sessionID: SessionID; description: string; state: "completed" | "error"; text: string }) {
+  const tag = input.state === "completed" ? "task_result" : "task_error"
+  const title =
+    input.state === "completed"
+      ? `Background task completed: ${input.description}`
+      : `Background task failed: ${input.description}`
+  return [title, `task_id: ${input.sessionID}`, `state: ${input.state}`, `<${tag}>`, input.text, `</${tag}>`].join(
+    "\n",
+  )
+}
+
+function errorText(error: unknown) {
+  if (error instanceof Error) return error.message
+  return String(error)
+}
+
 export const TaskTool = Tool.define(
   id,
   Effect.gen(function* () {
     const agent = yield* Agent.Service
+    const bus = yield* Bus.Service
     const config = yield* Config.Service
     const sessions = yield* Session.Service
+    const status = yield* SessionStatus.Service
+    const jobs = yield* BackgroundJob.Service
+    const scope = yield* Scope.Scope
 
-    const run = Effect.fn("TaskTool.execute")(function* (
-      params: Schema.Schema.Type<typeof Parameters>,
-      ctx: Tool.Context,
-    ) {
+    const run = Effect.fn(
+      "TaskTool.execute",
+    )(function* (params: Schema.Schema.Type<typeof Parameters>, ctx: Tool.Context) {
       const cfg = yield* config.get()
 
       if (!ctx.extra?.bypassAgentCheck) {
@@ -62,7 +111,7 @@ export const TaskTool = Tool.define(
 
       const taskID = params.task_id
       const session = taskID
-        ? yield* sessions.get(SessionID.make(taskID)).pipe(Effect.catchCause(() => Effect.succeed(undefined)))
+        ? yield* sessions.get(taskID).pipe(Effect.catchCause(() => Effect.succeed(undefined)))
         : undefined
       const parent = yield* sessions.get(ctx.sessionID)
       const nextSession =
@@ -107,19 +156,134 @@ export const TaskTool = Tool.define(
         modelID: msg.info.modelID,
         providerID: msg.info.providerID,
       }
+      const parentModel = {
+        modelID: msg.info.modelID,
+        providerID: msg.info.providerID,
+      }
+      const background = params.background === true
+      if (background && !Flag.OPENCODE_EXPERIMENTAL) {
+        return yield* Effect.fail(new Error("Background tasks require OPENCODE_EXPERIMENTAL=true"))
+      }
+      if ((yield* jobs.get(nextSession.id))?.status === "running") {
+        return yield* Effect.fail(new Error(`Task ${nextSession.id} is already running`))
+      }
+
+      const metadata = {
+        sessionId: nextSession.id,
+        model,
+        ...(background ? { background: true } : {}),
+      }
 
       yield* ctx.metadata({
         title: params.description,
-        metadata: {
-          sessionId: nextSession.id,
-          model,
-        },
+        metadata,
       })
 
       const ops = ctx.extra?.promptOps as TaskPromptOps
       if (!ops) return yield* Effect.fail(new Error("TaskTool requires promptOps in ctx.extra"))
 
-      const messageID = MessageID.ascending()
+      const runTask = Effect.fn("TaskTool.runTask")(function* () {
+        const parts = yield* ops.resolvePromptParts(params.prompt)
+        const result = yield* ops.prompt({
+          messageID: MessageID.ascending(),
+          sessionID: nextSession.id,
+          model: {
+            modelID: model.modelID,
+            providerID: model.providerID,
+          },
+          agent: next.name,
+          tools: {
+            ...(canTodo ? {} : { todowrite: false }),
+            ...(canTask ? {} : { task: false }),
+            ...Object.fromEntries((cfg.experimental?.primary_tools ?? []).map((item) => [item, false])),
+          },
+          parts,
+        })
+        return result.parts.findLast((item) => item.type === "text")?.text ?? ""
+      })
+
+      const resumeParent: (input: {
+        userID: MessageID
+        state: "completed" | "error"
+        attempts?: number
+      }) => Effect.Effect<void> = Effect.fn("TaskTool.resumeParent")(function* (input) {
+        if ((yield* status.get(ctx.sessionID)).type !== "idle") {
+          if ((input.attempts ?? 0) >= 60) return
+          yield* bus.subscribe(SessionStatus.Event.Idle).pipe(
+            Stream.filter((event) => event.properties.sessionID === ctx.sessionID),
+            Stream.take(1),
+            Stream.runDrain,
+            Effect.timeoutOption("1 second"),
+          )
+          return yield* resumeParent({ ...input, attempts: (input.attempts ?? 0) + 1 })
+        }
+        const latest = yield* sessions.findMessage(ctx.sessionID, (item) => item.info.role === "user")
+        if (Option.isNone(latest)) return
+        if (latest.value.info.id !== input.userID) return
+        yield* bus.publish(TuiEvent.ToastShow, {
+          title: input.state === "completed" ? "Background task complete" : "Background task failed",
+          message:
+            input.state === "completed"
+              ? `Background task \"${params.description}\" finished. Resuming the main thread.`
+              : `Background task \"${params.description}\" failed. Resuming the main thread.`,
+          variant: input.state === "completed" ? "success" : "error",
+          duration: 5000,
+        })
+        yield* ops.loop({ sessionID: ctx.sessionID }).pipe(Effect.ignore)
+      })
+
+      if (background) {
+        const inject = Effect.fn("TaskTool.injectBackgroundResult")(function* (state: "completed" | "error", text: string) {
+          const message = yield* ops.prompt({
+            sessionID: ctx.sessionID,
+            noReply: true,
+            model: parentModel,
+            agent: ctx.agent,
+            parts: [
+              {
+                type: "text",
+                synthetic: true,
+                text: backgroundMessage({
+                  sessionID: nextSession.id,
+                  description: params.description,
+                  state,
+                  text,
+                }),
+              },
+            ],
+          })
+          yield* resumeParent({ userID: message.info.id, state }).pipe(Effect.ignore, Effect.forkIn(scope))
+        })
+
+        yield* jobs.start({
+          id: nextSession.id,
+          type: id,
+          title: params.description,
+          metadata: {
+            parentSessionID: ctx.sessionID,
+            sessionID: nextSession.id,
+            subagent: next.name,
+          },
+          run: runTask().pipe(
+            Effect.matchCauseEffect({
+              onSuccess: (text) => inject("completed", text).pipe(Effect.as(text)),
+              onFailure: (cause) => {
+                const text = errorText(Cause.squash(cause))
+                return inject("error", text).pipe(
+                  Effect.catchCause(() => Effect.void),
+                  Effect.andThen(Effect.failCause(cause)),
+                )
+              },
+            }),
+          ),
+        })
+
+        return {
+          title: params.description,
+          metadata,
+          output: backgroundOutput(nextSession.id),
+        }
+      }
 
       function cancel() {
         ops.cancel(nextSession.id)
@@ -131,36 +295,11 @@ export const TaskTool = Tool.define(
         }),
         () =>
           Effect.gen(function* () {
-            const parts = yield* ops.resolvePromptParts(params.prompt)
-            const result = yield* ops.prompt({
-              messageID,
-              sessionID: nextSession.id,
-              model: {
-                modelID: model.modelID,
-                providerID: model.providerID,
-              },
-              agent: next.name,
-              tools: {
-                ...(canTodo ? {} : { todowrite: false }),
-                ...(canTask ? {} : { task: false }),
-                ...Object.fromEntries((cfg.experimental?.primary_tools ?? []).map((item) => [item, false])),
-              },
-              parts,
-            })
-
+            const text = yield* runTask()
             return {
               title: params.description,
-              metadata: {
-                sessionId: nextSession.id,
-                model,
-              },
-              output: [
-                `task_id: ${nextSession.id} (for resuming to continue this task if needed)`,
-                "",
-                "<task_result>",
-                result.parts.findLast((item) => item.type === "text")?.text ?? "",
-                "</task_result>",
-              ].join("\n"),
+              metadata,
+              output: output(nextSession.id, text),
             }
           }),
         () =>
@@ -168,13 +307,12 @@ export const TaskTool = Tool.define(
             ctx.abort.removeEventListener("abort", cancel)
           }),
       )
-    })
+    }, Effect.orDie)
 
     return {
       description: DESCRIPTION,
       parameters: Parameters,
-      execute: (params: Schema.Schema.Type<typeof Parameters>, ctx: Tool.Context) =>
-        run(params, ctx).pipe(Effect.orDie),
+      execute: run,
     }
   }),
 )

packages/opencode/src/tool/task.txt

@@ -14,11 +14,13 @@ When NOT to use the Task tool:
 
 Usage notes:
 1. Launch multiple agents concurrently whenever possible, to maximize performance; to do that, use a single message with multiple tool uses
-2. When the agent is done, it will return a single message back to you. The result returned by the agent is not visible to the user. To show the user the result, you should send a text message back to the user with a concise summary of the result. The output includes a task_id you can reuse later to continue the same subagent session.
-3. Each agent invocation starts with a fresh context unless you provide task_id to resume the same subagent session (which continues with its previous messages and tool outputs). When starting fresh, your prompt should contain a highly detailed task description for the agent to perform autonomously and you should specify exactly what information the agent should return back to you in its final and only message to you.
-4. The agent's outputs should generally be trusted
-5. Clearly tell the agent whether you expect it to write code or just to do research (search, file reads, web fetches, etc.), since it is not aware of the user's intent. Tell it how to verify its work if possible (e.g., relevant test commands).
-6. If the agent description mentions that it should be used proactively, then you should try your best to use it without the user having to ask for it first. Use your judgement.
+2. By default, task waits for completion and returns the result immediately, along with a task_id you can reuse later to continue the same subagent session.
+3. Set background=true to launch asynchronously. In background mode, continue your current work without waiting.
+4. For background runs, use task_status(task_id=..., wait=false) to poll, or wait=true to block until done (optionally with timeout_ms).
+5. Each agent invocation starts with a fresh context unless you provide task_id to resume the same subagent session (which continues with its previous messages and tool outputs). When starting fresh, your prompt should contain a highly detailed task description for the agent to perform autonomously and you should specify exactly what information the agent should return back to you in its final and only message to you.
+6. The agent's outputs should generally be trusted
+7. Clearly tell the agent whether you expect it to write code or just to do research (search, file reads, web fetches, etc.), since it is not aware of the user's intent. Tell it how to verify its work if possible (e.g., relevant test commands).
+8. If the agent description mentions that it should be used proactively, then you should try your best to use it without the user having to ask for it first. Use your judgement.
 
 Example usage (NOTE: The agents below are fictional examples for illustration only - use the actual agents listed above):
 

packages/opencode/src/tool/task_status.ts

@@ -0,0 +1,197 @@
+import * as Tool from "./tool"
+import DESCRIPTION from "./task_status.txt"
+import { Session } from "@/session/session"
+import { SessionID } from "@/session/schema"
+import { MessageV2 } from "@/session/message-v2"
+import { SessionStatus } from "@/session/status"
+import { PositiveInt } from "@/util/schema"
+import { Effect, Option, Schema } from "effect"
+import { BackgroundJob } from "@/background/job"
+
+const DEFAULT_TIMEOUT = 60_000
+const POLL_MS = 300
+
+const Parameters = Schema.Struct({
+  task_id: SessionID.annotate({ description: "The task_id returned by the task tool" }),
+  wait: Schema.optional(Schema.Boolean).annotate({ description: "When true, wait until the task reaches a terminal state or timeout" }),
+  timeout_ms: Schema.optional(PositiveInt).annotate({
+    description: "Maximum milliseconds to wait when wait=true (default: 60000)",
+  }),
+})
+
+type State = "running" | "completed" | "error"
+type InspectResult = { state: State; text: string }
+
+function format(input: { taskID: SessionID; state: State; text: string }) {
+  return [`task_id: ${input.taskID}`, `state: ${input.state}`, "", "<task_result>", input.text, "</task_result>"].join(
+    "\n",
+  )
+}
+
+function errorText(error: NonNullable<MessageV2.Assistant["error"]>) {
+  const data = Reflect.get(error, "data")
+  const message = data && typeof data === "object" ? Reflect.get(data, "message") : undefined
+  if (typeof message === "string" && message) return message
+  return error.name
+}
+
+function jobResult(job: BackgroundJob.Info): InspectResult {
+  if (job.status === "running") {
+    return {
+      state: "running",
+      text: "Task is still running.",
+    }
+  }
+  if (job.status === "completed") {
+    return {
+      state: "completed",
+      text: job.output ?? "",
+    }
+  }
+  return {
+    state: "error",
+    text: job.error ?? `Task ${job.status}.`,
+  }
+}
+
+export const TaskStatusTool = Tool.define(
+  "task_status",
+  Effect.gen(function* () {
+    const sessions = yield* Session.Service
+    const status = yield* SessionStatus.Service
+    const jobs = yield* BackgroundJob.Service
+
+    const inspect: (taskID: SessionID) => Effect.Effect<InspectResult> = Effect.fn("TaskStatusTool.inspect")(function* (
+      taskID: SessionID,
+    ) {
+      const current = yield* status.get(taskID)
+      if (current.type === "busy" || current.type === "retry") {
+        return {
+          state: "running" as const,
+          text: current.type === "retry" ? `Task is retrying: ${current.message}` : "Task is still running.",
+        }
+      }
+
+      const latestAssistant = yield* sessions.findMessage(taskID, (item) => item.info.role === "assistant")
+      if (Option.isNone(latestAssistant)) {
+        return {
+          state: "running" as const,
+          text: "Task has started but has not produced output yet.",
+        }
+      }
+      if (latestAssistant.value.info.role !== "assistant") {
+        return {
+          state: "running" as const,
+          text: "Task has started but has not produced output yet.",
+        }
+      }
+
+      const latestUser = yield* sessions.findMessage(taskID, (item) => item.info.role === "user")
+      if (
+        Option.isSome(latestUser) &&
+        latestUser.value.info.role === "user" &&
+        latestUser.value.info.id > latestAssistant.value.info.id
+      ) {
+        return {
+          state: "running" as const,
+          text: "Task is starting.",
+        }
+      }
+
+      const text = latestAssistant.value.parts.findLast((part) => part.type === "text")?.text ?? ""
+      if (latestAssistant.value.info.error) {
+        return {
+          state: "error" as const,
+          text: text || errorText(latestAssistant.value.info.error),
+        }
+      }
+
+      const done =
+        !!latestAssistant.value.info.finish && !["tool-calls", "unknown"].includes(latestAssistant.value.info.finish)
+      if (done) {
+        return {
+          state: "completed" as const,
+          text,
+        }
+      }
+
+      return {
+        state: "running" as const,
+        text: text || "Task is still running.",
+      }
+    })
+
+    const waitForTerminal: (
+      taskID: SessionID,
+      timeout: number,
+    ) => Effect.Effect<{ result: InspectResult; timedOut: boolean }> = Effect.fn(
+      "TaskStatusTool.waitForTerminal",
+    )(function* (taskID: SessionID, timeout: number) {
+        const result = yield* inspect(taskID)
+        if (result.state !== "running") return { result, timedOut: false }
+        if (timeout <= 0) return { result, timedOut: true }
+        const sleep = Math.min(POLL_MS, timeout)
+        yield* Effect.sleep(sleep)
+        return yield* waitForTerminal(taskID, timeout - sleep)
+      })
+
+    const run = Effect.fn(
+      "TaskStatusTool.execute",
+    )(function* (params: Schema.Schema.Type<typeof Parameters>, _ctx: Tool.Context) {
+      yield* sessions.get(params.task_id)
+
+      const job = yield* jobs.get(params.task_id)
+      const waitedJob =
+        job && params.wait === true
+          ? yield* jobs.wait({ id: params.task_id, timeout: params.timeout_ms ?? DEFAULT_TIMEOUT })
+          : { info: job, timedOut: false }
+      if (waitedJob.info) {
+        const result = jobResult(waitedJob.info)
+        return {
+          title: "Task status",
+          metadata: {
+            task_id: params.task_id,
+            state: result.state,
+            timed_out: waitedJob.timedOut,
+          },
+          output: format({
+            taskID: params.task_id,
+            state: result.state,
+            text: waitedJob.timedOut
+              ? `Timed out after ${params.timeout_ms ?? DEFAULT_TIMEOUT}ms while waiting for task completion.`
+              : result.text,
+          }),
+        }
+      }
+
+      const waited =
+        params.wait === true
+          ? yield* waitForTerminal(params.task_id, params.timeout_ms ?? DEFAULT_TIMEOUT)
+          : { result: yield* inspect(params.task_id), timedOut: false }
+
+      const outputText = waited.timedOut
+        ? `Timed out after ${params.timeout_ms ?? DEFAULT_TIMEOUT}ms while waiting for task completion.`
+        : waited.result.text
+
+      return {
+        title: "Task status",
+        metadata: {
+          task_id: params.task_id,
+          state: waited.result.state,
+          timed_out: waited.timedOut,
+        },
+        output: format({
+          taskID: params.task_id,
+          state: waited.result.state,
+          text: outputText,
+        }),
+      }
+    }, Effect.orDie)
+
+    return {
+      description: DESCRIPTION,
+      parameters: Parameters,
+      execute: run,
+    }
+  }),
+)

packages/opencode/src/tool/task_status.txt

@@ -0,0 +1,13 @@
+Poll the status of a subagent task launched with the task tool.
+
+Use this to check background tasks started with `task(background=true)`.
+
+Parameters:
+- `task_id` (required): the task session id returned by the task tool
+- `wait` (optional): when true, wait for completion
+- `timeout_ms` (optional): max wait duration in milliseconds when `wait=true`
+
+Returns compact, parseable output:
+- `task_id`
+- `state` (`running`, `completed`, or `error`)
+- `<task_result>...</task_result>` containing final output, error summary, or current progress text

packages/opencode/test/background/job.test.ts

@@ -0,0 +1,49 @@
+import { describe, expect } from "bun:test"
+import { Deferred, Effect, Layer } from "effect"
+import { BackgroundJob } from "@/background/job"
+import { CrossSpawnSpawner } from "@opencode-ai/core/cross-spawn-spawner"
+import { provideTmpdirInstance } from "../fixture/fixture"
+import { testEffect } from "../lib/effect"
+
+const it = testEffect(Layer.mergeAll(BackgroundJob.defaultLayer, CrossSpawnSpawner.defaultLayer))
+
+describe("background.job", () => {
+  it.live("tracks started jobs through completion", () =>
+    provideTmpdirInstance(() =>
+      Effect.gen(function* () {
+        const jobs = yield* BackgroundJob.Service
+        const latch = yield* Deferred.make<void>()
+        const job = yield* jobs.start({
+          type: "test",
+          title: "test job",
+          run: Deferred.await(latch).pipe(Effect.as("done")),
+        })
+
+        expect(job.status).toBe("running")
+        yield* Deferred.succeed(latch, undefined)
+        const done = yield* jobs.wait({ id: job.id })
+
+        expect(done.info?.status).toBe("completed")
+        expect(done.info?.output).toBe("done")
+        expect((yield* jobs.list()).map((item) => item.id)).toEqual([job.id])
+      }),
+    ),
+  )
+
+  it.live("can cancel running jobs", () =>
+    provideTmpdirInstance(() =>
+      Effect.gen(function* () {
+        const jobs = yield* BackgroundJob.Service
+        const latch = yield* Deferred.make<void>()
+        const job = yield* jobs.start({
+          type: "test",
+          run: Deferred.await(latch).pipe(Effect.as("done")),
+        })
+
+        const cancelled = yield* jobs.cancel(job.id)
+
+        expect(cancelled?.status).toBe("cancelled")
+      }),
+    ),
+  )
+})

packages/opencode/test/session/prompt.test.ts

@@ -41,6 +41,7 @@ import * as Log from "@opencode-ai/core/util/log"
 import { CrossSpawnSpawner } from "@opencode-ai/core/cross-spawn-spawner"
 import { Ripgrep } from "../../src/file/ripgrep"
 import { Format } from "../../src/format"
+import { BackgroundJob } from "@/background/job"
 import { provideTmpdirInstance, provideTmpdirServer } from "../fixture/fixture"
 import { testEffect } from "../lib/effect"
 import { reply, TestLLMServer } from "../lib/llm-server"
@@ -177,6 +178,7 @@ function makeHttp() {
     Layer.provide(CrossSpawnSpawner.defaultLayer),
     Layer.provide(Ripgrep.defaultLayer),
     Layer.provide(Format.defaultLayer),
+    Layer.provide(BackgroundJob.defaultLayer),
     Layer.provideMerge(todo),
     Layer.provideMerge(question),
     Layer.provideMerge(deps),

packages/opencode/test/session/snapshot-tool-race.test.ts

@@ -55,6 +55,7 @@ import { AppFileSystem } from "@opencode-ai/core/filesystem"
 import { CrossSpawnSpawner } from "@opencode-ai/core/cross-spawn-spawner"
 import { Ripgrep } from "../../src/file/ripgrep"
 import { Format } from "../../src/format"
+import { BackgroundJob } from "@/background/job"
 
 void Log.init({ print: false })
 
@@ -130,6 +131,7 @@ function makeHttp() {
     Layer.provide(CrossSpawnSpawner.defaultLayer),
     Layer.provide(Ripgrep.defaultLayer),
     Layer.provide(Format.defaultLayer),
+    Layer.provide(BackgroundJob.defaultLayer),
     Layer.provideMerge(todo),
     Layer.provideMerge(question),
     Layer.provideMerge(deps),

packages/opencode/test/tool/__snapshots__/parameters.test.ts.snap

@@ -320,6 +320,10 @@ exports[`tool parameters JSON Schema (wire shape) task 1`] = `
 {
   "$schema": "https://json-schema.org/draft/2020-12/schema",
   "properties": {
+    "background": {
+      "description": "When true, launch the subagent in the background and return immediately",
+      "type": "boolean",
+    },
     "command": {
       "description": "The command that triggered this task",
       "type": "string",
@@ -338,6 +342,7 @@ exports[`tool parameters JSON Schema (wire shape) task 1`] = `
     },
     "task_id": {
       "description": "This should only be set if you mean to resume a previous task (you can pass a prior task_id and the task will continue the same subagent session as before instead of creating a fresh one)",
+      "pattern": "^ses.*",
       "type": "string",
     },
   },

packages/opencode/test/tool/parameters.test.ts

@@ -25,6 +25,7 @@ import { Parameters as Todo } from "../../src/tool/todo"
 import { Parameters as WebFetch } from "../../src/tool/webfetch"
 import { Parameters as WebSearch } from "../../src/tool/websearch"
 import { Parameters as Write } from "../../src/tool/write"
+import { SessionID } from "../../src/session/schema"
 
 const parse = <S extends Schema.Decoder<unknown>>(schema: S, input: unknown): S["Type"] =>
   Schema.decodeUnknownSync(schema)(input)
@@ -203,6 +204,19 @@ describe("tool parameters", () => {
       const parsed = parse(Task, { description: "d", prompt: "p", subagent_type: "general" })
       expect(parsed.subagent_type).toBe("general")
     })
+    test("accepts optional task_id + command + background", () => {
+      const parsed = parse(Task, {
+        description: "d",
+        prompt: "p",
+        subagent_type: "general",
+        task_id: SessionID.make("ses_test"),
+        command: "/cmd",
+        background: true,
+      })
+      expect(parsed.task_id).toBe(SessionID.make("ses_test"))
+      expect(parsed.command).toBe("/cmd")
+      expect(parsed.background).toBe(true)
+    })
     test("rejects missing prompt", () => {
       expect(accepts(Task, { description: "d", subagent_type: "general" })).toBe(false)
     })

packages/opencode/test/tool/task.test.ts

@@ -1,21 +1,28 @@
 import { afterEach, describe, expect } from "bun:test"
-import { Effect, Layer } from "effect"
+import { Cause, Deferred, Effect, Exit, Layer } from "effect"
 import { Agent } from "../../src/agent/agent"
+import { Bus } from "../../src/bus"
 import { Config } from "@/config/config"
 import { CrossSpawnSpawner } from "@opencode-ai/core/cross-spawn-spawner"
 import { Instance } from "../../src/project/instance"
 import { Session } from "@/session/session"
 import { MessageV2 } from "../../src/session/message-v2"
 import type { SessionPrompt } from "../../src/session/prompt"
-import { MessageID, PartID } from "../../src/session/schema"
+import { MessageID, PartID, SessionID } from "../../src/session/schema"
+import { SessionStatus } from "../../src/session/status"
 import { ModelID, ProviderID } from "../../src/provider/schema"
 import { TaskTool, type TaskPromptOps } from "../../src/tool/task"
 import { Truncate } from "@/tool/truncate"
 import { ToolRegistry } from "@/tool/registry"
+import { BackgroundJob } from "@/background/job"
+import { Flag } from "@opencode-ai/core/flag/flag"
 import { provideTmpdirInstance } from "../fixture/fixture"
 import { testEffect } from "../lib/effect"
 
+const originalExperimental = Flag.OPENCODE_EXPERIMENTAL
+
 afterEach(async () => {
+  Flag.OPENCODE_EXPERIMENTAL = originalExperimental
   await Instance.disposeAll()
 })
 
@@ -27,9 +34,12 @@ const ref = {
 const it = testEffect(
   Layer.mergeAll(
     Agent.defaultLayer,
+    Bus.defaultLayer,
     Config.defaultLayer,
     CrossSpawnSpawner.defaultLayer,
     Session.defaultLayer,
+    SessionStatus.defaultLayer,
+    BackgroundJob.defaultLayer,
     Truncate.defaultLayer,
     ToolRegistry.defaultLayer,
   ),
@@ -64,15 +74,62 @@ const seed = Effect.fn("TaskToolTest.seed")(function* (title = "Pinned") {
   return { chat, assistant }
 })
 
-function stubOps(opts?: { onPrompt?: (input: SessionPrompt.PromptInput) => void; text?: string }): TaskPromptOps {
+function stubOps(
+  session: Session.Interface,
+  opts?: { onPrompt?: (input: SessionPrompt.PromptInput) => void; text?: string; wait?: Effect.Effect<void> },
+): TaskPromptOps {
   return {
     cancel() {},
     resolvePromptParts: (template) => Effect.succeed([{ type: "text" as const, text: template }]),
     prompt: (input) =>
-      Effect.sync(() => {
+      Effect.gen(function* () {
         opts?.onPrompt?.(input)
-        return reply(input, opts?.text ?? "done")
+        if (opts?.wait) yield* opts.wait
+        const userID = input.messageID ?? MessageID.ascending()
+        const user: MessageV2.User = {
+          id: userID,
+          role: "user",
+          sessionID: input.sessionID,
+          agent: input.agent ?? "build",
+          model: input.model ?? ref,
+          tools: input.tools,
+          time: { created: Date.now() },
+        }
+        yield* session.updateMessage(user)
+
+        const parts = input.parts.map((part) => ({
+          ...part,
+          id: part.id ?? PartID.ascending(),
+          messageID: user.id,
+          sessionID: input.sessionID,
+        }))
+        yield* Effect.forEach(parts, (part) => session.updatePart(part), { discard: true })
+
+        if (input.noReply) {
+          return {
+            info: user,
+            parts,
+          }
+        }
+
+        const result = reply({ ...input, messageID: user.id }, opts?.text ?? "done")
+        yield* session.updateMessage(result.info)
+        yield* Effect.forEach(result.parts, (part) => session.updatePart(part), { discard: true })
+        return result
       }),
+    loop: (input) =>
+      Effect.sync(() =>
+        reply(
+          {
+            sessionID: input.sessionID,
+            messageID: MessageID.ascending(),
+            agent: "build",
+            model: ref,
+            parts: [],
+          },
+          opts?.text ?? "done",
+        ),
+      ),
   }
 }
 
@@ -195,7 +252,7 @@ describe("tool.task", () => {
         const tool = yield* TaskTool
         const def = yield* tool.init()
         let seen: SessionPrompt.PromptInput | undefined
-        const promptOps = stubOps({ text: "resumed", onPrompt: (input) => (seen = input) })
+        const promptOps = stubOps(sessions, { text: "resumed", onPrompt: (input) => (seen = input) })
 
         const result = yield* def.execute(
           {
@@ -229,11 +286,12 @@ describe("tool.task", () => {
   it.live("execute asks by default and skips checks when bypassed", () =>
     provideTmpdirInstance(() =>
       Effect.gen(function* () {
+        const sessions = yield* Session.Service
         const { chat, assistant } = yield* seed()
         const tool = yield* TaskTool
         const def = yield* tool.init()
         const calls: unknown[] = []
-        const promptOps = stubOps()
+        const promptOps = stubOps(sessions)
 
         const exec = (extra?: Record<string, any>) =>
           def.execute(
@@ -282,14 +340,14 @@ describe("tool.task", () => {
         const tool = yield* TaskTool
         const def = yield* tool.init()
         let seen: SessionPrompt.PromptInput | undefined
-        const promptOps = stubOps({ text: "created", onPrompt: (input) => (seen = input) })
+        const promptOps = stubOps(sessions, { text: "created", onPrompt: (input) => (seen = input) })
 
         const result = yield* def.execute(
           {
             description: "inspect bug",
             prompt: "look into the cache key path",
             subagent_type: "general",
-            task_id: "ses_missing",
+            task_id: SessionID.make("ses_missing"),
           },
           {
             sessionID: chat.id,
@@ -322,7 +380,7 @@ describe("tool.task", () => {
           const tool = yield* TaskTool
           const def = yield* tool.init()
           let seen: SessionPrompt.PromptInput | undefined
-          const promptOps = stubOps({ onPrompt: (input) => (seen = input) })
+          const promptOps = stubOps(sessions, { onPrompt: (input) => (seen = input) })
 
           const result = yield* def.execute(
             {
@@ -384,4 +442,273 @@ describe("tool.task", () => {
       },
     ),
   )
+
+  it.live("execute launches background tasks without waiting for completion", () =>
+    provideTmpdirInstance(() =>
+      Effect.gen(function* () {
+        Flag.OPENCODE_EXPERIMENTAL = true
+        const sessions = yield* Session.Service
+        const jobs = yield* BackgroundJob.Service
+        const { chat, assistant } = yield* seed()
+        const tool = yield* TaskTool
+        const def = yield* tool.init()
+        const latch = yield* Deferred.make<void>()
+
+        const result = yield* def.execute(
+          {
+            description: "inspect bug",
+            prompt: "look into the cache key path",
+            subagent_type: "general",
+            background: true,
+          },
+          {
+            sessionID: chat.id,
+            messageID: assistant.id,
+            agent: "build",
+            abort: new AbortController().signal,
+            extra: {
+              promptOps: stubOps(sessions, { wait: Deferred.await(latch) }),
+            },
+            messages: [],
+            metadata: () => Effect.void,
+            ask: () => Effect.void,
+          },
+        )
+
+        expect(result.metadata.sessionId).toBeDefined()
+        expect(result.metadata.background).toBe(true)
+        expect(result.output).toContain(`task_id: ${result.metadata.sessionId}`)
+        expect(result.output).toContain("state: running")
+        expect((yield* jobs.get(result.metadata.sessionId))?.status).toBe("running")
+
+        yield* Deferred.succeed(latch, undefined)
+        expect((yield* jobs.wait({ id: result.metadata.sessionId })).info?.status).toBe("completed")
+      }),
+    ),
+  )
+
+  it.live("background tasks inject completion into the parent session and resume when idle", () =>
+    provideTmpdirInstance(() =>
+      Effect.gen(function* () {
+        Flag.OPENCODE_EXPERIMENTAL = true
+        const sessions = yield* Session.Service
+        const jobs = yield* BackgroundJob.Service
+        const { chat, assistant } = yield* seed()
+        const tool = yield* TaskTool
+        const def = yield* tool.init()
+        const loops: string[] = []
+        const resumed = yield* Deferred.make<void>()
+
+        const result = yield* def.execute(
+          {
+            description: "inspect bug",
+            prompt: "look into the cache key path",
+            subagent_type: "general",
+            background: true,
+          },
+          {
+            sessionID: chat.id,
+            messageID: assistant.id,
+            agent: "build",
+            abort: new AbortController().signal,
+            extra: {
+              promptOps: {
+                ...stubOps(sessions, { text: "background done" }),
+                loop(input) {
+                  loops.push(input.sessionID)
+                  return Deferred.succeed(resumed, undefined).pipe(
+                    Effect.andThen(
+                      Effect.sync(() =>
+                        reply(
+                          {
+                            sessionID: input.sessionID,
+                            messageID: MessageID.ascending(),
+                            agent: "build",
+                            model: ref,
+                            parts: [],
+                          },
+                          "looped",
+                        ),
+                      ),
+                    ),
+                  )
+                },
+              } satisfies TaskPromptOps,
+            },
+            messages: [],
+            metadata: () => Effect.void,
+            ask: () => Effect.void,
+          },
+        )
+
+        expect((yield* jobs.wait({ id: result.metadata.sessionId })).info?.status).toBe("completed")
+        yield* Deferred.await(resumed).pipe(Effect.timeout("1 second"))
+
+        const parent = yield* sessions.findMessage(chat.id, (msg) => msg.info.role === "user")
+        expect(parent._tag).toBe("Some")
+        if (parent._tag !== "Some") return
+        expect(parent.value.parts.find((part) => part.type === "text")?.text).toContain("Background task completed")
+        expect(parent.value.parts.find((part) => part.type === "text")?.text).toContain("background done")
+        expect(loops).toEqual([chat.id])
+
+        const child = yield* sessions.findMessage(result.metadata.sessionId, (msg) => msg.info.role === "assistant")
+        expect(child._tag).toBe("Some")
+        if (child._tag !== "Some") return
+        expect(child.value.parts.find((part) => part.type === "text")?.text).toBe("background done")
+      }),
+    ),
+  )
+
+  it.live("background task resumes parent after it becomes idle", () =>
+    provideTmpdirInstance(() =>
+      Effect.gen(function* () {
+        Flag.OPENCODE_EXPERIMENTAL = true
+        const sessions = yield* Session.Service
+        const status = yield* SessionStatus.Service
+        const jobs = yield* BackgroundJob.Service
+        const { chat, assistant } = yield* seed()
+        const tool = yield* TaskTool
+        const def = yield* tool.init()
+        const loops: string[] = []
+        const resumed = yield* Deferred.make<void>()
+
+        yield* status.set(chat.id, { type: "busy" })
+
+        const result = yield* def.execute(
+          {
+            description: "inspect bug",
+            prompt: "look into the cache key path",
+            subagent_type: "general",
+            background: true,
+          },
+          {
+            sessionID: chat.id,
+            messageID: assistant.id,
+            agent: "build",
+            abort: new AbortController().signal,
+            extra: {
+              promptOps: {
+                ...stubOps(sessions, { text: "background done" }),
+                loop(input) {
+                  loops.push(input.sessionID)
+                  return Effect.sync(() =>
+                    reply(
+                      {
+                        sessionID: input.sessionID,
+                        messageID: MessageID.ascending(),
+                        agent: "build",
+                        model: ref,
+                        parts: [],
+                      },
+                      "looped",
+                    ),
+                  ).pipe(Effect.tap(() => Deferred.succeed(resumed, undefined)))
+                },
+              } satisfies TaskPromptOps,
+            },
+            messages: [],
+            metadata: () => Effect.void,
+            ask: () => Effect.void,
+          },
+        )
+
+        expect((yield* jobs.wait({ id: result.metadata.sessionId })).info?.status).toBe("completed")
+        expect(loops).toEqual([])
+        yield* status.set(chat.id, { type: "idle" })
+        yield* Deferred.await(resumed).pipe(Effect.timeout("1 second"))
+        expect(loops).toEqual([chat.id])
+      }),
+    ),
+  )
+
+  it.live("background resume fails while task is already running", () =>
+    provideTmpdirInstance(() =>
+      Effect.gen(function* () {
+        Flag.OPENCODE_EXPERIMENTAL = true
+        const sessions = yield* Session.Service
+        const { chat, assistant } = yield* seed()
+        const tool = yield* TaskTool
+        const def = yield* tool.init()
+        const latch = yield* Deferred.make<void>()
+
+        const result = yield* def.execute(
+          {
+            description: "inspect bug",
+            prompt: "look into the cache key path",
+            subagent_type: "general",
+            background: true,
+          },
+          {
+            sessionID: chat.id,
+            messageID: assistant.id,
+            agent: "build",
+            abort: new AbortController().signal,
+            extra: {
+              promptOps: stubOps(sessions, { wait: Deferred.await(latch) }),
+            },
+            messages: [],
+            metadata: () => Effect.void,
+            ask: () => Effect.void,
+          },
+        )
+
+        const exit = yield* def
+          .execute(
+            {
+              description: "inspect bug again",
+              prompt: "second prompt",
+              subagent_type: "general",
+              task_id: result.metadata.sessionId,
+              background: true,
+            },
+            {
+              sessionID: chat.id,
+              messageID: assistant.id,
+              agent: "build",
+              abort: new AbortController().signal,
+              extra: { promptOps: stubOps(sessions) },
+              messages: [],
+              metadata: () => Effect.void,
+              ask: () => Effect.void,
+            },
+          )
+          .pipe(Effect.exit)
+
+        expect(Exit.isFailure(exit)).toBe(true)
+        if (Exit.isFailure(exit)) {
+          const error = Cause.squash(exit.cause)
+          expect(error instanceof Error ? error.message : String(error)).toContain("is already running")
+        }
+
+        const foregroundExit = yield* def
+          .execute(
+            {
+              description: "inspect bug again",
+              prompt: "second prompt",
+              subagent_type: "general",
+              task_id: result.metadata.sessionId,
+            },
+            {
+              sessionID: chat.id,
+              messageID: assistant.id,
+              agent: "build",
+              abort: new AbortController().signal,
+              extra: { promptOps: stubOps(sessions) },
+              messages: [],
+              metadata: () => Effect.void,
+              ask: () => Effect.void,
+            },
+          )
+          .pipe(Effect.exit)
+
+        expect(Exit.isFailure(foregroundExit)).toBe(true)
+        if (Exit.isFailure(foregroundExit)) {
+          const error = Cause.squash(foregroundExit.cause)
+          expect(error instanceof Error ? error.message : String(error)).toContain("is already running")
+        }
+
+        yield* Deferred.succeed(latch, undefined)
+      }),
+    ),
+  )
 })

packages/opencode/test/tool/task_status.test.ts

@@ -0,0 +1,280 @@
+import { afterEach, describe, expect } from "bun:test"
+import { Effect, Layer, Scope } from "effect"
+import { Agent } from "../../src/agent/agent"
+import { CrossSpawnSpawner } from "@opencode-ai/core/cross-spawn-spawner"
+import { Instance } from "../../src/project/instance"
+import { Session } from "@/session/session"
+import { MessageV2 } from "../../src/session/message-v2"
+import { MessageID, PartID } from "../../src/session/schema"
+import { SessionStatus } from "../../src/session/status"
+import { TaskStatusTool } from "../../src/tool/task_status"
+import { Truncate } from "@/tool/truncate"
+import { ModelID, ProviderID } from "../../src/provider/schema"
+import { BackgroundJob } from "@/background/job"
+import { provideTmpdirInstance } from "../fixture/fixture"
+import { testEffect } from "../lib/effect"
+
+afterEach(async () => {
+  await Instance.disposeAll()
+})
+
+const ref = {
+  providerID: ProviderID.make("test"),
+  modelID: ModelID.make("test-model"),
+}
+
+const it = testEffect(
+  Layer.mergeAll(
+    Agent.defaultLayer,
+    CrossSpawnSpawner.defaultLayer,
+    Session.defaultLayer,
+    SessionStatus.defaultLayer,
+    BackgroundJob.defaultLayer,
+    Truncate.defaultLayer,
+  ),
+)
+
+const seedUser = Effect.fn("TaskStatusToolTest.seedUser")(function* (sessionID: Session.Info["id"]) {
+  const session = yield* Session.Service
+  return yield* session.updateMessage({
+    id: MessageID.ascending(),
+    role: "user",
+    sessionID,
+    agent: "build",
+    model: ref,
+    time: { created: Date.now() },
+  })
+})
+
+const seedAssistant = Effect.fn("TaskStatusToolTest.seedAssistant")(function* (input: {
+  sessionID: Session.Info["id"]
+  text: string
+  error?: string
+}) {
+  const session = yield* Session.Service
+  const user = yield* seedUser(input.sessionID)
+  const message = yield* session.updateMessage({
+    id: MessageID.ascending(),
+    role: "assistant",
+    parentID: user.id,
+    sessionID: input.sessionID,
+    mode: "build",
+    agent: "build",
+    cost: 0,
+    path: { cwd: "/tmp", root: "/tmp" },
+    tokens: { input: 0, output: 0, reasoning: 0, cache: { read: 0, write: 0 } },
+    modelID: ref.modelID,
+    providerID: ref.providerID,
+    time: { created: Date.now(), completed: Date.now() },
+    finish: "stop",
+    ...(input.error
+      ? {
+          error: new MessageV2.APIError({
+            message: input.error,
+            isRetryable: false,
+          }).toObject(),
+        }
+      : {}),
+  })
+
+  yield* session.updatePart({
+    id: PartID.ascending(),
+    messageID: message.id,
+    sessionID: input.sessionID,
+    type: "text",
+    text: input.text,
+  })
+})
+
+describe("tool.task_status", () => {
+  it.live("returns running while session status is busy", () =>
+    provideTmpdirInstance(() =>
+      Effect.gen(function* () {
+        const sessions = yield* Session.Service
+        const status = yield* SessionStatus.Service
+        const tool = yield* TaskStatusTool
+        const def = yield* tool.init()
+        const chat = yield* sessions.create({})
+
+        yield* status.set(chat.id, { type: "busy" })
+        const result = yield* def.execute(
+          { task_id: chat.id },
+          {
+            sessionID: chat.id,
+            messageID: MessageID.ascending(),
+            agent: "build",
+            abort: new AbortController().signal,
+            messages: [],
+            metadata: () => Effect.void,
+            ask: () => Effect.void,
+          },
+        )
+
+        expect(result.output).toContain("state: running")
+      }),
+    ),
+  )
+
+  it.live("returns completed with final task output", () =>
+    provideTmpdirInstance(() =>
+      Effect.gen(function* () {
+        const sessions = yield* Session.Service
+        const tool = yield* TaskStatusTool
+        const def = yield* tool.init()
+        const chat = yield* sessions.create({})
+
+        yield* seedAssistant({ sessionID: chat.id, text: "all done" })
+
+        const result = yield* def.execute(
+          { task_id: chat.id },
+          {
+            sessionID: chat.id,
+            messageID: MessageID.ascending(),
+            agent: "build",
+            abort: new AbortController().signal,
+            messages: [],
+            metadata: () => Effect.void,
+            ask: () => Effect.void,
+          },
+        )
+
+        expect(result.output).toContain("state: completed")
+        expect(result.output).toContain("all done")
+      }),
+    ),
+  )
+
+  it.live("wait=true blocks until terminal status", () =>
+    provideTmpdirInstance(() =>
+      Effect.gen(function* () {
+        const sessions = yield* Session.Service
+        const status = yield* SessionStatus.Service
+        const tool = yield* TaskStatusTool
+        const def = yield* tool.init()
+        const chat = yield* sessions.create({})
+        const scope = yield* Scope.Scope
+
+        yield* status.set(chat.id, { type: "busy" })
+        yield* Effect.gen(function* () {
+          yield* Effect.sleep("150 millis")
+          yield* status.set(chat.id, { type: "idle" })
+          yield* seedAssistant({ sessionID: chat.id, text: "finished later" })
+        }).pipe(Effect.forkIn(scope))
+
+        const result = yield* def.execute(
+          {
+            task_id: chat.id,
+            wait: true,
+            timeout_ms: 4_000,
+          },
+          {
+            sessionID: chat.id,
+            messageID: MessageID.ascending(),
+            agent: "build",
+            abort: new AbortController().signal,
+            messages: [],
+            metadata: () => Effect.void,
+            ask: () => Effect.void,
+          },
+        )
+
+        expect(result.output).toContain("state: completed")
+        expect(result.output).toContain("finished later")
+      }),
+    ),
+  )
+
+  it.live("returns error when child run fails", () =>
+    provideTmpdirInstance(() =>
+      Effect.gen(function* () {
+        const sessions = yield* Session.Service
+        const tool = yield* TaskStatusTool
+        const def = yield* tool.init()
+        const chat = yield* sessions.create({})
+
+        yield* seedAssistant({ sessionID: chat.id, text: "", error: "child failed" })
+
+        const result = yield* def.execute(
+          { task_id: chat.id },
+          {
+            sessionID: chat.id,
+            messageID: MessageID.ascending(),
+            agent: "build",
+            abort: new AbortController().signal,
+            messages: [],
+            metadata: () => Effect.void,
+            ask: () => Effect.void,
+          },
+        )
+
+        expect(result.output).toContain("state: error")
+        expect(result.output).toContain("child failed")
+        expect(result.metadata.state).toBe("error")
+      }),
+    ),
+  )
+
+  it.live("wait=true times out with timed_out metadata", () =>
+    provideTmpdirInstance(() =>
+      Effect.gen(function* () {
+        const sessions = yield* Session.Service
+        const status = yield* SessionStatus.Service
+        const tool = yield* TaskStatusTool
+        const def = yield* tool.init()
+        const chat = yield* sessions.create({})
+
+        yield* status.set(chat.id, { type: "busy" })
+        const result = yield* def.execute(
+          {
+            task_id: chat.id,
+            wait: true,
+            timeout_ms: 80,
+          },
+          {
+            sessionID: chat.id,
+            messageID: MessageID.ascending(),
+            agent: "build",
+            abort: new AbortController().signal,
+            messages: [],
+            metadata: () => Effect.void,
+            ask: () => Effect.void,
+          },
+        )
+
+        expect(result.output).toContain("Timed out after 80ms")
+        expect(result.metadata.timed_out).toBe(true)
+        expect(result.metadata.state).toBe("running")
+      }),
+    ),
+  )
+
+  it.live("returns running for resumed task with a newer user turn", () =>
+    provideTmpdirInstance(() =>
+      Effect.gen(function* () {
+        const sessions = yield* Session.Service
+        const tool = yield* TaskStatusTool
+        const def = yield* tool.init()
+        const chat = yield* sessions.create({})
+
+        yield* seedAssistant({ sessionID: chat.id, text: "old done" })
+        yield* seedUser(chat.id)
+
+        const result = yield* def.execute(
+          { task_id: chat.id },
+          {
+            sessionID: chat.id,
+            messageID: MessageID.ascending(),
+            agent: "build",
+            abort: new AbortController().signal,
+            messages: [],
+            metadata: () => Effect.void,
+            ask: () => Effect.void,
+          },
+        )
+
+        expect(result.output).toContain("state: running")
+        expect(result.output).toContain("Task is starting.")
+      }),
+    ),
+  )
+})

packages/ui/src/components/message-part.tsx

@@ -1719,9 +1719,11 @@ ToolRegistry.register({
     const title = createMemo(() => agent().name ?? i18n.t("ui.tool.agent.default"))
     const tone = createMemo(() => agent().color)
     const subtitle = createMemo(() => {
-      const value = props.input.description
-      if (typeof value === "string" && value) return value
-      return childSessionId()
+      const value =
+        typeof props.input.description === "string" && props.input.description ? props.input.description : childSessionId()
+      if (!value) return value
+      if (props.metadata.background === true) return `${value} (background)`
+      return value
     })
     const running = createMemo(() => props.status === "pending" || props.status === "running")