diff --git a/packages/opencode/src/background/job.ts b/packages/opencode/src/background/job.ts
index 3ea228f04..8179c7328 100644
--- a/packages/opencode/src/background/job.ts
+++ b/packages/opencode/src/background/job.ts
@@ -1,6 +1,6 @@
 import { InstanceState } from "@/effect/instance-state"
 import { Identifier } from "@/id/id"
-import { Cause, Clock, Context, Deferred, Effect, Fiber, Layer, Scope, SynchronizedRef } from "effect"
+import { Cause, Clock, Context, Deferred, Effect, Exit, Layer, Scope, SynchronizedRef } from "effect"
 
 export type Status = "running" | "completed" | "error" | "cancelled"
 
@@ -19,7 +19,11 @@ export type Info = {
 type Active = {
   info: Info
   done: Deferred.Deferred<Info>
-  fiber?: Fiber.Fiber<void, unknown>
+  scope: Scope.Closeable
+  token: object
+  pending: number
+  next: number
+  output?: { sequence: number; text: string }
 }
 
 type State = {
@@ -30,6 +34,7 @@ type State = {
 type FinishResult = {
   info?: Info
   done?: Deferred.Deferred<Info>
+  scope?: Scope.Closeable
 }
 
 export type StartInput = {
@@ -40,6 +45,11 @@ export type StartInput = {
   run: Effect.Effect<string, unknown>
 }
 
+export type ExtendInput = {
+  id: string
+  run: Effect.Effect<string, unknown>
+}
+
 export type WaitInput = {
   id: string
   timeout?: number
@@ -54,6 +64,7 @@ export interface Interface {
   readonly list: () => Effect.Effect<Info[]>
   readonly get: (id: string) => Effect.Effect<Info | undefined>
   readonly start: (input: StartInput) => Effect.Effect<Info>
+  readonly extend: (input: ExtendInput) => Effect.Effect<boolean>
   readonly wait: (input: WaitInput) => Effect.Effect<WaitResult>
   readonly cancel: (id: string) => Effect.Effect<Info | undefined>
 }
@@ -84,36 +95,75 @@ export const layer = Layer.effect(
       }),
     )
 
-    const finish = Effect.fn("BackgroundJob.finish")(function* (
+    const settle = Effect.fn("BackgroundJob.settle")(function* (
       id: string,
-      status: Exclude<Status, "running">,
-      data?: { output?: string; error?: string },
+      token: object,
+      sequence: number,
+      exit: Exit.Exit<string, unknown>,
     ) {
       const completed_at = yield* Clock.currentTimeMillis
+      const s = yield* InstanceState.get(state)
       const result = yield* SynchronizedRef.modify(
-        (yield* InstanceState.get(state)).jobs,
+        s.jobs,
         (jobs): readonly [FinishResult, Map<string, Active>] => {
           const job = jobs.get(id)
           if (!job) return [{}, jobs]
+          if (job.token !== token) return [{}, jobs]
           if (job.info.status !== "running") return [{ info: snapshot(job) }, jobs]
+          const pending = job.pending - 1
+          const output = Exit.isSuccess(exit) && (!job.output || sequence > job.output.sequence)
+            ? { sequence, text: exit.value }
+            : job.output
+          if (Exit.isSuccess(exit) && pending > 0) {
+            return [{}, new Map(jobs).set(id, { ...job, pending, output })]
+          }
+          const status: Exclude<Status, "running"> = Exit.isSuccess(exit)
+            ? "completed"
+            : Cause.hasInterruptsOnly(exit.cause)
+              ? "cancelled"
+              : "error"
           const next = {
             ...job,
-            fiber: undefined,
+            pending: 0,
+            output,
             info: {
               ...job.info,
               status,
               completed_at,
-              ...(data?.output !== undefined ? { output: data.output } : {}),
-              ...(data?.error !== undefined ? { error: data.error } : {}),
+              ...(output ? { output: output.text } : {}),
+              ...(Exit.isFailure(exit) ? { error: errorText(Cause.squash(exit.cause)) } : {}),
             },
           }
-          return [{ info: snapshot(next), done: job.done }, new Map(jobs).set(id, next)]
+          return [
+            { info: snapshot(next), done: job.done, scope: job.scope },
+            new Map(jobs).set(id, next),
+          ]
         },
       )
       if (result.info && result.done) yield* Deferred.succeed(result.done, result.info).pipe(Effect.ignore)
+      if (result.scope) {
+        yield* Scope.close(result.scope, Exit.void).pipe(Effect.forkIn(s.scope, { startImmediately: true }))
+      }
       return result.info
     })
 
+    const fork = Effect.fn("BackgroundJob.fork")(function* (
+      scope: Scope.Scope,
+      id: string,
+      token: object,
+      sequence: number,
+      run: Effect.Effect<string, unknown>,
+    ) {
+      return yield* run.pipe(
+        Effect.matchCauseEffect({
+          onSuccess: (output) => settle(id, token, sequence, Exit.succeed(output)),
+          onFailure: (cause) => settle(id, token, sequence, Exit.failCause(cause)),
+        }),
+        Effect.asVoid,
+        Effect.forkIn(scope, { startImmediately: true }),
+      )
+    })
+
     const list: Interface["list"] = Effect.fn("BackgroundJob.list")(function* () {
       return Array.from((yield* SynchronizedRef.get((yield* InstanceState.get(state)).jobs)).values())
         .map(snapshot)
@@ -138,17 +188,9 @@ export const layer = Layer.effect(
             Effect.fnUntraced(function* (jobs) {
               const existing = jobs.get(id)
               if (existing?.info.status === "running") return [snapshot(existing), jobs] as const
-              const fiber = yield* restore(input.run).pipe(
-                Effect.matchCauseEffect({
-                  onSuccess: (output) => finish(id, "completed", { output }),
-                  onFailure: (cause) =>
-                    finish(id, Cause.hasInterruptsOnly(cause) ? "cancelled" : "error", {
-                      error: errorText(Cause.squash(cause)),
-                    }),
-                }),
-                Effect.asVoid,
-                Effect.forkIn(s.scope, { startImmediately: true }),
-              )
+              const scope = yield* Scope.fork(s.scope, "parallel")
+              const token = {}
+              yield* fork(scope, id, token, 0, restore(input.run))
               const job = {
                 info: {
                   id,
@@ -159,7 +201,10 @@ export const layer = Layer.effect(
                   metadata: input.metadata,
                 },
                 done,
-                fiber,
+                scope,
+                token,
+                pending: 1,
+                next: 1,
               }
               return [snapshot(job), new Map(jobs).set(id, job)] as const
             }),
@@ -168,6 +213,30 @@ export const layer = Layer.effect(
       )
     })
 
+    const extend: Interface["extend"] = Effect.fn("BackgroundJob.extend")(function* (input) {
+      return yield* Effect.uninterruptibleMask((restore) =>
+        Effect.gen(function* () {
+          const s = yield* InstanceState.get(state)
+          return yield* SynchronizedRef.modifyEffect(
+            s.jobs,
+            Effect.fnUntraced(function* (jobs) {
+              const job = jobs.get(input.id)
+              if (!job || job.info.status !== "running") return [false, jobs] as const
+              yield* fork(job.scope, input.id, job.token, job.next, restore(input.run))
+              return [
+                true,
+                new Map(jobs).set(input.id, {
+                  ...job,
+                  pending: job.pending + 1,
+                  next: job.next + 1,
+                }),
+              ] as const
+            }),
+          )
+        }),
+      )
+    })
+
     const wait: Interface["wait"] = Effect.fn("BackgroundJob.wait")(function* (input) {
       const job = (yield* SynchronizedRef.get((yield* InstanceState.get(state)).jobs)).get(input.id)
       if (!job) return { timedOut: false }
@@ -180,18 +249,34 @@ export const layer = Layer.effect(
     })
 
     const cancel: Interface["cancel"] = Effect.fn("BackgroundJob.cancel")(function* (id) {
-      const job = (yield* SynchronizedRef.get((yield* InstanceState.get(state)).jobs)).get(id)
-      if (!job) return
-      if (job.info.status !== "running") return snapshot(job)
-      if (job.fiber) {
-        yield* Fiber.interrupt(job.fiber).pipe(Effect.ignore)
-        yield* Fiber.await(job.fiber).pipe(Effect.ignore)
-      }
-      const info = yield* finish(id, "cancelled")
-      return info
+      const completed_at = yield* Clock.currentTimeMillis
+      const result = yield* SynchronizedRef.modify(
+        (yield* InstanceState.get(state)).jobs,
+        (jobs): readonly [FinishResult, Map<string, Active>] => {
+          const job = jobs.get(id)
+          if (!job) return [{}, jobs]
+          if (job.info.status !== "running") return [{ info: snapshot(job) }, jobs]
+          const next = {
+            ...job,
+            pending: 0,
+            info: {
+              ...job.info,
+              status: "cancelled" as const,
+              completed_at,
+            },
+          }
+          return [
+            { info: snapshot(next), done: job.done, scope: job.scope },
+            new Map(jobs).set(id, next),
+          ]
+        },
+      )
+      if (result.info && result.done) yield* Deferred.succeed(result.done, result.info).pipe(Effect.ignore)
+      if (result.scope) yield* Scope.close(result.scope, Exit.void)
+      return result.info
     })
 
-    return Service.of({ list, get, start, wait, cancel })
+    return Service.of({ list, get, start, extend, wait, cancel })
   }),
 )
 
diff --git a/packages/opencode/src/tool/task.ts b/packages/opencode/src/tool/task.ts
index d4affaec4..2324f79ff 100644
--- a/packages/opencode/src/tool/task.ts
+++ b/packages/opencode/src/tool/task.ts
@@ -10,7 +10,7 @@ import { Agent } from "../agent/agent"
 import { deriveSubagentSessionPermission } from "../agent/subagent-permissions"
 import type { SessionPrompt } from "../session/prompt"
 import { Config } from "@/config/config"
-import { Cause, Effect, Exit, Schema, Scope } from "effect"
+import { Effect, Exit, Schema, Scope } from "effect"
 import { EffectBridge } from "@/effect/bridge"
 import { RuntimeFlags } from "@/effect/runtime-flags"
 import { Database } from "@opencode-ai/core/database/database"
@@ -69,6 +69,17 @@ function backgroundOutput(sessionID: SessionID) {
   ].join("\n")
 }
 
+function backgroundUpdateOutput(sessionID: SessionID) {
+  return [
+    `<task id="${sessionID}" state="running">`,
+    "<summary>Background task updated</summary>",
+    "<task_result>",
+    "Additional context sent to the background task.",
+    "</task_result>",
+    "</task>",
+  ].join("\n")
+}
+
 function backgroundMessage(input: {
   sessionID: SessionID
   description: string
@@ -90,11 +101,6 @@ function backgroundMessage(input: {
   ].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* () {
@@ -231,9 +237,16 @@ export const TaskTool = Tool.define(
           .pipe(Effect.ignore, Effect.forkIn(scope, { startImmediately: true }))
       })
 
-      const existing = yield* background.get(nextSession.id)
-      if (existing?.status === "running") {
-        return yield* Effect.fail(new Error(`Task ${nextSession.id} is already running.`))
+      if (yield* background.extend({ id: nextSession.id, run: runTask() })) {
+        return {
+          title: params.description,
+          metadata: {
+            ...metadata,
+            background: true,
+            jobId: nextSession.id,
+          },
+          output: backgroundUpdateOutput(nextSession.id),
+        }
       }
 
       if (runInBackground) {
@@ -242,16 +255,18 @@ export const TaskTool = Tool.define(
           type: id,
           title: params.description,
           metadata,
-          run: runTask().pipe(
-            Effect.tap((text) => inject("completed", text).pipe(Effect.ignore)),
-            Effect.catchCause((cause) =>
-              (Cause.hasInterruptsOnly(cause)
-                ? Effect.void
-                : inject("error", errorText(Cause.squash(cause))).pipe(Effect.ignore)
-              ).pipe(Effect.andThen(Effect.failCause(cause))),
-            ),
-          ),
+          run: runTask(),
         })
+        yield* background
+          .wait({ id: info.id })
+          .pipe(
+            Effect.flatMap((result) => {
+              if (result.info?.status === "completed") return inject("completed", result.info.output ?? "")
+              if (result.info?.status === "error") return inject("error", result.info.error ?? "")
+              return Effect.void
+            }),
+            Effect.forkIn(scope, { startImmediately: true }),
+          )
 
         return {
           title: params.description,
diff --git a/packages/opencode/test/background/job.test.ts b/packages/opencode/test/background/job.test.ts
index afc7260bb..667f9f5db 100644
--- a/packages/opencode/test/background/job.test.ts
+++ b/packages/opencode/test/background/job.test.ts
@@ -78,6 +78,38 @@ describe("background.job", () => {
     }),
   )
 
+  it.instance("waits for extensions before completing a running job", () =>
+    Effect.gen(function* () {
+      const jobs = yield* BackgroundJob.Service
+      const first = yield* Deferred.make<void>()
+      const second = yield* Deferred.make<void>()
+      const job = yield* jobs.start({
+        type: "test",
+        run: Deferred.await(first).pipe(Effect.as("first")),
+      })
+
+      expect(yield* jobs.extend({ id: job.id, run: Deferred.await(second).pipe(Effect.as("second")) })).toBe(true)
+      yield* Deferred.succeed(first, undefined)
+      expect((yield* jobs.get(job.id))?.status).toBe("running")
+
+      yield* Deferred.succeed(second, undefined)
+      const done = yield* jobs.wait({ id: job.id })
+      expect(done.info?.status).toBe("completed")
+      expect(done.info?.output).toBe("second")
+    }),
+  )
+
+  it.instance("rejects extensions after a job completes", () =>
+    Effect.gen(function* () {
+      const jobs = yield* BackgroundJob.Service
+      const job = yield* jobs.start({ type: "test", run: Effect.succeed("done") })
+      yield* jobs.wait({ id: job.id })
+
+      expect(yield* jobs.extend({ id: job.id, run: Effect.succeed("late") })).toBe(false)
+      expect((yield* jobs.get(job.id))?.output).toBe("done")
+    }),
+  )
+
   it.instance("records failed jobs", () =>
     Effect.gen(function* () {
       const jobs = yield* BackgroundJob.Service
@@ -93,19 +125,56 @@ describe("background.job", () => {
     }),
   )
 
+  it.instance("ignores stale settlements after restarting a failed job", () =>
+    Effect.gen(function* () {
+      const jobs = yield* BackgroundJob.Service
+      const fail = yield* Deferred.make<void>()
+      const interrupted = yield* Deferred.make<void>()
+      const release = yield* Deferred.make<void>()
+      const id = "job_test"
+      yield* jobs.start({
+        id,
+        type: "test",
+        run: Deferred.await(fail).pipe(Effect.andThen(Effect.fail(new Error("boom")))),
+      })
+      yield* jobs.extend({
+        id,
+        run: Effect.never.pipe(
+          Effect.ensuring(Deferred.succeed(interrupted, undefined).pipe(Effect.andThen(Deferred.await(release)))),
+        ),
+      })
+
+      yield* Deferred.succeed(fail, undefined)
+      expect((yield* jobs.wait({ id })).info?.status).toBe("error")
+      yield* Deferred.await(interrupted)
+      yield* jobs.start({ id, type: "test", run: Effect.never })
+
+      yield* Deferred.succeed(release, undefined)
+      yield* Effect.yieldNow
+      expect((yield* jobs.get(id))?.status).toBe("running")
+      yield* jobs.cancel(id)
+    }),
+  )
+
   it.instance("can cancel running jobs", () =>
     Effect.gen(function* () {
       const jobs = yield* BackgroundJob.Service
       const interrupted = yield* Deferred.make<void>()
+      const extendedInterrupted = yield* Deferred.make<void>()
       const job = yield* jobs.start({
         type: "test",
         run: Effect.never.pipe(Effect.ensuring(Deferred.succeed(interrupted, undefined))),
       })
+      yield* jobs.extend({
+        id: job.id,
+        run: Effect.never.pipe(Effect.ensuring(Deferred.succeed(extendedInterrupted, undefined))),
+      })
 
       const cancelled = yield* jobs.cancel(job.id)
 
       expect(cancelled?.status).toBe("cancelled")
       yield* Deferred.await(interrupted).pipe(Effect.timeout("1 second"))
+      yield* Deferred.await(extendedInterrupted).pipe(Effect.timeout("1 second"))
       expect((yield* jobs.get(job.id))?.status).toBe("cancelled")
     }),
   )
diff --git a/packages/opencode/test/tool/task.test.ts b/packages/opencode/test/tool/task.test.ts
index 3e268d586..b8d2accf9 100644
--- a/packages/opencode/test/tool/task.test.ts
+++ b/packages/opencode/test/tool/task.test.ts
@@ -518,6 +518,79 @@ describe("tool.task", () => {
     }),
   )
 
+  background.instance("background task completion waits for running updates", () =>
+    Effect.gen(function* () {
+      const jobs = yield* BackgroundJob.Service
+      const { chat, assistant } = yield* seed()
+      const tool = yield* TaskTool
+      const def = yield* tool.init()
+      const first = defer<void>()
+      const second = defer<void>()
+      const updated = defer<SessionPrompt.PromptInput>()
+      const injected = defer<SessionPrompt.PromptInput>()
+      let prompts = 0
+      const promptOps: TaskPromptOps = {
+        ...stubOps(),
+        prompt: (input) => {
+          if (input.sessionID === chat.id) {
+            injected.resolve(input)
+            return Effect.succeed(reply(input, "done"))
+          }
+          prompts++
+          if (prompts === 1) return Effect.promise(() => first.promise).pipe(Effect.as(reply(input, "first done")))
+          updated.resolve(input)
+          return Effect.promise(() => second.promise).pipe(Effect.as(reply(input, "second done")))
+        },
+      }
+      const context = {
+        sessionID: chat.id,
+        messageID: assistant.id,
+        agent: "build",
+        abort: new AbortController().signal,
+        extra: { promptOps },
+        messages: [],
+        metadata: () => Effect.void,
+        ask: () => Effect.void,
+      }
+
+      const started = yield* def.execute(
+        {
+          description: "inspect bug",
+          prompt: "look into the cache key path",
+          subagent_type: "general",
+          background: true,
+        },
+        context,
+      )
+      const result = yield* def.execute(
+        {
+          description: "add investigation scope",
+          prompt: "also inspect cancellation",
+          subagent_type: "general",
+          task_id: started.metadata.sessionId,
+        },
+        context,
+      )
+
+      expect((yield* Effect.promise(() => updated.promise)).parts).toEqual([
+        { type: "text", text: "also inspect cancellation" },
+      ])
+      expect(result.metadata.sessionId).toBe(started.metadata.sessionId)
+      expect(result.metadata.background).toBe(true)
+      expect(result.output).toContain("Background task updated")
+      first.resolve()
+      expect((yield* jobs.get(started.metadata.sessionId))?.status).toBe("running")
+
+      second.resolve()
+      const waited = yield* jobs.wait({ id: started.metadata.sessionId, timeout: 1_000 })
+      expect(waited.info?.status).toBe("completed")
+      expect(waited.info?.output).toBe("second done")
+      const notification = yield* Effect.promise(() => injected.promise)
+      expect(notification.parts[0]?.type).toBe("text")
+      if (notification.parts[0]?.type === "text") expect(notification.parts[0].text).toContain("second done")
+    }),
+  )
+
   background.instance("background tasks complete through the background job service", () =>
     Effect.gen(function* () {
       const jobs = yield* BackgroundJob.Service