feat(background-task): track retry attempts across sessions

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

src/features/background-agent/attempt-lifecycle.ts

@@ -0,0 +1,174 @@
+import type { DelegatedModelConfig } from "../../shared/model-resolution-types"
+import type { BackgroundTask, BackgroundTaskAttempt, BackgroundTaskStatus } from "./types"
+
+type TerminalAttemptStatus = Extract<BackgroundTaskStatus, "completed" | "error" | "cancelled" | "interrupt">
+
+function toAttemptModel(model: DelegatedModelConfig | undefined): Pick<BackgroundTaskAttempt, "providerID" | "modelID" | "variant"> {
+  return {
+    providerID: model?.providerID,
+    modelID: model?.modelID,
+    variant: model?.variant,
+  }
+}
+
+function toTaskModel(attempt: BackgroundTaskAttempt): DelegatedModelConfig | undefined {
+  if (!attempt.providerID || !attempt.modelID) {
+    return undefined
+  }
+
+  return {
+    providerID: attempt.providerID,
+    modelID: attempt.modelID,
+    ...(attempt.variant ? { variant: attempt.variant } : {}),
+  }
+}
+
+function getAttemptIndex(task: BackgroundTask, attemptID: string): number {
+  return task.attempts?.findIndex((attempt) => attempt.attemptID === attemptID) ?? -1
+}
+
+function getAttempt(task: BackgroundTask, attemptID: string): BackgroundTaskAttempt | undefined {
+  const index = getAttemptIndex(task, attemptID)
+  return index === -1 ? undefined : task.attempts?.[index]
+}
+
+function isTerminalStatus(status: BackgroundTaskStatus): status is TerminalAttemptStatus {
+  return status === "completed" || status === "error" || status === "cancelled" || status === "interrupt"
+}
+
+export function getCurrentAttempt(task: BackgroundTask): BackgroundTaskAttempt | undefined {
+  if (!task.currentAttemptID) {
+    return undefined
+  }
+
+  return getAttempt(task, task.currentAttemptID)
+}
+
+export function ensureCurrentAttempt(
+  task: BackgroundTask,
+  model: DelegatedModelConfig | undefined = task.model,
+): BackgroundTaskAttempt {
+  const existingAttempt = getCurrentAttempt(task)
+  if (existingAttempt) {
+    return existingAttempt
+  }
+
+  const attempt: BackgroundTaskAttempt = {
+    attemptID: `att_${crypto.randomUUID().slice(0, 8)}`,
+    attemptNumber: (task.attempts?.length ?? 0) + 1,
+    sessionID: task.sessionID,
+    ...toAttemptModel(model),
+    status: task.status,
+    error: task.error,
+    startedAt: task.startedAt,
+    completedAt: task.completedAt,
+  }
+
+  task.attempts = [...(task.attempts ?? []), attempt]
+  task.currentAttemptID = attempt.attemptID
+  return attempt
+}
+
+export function projectTaskFromCurrentAttempt(task: BackgroundTask): BackgroundTask {
+  const currentAttempt = getCurrentAttempt(task)
+  if (!currentAttempt) {
+    return task
+  }
+
+  task.status = currentAttempt.status
+  task.sessionID = currentAttempt.sessionID
+  task.startedAt = currentAttempt.startedAt
+  task.completedAt = currentAttempt.completedAt
+  task.error = currentAttempt.error
+  task.model = toTaskModel(currentAttempt)
+
+  return task
+}
+
+export function startAttempt(task: BackgroundTask, model: DelegatedModelConfig | undefined): BackgroundTaskAttempt {
+  const attempt: BackgroundTaskAttempt = {
+    attemptID: `att_${crypto.randomUUID().slice(0, 8)}`,
+    attemptNumber: (task.attempts?.length ?? 0) + 1,
+    ...toAttemptModel(model),
+    status: "pending",
+  }
+
+  task.attempts = [...(task.attempts ?? []), attempt]
+  task.currentAttemptID = attempt.attemptID
+  task.status = "pending"
+  task.sessionID = undefined
+  task.startedAt = undefined
+  task.completedAt = undefined
+  task.error = undefined
+  task.model = model
+
+  return attempt
+}
+
+export function bindAttemptSession(
+  task: BackgroundTask,
+  attemptID: string,
+  sessionID: string,
+  model: DelegatedModelConfig | undefined,
+): BackgroundTaskAttempt | undefined {
+  ensureCurrentAttempt(task, model)
+  if (task.currentAttemptID !== attemptID) {
+    return undefined
+  }
+
+  const attempt = getAttempt(task, attemptID)
+  if (!attempt || isTerminalStatus(attempt.status)) {
+    return undefined
+  }
+
+  attempt.sessionID = sessionID
+  attempt.status = "running"
+  attempt.startedAt = new Date()
+  attempt.completedAt = undefined
+  attempt.error = undefined
+  attempt.providerID = model?.providerID ?? attempt.providerID
+  attempt.modelID = model?.modelID ?? attempt.modelID
+  attempt.variant = model?.variant ?? attempt.variant
+
+  return getCurrentAttempt(projectTaskFromCurrentAttempt(task))
+}
+
+export function finalizeAttempt(
+  task: BackgroundTask,
+  attemptID: string,
+  status: TerminalAttemptStatus,
+  error?: string,
+): BackgroundTaskAttempt | undefined {
+  const attempt = getAttempt(task, attemptID)
+  if (!attempt) {
+    return undefined
+  }
+
+  attempt.status = status
+  attempt.completedAt = new Date()
+  attempt.error = error
+
+  if (task.currentAttemptID === attemptID) {
+    projectTaskFromCurrentAttempt(task)
+  }
+
+  return attempt
+}
+
+export function scheduleRetryAttempt(
+  task: BackgroundTask,
+  failedAttemptID: string,
+  nextModel: DelegatedModelConfig,
+  error?: string,
+): BackgroundTaskAttempt | undefined {
+  const failedAttempt = finalizeAttempt(task, failedAttemptID, "error", error)
+  if (!failedAttempt || task.currentAttemptID !== failedAttemptID) {
+    return undefined
+  }
+
+  return startAttempt(task, nextModel)
+}
+
+export function findAttemptBySession(task: BackgroundTask, sessionID: string): BackgroundTaskAttempt | undefined {
+  return task.attempts?.find((attempt) => attempt.sessionID === sessionID)
+}

src/features/background-agent/constants.ts

@@ -45,6 +45,7 @@ export interface Todo {
 }
 
 export interface QueueItem {
+  attemptID: string
   task: BackgroundTask
   input: LaunchInput
 }

src/features/background-agent/fallback-retry-handler.test.ts

@@ -259,6 +259,57 @@ describe("tryFallbackRetry", () => {
       expect(queue![0].task).toBe(args.task)
       expect(args.processKey).toHaveBeenCalledWith(key)
     })
+
+    test("finalizes the failed attempt, creates a new pending attempt, and enqueues its explicit attemptID", async () => {
+      const args = createDefaultArgs({
+        status: "running",
+        sessionID: "session-attempt-1",
+        startedAt: new Date("2026-04-27T00:00:00.000Z"),
+        attempts: [
+          {
+            attemptID: "attempt-1",
+            attemptNumber: 1,
+            sessionID: "session-attempt-1",
+            providerID: "provider-a",
+            modelID: "original-model",
+            status: "running",
+            startedAt: new Date("2026-04-27T00:00:00.000Z"),
+          },
+        ],
+        currentAttemptID: "attempt-1",
+      })
+
+      await tryFallbackRetry(args)
+
+      expect(args.task.attempts).toHaveLength(2)
+      expect(args.task.attempts?.[0]).toMatchObject({
+        attemptID: "attempt-1",
+        sessionID: "session-attempt-1",
+        status: "error",
+        error: "model overloaded",
+      })
+      expect(args.task.attempts?.[0]?.completedAt).toBeInstanceOf(Date)
+
+      const nextAttempt = args.task.attempts?.[1]
+      expect(nextAttempt).toBeDefined()
+      expect(nextAttempt?.attemptNumber).toBe(2)
+      expect(nextAttempt?.providerID).toBe("provider-a")
+      expect(nextAttempt?.modelID).toBe("fallback-model-1")
+      expect(nextAttempt?.status).toBe("pending")
+
+      expect(args.task.currentAttemptID).toBe(nextAttempt?.attemptID)
+      expect(args.task.status).toBe("pending")
+      expect(args.task.model).toEqual({
+        providerID: "provider-a",
+        modelID: "fallback-model-1",
+        variant: undefined,
+      })
+
+      const key = `${args.task.model!.providerID}/${args.task.model!.modelID}`
+      const queue = args.queuesByKey.get(key)
+      expect(queue).toBeDefined()
+      expect((queue?.[0] as QueueItem & { attemptID?: string })?.attemptID).toBe(nextAttempt?.attemptID)
+    })
   })
 
   describe("#given non-retryable error", () => {
@@ -343,6 +394,25 @@ describe("tryFallbackRetry", () => {
     })
   })
 
+  describe("#given first fallback is a no-op for the current model", () => {
+    test("skips the no-op fallback and advances to the next distinct model", async () => {
+      const args = createDefaultArgs({
+        model: { providerID: "provider-a", modelID: "fallback-model-1" },
+        fallbackChain: [
+          { model: "fallback-model-1", providers: ["provider-a"], variant: undefined },
+          { model: "fallback-model-2", providers: ["provider-b"], variant: undefined },
+        ],
+      })
+
+      const result = await tryFallbackRetry(args)
+
+      expect(result).toBe(true)
+      expect(args.task.model?.providerID).toBe("provider-b")
+      expect(args.task.model?.modelID).toBe("fallback-model-2")
+      expect(args.task.attemptCount).toBe(2)
+    })
+  })
+
   describe("#given disconnected fallback providers with connected preferred provider", () => {
     test("keeps fallback entry and selects connected preferred provider", async () => {
       ;(readProviderModelsCache as any).mockReturnValueOnce({ connected: ["provider-a"] })

src/features/background-agent/fallback-retry-handler.ts

@@ -11,6 +11,11 @@ import {
 } from "../../shared/model-error-classifier"
 import { transformModelForProvider } from "../../shared/provider-model-id-transform"
 import { abortWithTimeout } from "./abort-with-timeout"
+import { ensureCurrentAttempt, scheduleRetryAttempt } from "./attempt-lifecycle"
+
+function canonicalizeModelID(modelID: string): string {
+  return modelID.toLowerCase().replace(/\./g, "-")
+}
 
 export async function tryFallbackRetry(args: {
   task: BackgroundTask
@@ -21,8 +26,16 @@ export async function tryFallbackRetry(args: {
   idleDeferralTimers: Map<string, ReturnType<typeof setTimeout>>
   queuesByKey: Map<string, QueueItem[]>
   processKey: (key: string) => void
+  onRetrying?: (details: {
+    task: BackgroundTask
+    source: string
+    previousSessionID?: string
+    failedModel?: string
+    failedError?: string
+    nextModel: string
+  }) => void
 }): Promise<boolean> {
-  const { task, errorInfo, source, concurrencyManager, client, idleDeferralTimers, queuesByKey, processKey } = args
+  const { task, errorInfo, source, concurrencyManager, client, idleDeferralTimers, queuesByKey, processKey, onRetrying } = args
   const fallbackChain = task.fallbackChain
   const canRetry =
     shouldRetryError(errorInfo) &&
@@ -48,6 +61,7 @@ export async function tryFallbackRetry(args: {
 
   let selectedAttemptCount = attemptCount
   let nextFallback: FallbackEntry | undefined
+  let nextProviderID: string | undefined
   while (fallbackChain && selectedAttemptCount < fallbackChain.length) {
     const candidate = getNextFallback(fallbackChain, selectedAttemptCount)
     if (!candidate) break
@@ -61,12 +75,31 @@ export async function tryFallbackRetry(args: {
       })
       continue
     }
+    const candidateProviderID = selectFallbackProvider(
+      candidate.providers,
+      task.model?.providerID,
+    )
+    const candidateModelID = transformModelForProvider(candidateProviderID, candidate.model)
+    const isNoOpFallback =
+      !!task.model &&
+      candidateProviderID.toLowerCase() === task.model.providerID.toLowerCase() &&
+      canonicalizeModelID(candidateModelID) === canonicalizeModelID(task.model.modelID)
+    if (isNoOpFallback) {
+      log("[background-agent] Skipping no-op fallback:", {
+        taskId: task.id,
+        source,
+        model: candidate.model,
+        providers: candidate.providers,
+      })
+      continue
+    }
     nextFallback = candidate
+    nextProviderID = candidateProviderID
     break
   }
   if (!nextFallback) return false
 
-  const providerID = selectFallbackProvider(
+  const providerID = nextProviderID ?? selectFallbackProvider(
     nextFallback.providers,
     task.model?.providerID,
   )
@@ -92,19 +125,39 @@ export async function tryFallbackRetry(args: {
   }
 
   const previousSessionID = task.sessionID
+  const previousModel = task.model
 
-  task.attemptCount = selectedAttemptCount
   const transformedModelId = transformModelForProvider(providerID, nextFallback.model)
-  task.model = {
+  const nextModel = {
     providerID,
     modelID: transformedModelId,
     variant: nextFallback.variant,
   }
-  task.status = "pending"
-  task.sessionID = undefined
-  task.startedAt = undefined
+  task.attemptCount = selectedAttemptCount
+  const failedAttemptID = ensureCurrentAttempt(task, previousModel).attemptID
+  const nextAttempt = failedAttemptID
+    ? scheduleRetryAttempt(task, failedAttemptID, nextModel, errorInfo.message)
+    : undefined
+  if (!nextAttempt) {
+    return false
+  }
+
   task.queuedAt = new Date()
-  task.error = undefined
+  task.retryNotification = {
+    previousSessionID,
+    failedModel: previousModel ? `${previousModel.providerID}/${previousModel.modelID}` : undefined,
+    failedError: errorInfo.message,
+    nextModel: `${providerID}/${transformedModelId}`,
+  }
+
+  onRetrying?.({
+    task,
+    source,
+    previousSessionID,
+    failedModel: task.retryNotification.failedModel,
+    failedError: errorInfo.message,
+    nextModel: `${providerID}/${transformedModelId}`,
+  })
 
   const key = task.model ? `${task.model.providerID}/${task.model.modelID}` : task.agent
   const queue = queuesByKey.get(key) ?? []
@@ -117,7 +170,7 @@ export async function tryFallbackRetry(args: {
     parentModel: task.parentModel,
     parentAgent: task.parentAgent,
     parentTools: task.parentTools,
-    model: task.model,
+    model: nextModel,
     fallbackChain: task.fallbackChain,
     category: task.category,
     isUnstableAgent: task.isUnstableAgent,
@@ -127,7 +180,7 @@ export async function tryFallbackRetry(args: {
     await abortWithTimeout(client, previousSessionID).catch(() => {})
   }
 
-  queue.push({ task, input: retryInput })
+  queue.push({ task, input: retryInput, attemptID: nextAttempt.attemptID })
   queuesByKey.set(key, queue)
   processKey(key)
   return true

src/features/background-agent/manager.test.ts

@@ -172,7 +172,7 @@ class MockBackgroundManager {
   }
 }
 
-function createMockTask(overrides: Partial<BackgroundTask> & { id: string; sessionID: string; parentSessionID: string }): BackgroundTask {
+function createMockTask(overrides: Partial<BackgroundTask> & { id: string; parentSessionID: string; sessionID?: string }): BackgroundTask {
   return {
     parentMessageID: "mock-message-id",
     description: "test task",
@@ -195,6 +195,21 @@ function createBackgroundManager(): BackgroundManager {
   return new BackgroundManager({ client, directory: tmpdir() } as unknown as PluginInput)
 }
 
+function createBackgroundManagerWithOptions(options: unknown): BackgroundManager {
+  const client = {
+    session: {
+      prompt: async () => ({}),
+      promptAsync: async () => ({}),
+      abort: async () => ({}),
+    },
+  }
+  return new BackgroundManager(
+    { client, directory: tmpdir() } as unknown as PluginInput,
+    undefined,
+    options as ConstructorParameters<typeof BackgroundManager>[2],
+  )
+}
+
 function getConcurrencyManager(manager: BackgroundManager): ConcurrencyManager {
   return (manager as unknown as { concurrencyManager: ConcurrencyManager }).concurrencyManager
 }
@@ -271,6 +286,325 @@ function createToastRemoveTaskTracker(): { removeTaskCalls: string[]; resetToast
   }
 }
 
+describe("BackgroundManager session.error fallback hydration", () => {
+  test("hydrates fallbackChain from session fallback state before retrying sync child-session errors", async () => {
+    //#given
+    const fallbackChain = [
+      { model: "fallback-model-1", providers: ["provider-a"], variant: undefined },
+    ]
+    const getSessionFallbackChain = mock((sessionID: string) =>
+      sessionID === "child-session" ? fallbackChain : undefined,
+    )
+    const manager = createBackgroundManagerWithOptions({
+      modelFallbackControllerAccessor: {
+        getSessionFallbackChain,
+      },
+    })
+    const task = createMockTask({
+      id: "task-sync-fallback",
+      sessionID: "child-session",
+      parentSessionID: "parent-session",
+      fallbackChain: undefined,
+    })
+    let capturedFallbackChain: BackgroundTask["fallbackChain"]
+    ;(manager as unknown as {
+      tryFallbackRetry: (task: BackgroundTask, errorInfo: { name?: string; message?: string }, source: string) => Promise<boolean>
+    }).tryFallbackRetry = async (retryTask) => {
+      capturedFallbackChain = retryTask.fallbackChain
+      return true
+    }
+
+    //#when
+    await (manager as unknown as {
+      handleSessionErrorEvent: (args: {
+        task: BackgroundTask
+        errorInfo: { name?: string; message?: string }
+        errorName: string | undefined
+        errorMessage: string | undefined
+      }) => Promise<void>
+    }).handleSessionErrorEvent({
+      task,
+      errorInfo: {
+        name: "APIError",
+        message: "Forbidden: Selected provider is forbidden",
+      },
+      errorName: "APIError",
+      errorMessage: "Forbidden: Selected provider is forbidden",
+    })
+
+    //#then
+    expect(getSessionFallbackChain).toHaveBeenCalledWith("child-session")
+    expect(task.fallbackChain).toEqual(fallbackChain)
+    expect(capturedFallbackChain).toEqual(fallbackChain)
+  })
+})
+
+describe("BackgroundManager prompt rejection fallback routing", () => {
+  test("routes launch-time prompt rejections into tryFallbackRetry before marking interrupt", async () => {
+    //#given
+    const promptError = {
+      name: "APIError",
+      data: { message: "Forbidden: Selected provider is forbidden" },
+    }
+    const client = {
+      session: {
+        get: async () => ({ data: { directory: tmpdir() } }),
+        create: async () => ({ data: { id: "ses_launch_retry" } }),
+        promptAsync: async () => {
+          throw promptError
+        },
+        abort: async () => ({}),
+      },
+    }
+    const manager = new BackgroundManager({ client, directory: tmpdir() } as unknown as PluginInput)
+    stubNotifyParentSession(manager)
+    ;(manager as unknown as {
+      reserveSubagentSpawn: () => Promise<{
+        spawnContext: { rootSessionID: string; parentDepth: number; childDepth: number }
+        descendantCount: number
+        commit: () => number
+        rollback: () => void
+      }>
+    }).reserveSubagentSpawn = async () => ({
+      spawnContext: { rootSessionID: "parent-session", parentDepth: 0, childDepth: 1 },
+      descendantCount: 1,
+      commit: () => 1,
+      rollback: () => {},
+    })
+    const retried: Array<{ taskId: string; errorInfo: { name?: string; message?: string }; source: string }> = []
+    ;(manager as unknown as {
+      tryFallbackRetry: (task: BackgroundTask, errorInfo: { name?: string; message?: string }, source: string) => Promise<boolean>
+    }).tryFallbackRetry = async (task, errorInfo, source) => {
+      retried.push({ taskId: task.id, errorInfo, source })
+      task.status = "pending"
+      task.error = undefined
+      return true
+    }
+
+    //#when
+    const launchedTask = await manager.launch({
+      description: "background retry test",
+      prompt: "say hi",
+      agent: "sisyphus-junior",
+      parentSessionID: "parent-session",
+      parentMessageID: "parent-message",
+      model: { providerID: "genai-proxy-openai", modelID: "gpt-5.4-mini" },
+      fallbackChain: [{ model: "claude-haiku-4-5", providers: ["anthropic"] }],
+    })
+    await flushBackgroundNotifications()
+
+    //#then
+    const storedTask = getTaskMap(manager).get(launchedTask.id)
+    expect(retried).toHaveLength(1)
+    expect(retried[0]?.source).toBe("promptAsync.launch")
+    expect(retried[0]?.errorInfo).toEqual({
+      name: "APIError",
+      message: "Forbidden: Selected provider is forbidden",
+    })
+    expect(storedTask?.status).toBe("pending")
+  })
+
+  test("routes resume-time prompt rejections into tryFallbackRetry before marking interrupt", async () => {
+    //#given
+    const promptError = {
+      name: "APIError",
+      data: { message: "Forbidden: Selected provider is forbidden" },
+    }
+    const client = {
+      session: {
+        promptAsync: async () => {
+          throw promptError
+        },
+        abort: async () => ({}),
+      },
+    }
+    const manager = new BackgroundManager({ client, directory: tmpdir() } as unknown as PluginInput)
+    stubNotifyParentSession(manager)
+    const task: BackgroundTask = {
+      id: "bg_resume_retry",
+      sessionID: "ses_resume_retry",
+      parentSessionID: "parent-session",
+      parentMessageID: "parent-message",
+      description: "resume retry test",
+      prompt: "say hi",
+      agent: "sisyphus-junior",
+      status: "completed",
+      startedAt: new Date(),
+      completedAt: new Date(),
+      model: { providerID: "genai-proxy-openai", modelID: "gpt-5.4-mini" },
+      fallbackChain: [{ model: "claude-haiku-4-5", providers: ["anthropic"] }],
+      concurrencyGroup: "genai-proxy-openai/gpt-5.4-mini",
+    }
+    getTaskMap(manager).set(task.id, task)
+    const retried: Array<{ taskId: string; errorInfo: { name?: string; message?: string }; source: string }> = []
+    ;(manager as unknown as {
+      tryFallbackRetry: (task: BackgroundTask, errorInfo: { name?: string; message?: string }, source: string) => Promise<boolean>
+    }).tryFallbackRetry = async (retryTask, errorInfo, source) => {
+      retried.push({ taskId: retryTask.id, errorInfo, source })
+      retryTask.status = "pending"
+      retryTask.error = undefined
+      return true
+    }
+
+    //#when
+    await manager.resume({
+      sessionId: "ses_resume_retry",
+      prompt: "continue",
+      parentSessionID: "parent-session",
+      parentMessageID: "parent-message-2",
+    })
+    await flushBackgroundNotifications()
+
+    //#then
+    const storedTask = getTaskMap(manager).get(task.id)
+    expect(retried).toHaveLength(1)
+    expect(retried[0]?.source).toBe("promptAsync.resume")
+    expect(retried[0]?.errorInfo).toEqual({
+      name: "APIError",
+      message: "Forbidden: Selected provider is forbidden",
+    })
+    expect(storedTask?.status).toBe("pending")
+  })
+})
+
+describe("BackgroundManager retry observability", () => {
+  test("queues a parent-visible retry notification when fallback retry is scheduled", async () => {
+    //#given
+    const client = {
+      session: {
+        abort: async () => ({}),
+      },
+    }
+    const manager = new BackgroundManager({ client, directory: tmpdir() } as unknown as PluginInput)
+    const task = createMockTask({
+      id: "bg_retry_observable",
+      parentSessionID: "parent-session",
+      fallbackChain: [{ model: "claude-haiku-4-5", providers: ["anthropic"] }],
+      attemptCount: 0,
+      status: "running",
+      attempts: [
+        {
+          attemptID: "att_retry_visibility",
+          attemptNumber: 1,
+          sessionID: "ses_retry_visibility",
+          providerID: "genai-proxy-openai",
+          modelID: "gpt-5.4-mini",
+          status: "running",
+        },
+      ],
+      currentAttemptID: "att_retry_visibility",
+    })
+    getTaskMap(manager).set(task.id, task)
+    const queuePendingNotification = mock(() => {})
+    ;(manager as unknown as {
+      queuePendingNotification: (sessionID: string | undefined, notification: string) => void
+    }).queuePendingNotification = queuePendingNotification
+
+    //#when
+    await (manager as unknown as {
+      tryFallbackRetry: (task: BackgroundTask, errorInfo: { name?: string; message?: string }, source: string) => Promise<boolean>
+    }).tryFallbackRetry(task, {
+      name: "APIError",
+      message: "Forbidden: Selected provider is forbidden",
+    }, "promptAsync.launch")
+
+    //#then
+    expect(queuePendingNotification).toHaveBeenCalledTimes(1)
+    const [sessionID, notification] = queuePendingNotification.mock.calls[0]
+    expect(sessionID).toBe("parent-session")
+    expect(notification).toContain("[BACKGROUND TASK RETRYING]")
+    expect(notification).toContain("ses_retry_visibility")
+    expect(notification).toContain("genai-proxy-openai/gpt-5.4-mini")
+    expect(notification).toContain("anthropic/claude-haiku-4.5")
+  })
+
+  test("queues a second parent-visible notification once the retry session ID is created", async () => {
+    //#given
+    const queuePendingNotification = mock(() => {})
+    const client = {
+      session: {
+        get: async () => ({ data: { directory: tmpdir() } }),
+        create: async () => ({ data: { id: "ses_retry_created" } }),
+        promptAsync: async () => ({}),
+      },
+    }
+    const manager = new BackgroundManager({ client, directory: tmpdir() } as unknown as PluginInput)
+    ;(manager as unknown as {
+      queuePendingNotification: (sessionID: string | undefined, notification: string) => void
+    }).queuePendingNotification = queuePendingNotification
+    const task = createMockTask({
+      id: "bg_retry_ready",
+      parentSessionID: "parent-session",
+      status: "pending",
+      attemptCount: 1,
+      queuedAt: new Date(),
+      model: { providerID: "anthropic", modelID: "claude-haiku-4.5" },
+      fallbackChain: [{ model: "claude-haiku-4-5", providers: ["anthropic"] }],
+      concurrencyGroup: "anthropic/claude-haiku-4.5",
+      retryNotification: {
+        nextModel: "anthropic/claude-haiku-4.5",
+      },
+      attempts: [
+        {
+          attemptID: "att_retry_failed",
+          attemptNumber: 1,
+          sessionID: "ses_retry_visibility",
+          providerID: "genai-proxy-openai",
+          modelID: "gpt-5.4-mini",
+          status: "error",
+          error: "Forbidden: Selected provider is forbidden",
+        },
+        {
+          attemptID: "att_retry_ready",
+          attemptNumber: 2,
+          providerID: "anthropic",
+          modelID: "claude-haiku-4.5",
+          status: "pending",
+        },
+      ],
+      currentAttemptID: "att_retry_ready",
+    })
+    getTaskMap(manager).set(task.id, task)
+    const taskInput = {
+      description: task.description,
+      prompt: task.prompt,
+      agent: task.agent,
+      parentSessionID: task.parentSessionID,
+      parentMessageID: task.parentMessageID,
+      model: task.model,
+      fallbackChain: task.fallbackChain,
+      category: task.category,
+    }
+    type RetryReadyQueueItem = {
+      task: BackgroundTask
+      input: typeof taskInput
+      attemptID: string
+    }
+    const item: RetryReadyQueueItem = {
+      task,
+      input: taskInput,
+      attemptID: task.currentAttemptID ?? "att_retry_ready",
+    }
+
+    //#when
+    await (manager as unknown as {
+      startTask: (queueItem: RetryReadyQueueItem) => Promise<void>
+    }).startTask(item)
+
+    //#then
+    const notifications = queuePendingNotification.mock.calls.map((call) => call[1])
+    const retryReadyNotification = notifications.find((notification) => notification.includes("[BACKGROUND TASK RETRY SESSION READY]"))
+    const expectedRetryLink = `http://127.0.0.1:4096/${Buffer.from(tmpdir()).toString("base64url")}/session/ses_retry_created`
+    expect(retryReadyNotification).toBeDefined()
+    expect(retryReadyNotification).toContain("**Retry attempt:** 2")
+    expect(retryReadyNotification).toContain("ses_retry_created")
+    expect(retryReadyNotification).toContain(expectedRetryLink)
+    expect(retryReadyNotification).toContain("ses_retry_visibility")
+    expect(retryReadyNotification).toContain("genai-proxy-openai/gpt-5.4-mini")
+    expect(retryReadyNotification).toContain("Forbidden: Selected provider is forbidden")
+  })
+})
+
 function getCleanupSignals(): Array<NodeJS.Signals | "beforeExit" | "exit"> {
   const signals: Array<NodeJS.Signals | "beforeExit" | "exit"> = ["SIGINT", "SIGTERM", "beforeExit", "exit"]
   if (process.platform === "win32") {
@@ -2037,6 +2371,43 @@ describe("BackgroundManager - Non-blocking Queue Integration", () => {
       expect(task.sessionID).toBeUndefined()
     })
 
+    test("should initialize attempt state for a newly launched task", async () => {
+      // given
+      const input = {
+        description: "Test task",
+        prompt: "Do something",
+        agent: "test-agent",
+        parentSessionID: "parent-session",
+        parentMessageID: "parent-message",
+        model: {
+          providerID: "openai",
+          modelID: "gpt-5.4-mini",
+          variant: "medium",
+        },
+      }
+
+      // when
+      const task = await manager.launch(input)
+
+      // then
+      expect(task.attempts).toHaveLength(1)
+      expect(task.currentAttemptID).toBe(task.attempts?.[0]?.attemptID)
+      expect(task.attempts?.[0]).toEqual({
+        attemptID: task.currentAttemptID,
+        attemptNumber: 1,
+        providerID: "openai",
+        modelID: "gpt-5.4-mini",
+        variant: "medium",
+        status: "pending",
+      })
+
+      expect(task.status).toBe("pending")
+      expect(task.model).toEqual(input.model)
+      expect(task.queuedAt).toBeInstanceOf(Date)
+      expect(task.startedAt).toBeUndefined()
+      expect(task.sessionID).toBeUndefined()
+    })
+
   test("should return immediately even with concurrency limit", async () => {
     // given
     const config = { defaultConcurrency: 1 }
@@ -5545,3 +5916,206 @@ describe("BackgroundManager - tool permission spread order", () => {
     manager.shutdown()
   })
 })
+
+describe("BackgroundManager.launch - attempt state initialization", () => {
+  test("newly launched task has attempt state with attemptNumber 1 and currentAttemptID pointing at it", async () => {
+    //#given
+    const manager = createBackgroundManager()
+    ;(manager as unknown as {
+      reserveSubagentSpawn: () => Promise<{
+        spawnContext: { rootSessionID: string; parentDepth: number; childDepth: number }
+        descendantCount: number
+        commit: () => number
+        rollback: () => void
+      }>
+    }).reserveSubagentSpawn = async () => ({
+      spawnContext: { rootSessionID: "parent-session", parentDepth: 0, childDepth: 1 },
+      descendantCount: 1,
+      commit: () => 1,
+      rollback: () => {},
+    })
+
+    //#when
+    const task = await manager.launch({
+      description: "attempt state test",
+      prompt: "do something",
+      agent: "explore",
+      parentSessionID: "parent-session",
+      parentMessageID: "parent-message",
+      model: { providerID: "anthropic", modelID: "claude-haiku-4.5" },
+    })
+
+    //#then
+    const stored = getTaskMap(manager).get(task.id)
+
+    expect(stored?.attempts).toBeDefined()
+    expect(stored?.attempts).toHaveLength(1)
+
+    const firstAttempt = stored?.attempts?.[0]
+    expect(firstAttempt?.attemptNumber).toBe(1)
+    expect(firstAttempt?.status).toBe("pending")
+    expect(firstAttempt?.providerID).toBe("anthropic")
+    expect(firstAttempt?.modelID).toBe("claude-haiku-4.5")
+
+    expect(stored?.currentAttemptID).toBeDefined()
+    expect(stored?.currentAttemptID).toBe(firstAttempt?.attemptID)
+
+    expect(stored?.status).toBeDefined()
+    expect(stored?.model).toBeDefined()
+    expect(stored?.parentSessionID).toBe("parent-session")
+
+    manager.shutdown()
+  })
+})
+
+describe("BackgroundManager attempt lifecycle bindings", () => {
+  test("startTask binds the created session to the queued attempt ID and mirrors task projection", async () => {
+    //#given
+    const client = {
+      session: {
+        get: async () => ({ data: { directory: "/test/dir" } }),
+        create: async () => ({ data: { id: "session-attempt-2" } }),
+        promptAsync: async () => ({}),
+        abort: async () => ({}),
+      },
+    }
+    const manager = new BackgroundManager({ client, directory: tmpdir() } as unknown as PluginInput)
+    const task: BackgroundTask = {
+      id: "task-attempt-binding",
+      status: "pending",
+      queuedAt: new Date(),
+      description: "retry binding task",
+      prompt: "continue",
+      agent: "sisyphus-junior",
+      parentSessionID: "parent-session",
+      parentMessageID: "parent-message",
+      model: { providerID: "anthropic", modelID: "claude-haiku-4.5", variant: "max" },
+      attempts: [
+        {
+          attemptID: "attempt-1",
+          attemptNumber: 1,
+          sessionID: "session-attempt-1",
+          providerID: "openai",
+          modelID: "gpt-5.4-mini",
+          status: "error",
+          error: "first attempt failed",
+          startedAt: new Date("2026-04-27T00:00:00.000Z"),
+          completedAt: new Date("2026-04-27T00:00:05.000Z"),
+        },
+        {
+          attemptID: "attempt-2",
+          attemptNumber: 2,
+          providerID: "anthropic",
+          modelID: "claude-haiku-4.5",
+          variant: "max",
+          status: "pending",
+        },
+      ],
+      currentAttemptID: "attempt-2",
+      attemptCount: 1,
+    }
+    const input: import("./types").LaunchInput = {
+      description: task.description,
+      prompt: task.prompt,
+      agent: task.agent,
+      parentSessionID: task.parentSessionID,
+      parentMessageID: task.parentMessageID,
+      model: task.model,
+    }
+
+    //#when
+    await (manager as unknown as {
+      startTask: (item: { task: BackgroundTask; input: import("./types").LaunchInput; attemptID: string }) => Promise<void>
+    }).startTask({ task, input, attemptID: "attempt-2" })
+
+    //#then
+    const activeAttempt = task.attempts?.find((attempt) => attempt.attemptID === "attempt-2")
+    expect(activeAttempt).toBeDefined()
+    expect(activeAttempt?.sessionID).toBe("session-attempt-2")
+    expect(activeAttempt?.status).toBe("running")
+    expect(activeAttempt?.startedAt).toBeInstanceOf(Date)
+    expect(task.currentAttemptID).toBe("attempt-2")
+    expect(task.sessionID).toBe("session-attempt-2")
+    expect(task.status).toBe("running")
+    expect(task.attempts?.[0]).toMatchObject({
+      attemptID: "attempt-1",
+      sessionID: "session-attempt-1",
+      status: "error",
+      error: "first attempt failed",
+    })
+
+    manager.shutdown()
+  })
+
+  test("historical attempt session IDs resolve to the task while stale session.error events leave the current attempt unchanged", async () => {
+    //#given
+    const manager = createBackgroundManager()
+    const task: BackgroundTask = {
+      id: "task-stale-session-event",
+      status: "running",
+      queuedAt: new Date("2026-04-27T00:00:00.000Z"),
+      startedAt: new Date("2026-04-27T00:00:10.000Z"),
+      sessionID: "session-attempt-2",
+      description: "ignore stale retry events",
+      prompt: "continue",
+      agent: "explore",
+      parentSessionID: "parent-session",
+      parentMessageID: "parent-message",
+      model: { providerID: "anthropic", modelID: "claude-haiku-4.5" },
+      attempts: [
+        {
+          attemptID: "attempt-1",
+          attemptNumber: 1,
+          sessionID: "session-attempt-1",
+          providerID: "openai",
+          modelID: "gpt-5.4-mini",
+          status: "error",
+          error: "first attempt failed",
+          startedAt: new Date("2026-04-27T00:00:00.000Z"),
+          completedAt: new Date("2026-04-27T00:00:05.000Z"),
+        },
+        {
+          attemptID: "attempt-2",
+          attemptNumber: 2,
+          sessionID: "session-attempt-2",
+          providerID: "anthropic",
+          modelID: "claude-haiku-4.5",
+          status: "running",
+          startedAt: new Date("2026-04-27T00:00:10.000Z"),
+        },
+      ],
+      currentAttemptID: "attempt-2",
+    }
+    getTaskMap(manager).set(task.id, task)
+
+    //#when
+    const resolvedTask = manager.findBySession("session-attempt-1")
+    manager.handleEvent({
+      type: "session.error",
+      properties: {
+        sessionID: "session-attempt-1",
+        error: { name: "UnknownError", message: "late event from old session" },
+      },
+    })
+    await flushBackgroundNotifications()
+
+    //#then
+    expect(resolvedTask?.id).toBe(task.id)
+    expect(task.currentAttemptID).toBe("attempt-2")
+    expect(task.sessionID).toBe("session-attempt-2")
+    expect(task.status).toBe("running")
+    expect(task.error).toBeUndefined()
+    expect(task.attempts?.[0]).toMatchObject({
+      attemptID: "attempt-1",
+      status: "error",
+      error: "first attempt failed",
+    })
+    expect(task.attempts?.[1]).toMatchObject({
+      attemptID: "attempt-2",
+      sessionID: "session-attempt-2",
+      status: "running",
+    })
+
+    manager.shutdown()
+  })
+})

src/features/background-agent/manager.ts

@@ -1,8 +1,10 @@
 
 import type { PluginInput } from "@opencode-ai/plugin"
+import type { ModelFallbackControllerAccessor } from "../../hooks/model-fallback"
 import { isAgentNotFoundError, FALLBACK_AGENT, buildFallbackBody } from "./spawner"
 import type {
   BackgroundTask,
+  BackgroundTaskAttempt,
   LaunchInput,
   ResumeInput,
 } from "./types"
@@ -30,6 +32,7 @@ import {
   POLLING_INTERVAL_MS,
   TASK_CLEANUP_DELAY_MS,
   TASK_TTL_MS,
+  type QueueItem,
 } from "./constants"
 
 import { subagentSessions } from "../claude-code-session-state"
@@ -47,6 +50,14 @@ import {
   isRecord,
 } from "./error-classifier"
 import { tryFallbackRetry } from "./fallback-retry-handler"
+import {
+  bindAttemptSession,
+  ensureCurrentAttempt,
+  findAttemptBySession,
+  finalizeAttempt,
+  getCurrentAttempt,
+  startAttempt,
+} from "./attempt-lifecycle"
 import { registerManagerForCleanup, unregisterManagerForCleanup } from "./process-cleanup"
 import {
   findNearestMessageExcludingCompaction,
@@ -119,9 +130,38 @@ interface Todo {
   id: string
 }
 
-interface QueueItem {
-  task: BackgroundTask
-  input: LaunchInput
+function formatAttemptModelSummary(attempt: Pick<BackgroundTaskAttempt, "providerID" | "modelID"> | undefined): string | undefined {
+  if (!attempt?.providerID || !attempt.modelID) {
+    return undefined
+  }
+
+  return `${attempt.providerID}/${attempt.modelID}`
+}
+
+function getPreviousAttempt(task: BackgroundTask, attemptID: string | undefined): BackgroundTaskAttempt | undefined {
+  if (!attemptID || !task.attempts || task.attempts.length === 0) {
+    return undefined
+  }
+
+  const attemptIndex = task.attempts.findIndex((attempt) => attempt.attemptID === attemptID)
+  if (attemptIndex <= 0) {
+    return undefined
+  }
+
+  return task.attempts[attemptIndex - 1]
+}
+
+function cloneAttempts(task: BackgroundTask): BackgroundTaskAttempt[] | undefined {
+  if (!task.attempts) {
+    return undefined
+  }
+
+  return task.attempts.map((attempt) => ({ ...attempt }))
+}
+
+function buildLocalSessionUrl(directory: string, sessionID: string): string {
+  const encodedDirectory = Buffer.from(directory).toString("base64url")
+  return `http://127.0.0.1:4096/${encodedDirectory}/session/${sessionID}`
 }
 
 export interface SubagentSessionCreatedEvent {
@@ -163,6 +203,7 @@ export class BackgroundManager {
   private rootDescendantCounts: Map<string, number>
   private preStartDescendantReservations: Set<string>
   private enableParentSessionNotifications: boolean
+  private modelFallbackControllerAccessor?: ModelFallbackControllerAccessor
   readonly taskHistory = new TaskHistory()
   private cachedCircuitBreakerSettings?: CircuitBreakerSettings
 
@@ -174,6 +215,7 @@ export class BackgroundManager {
       onSubagentSessionCreated?: OnSubagentSessionCreated
       onShutdown?: () => void | Promise<void>
       enableParentSessionNotifications?: boolean
+      modelFallbackControllerAccessor?: ModelFallbackControllerAccessor
     }
   ) {
     this.tasks = new Map()
@@ -190,6 +232,7 @@ export class BackgroundManager {
     this.rootDescendantCounts = new Map()
     this.preStartDescendantReservations = new Set()
     this.enableParentSessionNotifications = options?.enableParentSessionNotifications ?? true
+    this.modelFallbackControllerAccessor = options?.modelFallbackControllerAccessor
     this.registerProcessCleanup()
   }
 
@@ -324,6 +367,7 @@ export class BackgroundManager {
         attemptCount: 0,
         category: input.category,
       }
+      const firstAttempt = startAttempt(task, input.model)
 
       this.tasks.set(task.id, task)
       this.taskHistory.record(input.parentSessionID, { id: task.id, agent: input.agent, description: input.description, status: "pending", category: input.category })
@@ -338,7 +382,7 @@ export class BackgroundManager {
       // Add to queue
       const key = this.getConcurrencyKeyFromInput(input)
       const queue = this.queuesByKey.get(key) ?? []
-      queue.push({ task, input })
+      queue.push({ task, input, attemptID: firstAttempt.attemptID })
       this.queuesByKey.set(key, queue)
 
       log("[background-agent] Task queued:", { taskId: task.id, key, queueLength: queue.length })
@@ -399,9 +443,13 @@ export class BackgroundManager {
 
           // Mark task as error so the parent polling loop detects the failure
           // instead of leaving it in a zombie "running" state with no prompt sent
-          item.task.status = "error"
-          item.task.error = error instanceof Error ? error.message : String(error)
-          item.task.completedAt = new Date()
+          if (item.task.currentAttemptID) {
+            finalizeAttempt(item.task, item.task.currentAttemptID, "error", error instanceof Error ? error.message : String(error))
+          } else {
+            item.task.status = "error"
+            item.task.error = error instanceof Error ? error.message : String(error)
+            item.task.completedAt = new Date()
+          }
 
           if (item.task.concurrencyKey) {
             this.concurrencyManager.release(item.task.concurrencyKey)
@@ -430,6 +478,7 @@ export class BackgroundManager {
 
   private async startTask(item: QueueItem): Promise<void> {
     const { task, input } = item
+    const attemptID = item.attemptID ?? ensureCurrentAttempt(task, input.model).attemptID
 
     log("[background-agent] Starting task:", {
       taskId: task.id,
@@ -512,9 +561,17 @@ export class BackgroundManager {
       return
     }
 
-    task.status = "running"
-    task.startedAt = new Date()
-    task.sessionID = sessionID
+    const boundAttempt = bindAttemptSession(task, attemptID, sessionID, input.model)
+    if (!boundAttempt) {
+      await this.abortSessionWithLogging(sessionID, "stale attempt binding cleanup")
+      subagentSessions.delete(sessionID)
+      if (task.rootSessionID) {
+        this.unregisterRootDescendant(task.rootSessionID)
+      }
+      this.concurrencyManager.release(concurrencyKey)
+      return
+    }
+
     task.progress = {
       toolCalls: 0,
       lastUpdate: new Date(),
@@ -522,6 +579,39 @@ export class BackgroundManager {
     task.concurrencyKey = concurrencyKey
     task.concurrencyGroup = concurrencyKey
 
+    if (task.retryNotification) {
+      const attemptNumber = boundAttempt.attemptNumber
+      const retrySessionUrl = buildLocalSessionUrl(this.directory, sessionID)
+      const previousAttempt = getPreviousAttempt(task, boundAttempt.attemptID)
+      const failedSessionID = previousAttempt?.sessionID ?? task.retryNotification.previousSessionID
+      const failedSessionLine = failedSessionID
+        ? `\n- Failed session: \`${failedSessionID}\``
+        : ""
+      const failedModel = formatAttemptModelSummary(previousAttempt) ?? task.retryNotification.failedModel
+      const failedModelLine = failedModel
+        ? `\n- Failed model: \`${failedModel}\``
+        : ""
+      const failedError = previousAttempt?.error ?? task.retryNotification.failedError
+      const failedErrorLine = failedError
+        ? `\n- Error: ${failedError}`
+        : ""
+      const retryModel = formatAttemptModelSummary(boundAttempt) ?? task.retryNotification.nextModel
+      this.queuePendingNotification(
+        task.parentSessionID,
+        `<system-reminder>
+[BACKGROUND TASK RETRY SESSION READY]
+**ID:** \`${task.id}\`
+**Description:** ${task.description}
+**Retry attempt:** ${attemptNumber}
+**Retry session:** \`${sessionID}\`
+**Retry link:** ${retrySessionUrl}${failedSessionLine}${failedModelLine}${failedErrorLine}${retryModel ? `\n- Model: \`${retryModel}\`` : ""}
+
+The fallback retry session is now created and can be inspected directly.
+</system-reminder>`
+      )
+      task.retryNotification = undefined
+    }
+
     this.taskHistory.record(input.parentSessionID, { id: task.id, sessionID, agent: input.agent, description: input.description, status: "running", category: input.category, startedAt: task.startedAt })
     this.startPolling()
 
@@ -601,14 +691,25 @@ export class BackgroundManager {
       log("[background-agent] promptAsync error:", error)
       const existingTask = this.findBySession(sessionID)
       if (existingTask) {
-        existingTask.status = "interrupt"
-        const errorMessage = error instanceof Error ? error.message : String(error)
-        if (errorMessage.includes("agent.name") || errorMessage.includes("undefined") || isAgentNotFoundError(error)) {
-          existingTask.error = `Agent "${input.agent}" not found. Make sure the agent is registered in your opencode.json or provided by a plugin.`
-        } else {
-          existingTask.error = errorMessage
+        const errorInfo = {
+          name: extractErrorName(error),
+          message: extractErrorMessage(error),
+        }
+        if (await this.tryFallbackRetry(existingTask, errorInfo, "promptAsync.launch")) {
+          return
+        }
+
+        const errorMessage = errorInfo.message ?? (error instanceof Error ? error.message : String(error))
+        const terminalError = errorMessage.includes("agent.name") || errorMessage.includes("undefined") || isAgentNotFoundError(error)
+          ? `Agent "${input.agent}" not found. Make sure the agent is registered in your opencode.json or provided by a plugin.`
+          : errorMessage
+        if (existingTask.currentAttemptID) {
+          finalizeAttempt(existingTask, existingTask.currentAttemptID, "interrupt", terminalError)
+        } else {
+          existingTask.status = "interrupt"
+          existingTask.error = terminalError
+          existingTask.completedAt = new Date()
         }
-        existingTask.completedAt = new Date()
         if (existingTask.rootSessionID) {
           this.unregisterRootDescendant(existingTask.rootSessionID)
         }
@@ -665,10 +766,35 @@ export class BackgroundManager {
       if (task.sessionID === sessionID) {
         return task
       }
+      if (findAttemptBySession(task, sessionID)) {
+        return task
+      }
     }
     return undefined
   }
 
+  private resolveTaskAttemptBySession(sessionID: string): { task: BackgroundTask; attemptID?: string; isCurrent: boolean } | undefined {
+    const task = this.findBySession(sessionID)
+    if (!task) {
+      return undefined
+    }
+
+    const attempt = findAttemptBySession(task, sessionID)
+    if (!attempt) {
+      return {
+        task,
+        attemptID: undefined,
+        isCurrent: task.sessionID === sessionID,
+      }
+    }
+
+    return {
+      task,
+      attemptID: attempt.attemptID,
+      isCurrent: task.currentAttemptID === attempt.attemptID,
+    }
+  }
+
   private getConcurrencyKeyFromInput(input: LaunchInput): string {
     if (input.model) {
       return `${input.model.providerID}/${input.model.modelID}`
@@ -883,8 +1009,16 @@ export class BackgroundManager {
       },
     }).catch(async (error) => {
       log("[background-agent] resume prompt error:", error)
+      const errorInfo = {
+        name: extractErrorName(error),
+        message: extractErrorMessage(error),
+      }
+      if (await this.tryFallbackRetry(existingTask, errorInfo, "promptAsync.resume")) {
+        return
+      }
+
       existingTask.status = "interrupt"
-      const errorMessage = error instanceof Error ? error.message : String(error)
+      const errorMessage = errorInfo.message ?? (error instanceof Error ? error.message : String(error))
       existingTask.error = errorMessage
       existingTask.completedAt = new Date()
       if (existingTask.rootSessionID) {
@@ -986,8 +1120,11 @@ export class BackgroundManager {
 
       if (role !== "assistant") return
 
-      const task = this.findBySession(sessionID)
-      if (!task || task.status !== "running") return
+      const resolved = this.resolveTaskAttemptBySession(sessionID)
+      if (!resolved?.isCurrent) return
+
+      const { task } = resolved
+      if (task.status !== "running") return
 
       const assistantError = (info as Record<string, unknown>)["error"]
       if (!assistantError) return
@@ -1009,8 +1146,10 @@ export class BackgroundManager {
       const sessionID = partInfo?.sessionID
       if (!sessionID) return
 
-      const task = this.findBySession(sessionID)
-      if (!task) return
+      const resolved = this.resolveTaskAttemptBySession(sessionID)
+      if (!resolved?.isCurrent) return
+
+      const { task } = resolved
 
       if (this.hasOutputSignalFromPart(partInfo)) {
         this.markSessionOutputObserved(sessionID)
@@ -1113,7 +1252,10 @@ export class BackgroundManager {
       if (!props || typeof props !== "object") return
       handleSessionIdleBackgroundEvent({
         properties: props as Record<string, unknown>,
-        findBySession: (id) => this.findBySession(id),
+        findBySession: (id) => {
+          const resolved = this.resolveTaskAttemptBySession(id)
+          return resolved?.isCurrent ? resolved.task : undefined
+        },
         idleDeferralTimers: this.idleDeferralTimers,
         validateSessionHasOutput: (id) => this.validateSessionHasOutput(id),
         checkSessionTodos: (id) => this.checkSessionTodos(id),
@@ -1126,8 +1268,11 @@ export class BackgroundManager {
       const sessionID = typeof props?.sessionID === "string" ? props.sessionID : undefined
       if (!sessionID) return
 
-      const task = this.findBySession(sessionID)
-      if (!task || task.status !== "running") return
+      const resolved = this.resolveTaskAttemptBySession(sessionID)
+      if (!resolved?.isCurrent) return
+
+      const { task } = resolved
+      if (task.status !== "running") return
 
       const errorObj = props?.error as { name?: string; message?: string } | undefined
       const errorName = errorObj?.name
@@ -1156,9 +1301,9 @@ export class BackgroundManager {
       this.clearSessionTodoObservation(sessionID)
 
       const tasksToCancel = new Map<string, BackgroundTask>()
-      const directTask = this.findBySession(sessionID)
-      if (directTask) {
-        tasksToCancel.set(directTask.id, directTask)
+      const directTask = this.resolveTaskAttemptBySession(sessionID)
+      if (directTask?.isCurrent) {
+        tasksToCancel.set(directTask.task.id, directTask.task)
       }
       for (const descendant of this.getAllDescendantTasks(sessionID)) {
         tasksToCancel.set(descendant.id, descendant)
@@ -1213,8 +1358,11 @@ export class BackgroundManager {
       const status = props?.status as { type?: string; message?: string } | undefined
       if (!sessionID || status?.type !== "retry") return
 
-      const task = this.findBySession(sessionID)
-      if (!task || task.status !== "running") return
+      const resolved = this.resolveTaskAttemptBySession(sessionID)
+      if (!resolved?.isCurrent) return
+
+      const { task } = resolved
+      if (task.status !== "running") return
 
       const errorMessage = typeof status.message === "string" ? status.message : undefined
       const errorInfo = { name: "SessionRetry", message: errorMessage }
@@ -1235,6 +1383,13 @@ export class BackgroundManager {
   }): Promise<void> {
     const { task, errorInfo, errorMessage, errorName } = args
 
+    if (!task.fallbackChain && task.sessionID) {
+      const sessionFallbackChain = this.modelFallbackControllerAccessor?.getSessionFallbackChain(task.sessionID)
+      if (sessionFallbackChain?.length) {
+        task.fallbackChain = sessionFallbackChain
+      }
+    }
+
     // Agent-not-found errors are handled by the prompt catch block with agent fallback.
     // Do not also trigger model fallback retry — that would race with the agent retry.
     if (isAgentNotFoundError({ message: errorInfo.message } as Error)) {
@@ -1262,9 +1417,13 @@ export class BackgroundManager {
       canRetry,
     })
 
-    task.status = "error"
-    task.error = errorMsg
-    task.completedAt = new Date()
+    if (task.currentAttemptID) {
+      finalizeAttempt(task, task.currentAttemptID, "error", errorMsg)
+    } else {
+      task.status = "error"
+      task.error = errorMsg
+      task.completedAt = new Date()
+    }
     if (task.rootSessionID) {
       this.unregisterRootDescendant(task.rootSessionID)
     }
@@ -1319,6 +1478,26 @@ export class BackgroundManager {
       idleDeferralTimers: this.idleDeferralTimers,
       queuesByKey: this.queuesByKey,
       processKey: (key: string) => this.processKey(key),
+      onRetrying: ({ task, source }) => {
+        const currentAttempt = getCurrentAttempt(task)
+        const previousAttempt = getPreviousAttempt(task, currentAttempt?.attemptID)
+        const sourceText = source ? ` via ${source}` : ""
+        const failedSessionLine = previousAttempt?.sessionID ? `\n- Failed session: \`${previousAttempt.sessionID}\`` : ""
+        const failedModel = formatAttemptModelSummary(previousAttempt)
+        const failedModelLine = failedModel ? `\n- Failed model: \`${failedModel}\`` : ""
+        const failedErrorLine = previousAttempt?.error ? `\n- Error: ${previousAttempt.error}` : ""
+        const nextModel = formatAttemptModelSummary(currentAttempt)
+        this.queuePendingNotification(
+          task.parentSessionID,
+          `<system-reminder>
+[BACKGROUND TASK RETRYING]
+**ID:** \`${task.id}\`
+**Description:** ${task.description}${sourceText}${failedSessionLine}${failedModelLine}${failedErrorLine}${nextModel ? `\n- Next model: \`${nextModel}\`` : ""}
+
+The task was re-queued on a fallback model after a retryable failure.
+</system-reminder>`
+        )
+      },
     })
     return result.then((retried) => {
       if (retried && previousSessionID) {
@@ -1537,14 +1716,18 @@ export class BackgroundManager {
     }
 
     const wasRunning = task.status === "running"
-    task.status = "cancelled"
-    task.completedAt = new Date()
+    if (task.currentAttemptID) {
+      finalizeAttempt(task, task.currentAttemptID, "cancelled", reason)
+    } else {
+      task.status = "cancelled"
+      task.completedAt = new Date()
+      if (reason) {
+        task.error = reason
+      }
+    }
     if (wasRunning && task.rootSessionID) {
       this.unregisterRootDescendant(task.rootSessionID)
     }
-    if (reason) {
-      task.error = reason
-    }
     this.taskHistory.record(task.parentSessionID, { id: task.id, sessionID: task.sessionID, agent: task.agent, description: task.description, status: "cancelled", category: task.category, startedAt: task.startedAt, completedAt: task.completedAt })
 
     if (task.concurrencyKey) {
@@ -1657,8 +1840,12 @@ export class BackgroundManager {
     }
 
     // Atomically mark as completed to prevent race conditions
-    task.status = "completed"
-    task.completedAt = new Date()
+    if (task.currentAttemptID) {
+      finalizeAttempt(task, task.currentAttemptID, "completed")
+    } else {
+      task.status = "completed"
+      task.completedAt = new Date()
+    }
     this.taskHistory.record(task.parentSessionID, { id: task.id, sessionID: task.sessionID, agent: task.agent, description: task.description, status: "completed", category: task.category, startedAt: task.startedAt, completedAt: task.completedAt })
 
     if (task.rootSessionID) {
@@ -1722,6 +1909,7 @@ export class BackgroundManager {
       description: task.description,
       status: task.status,
       error: task.error,
+      attempts: cloneAttempts(task),
     })
 
     // Update pending tracking and check if all tasks complete
@@ -1743,7 +1931,7 @@ export class BackgroundManager {
     }
 
     const completedTasks = allComplete
-      ? (this.completedTaskSummaries.get(task.parentSessionID) ?? [{ id: task.id, description: task.description, status: task.status, error: task.error }])
+      ? (this.completedTaskSummaries.get(task.parentSessionID) ?? [{ id: task.id, description: task.description, status: task.status, error: task.error, attempts: cloneAttempts(task) }])
       : []
 
     if (allComplete) {
@@ -1950,9 +2138,13 @@ export class BackgroundManager {
   }
 
   private async failCrashedTask(task: BackgroundTask, errorMessage: string): Promise<void> {
-    task.status = "error"
-    task.error = errorMessage
-    task.completedAt = new Date()
+    if (task.currentAttemptID) {
+      finalizeAttempt(task, task.currentAttemptID, "error", errorMessage)
+    } else {
+      task.status = "error"
+      task.error = errorMessage
+      task.completedAt = new Date()
+    }
     if (task.rootSessionID) {
       this.unregisterRootDescendant(task.rootSessionID)
     }

src/features/background-agent/spawner.test.ts

@@ -378,7 +378,7 @@ describe("background-agent spawner fallback model promotion", () => {
 
     //#when
     await startTask(
-      { task, input },
+      { task, input, attemptID: "att_test123" },
       {
         client,
         directory: "/tmp/test",

src/features/background-agent/types.ts

@@ -26,6 +26,21 @@ export interface TaskProgress {
   lastMessageAt?: Date
 }
 
+export type BackgroundTaskAttemptStatus = BackgroundTaskStatus
+
+export interface BackgroundTaskAttempt {
+  attemptID: string
+  attemptNumber: number
+  sessionID?: string
+  providerID?: string
+  modelID?: string
+  variant?: string
+  status: BackgroundTaskAttemptStatus
+  error?: string
+  startedAt?: Date
+  completedAt?: Date
+}
+
 export interface BackgroundTask {
   id: string
   sessionID?: string
@@ -61,6 +76,18 @@ export interface BackgroundTask {
   isUnstableAgent?: boolean
   /** Category used for this task (e.g., 'quick', 'visual-engineering') */
   category?: string
+  /** Pending retry notification details for the next spawned retry session */
+  retryNotification?: {
+    previousSessionID?: string
+    failedModel?: string
+    failedError?: string
+    nextModel: string
+  }
+
+  /** Structured attempt history for retry observability */
+  attempts?: BackgroundTaskAttempt[]
+  /** ID of the currently active attempt */
+  currentAttemptID?: string
 
   /** Last message count for stability detection */
   lastMsgCount?: number