Summary

• fix delegated child-session fallback recovery when the first prompt fails before session history is persisted
• introduce a shared bootstrap context so background and sync delegated flows register retry payloads and fallback metadata before first prompt dispatch
• add cleanup and regression coverage for exhaustion, non-retryable failures, isolation, and terminal-path cleanup

Background

In the delegated child-session flow, a subagent session can fail on the very first promptAsync call because of a provider or model error.

Before this change, that path had two related problems:

1. background child-session fallback context registration could race behind first-prompt dispatch
2. runtime fallback depended on session.messages to recover the last user prompt, so empty-history first-prompt failures could not retry

That left delegated child sessions stuck in early failure without correctly advancing to fallback models.

First-prompt early-failure flow after this change

1. A delegated child session is created from the parent session
2. Before the first prompt is sent, the system registers delegated bootstrap context containing:
   • retry prompt parts
   • fallback chain
   • category metadata
3. The first prompt is dispatched
4. If the first prompt fails with a retryable error:
   • background and sync delegated paths enter fallback handling
   • runtime fallback first tries persisted session.messages
   • if history is still empty and the session is a delegated child session, it falls back to the bootstrap retry parts
5. Fallback models are attempted in order with existing exhaustion semantics preserved
6. On retry handoff, completion, cancellation, crash/error, session deletion, shutdown, or launch-time terminal interrupt, the session-scoped bootstrap and fallback metadata are cleared

flowchart TD
    A[Parent session delegates child task] --> B[Create child session]
    B --> C[Register delegated bootstrap context before first prompt]
    C --> D[Send first prompt]

    D -->|Success| E[Continue normal background or sync flow]
    D -->|Failure| F{Retryable error?}

    F -->|No| G[Enter terminal path]
    F -->|Yes| H{session.messages has user parts?}

    H -->|Yes| I[Use persisted user parts for retry]
    H -->|No| J{Delegated bootstrap exists?}

    J -->|No| G
    J -->|Yes| K[Use bootstrap retry parts]

    I --> L[Pick next fallback model]
    K --> L
    L --> M{Fallback chain exhausted?}

    M -->|No| N[Retry with next fallback model]
    M -->|Yes| G

    N --> O{Retry succeeds?}
    O -->|Yes| P[Continue task execution]
    O -->|No| F

    E --> Q[Terminal cleanup]
    P --> Q
    G --> Q

    Q --> R[Clear bootstrap prompt context]
    Q --> S[Clear fallback-chain registration]
    Q --> T[Clear category registration]

What changed

Shared delegated bootstrap contract

• add a shared bootstrap state to preserve delegated child-session retry parts before first prompt dispatch
• register fallback chain and category context before the first prompt in both background and sync delegated paths

Background path atomic registration

• move background fallback-context registration into the launch path itself
• remove the dependency on delayed polling or backfill to make critical fallback metadata available

Runtime fallback support for empty-history retry

• preserve the existing history-first retry path
• fall back to delegated bootstrap retry parts only when the session is a delegated child session and persisted history is still empty

Sync path alignment and cleanup

• align sync delegated execution with the shared bootstrap contract
• ensure retry-created sessions and terminal paths clear bootstrap and fallback context so session-scoped state does not leak

Regression coverage

• background first-prompt retryable failure
• sync first-prompt fallback ordering
• delegated empty-history runtime retry
• exhaustion behavior
• non-retryable failures do not retry
• concurrent delegated session isolation
• background and sync terminal cleanup

Affected areas

• src/features/background-agent/manager.ts
• src/tools/delegate-task/background-task.ts
• src/tools/delegate-task/sync-task.ts
• src/tools/delegate-task/sync-prompt-sender.ts
• src/hooks/runtime-fallback/auto-retry.ts
• src/hooks/runtime-fallback/last-user-retry-parts.ts
• src/shared/delegated-child-session-bootstrap.ts

Verification

bun test src/features/background-agent/manager.test.ts src/features/background-agent/fallback-retry-handler.test.ts src/features/background-agent/spawner.test.ts src/hooks/runtime-fallback/index.test.ts src/tools/delegate-task/sync-task.test.ts
bun run typecheck

• 256 pass, 0 fail
• typecheck passed

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Summary by cubic

Fixes delegated child-session fallback when the first prompt fails before any history exists. Captures the first-prompt retry payload at launch and cleans up on every terminal path to keep state consistent and isolated.

• Bug Fixes
  • Introduced shared/delegated-child-session-bootstrap to register retry parts, fallback chain, and category before the first prompt in both background and sync flows.
  • Background manager registers the bootstrap at launch and centralizes cleanup (clears bootstrap, session fallback chain, and category) on completion, cancellation, retry handoff, deletion, shutdown, and terminal launch failures; preserves late wiring when the parent aborts early.
  • Runtime auto-retry now passes sessionID to last-user lookup and uses bootstrap retry parts when history is empty; does not invent payloads if no bootstrap exists.
  • Sync path aligns with the bootstrap contract, injects the built prompt text into sendSyncPrompt, and clears per-session bootstrap/fallback context on retries and finish; concurrent sessions remain isolated.
  • Merged latest dev and resolved manager/runtime-fallback conflicts while keeping delegated bootstrap wiring and upstream poll-recovery; expanded tests for empty-history retries, terminal launch cleanup, scheduled-retry cleanup, cancellation/completion cleanup, isolation, exhaustion, and non-retryable errors.

^{Written for commit 12f5233. Summary will update on new commits.}