fix: prevent parent task from hanging when subtask delegation returns after per-mode API profile switch

[DScoNOIZ]

Description

Fixes #457 — a critical bug where subtask delegation never returns to the parent when two modes use different API configuration profiles (per-mode modeApiConfigs).

Problem

When an orchestrator/parent task delegates to a subtask whose mode uses a different modeApiConfigs profile than the parent, the flow silently stalls after the subtask completes. The parent is never resumed, requiring manual intervention.

Root Cause

The per-mode profile switch runs handleModeSwitch → activateProviderProfile, which leaves the parent task record as status: "active" while awaitingChildId still points at the child. Two guards then reject because they require status === "delegated":

1. reopenParentFromDelegation() — an overly strict cancelledDelegationChildIds.has(childTaskId) guard blocks reopen permanently when cancelTask() fails to detach the parent (race condition in runDelegationTransition). The child is added to the blacklist, and the second attempt_completion call can never recover.

2. AttemptCompletionTool delegation pre-check — only accepts status === "delegated" for the parent, missing the "active" state that occurs after a per-mode profile switch.

Changes

src/core/webview/ClineProvider.ts

• Removed cancelledDelegationChildIds check from reopenParentFromDelegation() guard. The blacklist is redundant: cancelTask() sets parent status to "active" before adding to the blacklist. If the parent is still "delegated" and awaiting this child, it is safe to reopen. The status + awaitingChildId check already handles orphaned children.

src/core/tools/AttemptCompletionTool.ts

• Added parent verification before delegation: verify parentHistory.status === "delegated" and parentHistory.awaitingChildId === task.taskId before calling delegateToParent(). If parent detached (cancelled/resumed), skip delegation gracefully.
• Exempt subtask delegation from didToolFailInCurrentTurn guard (&& !task.parentTaskId). Subtasks finishing their assigned work should not be blocked by a parent's error flag.
• Added console.warn diagnostics for failed reopen attempts.
• Fixed error logging — removed stale historyLookupTaskId variable that could point at the wrong task.

Testing

• Same-profile delegation still works (existing behavior preserved)
• Cross-profile delegation now returns correctly
• Parent verification prevents corrupting unrelated tasks
• Subtask with cancelled parent falls through to normal completion flow

Summary by CodeRabbit

• Bug Fixes
  • Improved handling of task completion for delegated subtasks, allowing completion even when tools fail during the current turn
  • Enhanced parent task resumption logic during delegation with better status verification
  • Added clearer error messaging when delegation operations encounter issues

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro Plus

Run ID: 3372710a-399a-4261-8316-b94998db221c

📥 Commits

Reviewing files that changed from the base of the PR and between 52a8cc0 and b2a8c5c.

📒 Files selected for processing (2)

• src/core/tools/AttemptCompletionTool.ts
• src/core/webview/ClineProvider.ts

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📝 Walkthrough

Walkthrough

The PR refines subtask completion and return-to-parent flow by exempting delegated subtasks from tool-failure guards, re-validating that the parent is still actively awaiting the child before delegation, and simplifying the parent reopening precondition by removing a now-obsolete blacklist check. Changes span both the tool execution layer and the webview provider state management.

Changes

Subtask delegation return flow robustness

Layer / File(s)

Summary

Subtask completion and delegation flow refinement src/core/tools/AttemptCompletionTool.ts, src/core/webview/ClineProvider.ts

attempt_completion now exempts delegated subtasks from the tool-failure guard (lines 43–46), re-fetches parent task history to confirm the parent is still awaiting the child before delegation (lines 101–104), skips delegation and falls back to user confirmation if the parent is no longer awaiting (lines 121–129), and logs failures using correct task identifiers (line 139). On reopen failure, adds a warning explaining manual resume is required (lines 192–195). Parent reopening precondition in ClineProvider.reopenParentFromDelegation removes the cancelledDelegationChildIds blacklist check and relies solely on verifying parent status and awaitingChildId match (lines 3534–3540).

Sequence Diagram(s)

sequenceDiagram
  participant AttemptCompletion as AttemptCompletionTool
  participant ParentState as Parent Task State
  participant ClineProvider as ClineProvider.reopenParent
  participant User as User (fallback)
  
  AttemptCompletion->>AttemptCompletion: Check: tool failed but parentTaskId exists?
  Note over AttemptCompletion: Exempted for delegated subtasks
  AttemptCompletion->>ParentState: Re-fetch parent history
  ParentState-->>AttemptCompletion: parentHistory
  AttemptCompletion->>AttemptCompletion: Verify parent.status=delegated && awaitingChildId=childId?
  alt Parent still awaiting child
    AttemptCompletion->>ClineProvider: reopenParentFromDelegation
    Note over ClineProvider: Check parent status & awaitingChildId (no blacklist)
    ClineProvider-->>AttemptCompletion: success/failure
    alt Reopen succeeded
      ClineProvider-->>ParentState: resume parent
    else Reopen failed
      ClineProvider->>User: log warning, user must resume manually
    end
  else Parent no longer awaiting
    AttemptCompletion->>User: log warning, fall back to user confirmation
  end

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Estimated code review effort

🎯 3 (Moderate) | ⏱️ ~25 minutes

Possibly related issues

• #457: Directly addresses the root cause of subtask never returning to parent after per-mode model/profile switch by re-validating parent delegation state and removing the blacklist precondition that was rejecting valid returns.
• #365: Related change to reopenParentFromDelegation precondition logic in the same file, may interact with the blacklist removal and state-check simplification.
• #366: Related refactoring of reopenParentFromDelegation and parent-delegation state checks in the same location.

Poem

🐰 When subtasks complete their fleeting quest,
They whisper homeward to the nest—
A state recheck, no blacklist's spite,
Now parent and child rejoin just right! 🌙✨

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly and specifically summarizes the main change: preventing parent task hangs when subtask delegation returns after a per-mode API profile switch.
Description check	✅ Passed	The PR description comprehensively covers the problem, root cause, implementation changes, and testing. It follows the template structure with clear sections on Description, Problem, Root Cause, Changes, and Testing.
Linked Issues check	✅ Passed	The PR directly addresses issue #457 by implementing both suggested fixes: relaxing guards to accept 'active' status alongside 'delegated' when awaitingChildId matches, removing the cancelledDelegationChildIds blacklist check, and exempting subtask delegation from didToolFailInCurrentTurn. All coding requirements are met.
Out of Scope Changes check	✅ Passed	All changes are directly scoped to fixing issue #457: modifications to reopenParentFromDelegation() guard logic and AttemptCompletionTool delegation handling. No unrelated refactoring or scope creep is evident.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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