fix: Fixes race conditions and missing error handling that cause subagents to hang indefinitely when errors occur.

[huyusong10]

Issue for this PR

Closes #18378

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

Fixes race conditions and missing error handling that cause subagents to hang indefinitely when errors occur.

Changes Made

1. prompt.ts - session/prompt.ts

• Add state existence check before adding callbacks to prevent race conditions
• Reject promise immediately if session state is not found
• Prevents "hanging" when concurrent operations delete/recreate session state

if (!abort) {
  return new Promise<MessageV2.WithParts>((resolve, reject) => {
    const current = state()
    const sessionState = current[sessionID]
    if (!sessionState) {
      reject(new DOMException("Session state not found", "AbortError"))
      return
    }
    sessionState.callbacks.push({ resolve, reject })
  })
}

2. task.ts - tool/task.ts

• Add try-catch around SessionPrompt.prompt() call
• Ensure errors are properly propagated to parent agent
• Provide meaningful error messages for debugging
  let result: MessageV2.WithParts
  try {
  result = await SessionPrompt.prompt({ /* ... */ })
  } catch (error) {
  throw new Error(Subagent execution failed: ${error instanceof Error ? error.message : String(error)})
  }

Problem Solved
Before: Parent agent shows subagent as "running" forever, even though subagent crashed/cancelled
After: Parent agent receives proper error notification and can handle failure gracefully

How did you verify your code works?

• Ran bun run typecheck - all checks passed
• Code review confirms state check prevents race condition
• Error handling path ensures parent receives notification

Screenshots / recordings

Not applicable (backend error handling fix)

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

[github-actions]

Hey! Your PR title Fix/subagent error handling doesn't follow conventional commit format.

Please update it to start with one of:

• feat: or feat(scope): new feature
• fix: or fix(scope): bug fix
• docs: or docs(scope): documentation changes
• chore: or chore(scope): maintenance tasks
• refactor: or refactor(scope): code refactoring
• test: or test(scope): adding or updating tests

Where scope is the package name (e.g., app, desktop, opencode).

See CONTRIBUTING.md for details.

[github-actions]

The following comment was made by an LLM, it may be inaccurate:

Based on the search results, I found the following potentially related PRs:

Potentially Related PRs

1. fix: robust subagent completion propagation #13321 - "fix: robust subagent completion propagation"

   • fix: robust subagent completion propagation #13321
   • Related to ensuring subagent completions propagate correctly to parent agents

2. fix(opencode): propagate subagent errors to parent session #13422 - "fix(opencode): propagate subagent errors to parent session"

   • fix(opencode): propagate subagent errors to parent session #13422
   • Directly addresses propagating subagent errors, which is a core issue in PR fix: Fixes race conditions and missing error handling that cause subagents to hang indefinitely when errors occur. #18389

3. fix(run): prevent subagent question tool hang in non-interactive mode #13974 - "fix(run): prevent subagent question tool hang in non-interactive mode"

   • fix(run): prevent subagent question tool hang in non-interactive mode #13974
   • Addresses subagent hanging issues in specific scenarios

4. fix(acp): handle question.asked event to prevent hanging #17921 - "fix(acp): handle question.asked event to prevent hanging"

   • fix(acp): handle question.asked event to prevent hanging #17921
   • Recent fix related to preventing hanging, may address similar async/error handling concerns

5. fix: improve plugin system robustness — agent/command resolution, async errors, hook timing, two-phase init #18280 - "fix: improve plugin system robustness — agent/command resolution, async errors, hook timing, two-phase init"

   • fix: improve plugin system robustness — agent/command resolution, async errors, hook timing, two-phase init #18280
   • Recent PR addressing async error handling and timing issues in agent systems

Most relevant: PR #13422 appears to be the most directly related, as it specifically addresses propagating subagent errors to the parent session, which is the core issue being fixed in PR #18389.