fix: support ACP question prompts via extMethod

[hikaruczl]

Issue for this PR

Closes #17920

Type of change

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

What does this PR do?

ACP mode was still exposing a gap around the question tool: the tool can be opt-in enabled, but ACP had no bridge for question.asked, so ACP sessions could not complete the question/reply flow.

This PR keeps the default ACP behavior unchanged and only enables ACP question prompts when both of these are true:

• OPENCODE_ENABLE_QUESTION_TOOL=1
• the ACP client advertises _meta["opencode/question"]

When that capability is present, packages/opencode/src/acp/agent.ts forwards question.asked to the ACP client through extMethod("opencode/question", ...), then maps the response back to the existing sdk.question.reply(...) / sdk.question.reject(...) APIs.

I also updated the ACP README with the extension contract and added targeted event-subscription coverage for both reply and reject flows.

I chose the ACP extension route instead of requestPermission() because question prompts are not the same thing as permission prompts, and I wanted the ACP bridge to use an explicit, capability-gated question path.

How did you verify your code works?

I tested locally with:

• bun test ./test/acp/event-subscription.test.ts
• bun test ./test/tool/question.test.ts
• bun test ./test/acp/agent-interface.test.ts
• bun run typecheck

Screenshots / recordings

N/A

Checklist

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

[github-actions]

Thanks for your contribution!

This PR doesn't have a linked issue. All PRs must reference an existing issue.

Please:

1. Open an issue describing the bug/feature (if one doesn't exist)
2. Add Fixes #<number> or Closes #<number> to this PR description

See CONTRIBUTING.md for details.

[github-actions]

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

Potential Related PR Found:

• PR fix(acp): unblock question tool in ACP mode #18657: fix(acp): unblock question tool in ACP mode

Relationship: This earlier PR addresses question tool support in ACP mode. The current PR (20017) builds upon this by adding support for ACP question prompts via the extMethod mechanism and bridging question.asked events to ACP clients. They both relate to enabling question functionality in ACP contexts, though they appear to address different aspects of the feature.

[hikaruczl]

Thanks — I checked #18657 and it does overlap with the same ACP question.asked gap.

The main difference is approach:

• fix(acp): unblock question tool in ACP mode #18657 maps question prompts onto ACP requestPermission()
• this PR uses a capability-gated ACP extension via extMethod("opencode/question", ...)

I took the extension-based approach intentionally because question is semantically different from permission, and the existing question model can include richer interactions that do not map cleanly onto requestPermission().

So this PR is meant as an alternative design for the same bug, not an independent feature. If maintainers prefer the minimal requestPermission() bridge from #18657, I am happy to close this in favor of that approach.

[github-actions]

Thanks for updating your PR! It now meets our contributing guidelines. 👍

[kennylbj]

I prefer extension-based way because it allows user to input custom text compared to requestPermission

hope this pr could be merged ASAP