ACP: support an ask_user / ask_question style extension method

[TristanVII]

Summary

Please consider adding ACP support for an ask_user / ask_question style extension method so custom ACP clients can surface clarifying questions and return structured answers to Copilot CLI.

Today ACP support in Copilot CLI includes session/request_permission, but that only covers allow/deny tool approvals.

Why this would help

Custom ACP frontends can already render streaming messages, plans, tool calls, and permission prompts. The missing piece is a first-class way for the agent to ask the human a question and wait for a reply.

That makes it hard to support workflows that depend on interactive clarification, even though GitHub Copilot itself supports this kind of host-side interaction in some environments.

A small extension method would make custom ACP frontends much more capable without requiring each client to invent incompatible workarounds.

Suggestion

A Copilot-specific ACP extension would already be helpful, for example something like:

• request method: copilot/ask_user (or similar)
• params:
  • sessionId
  • question
  • optional choices: string[]
  • optional allowFreeform / allow_freeform: boolean
• result:
  • { outcome: "answered", answer: string }
  • or { outcome: "cancelled" }

Cursor has cursor/ask_question Source: https://cursor.com/docs/cli/acp#cursor-extension-methods