Xcode ACP custom agent: session/prompt always fails with "failed to produce a response" despite a working CLI and valid auth

Goal: Set up GitHub Copilot CLI as a custom agent in Xcode's Coding Intelligence settings via the Agent Client Protocol ( --acp ).

Setup:

• Copilot CLI 1.0.73, installed via Homebrew ( /opt/homebrew/bin/copilot )
• Xcode Coding Intelligence → custom agent config:
• Executable: /opt/homebrew/bin/copilot
• Arguments: --acp
• Interpreter: blank
• Environment Variables: blank (later also tried setting these — see below)
• copilot works fine standalone in Terminal (chat, auth all functional)

Symptom:
As soon as I send any prompt in Xcode (even plain text like "test" ), it fails with:

JSON-RPC request with ID JSONRPCID(rawValue: "D1C50172-B98A-4ABD-8550-6E06581D4B42", intValue: nil) failed to produce a response.

initialize , authenticate , and session/new all appear to succeed — only the prompt request fails, immediately, every time.

Troubleshooting already attempted (no change in any case):

1. Fully quit and relaunched Xcode.
2. Removed and re-added the custom agent entry.
3. Set GITHUB_TOKEN and COPILOT_GITHUB_TOKEN explicitly in Xcode's agent Environment Variables field (I have these set in my shell profile normally, suspecting Xcode's spawned process doesn't inherit them — no effect either way).
4. Confirmed Xcode's agent config only supports Executable + Arguments (stdio) — no URL/host/port field available to try TCP mode ( --acp --port ) as a workaround.
5. Don't have admin access, so can't use Console.app to pull crash/error logs for the copilot process.

Independent verification (works fine outside Xcode):
I manually replayed the full ACP handshake and prompt flow over raw stdio JSON-RPC against the same CLI binary/version:
initialize → authenticate → session/new → session/prompt , using both plain integer IDs and UUID-string IDs (matching the format Xcode uses). All four requests completed successfully, including a real streamed model response and stopReason: "end_turn" .

This strongly suggests the CLI's ACP server implementation is functioning correctly, and the issue lies in Xcode's ACP client — possibly around stdio pipe handling, a client-side capability/callback (e.g., permission requests) that isn't being answered, or some other difference between how Xcode spawns the process versus how Terminal/a manual harness does.

Ask: Any guidance on Xcode-side ACP client debugging, or known issues with this specific integration path, would be appreciated.

Start with Xcode Coding Intelligence's custom agent configuration and the /opt/homebrew/bin/copilot --acp entry point. Reproduce session/prompt in Xcode while comparing it with the successful raw stdio JSON-RPC flow; done means Xcode receives the streamed response and end_turn result instead of the failed-to-produce-a-response error.

Written by the indexing model from the issue text.