feat: add copilot-acp provider

[codefromthecrypt]

Summary

Add an ACP provider for GitHub's Copilot CLI. Copilot CLI supports ACP natively via its --acp flag.

Notable limitations

• MCP not yet supported: copilot ignores mcpServers in session/new (github/copilot-cli#1040)
• No session/close
• No terminal or fs delegation: all execution is local

Type of Change

• Feature
• Tests

AI Assistance

• This PR was created or reviewed with AI assistance

Testing

Provider integration tests

$ source bin/activate-hermit
$ npm install -g @github/copilot
$ cargo test --test providers -- test_copilot_acp_provider --nocapture
running 1 test
=== copilot-acp::model_listing ===
&models = [
    "claude-sonnet-4.6", "claude-sonnet-4.5", "claude-haiku-4.5",
    "claude-opus-4.6", "claude-opus-4.5", "claude-sonnet-4",
    "gpt-5.4", "gpt-5.3-codex", "gpt-5.2-codex", "gpt-5.2",
    "gpt-5.1-codex-max", "gpt-5.1-codex", "gpt-5.1",
    "gpt-5.4-mini", "gpt-5.1-codex-mini", "gpt-5-mini", "gpt-4.1",
]
=== copilot-acp::basic_response === Hello! 👋 How can I help you today?
=== copilot-acp::model_switch (claude-sonnet-4.6 -> gpt-4.1) === Hello! 👋 How can I help you today?
=== copilot-acp::permission_allow ===
=== copilot-acp::permission_deny ===
test test_copilot_acp_provider ... ok

test result: ok. 1 passed; 0 failed; 0 ignored; 0 measured; 19 filtered out; finished in 36.69s

============== Providers ==============
✅ copilot-acp
=======================================

MCP tool tests skipped — github/copilot-cli#1040.

Goose CLI

$ GOOSE_PROVIDER=copilot-acp GOOSE_MODEL=current RUST_LOG=debug,sacp=trace target/release/goose run -t 'say hello'

    __( O)>  ● new session · copilot-acp current
   \____)    20260327_1
     L L     goose is ready
Hello! How can I help you with the goose project today?

$ grep -rh 'resolved ACP model' ~/.local/state/goose/logs/
{"timestamp":"2026-03-27T15:00:00.309339Z","level":"INFO","fields":{"message":"resolved ACP model","from":"current","to":"claude-sonnet-4.6"},"target":"goose::acp::provider"}

Goose CLI — permission routing (approve mode)

$ rm -rf ~/.local/state/goose/logs ~/.local/share/goose/sessions
$ GOOSE_PROVIDER=copilot-acp GOOSE_MODEL=current GOOSE_MODE=approve RUST_LOG=debug,sacp=trace target/release/goose run -t 'Write the word hello to /tmp/test-copilot-acp.txt'

    __( O)>  ● new session · copilot-acp current
   \____)    20260327_1
     L L     goose is ready

$ grep -rh 'request_permission' ~/.local/state/goose/logs/ | head -1
{"timestamp":"2026-03-27T15:01:36.152998Z","level":"TRACE","fields":{"message":"Received JSON-RPC message","message":"{\"jsonrpc\":\"2.0\",\"id\":0,\"method\":\"session/request_permission\",\"params\":{\"sessionId\":\"8c5ef659-503a-4e47-bfda-d19598962e12\",\"toolCall\":{\"toolCallId\":\"shell-permission\",\"title\":\"Write hello to file\",\"kind\":\"execute\",\"status\":\"pending\",\"rawInput\":{\"command\":\"echo \\\"hello\\\" > /tmp/test-copilot-acp.txt\",\"commands\":[\"echo \\\"hello\\\" > /tmp/test-copilot-acp.txt\"]}},\"options\":[{\"optionId\":\"allow_once\",\"kind\":\"allow_once\",\"name\":\"Allow once\"},{\"optionId\":\"allow_always\",\"kind\":\"allow_always\",\"name\":\"Always allow\"},{\"optionId\":\"reject_once\",\"kind\":\"reject_once\",\"name\":\"Deny\"}]}}"},"target":"sacp::jsonrpc::transport_actor"}

Copilot-side logs (~/.copilot/logs/)

$ cat $(ls -t ~/.copilot/logs/process-*.log | head -1)
2026-03-27T15:01:26.901Z [INFO] Starting ACP server (stdio)
2026-03-27T15:01:28.861Z [INFO] MCP config prepared for ACP mode
2026-03-27T15:01:28.861Z [INFO] ACP server started (stdio mode)
2026-03-27T15:01:28.862Z [INFO] ACP initialize request from client: unknown
2026-03-27T15:01:30.139Z [INFO] Created ACP session: bc0c6272-e52b-4683-a747-9c620a34a44c
2026-03-27T15:01:31.084Z [INFO] Created ACP session: 8c5ef659-503a-4e47-bfda-d19598962e12
2026-03-27T15:01:31.663Z [INFO] Created ACP session: 0780520e-8672-49da-8365-5959f960752c
2026-03-27T15:01:31.667Z [INFO] Using default model: claude-sonnet-4.6

[codefromthecrypt]

@michaelneale @DOsinga @alexhancock FYI this is the only ACP provider so far that eagerly persists ACP sessions on session/new. the other 3 only persist after first prompt.

What this means is that for goose term which expects a session to survive before a prompt is sent, all but this ACP provider will have a glitch we need to think through.

[codefromthecrypt]

oops was wrong.. copilot does write logs even before a session has a prompt, but it doesn't support load really as everything is process-local. github/copilot-cli#1767

[DOsinga]

Looks good — clean, follows the existing ACP provider pattern exactly, CI passes.

One thought for a follow-up: now that we have three ACP providers (claude-acp, codex-acp, copilot-acp) that are structurally almost identical — same from_env skeleton, same ProviderDef impl, just different constants and mode/permission mappings — would it make sense to turn these into a declarative ACP provider type? Something like the existing declarative OpenAI-compatible providers, where each ACP provider is just a config struct (binary name, mode mapping, permission mapping, setup steps, etc.) and the shared from_env logic lives in one place.

Not blocking this PR on that — just curious what you think. You built the ACP infra so you'd know best whether the remaining differences (codex-acp's sandbox args, claude-acp's env_remove, etc.) make that awkward or whether they'd fit cleanly into a config-driven approach.

[codefromthecrypt]

@DOsinga I completely agree. I will raise a pr on this