feat(acp): add model selection support for session/new and session/set_model

[codefromthecrypt]

Summary

Add model selection support to the ACP server (goose acp). ACP clients like Zed show a model picker when the agent returns models in the session/new response, and switch models mid-session via session/set_model. This adds both so clients can see and switch between the configured provider's available models.

Clients that don't support model selection continue to work unchanged -- the models field is optional and session/set_model is never sent by those clients.

The sacp crate is at the latest version, v10.1.0, but is out-of-date and even main lacks model support. HACK comments apply until sorted.

Type of Change

• Feature
• Tests

AI Assistance

• This PR was created or reviewed with AI assistance

Testing

Manual: Zed 0.222.4 + goose (this branch):

1. cargo build --release -p goose-cli
2. Open Zed
3. Click Zed, Settings, Open Settings File, enter below and Save

  "agent_servers": {
    "goose": {
      "type": "custom",
      "command": "/Users/codefromthecrypt/oss/goose/target/release/goose",
      "args": ["acp"],
      "env": {},
    },
  },

4. Click the Agent (stars) icon in the bottom left, then Click + on New Thread and choose goose
5. In the bottom left, click the model selector, and switch to a different model.
6. Send a prompt and verify the agent responds using the new model

Integration tests (cargo test -p goose-acp --test server_test -- --nocapture):

• test_model_list -- verifies session/new response includes SessionModelState with current model and full available model list
• test_set_model -- switches to o4-mini, verifies the model change reaches the OpenAI API via exchange pattern matching

Related Issues

• feat: ACP providers for claude code and codex #6605 — ACP providers (claude-code-acp, codex-acp) need the server to return models for the provider interface to work
• fix: remove Option from model listing return types, propagate errors #7074 — established fetch_recommended_models on the provider trait; this PR uses it to populate the ACP response
• test(providers): add model listing to live provider suite #7038 — model listing live test suite; this PR adds the server-side support that ACP providers will surface
• Support set_model when create a session agentclientprotocol/symposium-acp#118 -- sacp issue we're working around.

Screenshots/Demos

[Copilot]

Pull request overview

Adds ACP model selection support to goose acp by surfacing provider model lists in session/new / session/load responses and handling session/set_model to switch models mid-session (with integration tests and fixtures to validate behavior).

Changes:

• Include SessionModelState (models) in session/new and session/load responses using provider model discovery.
• Add (temporary) untyped JSON handling for session/set_model to switch the provider/model during a session.
• Add integration tests + OpenAI /v1/models fixture data to verify model listing and model switching behavior.

Reviewed changes

Copilot reviewed 11 out of 12 changed files in this pull request and generated 5 comments.

Show a summary per file

File	Description
crates/goose/src/providers/openai.rs	Adds gpt-5-nano to the known model/context-limit list.
crates/goose/src/providers/errors.rs	Makes ProviderError clonable (used by new tests/mocks).
crates/goose-test-support/src/session.rs	Introduces TEST_MODEL constant for tests/fixtures.
crates/goose-test-support/src/lib.rs	Re-exports TEST_MODEL.
crates/goose-acp/tests/test_data/openai_models.json	Adds fixture response for OpenAI model listing.
crates/goose-acp/tests/server_test.rs	Registers new integration tests for model list and set_model.
crates/goose-acp/tests/fixtures/server.rs	Captures NewSessionResponse.models and adds an untyped session/set_model sender for tests.
crates/goose-acp/tests/fixtures/mod.rs	Mocks /v1/models and wires config to use TEST_MODEL; extends Session test trait.
crates/goose-acp/tests/common_tests/mod.rs	Adds run_model_list + run_set_model common test helpers.
crates/goose-acp/src/server.rs	Implements model state building, attaches models to responses, and adds session/set_model handling.
crates/goose-acp/Cargo.toml	Enables schema feature (unstable_session_model) via direct dependency.
Cargo.lock	Records the new dependency for goose-acp.

[Copilot]

ModelConfig::new(model_id) failing is a client input error, but this is currently returned as internal_error; returning invalid_params (and similarly mapping “unknown session” to invalid_params) will give ACP clients correct feedback when they select an unsupported model or bad session id.

Suggested change

	sacp::Error::internal_error().data(format!("Invalid model config: {}", e))
	sacp::Error::invalid_params().data(format!("Invalid model config: {}", e))

[codefromthecrypt]

Aside: Copilot reviews are getting worse lately only 2/5 comments valid and another pr 2/4

[michaelneale]

yeah copilot was so believable but yeah, nice one.