Feature 8: Model Variants

Status: Deferred / out of scope for the current implementation pass.

Implementation approach: Enhance existing _sandboxagent/session/list_models extension

Summary

v1 had AgentModelInfo.variants, AgentModelInfo.defaultVariant, and CreateSessionRequest.variant. v1 already has _sandboxagent/session/list_models but the variant fields need to be verified and the session-creation variant selection needs to work end-to-end.

Current v1 State

From acp_runtime/mod.rs, _sandboxagent/session/list_models is implemented and returns:

availableModels[] with modelId, name, description
currentModelId
Fields for defaultVariant, variants[] are documented in rfds-vs-extensions.md

From v1 router.rs, model/variant types existed:

pub struct AgentModelsResponse {
    pub models: Vec<AgentModelInfo>,
    pub default_model: Option<String>,
}

pub struct AgentModelInfo {
    pub id: String,
    pub name: Option<String>,
    pub variants: Option<Vec<AgentModelVariant>>,
    pub default_variant: Option<String>,
}

pub struct AgentModelVariant {
    pub id: String,
    pub name: Option<String>,
}

v1 Usage

Pre-session Model Discovery

GET /v1/agents/{agent}/models

Returned AgentModelsResponse with full model list including variants.

Session Creation with Variant

POST /v1/sessions

Body included variant: Option<String> to select a specific model variant at session creation time.

Per-Agent Model Logic (from `router.rs`)

fn amp_models_response() -> AgentModelsResponse {
    AgentModelsResponse {
        models: vec![AgentModelInfo {
            id: "amp-default".to_string(),
            name: Some("Amp Default".to_string()),
            variants: None,
            default_variant: None,
        }],
        default_model: Some("amp-default".to_string()),
    }
}

fn mock_models_response() -> AgentModelsResponse {
    AgentModelsResponse {
        models: vec![AgentModelInfo {
            id: "mock".to_string(),
            name: Some("Mock".to_string()),
            variants: None,
            default_variant: None,
        }],
        default_model: Some("mock".to_string()),
    }
}

Claude and Codex models were fetched dynamically from the agent process.

Implementation Plan

Verify/Enrich `_sandboxagent/session/list_models`

The existing extension method already returns model data. Verify that:

variants array is included in each model entry when available
defaultVariant is included when available
The response shape matches the documented RFD shape

Add Variant to Session Creation

Session creation via session/new should accept a variant hint in _meta:

{
  "method": "session/new",
  "params": {
    "_meta": {
      "sandboxagent.dev": {
        "variant": "opus"
      }
    }
  }
}

The runtime should forward this variant to the agent process (e.g., as a model parameter in the spawn command or via session/set_model).

Files to Modify

File	Change
`server/packages/sandbox-agent/src/acp_runtime/mod.rs`	Verify `list_models` response includes `variants`/`defaultVariant`; extract and forward `variant` from `session/new` `_meta`
`server/packages/sandbox-agent/src/acp_runtime/mock.rs`	Add variant support to mock model listing
`sdks/typescript/src/client.ts`	Update `listModels` return type to include variants
`server/packages/sandbox-agent/tests/v1_api.rs`	Add model variants test

Docs to Update

Doc	Change
`docs/sdks/typescript.mdx`	Document variant support in model listing and session creation
`research/acp/spec.md`	Update `_sandboxagent/session/list_models` payload shape

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