Unstable support for model selection

Signed-off-by: Ben Brandt <benjamin.j.brandt@gmail.com>

Author: benbrandt

docs/protocol/schema.mdx

@@ -232,6 +232,14 @@ Response from loading an existing session.
 
 <ResponseField name="_meta" type={"object"} >
   Extension point for implementations
+</ResponseField>
+<ResponseField name="models" type={<><span><a href="#sessionmodelstate">SessionModelState</a></span><span> | null</span></>} >
+  **UNSTABLE**
+
+This capability is not part of the spec yet, and may be removed or changed at any point.
+
+Initial model state if supported by the Agent
+
 </ResponseField>
 <ResponseField name="modes" type={<><span><a href="#sessionmodestate">SessionModeState</a></span><span> | null</span></>} >
   Initial mode state if supported by the Agent
@@ -300,6 +308,14 @@ See protocol docs: [Creating a Session](https://agentclientprotocol.com/protocol
 
 <ResponseField name="_meta" type={"object"} >
   Extension point for implementations
+</ResponseField>
+<ResponseField name="models" type={<><span><a href="#sessionmodelstate">SessionModelState</a></span><span> | null</span></>} >
+  **UNSTABLE**
+
+This capability is not part of the spec yet, and may be removed or changed at any point.
+
+Initial model state if supported by the Agent
+
 </ResponseField>
 <ResponseField name="modes" type={<><span><a href="#sessionmodestate">SessionModeState</a></span><span> | null</span></>} >
   Initial mode state if supported by the Agent
@@ -439,6 +455,57 @@ Response to `session/set_mode` method.
 
 <ResponseField name="meta" type={"object"}></ResponseField>
 
+<a id="session-set_model"></a>
+### <span class="font-mono">session/set_model</span>
+
+**UNSTABLE**
+
+This capability is not part of the spec yet, and may be removed or changed at any point.
+
+Select a model for a given session.
+
+#### <span class="font-mono">SessionSetModelRequest</span>
+
+**UNSTABLE**
+
+This capability is not part of the spec yet, and may be removed or changed at any point.
+
+Request parameters for setting a session model.
+
+**Type:** Object
+
+**Properties:**
+
+<ResponseField name="_meta" type={"object"}>
+  Extension point for implementations
+</ResponseField>
+<ResponseField name="modelId" type={<a href="#modelid">ModelId</a>} required>
+  The ID of the model to set.
+</ResponseField>
+<ResponseField
+  name="sessionId"
+  type={<a href="#sessionid">SessionId</a>}
+  required
+>
+  The ID of the session to set the model for.
+</ResponseField>
+
+#### <span class="font-mono">SessionSetModelResponse</span>
+
+**UNSTABLE**
+
+This capability is not part of the spec yet, and may be removed or changed at any point.
+
+Response to `session/set_model` method.
+
+**Type:** Object
+
+**Properties:**
+
+<ResponseField name="_meta" type={"object"}>
+  Extension point for implementations
+</ResponseField>
+
 ## Client
 
 Defines the interface that ACP-compliant clients must implement.
@@ -541,7 +608,7 @@ Only available if the client supports the `fs.writeTextFile` capability.
 
 #### <span class="font-mono">WriteTextFileResponse</span>
 
-Response to fs/write_text_file
+Response to `fs/write_text_file`
 
 **Type:** Object
 
@@ -1093,7 +1160,7 @@ Information about a command.
   Input for the command if required
 </ResponseField>
 <ResponseField name="name" type={"string"} required>
-  Command name (e.g., "create_plan", "research_codebase").
+  Command name (e.g., `create_plan`, `research_codebase`).
 </ResponseField>
 
 ## <span class="font-mono">AvailableCommandInput</span>
@@ -1622,6 +1689,38 @@ All Agents MUST support this transport.
 </Expandable>
 </ResponseField>
 
+## <span class="font-mono">ModelId</span>
+
+**UNSTABLE**
+
+This capability is not part of the spec yet, and may be removed or changed at any point.
+
+A unique identifier for a model.
+
+**Type:** `string`
+
+## <span class="font-mono">ModelInfo</span>
+
+**UNSTABLE**
+
+This capability is not part of the spec yet, and may be removed or changed at any point.
+
+Information about a selectable model.
+
+**Type:** Object
+
+**Properties:**
+
+<ResponseField name="_meta" type={"object"}>
+  Extension point for implementations
+</ResponseField>
+<ResponseField name="modelId" type={<a href="#modelid">ModelId</a>} required>
+  Unique identifier for the model.
+</ResponseField>
+<ResponseField name="name" type={"string"} required>
+  Human-readable name of the model.
+</ResponseField>
+
 ## <span class="font-mono">PermissionOption</span>
 
 An option presented to the user when requesting permission.
@@ -1998,6 +2097,43 @@ The set of modes and the one currently active.
   The current mode the Agent is in.
 </ResponseField>
 
+## <span class="font-mono">SessionModelState</span>
+
+**UNSTABLE**
+
+This capability is not part of the spec yet, and may be removed or changed at any point.
+
+The set of models and the one currently active.
+
+**Type:** Object
+
+**Properties:**
+
+<ResponseField name="_meta" type={"object"}>
+  Extension point for implementations
+</ResponseField>
+<ResponseField
+  name="availableModels"
+  type={
+    <>
+      <span>
+        <a href="#modelinfo">ModelInfo</a>
+      </span>
+      <span>[]</span>
+    </>
+  }
+  required
+>
+  The set of models that the Agent can use
+</ResponseField>
+<ResponseField
+  name="currentModelId"
+  type={<a href="#modelid">ModelId</a>}
+  required
+>
+  The current model the Agent is in.
+</ResponseField>
+
 ## <span class="font-mono">SessionUpdate</span>
 
 Different types of updates that can be sent during session processing.

rust/acp.rs

@@ -234,6 +234,19 @@ impl Agent for ClientSideConnection {
         )
     }
 
+    #[cfg(feature = "unstable")]
+    async fn set_session_model(
+        &self,
+        args: SessionSetModelRequest,
+    ) -> Result<SessionSetModelResponse, Error> {
+        self.conn
+            .request(
+                SESSION_SET_MODEL_METHOD_NAME,
+                Some(ClientRequest::ModelSelectRequest(args)),
+            )
+            .await
+    }
+
     async fn ext_method(&self, args: ExtRequest) -> Result<ExtResponse, Error> {
         self.conn
             .request(
@@ -667,6 +680,11 @@ impl<T: Agent> MessageHandler<AgentSide> for T {
                 let response = self.set_session_mode(args).await?;
                 Ok(AgentResponse::SetSessionModeResponse(response))
             }
+            #[cfg(feature = "unstable")]
+            ClientRequest::ModelSelectRequest(args) => {
+                let response = self.set_session_model(args).await?;
+                Ok(AgentResponse::ModelSelectResponse(response))
+            }
             ClientRequest::ExtMethodRequest(args) => {
                 let response = self.ext_method(args).await?;
                 Ok(AgentResponse::ExtMethodResponse(response))