Unstable support for model selection

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

Author: benbrandt

docs/protocol/schema.mdx

@@ -88,7 +88,7 @@ See protocol docs: [Initialization](https://agentclientprotocol.com/protocol/ini
 <ResponseField name="clientCapabilities" type={<a href="#clientcapabilities">ClientCapabilities</a>} >
   Capabilities supported by the client.
 
-    - Default: `{"fs":{"readTextFile":false,"writeTextFile":false},"terminal":false}`
+    - Default: `{"fs":{"readTextFile":false,"writeTextFile":false},"modelSelection":false,"terminal":false}`
 
 </ResponseField>
 <ResponseField name="protocolVersion" type={<a href="#protocolversion">ProtocolVersion</a>} required>
@@ -113,7 +113,7 @@ See protocol docs: [Initialization](https://agentclientprotocol.com/protocol/ini
 <ResponseField name="agentCapabilities" type={<a href="#agentcapabilities">AgentCapabilities</a>} >
   Capabilities supported by the agent.
 
-    - Default: `{"loadSession":false,"mcpCapabilities":{"http":false,"sse":false},"promptCapabilities":{"audio":false,"embeddedContext":false,"image":false}}`
+    - Default: `{"loadSession":false,"mcpCapabilities":{"http":false,"sse":false},"modelSelection":false,"promptCapabilities":{"audio":false,"embeddedContext":false,"image":false}}`
 
 </ResponseField>
 <ResponseField name="authMethods" type={<><span><a href="#authmethod">AuthMethod</a></span><span>[]</span></>} >
@@ -130,6 +130,184 @@ The client should disconnect, if it doesn't support this version.
 
 </ResponseField>
 
+<a id="model-list"></a>
+### <span class="font-mono">model/list</span>
+
+**UNSTABLE**
+
+This capability is not part of the spec yet, and may be removed or changed at any point.
+
+List all available models for the session.
+
+#### <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">ModelListRequest</span>
+
+**UNSTABLE**
+
+This capability is not part of the spec yet, and may be removed or changed at any point.
+
+Request parameters for requesting available models from the agent.
+This request is per-session to allow for custom settings to affect the list of available models.
+
+**Type:** Object
+
+**Properties:**
+
+<ResponseField name="_meta" type={"object"}>
+  Extension point for implementations
+</ResponseField>
+<ResponseField
+  name="sessionId"
+  type={<a href="#sessionid">SessionId</a>}
+  required
+>
+  The ID of the session to send this request to.
+</ResponseField>
+
+#### <span class="font-mono">ModelListResponse</span>
+
+**UNSTABLE**
+
+This capability is not part of the spec yet, and may be removed or changed at any point.
+
+Response of available models from the agent.
+
+**Type:** Object
+
+**Properties:**
+
+<ResponseField name="_meta" type={"object"}>
+  Extension point for implementations
+</ResponseField>
+<ResponseField
+  name="models"
+  type={
+    <>
+      <span>
+        <a href="#modelinfo">ModelInfo</a>
+      </span>
+      <span>[]</span>
+    </>
+  }
+  required
+>
+  Models available to choose from.
+</ResponseField>
+
+<a id="model-select"></a>
+### <span class="font-mono">model/select</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">ModelSelectRequest</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">ModelSelectResponse</span>
+
+**UNSTABLE**
+
+This capability is not part of the spec yet, and may be removed or changed at any point.
+
+Response to `model/select` method.
+
+**Type:** Object
+
+**Properties:**
+
+<ResponseField name="_meta" type={"object"}>
+  Extension point for implementations
+</ResponseField>
+
+#### <span class="font-mono">ModelSelectedRequest</span>
+
+**UNSTABLE**
+
+This capability is not part of the spec yet, and may be removed or changed at any point.
+
+Request parameters for getting the currently selected model for a session.
+
+**Type:** Object
+
+**Properties:**
+
+<ResponseField name="_meta" type={"object"}>
+  Extension point for implementations
+</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">ModelSelectedResponse</span>
+
+**UNSTABLE**
+
+This capability is not part of the spec yet, and may be removed or changed at any point.
+
+Response to `model/selected` method.
+
+**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>
+
 <a id="session-cancel"></a>
 ### <span class="font-mono">session/cancel</span>
 
@@ -541,7 +719,7 @@ 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
 
@@ -986,6 +1164,16 @@ See protocol docs: [Agent Capabilities](https://agentclientprotocol.com/protocol
 
     - Default: `{"http":false,"sse":false}`
 
+</ResponseField>
+<ResponseField name="modelSelection" type={"boolean"} >
+  **UNSTABLE**
+
+This capability is not part of the spec yet, and may be removed or changed at any point.
+
+Whether the agent supports `model/*`.
+
+    - Default: `false`
+
 </ResponseField>
 <ResponseField name="promptCapabilities" type={<a href="#promptcapabilities">PromptCapabilities</a>} >
   Prompt capabilities supported by the agent.
@@ -1093,7 +1281,7 @@ 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>
@@ -1151,6 +1339,16 @@ Determines which file operations the agent can request.
 
     - Default: `{"readTextFile":false,"writeTextFile":false}`
 
+</ResponseField>
+<ResponseField name="modelSelection" type={"boolean"} >
+  **UNSTABLE**
+
+This capability is not part of the spec yet, and may be removed or changed at any point.
+
+Whether the Client supports `model/*` methods.
+
+    - Default: `false`
+
 </ResponseField>
 <ResponseField name="terminal" type={"boolean"} >
   Whether the Client support all `terminal/*` methods.
@@ -1622,6 +1820,40 @@ 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">ModelNotification</span>
+
+**UNSTABLE**
+
+This capability is not part of the spec yet, and may be removed or changed at any point.
+
+Notification that the agent's model list has been updated
+so that the client can requery it.
+
+**Type:** Object
+
+**Properties:**
+
+<ResponseField name="_meta" type={"object"}>
+  Extension point for implementations
+</ResponseField>
+<ResponseField
+  name="sessionId"
+  type={<a href="#sessionid">SessionId</a>}
+  required
+>
+  The session ID that was updated.
+</ResponseField>
+
 ## <span class="font-mono">PermissionOption</span>
 
 An option presented to the user when requesting permission.

rust/acp.rs

@@ -234,6 +234,39 @@ impl Agent for ClientSideConnection {
         )
     }
 
+    #[cfg(feature = "unstable")]
+    async fn list_models(&self, args: ModelListRequest) -> Result<ModelListResponse, Error> {
+        self.conn
+            .request(
+                MODEL_LIST_METHOD_NAME,
+                Some(ClientRequest::ModelListRequest(args)),
+            )
+            .await
+    }
+
+    #[cfg(feature = "unstable")]
+    async fn select_model(&self, args: ModelSelectRequest) -> Result<ModelSelectResponse, Error> {
+        self.conn
+            .request(
+                MODEL_SELECT_METHOD_NAME,
+                Some(ClientRequest::ModelSelectRequest(args)),
+            )
+            .await
+    }
+
+    #[cfg(feature = "unstable")]
+    async fn selected_model(
+        &self,
+        args: ModelSelectedRequest,
+    ) -> Result<ModelSelectedResponse, Error> {
+        self.conn
+            .request(
+                MODEL_SELECTED_METHOD_NAME,
+                Some(ClientRequest::ModelSelectedRequest(args)),
+            )
+            .await
+    }
+
     async fn ext_method(&self, args: ExtRequest) -> Result<ExtResponse, Error> {
         self.conn
             .request(
@@ -377,6 +410,10 @@ impl<T: Client> MessageHandler<ClientSide> for T {
             AgentNotification::SessionNotification(args) => {
                 self.session_notification(args).await?;
             }
+            #[cfg(feature = "unstable")]
+            AgentNotification::ModelNotification(args) => {
+                self.model_notification(args).await?;
+            }
             AgentNotification::ExtNotification(args) => {
                 self.ext_notification(args).await?;
             }
@@ -550,6 +587,14 @@ impl Client for AgentSideConnection {
         )
     }
 
+    #[cfg(feature = "unstable")]
+    async fn model_notification(&self, args: ModelNotification) -> Result<(), Error> {
+        self.conn.notify(
+            MODEL_UPDATE_NOTIFICATION,
+            Some(AgentNotification::ModelNotification(args)),
+        )
+    }
+
     async fn ext_method(&self, args: ExtRequest) -> Result<ExtResponse, Error> {
         self.conn
             .request(
@@ -667,6 +712,21 @@ impl<T: Agent> MessageHandler<AgentSide> for T {
                 let response = self.set_session_mode(args).await?;
                 Ok(AgentResponse::SetSessionModeResponse(response))
             }
+            #[cfg(feature = "unstable")]
+            ClientRequest::ModelListRequest(args) => {
+                let response = self.list_models(args).await?;
+                Ok(AgentResponse::ModelListResponse(response))
+            }
+            #[cfg(feature = "unstable")]
+            ClientRequest::ModelSelectRequest(args) => {
+                let response = self.select_model(args).await?;
+                Ok(AgentResponse::ModelSelectResponse(response))
+            }
+            #[cfg(feature = "unstable")]
+            ClientRequest::ModelSelectedRequest(args) => {
+                let response = self.selected_model(args).await?;
+                Ok(AgentResponse::ModelSelectedResponse(response))
+            }
             ClientRequest::ExtMethodRequest(args) => {
                 let response = self.ext_method(args).await?;
                 Ok(AgentResponse::ExtMethodResponse(response))