Introduce experimental session modes

docs/protocol/schema.mdx

@@ -250,6 +250,12 @@ See protocol docs: [Creating a Session](https://agentclientprotocol.com/protocol
 
 **Properties:**
 
+<ResponseField name="modes" type={<><span><a href="#sessionmodestate">SessionModeState</a></span><span> | null</span></>} >
+  **UNSTABLE**
+
+This field is not part of the spec, and may be removed or changed at any point.
+
+</ResponseField>
 <ResponseField name="sessionId" type={<a href="#sessionid">SessionId</a>} required>
   Unique identifier for the created session.
 
@@ -981,6 +987,21 @@ An image provided to or from an LLM.
 <ResponseField name="mimeType" type={"string"} required></ResponseField>
 <ResponseField name="uri" type={"string | null"}></ResponseField>
 
+## <span class="font-mono">LoadSessionResponse</span>
+
+Response from loading an existing session.
+
+**Type:** Object
+
+**Properties:**
+
+<ResponseField name="modes" type={<><span><a href="#sessionmodestate">SessionModeState</a></span><span> | null</span></>} >
+  **UNSTABLE**
+
+This field is not part of the spec, and may be removed or changed at any point.
+
+</ResponseField>
+
 ## <span class="font-mono">McpCapabilities</span>
 
 MCP capabilities supported by the agent
@@ -1431,6 +1452,60 @@ See protocol docs: [Session ID](https://agentclientprotocol.com/protocol/session
 
 **Type:** `string`
 
+## <span class="font-mono">SessionMode</span>
+
+**UNSTABLE**
+
+This type is not part of the spec, and may be removed or changed at any point.
+
+**Type:** Object
+
+**Properties:**
+
+<ResponseField name="description" type={"string | null"}></ResponseField>
+<ResponseField
+  name="id"
+  type={<a href="#sessionmodeid">SessionModeId</a>}
+  required
+></ResponseField>
+<ResponseField name="name" type={"string"} required></ResponseField>
+
+## <span class="font-mono">SessionModeId</span>
+
+**UNSTABLE**
+
+This type is not part of the spec, and may be removed or changed at any point.
+
+**Type:** `string`
+
+## <span class="font-mono">SessionModeState</span>
+
+**UNSTABLE**
+
+This type is not part of the spec, and may be removed or changed at any point.
+
+**Type:** Object
+
+**Properties:**
+
+<ResponseField
+  name="availableModes"
+  type={
+    <>
+      <span>
+        <a href="#sessionmode">SessionMode</a>
+      </span>
+      <span>[]</span>
+    </>
+  }
+  required
+></ResponseField>
+<ResponseField
+  name="currentModeId"
+  type={<a href="#sessionmodeid">SessionModeId</a>}
+  required
+></ResponseField>
+
 ## <span class="font-mono">SessionUpdate</span>
 
 Different types of updates that can be sent during session processing.
@@ -1648,6 +1723,29 @@ Available commands are ready or have changed
 </Expandable>
 </ResponseField>
 
+<ResponseField name="current_mode_update">
+The current mode of the session has changed
+
+<Expandable title="Properties">
+
+<ResponseField
+  name="currentModeId"
+  type={<a href="#sessionmodeid">SessionModeId</a>}
+  required
+></ResponseField>
+<ResponseField name="sessionUpdate" type={"string"} required></ResponseField>
+
+</Expandable>
+</ResponseField>
+
+## <span class="font-mono">SetSessionModeResponse</span>
+
+**UNSTABLE**
+
+This type is not part of the spec, and may be removed or changed at any point.
+
+**Type:** `object`
+
 ## <span class="font-mono">StopReason</span>
 
 Reasons why an agent stops processing a prompt turn.
@@ -1984,4 +2082,11 @@ See protocol docs: [Creating](https://agentclientprotocol.com/protocol/tool-call
 
 <ResponseField name="fetch">Retrieving external data.</ResponseField>
 
+<ResponseField name="switch_mode">
+**UNSTABLE**
+
+This tool kind is not part of the spec and may be removed at any point.
+
+</ResponseField>
+
 <ResponseField name="other">Other tool types (default).</ResponseField>

rust/acp.rs

@@ -192,7 +192,10 @@ impl Agent for ClientSideConnection {
             .await
     }
 
-    async fn load_session(&self, arguments: LoadSessionRequest) -> Result<(), Error> {
+    async fn load_session(
+        &self,
+        arguments: LoadSessionRequest,
+    ) -> Result<LoadSessionResponse, Error> {
         self.conn
             .request(
                 SESSION_LOAD_METHOD_NAME,
@@ -201,6 +204,19 @@ impl Agent for ClientSideConnection {
             .await
     }
 
+    #[cfg(feature = "unstable")]
+    async fn set_session_mode(
+        &self,
+        arguments: SetSessionModeRequest,
+    ) -> Result<SetSessionModeResponse, Error> {
+        self.conn
+            .request(
+                SESSION_SET_MODE_METHOD_NAME,
+                Some(ClientRequest::SetSessionModeRequest(arguments)),
+            )
+            .await
+    }
+
     async fn prompt(&self, arguments: PromptRequest) -> Result<PromptResponse, Error> {
         self.conn
             .request(
@@ -525,6 +541,10 @@ impl Side for AgentSide {
             SESSION_LOAD_METHOD_NAME => serde_json::from_str(params.get())
                 .map(ClientRequest::LoadSessionRequest)
                 .map_err(Into::into),
+            #[cfg(feature = "unstable")]
+            SESSION_SET_MODE_METHOD_NAME => serde_json::from_str(params.get())
+                .map(ClientRequest::SetSessionModeRequest)
+                .map_err(Into::into),
             SESSION_PROMPT_METHOD_NAME => serde_json::from_str(params.get())
                 .map(ClientRequest::PromptRequest)
                 .map_err(Into::into),
@@ -563,13 +583,18 @@ impl<T: Agent> MessageHandler<AgentSide> for T {
                 Ok(AgentResponse::NewSessionResponse(response))
             }
             ClientRequest::LoadSessionRequest(args) => {
-                self.load_session(args).await?;
-                Ok(AgentResponse::LoadSessionResponse)
+                let response = self.load_session(args).await?;
+                Ok(AgentResponse::LoadSessionResponse(response))
             }
             ClientRequest::PromptRequest(args) => {
                 let response = self.prompt(args).await?;
                 Ok(AgentResponse::PromptResponse(response))
             }
+            #[cfg(feature = "unstable")]
+            ClientRequest::SetSessionModeRequest(args) => {
+                let response = self.set_session_mode(args).await?;
+                Ok(AgentResponse::SetSessionModeResponse(response))
+            }
         }
     }
 

rust/agent.rs

@@ -75,7 +75,16 @@ pub trait Agent {
     fn load_session(
         &self,
         arguments: LoadSessionRequest,
-    ) -> impl Future<Output = Result<(), Error>>;
+    ) -> impl Future<Output = Result<LoadSessionResponse, Error>>;
+
+    /// **UNSTABLE**
+    ///
+    /// This method is not part of the spec, and may be removed or changed at any point.
+    #[cfg(feature = "unstable")]
+    fn set_session_mode(
+        &self,
+        arguments: SetSessionModeRequest,
+    ) -> impl Future<Output = Result<SetSessionModeResponse, Error>>;
 
     /// Processes a user prompt within a session.
     ///
@@ -204,6 +213,11 @@ pub struct NewSessionResponse {
     ///
     /// Used in all subsequent requests for this conversation.
     pub session_id: SessionId,
+    /// **UNSTABLE**
+    ///
+    /// This field is not part of the spec, and may be removed or changed at any point.
+    #[serde(default, skip_serializing_if = "Option::is_none")]
+    pub modes: Option<SessionModeState>,
 }
 
 // Load session
@@ -225,6 +239,72 @@ pub struct LoadSessionRequest {
     pub session_id: SessionId,
 }
 
+/// Response from loading an existing session.
+#[derive(Debug, Clone, Serialize, Deserialize, JsonSchema)]
+#[serde(rename_all = "camelCase")]
+pub struct LoadSessionResponse {
+    /// **UNSTABLE**
+    ///
+    /// This field is not part of the spec, and may be removed or changed at any point.
+    #[serde(default, skip_serializing_if = "Option::is_none")]
+    pub modes: Option<SessionModeState>,
+}
+
+// Session modes
+
+/// **UNSTABLE**
+///
+/// This type is not part of the spec, and may be removed or changed at any point.
+#[derive(Debug, Clone, Serialize, Deserialize, JsonSchema)]
+#[serde(rename_all = "camelCase")]
+pub struct SessionModeState {
+    pub current_mode_id: SessionModeId,
+    pub available_modes: Vec<SessionMode>,
+}
+
+/// **UNSTABLE**
+///
+/// This type is not part of the spec, and may be removed or changed at any point.
+#[derive(Debug, Clone, Serialize, Deserialize, JsonSchema)]
+#[serde(rename_all = "camelCase")]
+pub struct SessionMode {
+    pub id: SessionModeId,
+    pub name: String,
+    #[serde(default, skip_serializing_if = "Option::is_none")]
+    pub description: Option<String>,
+}
+
+/// **UNSTABLE**
+///
+/// This type is not part of the spec, and may be removed or changed at any point.
+#[derive(Debug, Clone, Serialize, Deserialize, JsonSchema, PartialEq, Eq, Hash)]
+#[serde(rename_all = "camelCase")]
+pub struct SessionModeId(pub Arc<str>);
+
+impl std::fmt::Display for SessionModeId {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        write!(f, "{}", self.0)
+    }
+}
+
+/// **UNSTABLE**
+///
+/// This type is not part of the spec, and may be removed or changed at any point.
+#[derive(Debug, Clone, Serialize, Deserialize, JsonSchema)]
+#[schemars(extend("x-docs-ignore" = true))]
+#[serde(rename_all = "camelCase")]
+pub struct SetSessionModeRequest {
+    pub session_id: SessionId,
+    pub mode_id: SessionModeId,
+}
+
+/// **UNSTABLE**
+///
+/// This type is not part of the spec, and may be removed or changed at any point.
+#[derive(Debug, Clone, Serialize, Deserialize, JsonSchema)]
+#[serde(rename_all = "camelCase")]
+pub struct SetSessionModeResponse {}
+
 // MCP
 
 /// Configuration for connecting to an MCP (Model Context Protocol) server.
@@ -440,6 +520,9 @@ pub struct AgentMethodNames {
     pub session_new: &'static str,
     /// Method for loading an existing session.
     pub session_load: &'static str,
+    /// Method for setting the mode for a session.
+    #[cfg(feature = "unstable")]
+    pub session_set_mode: &'static str,
     /// Method for sending a prompt to the agent.
     pub session_prompt: &'static str,
     /// Notification for cancelling operations.
@@ -452,6 +535,8 @@ pub const AGENT_METHOD_NAMES: AgentMethodNames = AgentMethodNames {
     authenticate: AUTHENTICATE_METHOD_NAME,
     session_new: SESSION_NEW_METHOD_NAME,
     session_load: SESSION_LOAD_METHOD_NAME,
+    #[cfg(feature = "unstable")]
+    session_set_mode: SESSION_SET_MODE_METHOD_NAME,
     session_prompt: SESSION_PROMPT_METHOD_NAME,
     session_cancel: SESSION_CANCEL_METHOD_NAME,
 };
@@ -464,6 +549,9 @@ pub(crate) const AUTHENTICATE_METHOD_NAME: &str = "authenticate";
 pub(crate) const SESSION_NEW_METHOD_NAME: &str = "session/new";
 /// Method name for loading an existing session.
 pub(crate) const SESSION_LOAD_METHOD_NAME: &str = "session/load";
+/// Method name for setting the mode for a session.
+#[cfg(feature = "unstable")]
+pub(crate) const SESSION_SET_MODE_METHOD_NAME: &str = "session/set_mode";
 /// Method name for sending a prompt.
 pub(crate) const SESSION_PROMPT_METHOD_NAME: &str = "session/prompt";
 /// Method name for the cancel notification.
@@ -483,6 +571,8 @@ pub enum ClientRequest {
     AuthenticateRequest(AuthenticateRequest),
     NewSessionRequest(NewSessionRequest),
     LoadSessionRequest(LoadSessionRequest),
+    #[cfg(feature = "unstable")]
+    SetSessionModeRequest(SetSessionModeRequest),
     PromptRequest(PromptRequest),
 }
 
@@ -499,7 +589,9 @@ pub enum AgentResponse {
     InitializeResponse(InitializeResponse),
     AuthenticateResponse,
     NewSessionResponse(NewSessionResponse),
-    LoadSessionResponse,
+    LoadSessionResponse(LoadSessionResponse),
+    #[cfg(feature = "unstable")]
+    SetSessionModeResponse(SetSessionModeResponse),
     PromptResponse(PromptResponse),
 }
 

rust/client.rs

@@ -9,6 +9,8 @@ use anyhow::Result;
 use schemars::JsonSchema;
 use serde::{Deserialize, Serialize};
 
+#[cfg(feature = "unstable")]
+use crate::SessionModeId;
 use crate::{ContentBlock, Error, Plan, SessionId, ToolCall, ToolCallUpdate};
 
 /// Defines the interface that ACP-compliant clients must implement.
@@ -165,6 +167,11 @@ pub enum SessionUpdate {
     AvailableCommandsUpdate {
         available_commands: Vec<AvailableCommand>,
     },
+    /// The current mode of the session has changed
+    #[cfg(feature = "unstable")]
+    #[serde(rename_all = "camelCase")]
+    #[schemars(extend("x-docs-ignore" = true))]
+    CurrentModeUpdate { current_mode_id: SessionModeId },
 }
 
 /// Information about a command.

rust/example_agent.rs

@@ -61,12 +61,16 @@ impl acp::Agent for ExampleAgent {
         self.next_session_id.set(session_id + 1);
         Ok(acp::NewSessionResponse {
             session_id: acp::SessionId(session_id.to_string().into()),
+            modes: None,
         })
     }
 
-    async fn load_session(&self, arguments: acp::LoadSessionRequest) -> Result<(), acp::Error> {
+    async fn load_session(
+        &self,
+        arguments: acp::LoadSessionRequest,
+    ) -> Result<acp::LoadSessionResponse, acp::Error> {
         log::info!("Received load session request {arguments:?}");
-        Err(acp::Error::method_not_found())
+        Ok(acp::LoadSessionResponse { modes: None })
     }
 
     async fn prompt(