feat(unstable): Add initial support for resuming sessions

src/agent-client-protocol/src/agent.rs

@@ -10,6 +10,8 @@ use agent_client_protocol_schema::{
 use agent_client_protocol_schema::{ForkSessionRequest, ForkSessionResponse};
 #[cfg(feature = "unstable_session_list")]
 use agent_client_protocol_schema::{ListSessionsRequest, ListSessionsResponse};
+#[cfg(feature = "unstable_session_resume")]
+use agent_client_protocol_schema::{ResumeSessionRequest, ResumeSessionResponse};
 #[cfg(feature = "unstable_session_model")]
 use agent_client_protocol_schema::{SetSessionModelRequest, SetSessionModelResponse};
 use serde_json::value::RawValue;
@@ -154,6 +156,21 @@ pub trait Agent {
         Err(Error::method_not_found())
     }
 
+    /// **UNSTABLE**
+    ///
+    /// This capability is not part of the spec yet, and may be removed or changed at any point.
+    ///
+    /// Resumes an existing session without replaying message history.
+    ///
+    /// This is similar to `load_session`, except it does not return previous messages.
+    /// Useful for agents that support continuing conversations but don't store full history.
+    ///
+    /// Only available if the Agent supports the `sessionCapabilities.resume` capability.
+    #[cfg(feature = "unstable_session_resume")]
+    async fn resume_session(&self, _args: ResumeSessionRequest) -> Result<ResumeSessionResponse> {
+        Err(Error::method_not_found())
+    }
+
     /// Handles extension method requests from the client.
     ///
     /// Extension methods provide a way to add custom functionality while maintaining
@@ -216,6 +233,10 @@ impl<T: Agent> Agent for Rc<T> {
     async fn fork_session(&self, args: ForkSessionRequest) -> Result<ForkSessionResponse> {
         self.as_ref().fork_session(args).await
     }
+    #[cfg(feature = "unstable_session_resume")]
+    async fn resume_session(&self, args: ResumeSessionRequest) -> Result<ResumeSessionResponse> {
+        self.as_ref().resume_session(args).await
+    }
     async fn ext_method(&self, args: ExtRequest) -> Result<ExtResponse> {
         self.as_ref().ext_method(args).await
     }
@@ -265,6 +286,10 @@ impl<T: Agent> Agent for Arc<T> {
     async fn fork_session(&self, args: ForkSessionRequest) -> Result<ForkSessionResponse> {
         self.as_ref().fork_session(args).await
     }
+    #[cfg(feature = "unstable_session_resume")]
+    async fn resume_session(&self, args: ResumeSessionRequest) -> Result<ResumeSessionResponse> {
+        self.as_ref().resume_session(args).await
+    }
     async fn ext_method(&self, args: ExtRequest) -> Result<ExtResponse> {
         self.as_ref().ext_method(args).await
     }

src/agent-client-protocol/src/lib.rs

@@ -175,6 +175,16 @@ impl Agent for ClientSideConnection {
             .await
     }
 
+    #[cfg(feature = "unstable_session_resume")]
+    async fn resume_session(&self, args: ResumeSessionRequest) -> Result<ResumeSessionResponse> {
+        self.conn
+            .request(
+                AGENT_METHOD_NAMES.session_resume,
+                Some(ClientRequest::ResumeSessionRequest(args)),
+            )
+            .await
+    }
+
     async fn ext_method(&self, args: ExtRequest) -> Result<ExtResponse> {
         self.conn
             .request(
@@ -546,6 +556,10 @@ impl Side for AgentSide {
             m if m == AGENT_METHOD_NAMES.session_fork => serde_json::from_str(params.get())
                 .map(ClientRequest::ForkSessionRequest)
                 .map_err(Into::into),
+            #[cfg(feature = "unstable_session_resume")]
+            m if m == AGENT_METHOD_NAMES.session_resume => serde_json::from_str(params.get())
+                .map(ClientRequest::ResumeSessionRequest)
+                .map_err(Into::into),
             m if m == AGENT_METHOD_NAMES.session_prompt => serde_json::from_str(params.get())
                 .map(ClientRequest::PromptRequest)
                 .map_err(Into::into),
@@ -625,6 +639,11 @@ impl<T: Agent> MessageHandler<AgentSide> for T {
                 let response = self.fork_session(args).await?;
                 Ok(AgentResponse::ForkSessionResponse(response))
             }
+            #[cfg(feature = "unstable_session_resume")]
+            ClientRequest::ResumeSessionRequest(args) => {
+                let response = self.resume_session(args).await?;
+                Ok(AgentResponse::ResumeSessionResponse(response))
+            }
             ClientRequest::ExtMethodRequest(args) => {
                 let response = self.ext_method(args).await?;
                 Ok(AgentResponse::ExtMethodResponse(response))

src/agent-client-protocol/src/rpc_tests.rs

@@ -240,6 +240,18 @@ impl Agent for TestAgent {
         ))
     }
 
+    #[cfg(feature = "unstable_session_resume")]
+    async fn resume_session(
+        &self,
+        args: agent_client_protocol_schema::ResumeSessionRequest,
+    ) -> Result<agent_client_protocol_schema::ResumeSessionResponse> {
+        // Check if session exists
+        if !self.sessions.lock().unwrap().contains_key(&args.session_id) {
+            return Err(Error::invalid_params());
+        }
+        Ok(agent_client_protocol_schema::ResumeSessionResponse::new())
+    }
+
     async fn ext_method(&self, args: ExtRequest) -> Result<ExtResponse> {
         dbg!();
         match dbg!(args.method.as_ref()) {
@@ -773,3 +785,37 @@ async fn test_list_sessions() {
         })
         .await;
 }
+
+#[cfg(feature = "unstable_session_resume")]
+#[tokio::test]
+async fn test_resume_session() {
+    let local_set = tokio::task::LocalSet::new();
+    local_set
+        .run_until(async {
+            let client = TestClient::new();
+            let agent = TestAgent::new();
+
+            let (agent_conn, _client_conn) = create_connection_pair(&client, &agent);
+
+            // First create a session
+            let new_session_response = agent_conn
+                .new_session(NewSessionRequest::new("/test"))
+                .await
+                .expect("new_session failed");
+
+            let session_id = new_session_response.session_id;
+
+            // Resume the session
+            let resume_response = agent_conn
+                .resume_session(agent_client_protocol_schema::ResumeSessionRequest::new(
+                    session_id.clone(),
+                    "/test",
+                ))
+                .await
+                .expect("resume_session failed");
+
+            // Verify we got a valid response (no modes by default in TestAgent)
+            assert!(resume_response.modes.is_none());
+        })
+        .await;
+}