[mcp-server] Add reply tool call (#1643)

## Summary
Adds a new mcp tool call, `codex-reply`, so we can continue existing
sessions. This is a first draft and does not yet support sessions from
previous processes.

## Testing
- [x] tested with mcp client

codex-rs/core/src/codex.rs

@@ -101,7 +101,7 @@ impl Codex {
     /// Spawn a new [`Codex`] and initialize the session. Returns the instance
     /// of `Codex` and the ID of the `SessionInitialized` event that was
     /// submitted to start the session.
-    pub async fn spawn(config: Config, ctrl_c: Arc<Notify>) -> CodexResult<(Codex, String)> {
+    pub async fn spawn(config: Config, ctrl_c: Arc<Notify>) -> CodexResult<(Codex, String, Uuid)> {
         // experimental resume path (undocumented)
         let resume_path = config.experimental_resume.clone();
         info!("resume_path: {resume_path:?}");
@@ -124,7 +124,12 @@ impl Codex {
         };
 
         let config = Arc::new(config);
-        tokio::spawn(submission_loop(config, rx_sub, tx_event, ctrl_c));
+
+        // Generate a unique ID for the lifetime of this Codex session.
+        let session_id = Uuid::new_v4();
+        tokio::spawn(submission_loop(
+            session_id, config, rx_sub, tx_event, ctrl_c,
+        ));
         let codex = Codex {
             next_id: AtomicU64::new(0),
             tx_sub,
@@ -132,7 +137,7 @@ impl Codex {
         };
         let init_id = codex.submit(configure_session).await?;
 
-        Ok((codex, init_id))
+        Ok((codex, init_id, session_id))
     }
 
     /// Submit the `op` wrapped in a `Submission` with a unique ID.
@@ -521,14 +526,12 @@ impl AgentTask {
 }
 
 async fn submission_loop(
+    mut session_id: Uuid,
     config: Arc<Config>,
     rx_sub: Receiver<Submission>,
     tx_event: Sender<Event>,
     ctrl_c: Arc<Notify>,
 ) {
-    // Generate a unique ID for the lifetime of this Codex session.
-    let mut session_id = Uuid::new_v4();
-
     let mut sess: Option<Arc<Session>> = None;
     // shorthand - send an event when there is no active session
     let send_no_session_event = |sub_id: String| async {

codex-rs/core/src/codex_wrapper.rs

@@ -6,15 +6,16 @@ use crate::protocol::Event;
 use crate::protocol::EventMsg;
 use crate::util::notify_on_sigint;
 use tokio::sync::Notify;
+use uuid::Uuid;
 
 /// Spawn a new [`Codex`] and initialize the session.
 ///
 /// Returns the wrapped [`Codex`] **and** the `SessionInitialized` event that
 /// is received as a response to the initial `ConfigureSession` submission so
 /// that callers can surface the information to the UI.
-pub async fn init_codex(config: Config) -> anyhow::Result<(Codex, Event, Arc<Notify>)> {
+pub async fn init_codex(config: Config) -> anyhow::Result<(Codex, Event, Arc<Notify>, Uuid)> {
     let ctrl_c = notify_on_sigint();
-    let (codex, init_id) = Codex::spawn(config, ctrl_c.clone()).await?;
+    let (codex, init_id, session_id) = Codex::spawn(config, ctrl_c.clone()).await?;
 
     // The first event must be `SessionInitialized`. Validate and forward it to
     // the caller so that they can display it in the conversation history.
@@ -33,5 +34,5 @@ pub async fn init_codex(config: Config) -> anyhow::Result<(Codex, Event, Arc<Not
         ));
     }
 
-    Ok((codex, event, ctrl_c))
+    Ok((codex, event, ctrl_c, session_id))
 }

codex-rs/core/tests/client.rs

@@ -75,7 +75,7 @@ async fn includes_session_id_and_model_headers_in_request() {
     let mut config = load_default_config_for_test(&codex_home);
     config.model_provider = model_provider;
     let ctrl_c = std::sync::Arc::new(tokio::sync::Notify::new());
-    let (codex, _init_id) = Codex::spawn(config, ctrl_c.clone()).await.unwrap();
+    let (codex, _init_id, _session_id) = Codex::spawn(config, ctrl_c.clone()).await.unwrap();
 
     codex
         .submit(Op::UserInput {

codex-rs/core/tests/live_agent.rs

@@ -49,7 +49,8 @@ async fn spawn_codex() -> Result<Codex, CodexErr> {
     let mut config = load_default_config_for_test(&codex_home);
     config.model_provider.request_max_retries = Some(2);
     config.model_provider.stream_max_retries = Some(2);
-    let (agent, _init_id) = Codex::spawn(config, std::sync::Arc::new(Notify::new())).await?;
+    let (agent, _init_id, _session_id) =
+        Codex::spawn(config, std::sync::Arc::new(Notify::new())).await?;
 
     Ok(agent)
 }

codex-rs/core/tests/previous_response_id.rs

@@ -113,7 +113,7 @@ async fn keeps_previous_response_id_between_tasks() {
     let mut config = load_default_config_for_test(&codex_home);
     config.model_provider = model_provider;
     let ctrl_c = std::sync::Arc::new(tokio::sync::Notify::new());
-    let (codex, _init_id) = Codex::spawn(config, ctrl_c.clone()).await.unwrap();
+    let (codex, _init_id, _session_id) = Codex::spawn(config, ctrl_c.clone()).await.unwrap();
 
     // Task 1 – triggers first request (no previous_response_id)
     codex

codex-rs/core/tests/stream_no_completed.rs

@@ -95,7 +95,7 @@ async fn retries_on_early_close() {
     let codex_home = TempDir::new().unwrap();
     let mut config = load_default_config_for_test(&codex_home);
     config.model_provider = model_provider;
-    let (codex, _init_id) = Codex::spawn(config, ctrl_c).await.unwrap();
+    let (codex, _init_id, _session_id) = Codex::spawn(config, ctrl_c).await.unwrap();
 
     codex
         .submit(Op::UserInput {