feat: fork conversation/thread (#8866)

## Summary
- add thread/conversation fork endpoints to the protocol (v1 + v2)
- implement fork handling in app-server using thread manager and config
overrides
- add fork coverage in app-server tests and document `thread/fork` usage

codex-rs/app-server/tests/suite/fork_thread.rs

@@ -0,0 +1,140 @@
+use anyhow::Result;
+use app_test_support::McpProcess;
+use app_test_support::create_fake_rollout;
+use app_test_support::to_response;
+use codex_app_server_protocol::ForkConversationParams;
+use codex_app_server_protocol::ForkConversationResponse;
+use codex_app_server_protocol::JSONRPCNotification;
+use codex_app_server_protocol::JSONRPCResponse;
+use codex_app_server_protocol::NewConversationParams; // reused for overrides shape
+use codex_app_server_protocol::RequestId;
+use codex_app_server_protocol::ServerNotification;
+use codex_app_server_protocol::SessionConfiguredNotification;
+use codex_core::protocol::EventMsg;
+use pretty_assertions::assert_eq;
+use tempfile::TempDir;
+use tokio::time::timeout;
+
+const DEFAULT_READ_TIMEOUT: std::time::Duration = std::time::Duration::from_secs(10);
+
+#[tokio::test(flavor = "multi_thread", worker_threads = 2)]
+async fn fork_conversation_creates_new_rollout() -> Result<()> {
+    let codex_home = TempDir::new()?;
+
+    let preview = "Hello A";
+    let conversation_id = create_fake_rollout(
+        codex_home.path(),
+        "2025-01-02T12-00-00",
+        "2025-01-02T12:00:00Z",
+        preview,
+        Some("openai"),
+        None,
+    )?;
+
+    let original_path = codex_home
+        .path()
+        .join("sessions")
+        .join("2025")
+        .join("01")
+        .join("02")
+        .join(format!(
+            "rollout-2025-01-02T12-00-00-{conversation_id}.jsonl"
+        ));
+    assert!(
+        original_path.exists(),
+        "expected original rollout to exist at {}",
+        original_path.display()
+    );
+    let original_contents = std::fs::read_to_string(&original_path)?;
+
+    let mut mcp = McpProcess::new(codex_home.path()).await?;
+    timeout(DEFAULT_READ_TIMEOUT, mcp.initialize()).await??;
+
+    let fork_req_id = mcp
+        .send_fork_conversation_request(ForkConversationParams {
+            path: Some(original_path.clone()),
+            conversation_id: None,
+            overrides: Some(NewConversationParams {
+                model: Some("o3".to_string()),
+                ..Default::default()
+            }),
+        })
+        .await?;
+
+    // Expect a sessionConfigured notification for the forked session.
+    let notification: JSONRPCNotification = timeout(
+        DEFAULT_READ_TIMEOUT,
+        mcp.read_stream_until_notification_message("sessionConfigured"),
+    )
+    .await??;
+    let session_configured: ServerNotification = notification.try_into()?;
+    let ServerNotification::SessionConfigured(SessionConfiguredNotification {
+        model,
+        session_id,
+        rollout_path,
+        initial_messages: session_initial_messages,
+        ..
+    }) = session_configured
+    else {
+        unreachable!("expected sessionConfigured notification");
+    };
+
+    assert_eq!(model, "o3");
+    assert_ne!(
+        session_id.to_string(),
+        conversation_id,
+        "expected a new conversation id when forking"
+    );
+    assert_ne!(
+        rollout_path, original_path,
+        "expected a new rollout path when forking"
+    );
+    assert!(
+        rollout_path.exists(),
+        "expected forked rollout to exist at {}",
+        rollout_path.display()
+    );
+
+    let session_initial_messages =
+        session_initial_messages.expect("expected initial messages when forking from rollout");
+    match session_initial_messages.as_slice() {
+        [EventMsg::UserMessage(message)] => {
+            assert_eq!(message.message, preview);
+        }
+        other => panic!("unexpected initial messages from rollout fork: {other:#?}"),
+    }
+
+    // Then the response for forkConversation.
+    let fork_resp: JSONRPCResponse = timeout(
+        DEFAULT_READ_TIMEOUT,
+        mcp.read_stream_until_response_message(RequestId::Integer(fork_req_id)),
+    )
+    .await??;
+    let ForkConversationResponse {
+        conversation_id: forked_id,
+        model: forked_model,
+        initial_messages: response_initial_messages,
+        rollout_path: response_rollout_path,
+    } = to_response::<ForkConversationResponse>(fork_resp)?;
+
+    assert_eq!(forked_model, "o3");
+    assert_eq!(response_rollout_path, rollout_path);
+    assert_ne!(forked_id.to_string(), conversation_id);
+
+    let response_initial_messages =
+        response_initial_messages.expect("expected initial messages in fork response");
+    match response_initial_messages.as_slice() {
+        [EventMsg::UserMessage(message)] => {
+            assert_eq!(message.message, preview);
+        }
+        other => panic!("unexpected initial messages in fork response: {other:#?}"),
+    }
+
+    let after_contents = std::fs::read_to_string(&original_path)?;
+    assert_eq!(
+        after_contents, original_contents,
+        "fork should not mutate the original rollout file"
+    );
+
+    Ok(())
+}

codex-rs/app-server/tests/suite/mod.rs

@@ -3,6 +3,7 @@ mod auth;
 mod codex_message_processor_flow;
 mod config;
 mod create_thread;
+mod fork_thread;
 mod fuzzy_file_search;
 mod interrupt;
 mod list_resume;

codex-rs/app-server/tests/suite/v2/mod.rs

@@ -5,6 +5,7 @@ mod output_schema;
 mod rate_limits;
 mod review;
 mod thread_archive;
+mod thread_fork;
 mod thread_list;
 mod thread_loaded_list;
 mod thread_resume;

codex-rs/app-server/tests/suite/v2/thread_fork.rs

@@ -0,0 +1,140 @@
+use anyhow::Result;
+use app_test_support::McpProcess;
+use app_test_support::create_fake_rollout;
+use app_test_support::create_mock_chat_completions_server;
+use app_test_support::to_response;
+use codex_app_server_protocol::JSONRPCNotification;
+use codex_app_server_protocol::JSONRPCResponse;
+use codex_app_server_protocol::RequestId;
+use codex_app_server_protocol::SessionSource;
+use codex_app_server_protocol::ThreadForkParams;
+use codex_app_server_protocol::ThreadForkResponse;
+use codex_app_server_protocol::ThreadItem;
+use codex_app_server_protocol::ThreadStartedNotification;
+use codex_app_server_protocol::TurnStatus;
+use codex_app_server_protocol::UserInput;
+use pretty_assertions::assert_eq;
+use std::path::Path;
+use tempfile::TempDir;
+use tokio::time::timeout;
+
+const DEFAULT_READ_TIMEOUT: std::time::Duration = std::time::Duration::from_secs(10);
+
+#[tokio::test]
+async fn thread_fork_creates_new_thread_and_emits_started() -> Result<()> {
+    let server = create_mock_chat_completions_server(vec![]).await;
+    let codex_home = TempDir::new()?;
+    create_config_toml(codex_home.path(), &server.uri())?;
+
+    let preview = "Saved user message";
+    let conversation_id = create_fake_rollout(
+        codex_home.path(),
+        "2025-01-05T12-00-00",
+        "2025-01-05T12:00:00Z",
+        preview,
+        Some("mock_provider"),
+        None,
+    )?;
+
+    let original_path = codex_home
+        .path()
+        .join("sessions")
+        .join("2025")
+        .join("01")
+        .join("05")
+        .join(format!(
+            "rollout-2025-01-05T12-00-00-{conversation_id}.jsonl"
+        ));
+    assert!(
+        original_path.exists(),
+        "expected original rollout to exist at {}",
+        original_path.display()
+    );
+    let original_contents = std::fs::read_to_string(&original_path)?;
+
+    let mut mcp = McpProcess::new(codex_home.path()).await?;
+    timeout(DEFAULT_READ_TIMEOUT, mcp.initialize()).await??;
+
+    let fork_id = mcp
+        .send_thread_fork_request(ThreadForkParams {
+            thread_id: conversation_id.clone(),
+            ..Default::default()
+        })
+        .await?;
+    let fork_resp: JSONRPCResponse = timeout(
+        DEFAULT_READ_TIMEOUT,
+        mcp.read_stream_until_response_message(RequestId::Integer(fork_id)),
+    )
+    .await??;
+    let ThreadForkResponse { thread, .. } = to_response::<ThreadForkResponse>(fork_resp)?;
+
+    let after_contents = std::fs::read_to_string(&original_path)?;
+    assert_eq!(
+        after_contents, original_contents,
+        "fork should not mutate the original rollout file"
+    );
+
+    assert_ne!(thread.id, conversation_id);
+    assert_eq!(thread.preview, preview);
+    assert_eq!(thread.model_provider, "mock_provider");
+    assert!(thread.path.is_absolute());
+    assert_ne!(thread.path, original_path);
+    assert!(thread.cwd.is_absolute());
+    assert_eq!(thread.source, SessionSource::VsCode);
+
+    assert_eq!(
+        thread.turns.len(),
+        1,
+        "expected forked thread to include one turn"
+    );
+    let turn = &thread.turns[0];
+    assert_eq!(turn.status, TurnStatus::Completed);
+    assert_eq!(turn.items.len(), 1, "expected user message item");
+    match &turn.items[0] {
+        ThreadItem::UserMessage { content, .. } => {
+            assert_eq!(
+                content,
+                &vec![UserInput::Text {
+                    text: preview.to_string()
+                }]
+            );
+        }
+        other => panic!("expected user message item, got {other:?}"),
+    }
+
+    // A corresponding thread/started notification should arrive.
+    let notif: JSONRPCNotification = timeout(
+        DEFAULT_READ_TIMEOUT,
+        mcp.read_stream_until_notification_message("thread/started"),
+    )
+    .await??;
+    let started: ThreadStartedNotification =
+        serde_json::from_value(notif.params.expect("params must be present"))?;
+    assert_eq!(started.thread, thread);
+
+    Ok(())
+}
+
+// Helper to create a config.toml pointing at the mock model server.
+fn create_config_toml(codex_home: &Path, server_uri: &str) -> std::io::Result<()> {
+    let config_toml = codex_home.join("config.toml");
+    std::fs::write(
+        config_toml,
+        format!(
+            r#"
+model = "mock-model"
+approval_policy = "never"
+sandbox_mode = "read-only"
+
+model_provider = "mock_provider"
+
+[model_providers.mock_provider]
+name = "Mock provider for test"
+base_url = "{server_uri}/v1"
+wire_api = "chat"
+request_max_retries = 0
+stream_max_retries = 0
+"#
+        ),
+    )
+}