enable-resume (#3537)

Adding the ability to resume conversations.
we have one verb `resume`. 

Behavior:

`tui`:
`codex resume`: opens session picker
`codex resume --last`: continue last message
`codex resume <session id>`: continue conversation with `session id`

`exec`:
`codex resume --last`: continue last conversation
`codex resume <session id>`: continue conversation with `session id`

Implementation:
- I added a function to find the path in `~/.codex/sessions/` with a
`UUID`. This is helpful in resuming with session id.
- Added the above mentioned flags
- Added lots of testing

codex-rs/core/tests/suite/cli_stream.rs

@@ -420,12 +420,6 @@ async fn integration_creates_and_checks_session_file() {
     // Second run: resume should update the existing file.
     let marker2 = format!("integration-resume-{}", Uuid::new_v4());
     let prompt2 = format!("echo {marker2}");
-    // Cross‑platform safe resume override.  On Windows, backslashes in a TOML string must be escaped
-    // or the parse will fail and the raw literal (including quotes) may be preserved all the way down
-    // to Config, which in turn breaks resume because the path is invalid. Normalize to forward slashes
-    // to sidestep the issue.
-    let resume_path_str = path.to_string_lossy().replace('\\', "/");
-    let resume_override = format!("experimental_resume=\"{resume_path_str}\"");
     let mut cmd2 = AssertCommand::new("cargo");
     cmd2.arg("run")
         .arg("-p")
@@ -434,11 +428,11 @@ async fn integration_creates_and_checks_session_file() {
         .arg("--")
         .arg("exec")
         .arg("--skip-git-repo-check")
-        .arg("-c")
-        .arg(&resume_override)
         .arg("-C")
         .arg(env!("CARGO_MANIFEST_DIR"))
-        .arg(&prompt2);
+        .arg(&prompt2)
+        .arg("resume")
+        .arg("--last");
     cmd2.env("CODEX_HOME", home.path())
         .env("OPENAI_API_KEY", "dummy")
         .env("CODEX_RS_SSE_FIXTURE", &fixture)

codex-rs/core/tests/suite/client.rs

@@ -236,20 +236,21 @@ async fn resume_includes_initial_messages_and_sends_prior_items() {
     let codex_home = TempDir::new().unwrap();
     let mut config = load_default_config_for_test(&codex_home);
     config.model_provider = model_provider;
-    config.experimental_resume = Some(session_path.clone());
     // Also configure user instructions to ensure they are NOT delivered on resume.
     config.user_instructions = Some("be nice".to_string());
 
     let conversation_manager =
         ConversationManager::with_auth(CodexAuth::from_api_key("Test API Key"));
+    let auth_manager =
+        codex_core::AuthManager::from_auth_for_testing(CodexAuth::from_api_key("Test API Key"));
     let NewConversation {
         conversation: codex,
         session_configured,
         ..
     } = conversation_manager
-        .new_conversation(config)
+        .resume_conversation_from_rollout(config, session_path.clone(), auth_manager)
         .await
-        .expect("create new conversation");
+        .expect("resume conversation");
 
     // 1) Assert initial_messages only includes existing EventMsg entries; response items are not converted
     let initial_msgs = session_configured

codex-rs/core/tests/suite/mod.rs

@@ -11,6 +11,7 @@ mod live_cli;
 mod model_overrides;
 mod prompt_caching;
 mod review;
+mod rollout_list_find;
 mod seatbelt;
 mod stream_error_allows_next_turn;
 mod stream_no_completed;

codex-rs/core/tests/suite/review.rs

@@ -379,13 +379,8 @@ async fn review_input_isolated_from_parent_history() {
             .await
             .unwrap();
     }
-    config.experimental_resume = Some(session_file);
-
-    let codex = new_conversation_for_server(&server, &codex_home, |cfg| {
-        // apply resume file
-        cfg.experimental_resume = config.experimental_resume.clone();
-    })
-    .await;
+    let codex =
+        resume_conversation_for_server(&server, &codex_home, session_file.clone(), |_| {}).await;
 
     // Submit review request; it must start fresh (no parent history in `input`).
     let review_prompt = "Please review only this".to_string();
@@ -546,3 +541,32 @@ where
         .expect("create conversation")
         .conversation
 }
+
+/// Create a conversation resuming from a rollout file, configured to talk to the provided mock server.
+#[expect(clippy::expect_used)]
+async fn resume_conversation_for_server<F>(
+    server: &MockServer,
+    codex_home: &TempDir,
+    resume_path: std::path::PathBuf,
+    mutator: F,
+) -> Arc<CodexConversation>
+where
+    F: FnOnce(&mut Config),
+{
+    let model_provider = ModelProviderInfo {
+        base_url: Some(format!("{}/v1", server.uri())),
+        ..built_in_model_providers()["openai"].clone()
+    };
+    let mut config = load_default_config_for_test(codex_home);
+    config.model_provider = model_provider;
+    mutator(&mut config);
+    let conversation_manager =
+        ConversationManager::with_auth(CodexAuth::from_api_key("Test API Key"));
+    let auth_manager =
+        codex_core::AuthManager::from_auth_for_testing(CodexAuth::from_api_key("Test API Key"));
+    conversation_manager
+        .resume_conversation_from_rollout(config, resume_path, auth_manager)
+        .await
+        .expect("resume conversation")
+        .conversation
+}

codex-rs/core/tests/suite/rollout_list_find.rs

@@ -0,0 +1,50 @@
+#![allow(clippy::unwrap_used, clippy::expect_used)]
+use std::io::Write;
+use std::path::PathBuf;
+
+use codex_core::find_conversation_path_by_id_str;
+use tempfile::TempDir;
+use uuid::Uuid;
+
+/// Create sessions/YYYY/MM/DD and write a minimal rollout file containing the
+/// provided conversation id in the SessionMeta line. Returns the absolute path.
+fn write_minimal_rollout_with_id(codex_home: &TempDir, id: Uuid) -> PathBuf {
+    let sessions = codex_home.path().join("sessions/2024/01/01");
+    std::fs::create_dir_all(&sessions).unwrap();
+
+    let file = sessions.join(format!("rollout-2024-01-01T00-00-00-{id}.jsonl"));
+    let mut f = std::fs::File::create(&file).unwrap();
+    // Minimal first line: session_meta with the id so content search can find it
+    writeln!(
+        f,
+        "{}",
+        serde_json::json!({
+            "timestamp": "2024-01-01T00:00:00.000Z",
+            "type": "session_meta",
+            "payload": {
+                "id": id,
+                "timestamp": "2024-01-01T00:00:00Z",
+                "instructions": null,
+                "cwd": ".",
+                "originator": "test",
+                "cli_version": "test"
+            }
+        })
+    )
+    .unwrap();
+
+    file
+}
+
+#[tokio::test]
+async fn find_locates_rollout_file_by_id() {
+    let home = TempDir::new().unwrap();
+    let id = Uuid::new_v4();
+    let expected = write_minimal_rollout_with_id(&home, id);
+
+    let found = find_conversation_path_by_id_str(home.path(), &id.to_string())
+        .await
+        .unwrap();
+
+    assert_eq!(found.unwrap(), expected);
+}