chore: unify history loading (#2736)

We have two ways of loading conversation with a previous history. Fork
conversation and the experimental resume that we had before. In this PR,
I am unifying their code path. The path is getting the history items and
recording them in a brand new conversation. This PR also constraint the
rollout recorder responsibilities to be only recording to the disk and
loading from the disk.

The PR also fixes a current bug when we have two forking in a row:
History 1:
<Environment Context>
UserMessage_1
UserMessage_2
UserMessage_3

**Fork with n = 1 (only remove one element)**
History 2:
<Environment Context>
UserMessage_1
UserMessage_2
<Environment Context>

**Fork with n = 1 (only remove one element)**
History 2:
<Environment Context>
UserMessage_1
UserMessage_2
**<Environment Context>**

This shouldn't happen but because we were appending the `<Environment
Context>` after each spawning and it's considered as _user message_.
Now, we don't add this message if restoring and old conversation.

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

@@ -388,7 +388,7 @@ async fn integration_creates_and_checks_session_file() {
         "No message found in session file containing the marker"
     );
 
-    // Second run: resume and append.
+    // Second run: resume should create a NEW session file that contains both old and new history.
     let orig_len = content.lines().count();
     let marker2 = format!("integration-resume-{}", Uuid::new_v4());
     let prompt2 = format!("echo {marker2}");
@@ -419,31 +419,58 @@ async fn integration_creates_and_checks_session_file() {
     let output2 = cmd2.output().unwrap();
     assert!(output2.status.success(), "resume codex-cli run failed");
 
-    // The rollout writer runs on a background async task; give it a moment to flush.
-    let mut new_len = orig_len;
-    let deadline = Instant::now() + Duration::from_secs(5);
-    let mut content2 = String::new();
-    while Instant::now() < deadline {
-        if let Ok(c) = std::fs::read_to_string(&path) {
-            let count = c.lines().count();
-            if count > orig_len {
-                content2 = c;
-                new_len = count;
+    // Find the new session file containing the resumed marker.
+    let deadline = Instant::now() + Duration::from_secs(10);
+    let mut resumed_path: Option<std::path::PathBuf> = None;
+    while Instant::now() < deadline && resumed_path.is_none() {
+        for entry in WalkDir::new(&sessions_dir) {
+            let entry = match entry {
+                Ok(e) => e,
+                Err(_) => continue,
+            };
+            if !entry.file_type().is_file() {
+                continue;
+            }
+            if !entry.file_name().to_string_lossy().ends_with(".jsonl") {
+                continue;
+            }
+            let p = entry.path();
+            let Ok(c) = std::fs::read_to_string(p) else {
+                continue;
+            };
+            if c.contains(&marker2) {
+                resumed_path = Some(p.to_path_buf());
                 break;
             }
         }
-        std::thread::sleep(Duration::from_millis(50));
+        if resumed_path.is_none() {
+            std::thread::sleep(Duration::from_millis(50));
+        }
     }
-    if content2.is_empty() {
-        // last attempt
-        content2 = std::fs::read_to_string(&path).unwrap();
-        new_len = content2.lines().count();
-    }
-    assert!(new_len > orig_len, "rollout file did not grow after resume");
-    assert!(content2.contains(&marker), "rollout lost original marker");
+
+    let resumed_path = resumed_path.expect("No resumed session file found containing the marker2");
+    // Resume should have written to a new file, not the original one.
+    assert_ne!(
+        resumed_path, path,
+        "resume should create a new session file"
+    );
+
+    let resumed_content = std::fs::read_to_string(&resumed_path).unwrap();
     assert!(
-        content2.contains(&marker2),
-        "rollout missing resumed marker"
+        resumed_content.contains(&marker),
+        "resumed file missing original marker"
+    );
+    assert!(
+        resumed_content.contains(&marker2),
+        "resumed file missing resumed marker"
+    );
+
+    // Original file should remain unchanged.
+    let content_after = std::fs::read_to_string(&path).unwrap();
+    assert_eq!(
+        content_after.lines().count(),
+        orig_len,
+        "original rollout file should not change on resume"
     );
 }
 

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

@@ -0,0 +1,154 @@
+use codex_core::ConversationManager;
+use codex_core::ModelProviderInfo;
+use codex_core::NewConversation;
+use codex_core::built_in_model_providers;
+use codex_core::protocol::ConversationHistoryResponseEvent;
+use codex_core::protocol::EventMsg;
+use codex_core::protocol::InputItem;
+use codex_core::protocol::Op;
+use codex_login::CodexAuth;
+use core_test_support::load_default_config_for_test;
+use core_test_support::wait_for_event;
+use tempfile::TempDir;
+use wiremock::Mock;
+use wiremock::MockServer;
+use wiremock::ResponseTemplate;
+use wiremock::matchers::method;
+use wiremock::matchers::path;
+
+/// Build minimal SSE stream with completed marker using the JSON fixture.
+fn sse_completed(id: &str) -> String {
+    core_test_support::load_sse_fixture_with_id("tests/fixtures/completed_template.json", id)
+}
+
+#[tokio::test(flavor = "multi_thread", worker_threads = 2)]
+async fn fork_conversation_twice_drops_to_first_message() {
+    // Start a mock server that completes three turns.
+    let server = MockServer::start().await;
+    let sse = sse_completed("resp");
+    let first = ResponseTemplate::new(200)
+        .insert_header("content-type", "text/event-stream")
+        .set_body_raw(sse.clone(), "text/event-stream");
+
+    // Expect three calls to /v1/responses – one per user input.
+    Mock::given(method("POST"))
+        .and(path("/v1/responses"))
+        .respond_with(first)
+        .expect(3)
+        .mount(&server)
+        .await;
+
+    // Configure Codex to use the mock server.
+    let model_provider = ModelProviderInfo {
+        base_url: Some(format!("{}/v1", server.uri())),
+        ..built_in_model_providers()["openai"].clone()
+    };
+
+    let home = TempDir::new().unwrap();
+    let mut config = load_default_config_for_test(&home);
+    config.model_provider = model_provider.clone();
+    let config_for_fork = config.clone();
+
+    let conversation_manager = ConversationManager::with_auth(CodexAuth::from_api_key("dummy"));
+    let NewConversation {
+        conversation: codex,
+        ..
+    } = conversation_manager
+        .new_conversation(config)
+        .await
+        .expect("create conversation");
+
+    // Send three user messages; wait for three completed turns.
+    for text in ["first", "second", "third"] {
+        codex
+            .submit(Op::UserInput {
+                items: vec![InputItem::Text {
+                    text: text.to_string(),
+                }],
+            })
+            .await
+            .unwrap();
+        let _ = wait_for_event(&codex, |ev| matches!(ev, EventMsg::TaskComplete(_))).await;
+    }
+
+    // Request history from the base conversation.
+    codex.submit(Op::GetHistory).await.unwrap();
+    let base_history =
+        wait_for_event(&codex, |ev| matches!(ev, EventMsg::ConversationHistory(_))).await;
+
+    // Capture entries from the base history and compute expected prefixes after each fork.
+    let entries_after_three = match &base_history {
+        EventMsg::ConversationHistory(ConversationHistoryResponseEvent { entries, .. }) => {
+            entries.clone()
+        }
+        _ => panic!("expected ConversationHistory event"),
+    };
+    // History layout for this test:
+    // [0] user instructions,
+    // [1] environment context,
+    // [2] "first" user message,
+    // [3] "second" user message,
+    // [4] "third" user message.
+
+    // Fork 1: drops the last user message and everything after.
+    let expected_after_first = vec![
+        entries_after_three[0].clone(),
+        entries_after_three[1].clone(),
+        entries_after_three[2].clone(),
+        entries_after_three[3].clone(),
+    ];
+
+    // Fork 2: drops the last user message and everything after.
+    // [0] user instructions,
+    // [1] environment context,
+    // [2] "first" user message,
+    let expected_after_second = vec![
+        entries_after_three[0].clone(),
+        entries_after_three[1].clone(),
+        entries_after_three[2].clone(),
+    ];
+
+    // Fork once with n=1 → drops the last user message and everything after.
+    let NewConversation {
+        conversation: codex_fork1,
+        ..
+    } = conversation_manager
+        .fork_conversation(entries_after_three.clone(), 1, config_for_fork.clone())
+        .await
+        .expect("fork 1");
+
+    codex_fork1.submit(Op::GetHistory).await.unwrap();
+    let fork1_history = wait_for_event(&codex_fork1, |ev| {
+        matches!(ev, EventMsg::ConversationHistory(_))
+    })
+    .await;
+    let entries_after_first_fork = match &fork1_history {
+        EventMsg::ConversationHistory(ConversationHistoryResponseEvent { entries, .. }) => {
+            assert!(matches!(
+                fork1_history,
+                EventMsg::ConversationHistory(ConversationHistoryResponseEvent { ref entries, .. }) if *entries == expected_after_first
+            ));
+            entries.clone()
+        }
+        _ => panic!("expected ConversationHistory event after first fork"),
+    };
+
+    // Fork again with n=1 → drops the (new) last user message, leaving only the first.
+    let NewConversation {
+        conversation: codex_fork2,
+        ..
+    } = conversation_manager
+        .fork_conversation(entries_after_first_fork.clone(), 1, config_for_fork.clone())
+        .await
+        .expect("fork 2");
+
+    codex_fork2.submit(Op::GetHistory).await.unwrap();
+    let fork2_history = wait_for_event(&codex_fork2, |ev| {
+        matches!(ev, EventMsg::ConversationHistory(_))
+    })
+    .await;
+    assert!(matches!(
+        fork2_history,
+        EventMsg::ConversationHistory(ConversationHistoryResponseEvent { ref entries, .. }) if *entries == expected_after_second
+    ));
+}

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

@@ -5,6 +5,7 @@ mod client;
 mod compact;
 mod exec;
 mod exec_stream_events;
+mod fork_conversation;
 mod live_cli;
 mod prompt_caching;
 mod seatbelt;