Reduce startup latency for ancestor discovery (#32106)

## Why

Serial metadata probes add remote round trips while Codex discovers project
instructions, repository skills, and project-root markers during startup.

## What changed

- Probe ancestor directories concurrently while preserving discovery order.
- Raise the shared find-up probe window to cover typical project hierarchies.
- Refresh project instructions in parallel with plugin and skill warmup.

## Testing

Add coverage that blocks one metadata request and verifies all expected project-root
and `AGENTS.md` ancestor probes have started before it is released.

GitOrigin-RevId: 11449837c7a212ec8ed385e7290476e926d93531

codex-rs/core-skills/src/loader.rs

@@ -146,6 +146,9 @@ const MAX_DEPENDENCY_URL_LEN: usize = MAX_DESCRIPTION_LEN;
 // Traversal depth from the skills root.
 const MAX_SCAN_DEPTH: usize = 6;
 const MAX_SKILLS_DIRS_PER_ROOT: usize = 2000;
+// Keep ancestor metadata probes within one remote round trip for typical project hierarchies while
+// leaving room for other startup discovery on the shared exec-server transport.
+const MAX_CONCURRENT_ANCESTOR_PROBES: usize = 256;
 
 struct ResolvedDiscoveredSkill {
     skill: DiscoveredSkill,
@@ -387,10 +390,19 @@ async fn repo_agents_skill_roots(
     let project_root = find_project_root(fs.as_ref(), cwd, &project_root_markers).await;
     let dirs = dirs_between_project_root_and_cwd(cwd, &project_root);
     let mut roots = Vec::new();
-    for dir in dirs {
-        let agents_skills = dir.join(AGENTS_DIR_NAME).join(SKILLS_DIR_NAME);
-        let agents_skills_uri = PathUri::from_abs_path(&agents_skills);
-        match fs.get_metadata(&agents_skills_uri, /*sandbox*/ None).await {
+    let mut results = futures::stream::iter(dirs)
+        .map(|dir| {
+            let fs = Arc::clone(&fs);
+            async move {
+                let agents_skills = dir.join(AGENTS_DIR_NAME).join(SKILLS_DIR_NAME);
+                let agents_skills_uri = PathUri::from_abs_path(&agents_skills);
+                let result = fs.get_metadata(&agents_skills_uri, /*sandbox*/ None).await;
+                (agents_skills, result)
+            }
+        })
+        .buffered(MAX_CONCURRENT_ANCESTOR_PROBES);
+    while let Some((agents_skills, result)) = results.next().await {
+        match result {
             Ok(metadata) if metadata.is_directory => roots.push(SkillRoot {
                 path: agents_skills,
                 scope: SkillScope::Repo,
@@ -443,19 +455,29 @@ async fn find_project_root(
         return cwd.clone();
     }
 
+    let mut probes = Vec::new();
     for ancestor in cwd.ancestors() {
         for marker in project_root_markers {
             let marker_path = ancestor.join(marker);
+            probes.push((ancestor.clone(), marker_path));
+        }
+    }
+    let mut results = futures::stream::iter(probes)
+        .map(|(ancestor, marker_path)| async move {
             let marker_path_uri = PathUri::from_abs_path(&marker_path);
-            match fs.get_metadata(&marker_path_uri, /*sandbox*/ None).await {
-                Ok(_) => return ancestor,
-                Err(err) if err.kind() == io::ErrorKind::NotFound => {}
-                Err(err) => {
-                    tracing::warn!(
-                        "failed to stat project root marker {}: {err:#}",
-                        marker_path.display()
-                    );
-                }
+            let result = fs.get_metadata(&marker_path_uri, /*sandbox*/ None).await;
+            (ancestor, marker_path, result)
+        })
+        .buffered(MAX_CONCURRENT_ANCESTOR_PROBES);
+    while let Some((ancestor, marker_path, result)) = results.next().await {
+        match result {
+            Ok(_) => return ancestor,
+            Err(err) if err.kind() == io::ErrorKind::NotFound => {}
+            Err(err) => {
+                tracing::warn!(
+                    "failed to stat project root marker {}: {err:#}",
+                    marker_path.display()
+                );
             }
         }
     }

codex-rs/core/src/agents_md.rs

@@ -29,6 +29,7 @@ use codex_file_system::FindUpErrorPolicy;
 use codex_file_system::find_nearest_ancestor_with_markers;
 use codex_utils_absolute_path::AbsolutePathBuf;
 use codex_utils_path_uri::PathUri;
+use futures::StreamExt;
 use std::io;
 use toml::Value as TomlValue;
 use tracing::error;
@@ -42,6 +43,11 @@ pub const LOCAL_AGENTS_MD_FILENAME: &str = "AGENTS.override.md";
 /// concatenated with the following separator.
 const AGENTS_MD_SEPARATOR: &str = "\n\n--- project-doc ---\n\n";
 
+// Metadata probes are cheap and the exec-server transport already bounds total in-flight calls.
+// This covers typical project hierarchies in one remote round trip without monopolizing that
+// transport when independent startup discovery runs concurrently.
+const MAX_CONCURRENT_ANCESTOR_PROBES: usize = 256;
+
 /// Loads project AGENTS.md content and combines it with host-provided user
 /// instructions.
 pub(crate) async fn load_project_instructions(
@@ -198,22 +204,28 @@ async fn agents_md_paths(
         vec![dir]
     };
 
-    let mut found = Vec::new();
     let candidate_filenames = candidate_filenames(config);
-    for directory in search_dirs {
-        for name in &candidate_filenames {
-            let candidate = directory
-                .join(name)
-                .map_err(|err| io::Error::new(io::ErrorKind::InvalidInput, err))?;
-            match fs.get_metadata(&candidate, /*sandbox*/ None).await {
-                Ok(metadata) if metadata.is_file => {
-                    found.push(candidate);
-                    break;
+    let candidate_filenames = &candidate_filenames;
+    let mut results = futures::stream::iter(search_dirs)
+        .map(|directory| async move {
+            for name in candidate_filenames {
+                let candidate = directory
+                    .join(name)
+                    .map_err(|err| io::Error::new(io::ErrorKind::InvalidInput, err))?;
+                match fs.get_metadata(&candidate, /*sandbox*/ None).await {
+                    Ok(metadata) if metadata.is_file => return Ok(Some(candidate)),
+                    Ok(_) => {}
+                    Err(err) if err.kind() == io::ErrorKind::NotFound => {}
+                    Err(err) => return Err(err),
                 }
-                Ok(_) => {}
-                Err(err) if err.kind() == io::ErrorKind::NotFound => {}
-                Err(err) => return Err(err),
             }
+            Ok(None)
+        })
+        .buffered(MAX_CONCURRENT_ANCESTOR_PROBES);
+    let mut found = Vec::new();
+    while let Some(result) = results.next().await {
+        if let Some(candidate) = result? {
+            found.push(candidate);
         }
     }
     Ok(found)

codex-rs/core/src/agents_md_tests.rs

@@ -719,9 +719,8 @@ async fn marker_search_does_not_wait_for_a_higher_ancestor() {
 }
 
 #[tokio::test]
-async fn project_root_marker_search_pipelines_bounded_window_and_continues() {
+async fn project_root_marker_search_starts_all_ancestor_probes() {
     const NESTING_DEPTH: usize = 9;
-    const CONCURRENCY_LIMIT: usize = 8;
 
     let tmp = tempfile::tempdir().expect("tempdir");
     fs::write(tmp.path().join(".git"), "").unwrap();
@@ -734,6 +733,7 @@ async fn project_root_marker_search_pipelines_bounded_window_and_continues() {
 
     let mut config = make_config(&tmp, /*limit*/ 4096, /*instructions*/ None).await;
     config.cwd = nested.abs();
+    let expected_probe_count = config.cwd.ancestors().count();
     let cwd = PathUri::from_abs_path(&config.cwd);
     let metadata_calls = Arc::new(MetadataCallCounts::default());
     let fs = FailingFileSystem {
@@ -752,7 +752,7 @@ async fn project_root_marker_search_pipelines_bounded_window_and_continues() {
                 .lock()
                 .expect("metadata paths lock")
                 .len()
-                >= CONCURRENCY_LIMIT
+                >= expected_probe_count
             {
                 break;
             }
@@ -767,7 +767,7 @@ async fn project_root_marker_search_pipelines_bounded_window_and_continues() {
             .lock()
             .expect("metadata paths lock")
             .len(),
-        CONCURRENCY_LIMIT
+        expected_probe_count
     );
 
     metadata_calls.release.notify_one();
@@ -785,6 +785,90 @@ async fn project_root_marker_search_pipelines_bounded_window_and_continues() {
     );
 }
 
+#[tokio::test]
+async fn agents_md_search_starts_all_directory_probes() {
+    const NESTING_DEPTH: usize = 9;
+
+    let tmp = tempfile::tempdir().expect("tempdir");
+    fs::write(tmp.path().join(".git"), "").unwrap();
+    fs::write(tmp.path().join("AGENTS.md"), "project doc").unwrap();
+    let mut nested = tmp.path().to_path_buf();
+    for depth in 0..NESTING_DEPTH {
+        nested.push(format!("nested-{depth}"));
+    }
+    fs::create_dir_all(&nested).unwrap();
+
+    let mut config = make_config(&tmp, /*limit*/ 4096, /*instructions*/ None).await;
+    config.cwd = nested.abs();
+    let cwd = PathUri::from_abs_path(&config.cwd);
+    let mut search_dirs = config
+        .cwd
+        .ancestors()
+        .take(NESTING_DEPTH + 1)
+        .collect::<Vec<_>>();
+    search_dirs.reverse();
+    let expected_probes = search_dirs
+        .into_iter()
+        .map(|directory| PathUri::from_abs_path(&directory.join(LOCAL_AGENTS_MD_FILENAME)))
+        .collect::<Vec<_>>();
+    let metadata_calls = Arc::new(MetadataCallCounts::default());
+    let fs = FailingFileSystem {
+        path: tmp.path().join(LOCAL_AGENTS_MD_FILENAME).abs(),
+        failure: InjectedFailure::MetadataBlocked,
+        metadata_calls: Arc::clone(&metadata_calls),
+    };
+
+    let search =
+        tokio::spawn(async move { super::agents_md_paths(&config.config, &cwd, &fs).await });
+    tokio::time::timeout(std::time::Duration::from_secs(5), async {
+        loop {
+            let started = metadata_calls.started.notified();
+            if expected_probes.iter().all(|candidate| {
+                metadata_calls
+                    .paths
+                    .lock()
+                    .expect("metadata paths lock")
+                    .contains(candidate)
+            }) {
+                break;
+            }
+            started.await;
+        }
+    })
+    .await
+    .expect("all directory probes should start");
+
+    let mut actual_probes = metadata_calls
+        .paths
+        .lock()
+        .expect("metadata paths lock")
+        .iter()
+        .filter(|path| expected_probes.contains(path))
+        .map(ToString::to_string)
+        .collect::<Vec<_>>();
+    actual_probes.sort();
+    let mut expected_probes = expected_probes
+        .into_iter()
+        .map(|path| path.to_string())
+        .collect::<Vec<_>>();
+    expected_probes.sort();
+    assert_eq!(actual_probes, expected_probes);
+
+    metadata_calls.release.notify_one();
+    let paths = tokio::time::timeout(std::time::Duration::from_secs(5), search)
+        .await
+        .expect("AGENTS.md search should complete")
+        .expect("AGENTS.md search task")
+        .expect("AGENTS.md discovery");
+
+    assert_eq!(
+        paths,
+        vec![PathUri::from_abs_path(
+            &tmp.path().join(DEFAULT_AGENTS_MD_FILENAME).abs()
+        )]
+    );
+}
+
 #[tokio::test]
 async fn empty_project_root_markers_only_probe_cwd_candidates() {
     let tmp = tempfile::tempdir().expect("tempdir");

codex-rs/core/src/session/session.rs

@@ -907,10 +907,7 @@ impl Session {
             turn_environments.update_selections(session_configuration.environment_selections());
             let resolved_environments = turn_environments.snapshot().await;
             let agents_md_manager = Arc::new(AgentsMdManager::new(user_instructions));
-            agents_md_manager
-                .refresh(config.as_ref(), &resolved_environments)
-                .await;
-            let plugin_skill_errors = warm_plugins_and_skills_for_session_init(
+            let plugin_skill_warmup = warm_plugins_and_skills_for_session_init(
                 Arc::clone(&config),
                 Arc::clone(&plugins_manager),
                 Arc::clone(&skills_service),
@@ -919,8 +916,11 @@ impl Session {
             .instrument(info_span!(
                 "session_init.plugin_skill_warmup",
                 otel.name = "session_init.plugin_skill_warmup",
-            ))
-            .await;
+            ));
+            let ((), plugin_skill_errors) = tokio::join!(
+                agents_md_manager.refresh(config.as_ref(), &resolved_environments),
+                plugin_skill_warmup,
+            );
             for err in &plugin_skill_errors {
                 error!(
                     "failed to load skill {}: {}",

codex-rs/file-system/src/find_up.rs

@@ -6,7 +6,9 @@ use codex_utils_path_uri::PathUri;
 use futures::StreamExt;
 use std::io;
 
-const MAX_CONCURRENT_PROBES: usize = 8;
+// Keep enough ordinary metadata calls in flight to cover typical ancestor chains in one remote
+// round trip, while leaving room for independent startup discovery to run at the same time.
+const MAX_CONCURRENT_PROBES: usize = 256;
 
 /// Controls how an upward marker search handles metadata errors other than `NotFound`.
 #[derive(Clone, Copy, Debug, Eq, PartialEq)]