diff --git a/codex-rs/core/src/session/session.rs b/codex-rs/core/src/session/session.rs
index 0330f6d0f4..61ae601171 100644
--- a/codex-rs/core/src/session/session.rs
+++ b/codex-rs/core/src/session/session.rs
@@ -1074,12 +1074,16 @@ impl Session {
                     InitialHistory::New | InitialHistory::Forked(_)
                 )
             );
+            let should_run_session_start =
+                !matches!(session_configuration.session_source, SessionSource::SubAgent(_));
 
             // record_initial_history can emit events. We record only after the SessionConfiguredEvent is emitted.
             sess.record_initial_history(initial_history).await;
             {
                 let mut state = sess.state.lock().await;
-                state.set_pending_session_start_source(Some(session_start_source));
+                state.set_pending_session_start_source(
+                    should_run_session_start.then_some(session_start_source),
+                );
                 state.set_pending_subagent_start(should_run_subagent_start);
             }
 
diff --git a/codex-rs/core/tests/suite/subagent_notifications.rs b/codex-rs/core/tests/suite/subagent_notifications.rs
index 633b9f00be..2992031045 100644
--- a/codex-rs/core/tests/suite/subagent_notifications.rs
+++ b/codex-rs/core/tests/suite/subagent_notifications.rs
@@ -114,6 +114,21 @@ fn write_home_skill(codex_home: &Path, dir: &str, name: &str, description: &str)
 }
 
 fn write_subagent_lifecycle_hooks(home: &Path, stop_prompts: &[&str]) -> Result<()> {
+    let session_start_script_path = home.join("session_start_hook.py");
+    let session_start_log_path = home.join("session_start_hook_log.jsonl");
+    let session_start_script = format!(
+        r#"import json
+from pathlib import Path
+import sys
+
+log_path = Path(r"{session_start_log_path}")
+payload = json.load(sys.stdin)
+with log_path.open("a", encoding="utf-8") as handle:
+    handle.write(json.dumps(payload) + "\n")
+"#,
+        session_start_log_path = session_start_log_path.display(),
+    );
+
     let start_script_path = home.join("subagent_start_hook.py");
     let start_log_path = home.join("subagent_start_hook_log.jsonl");
     let start_script = format!(
@@ -177,6 +192,13 @@ print(json.dumps({{"systemMessage": "root stop complete"}}))
 
     let hooks = serde_json::json!({
         "hooks": {
+            "SessionStart": [{
+                "matcher": "startup",
+                "hooks": [{
+                    "type": "command",
+                    "command": format!("python3 {}", session_start_script_path.display()),
+                }]
+            }],
             "SubagentStart": [{
                 "matcher": "worker",
                 "hooks": [{
@@ -200,6 +222,7 @@ print(json.dumps({{"systemMessage": "root stop complete"}}))
         }
     });
 
+    fs::write(&session_start_script_path, session_start_script)?;
     fs::write(&start_script_path, start_script)?;
     fs::write(&subagent_stop_script_path, subagent_stop_script)?;
     fs::write(&stop_script_path, stop_script)?;
@@ -485,6 +508,15 @@ async fn subagent_start_injects_context_once_for_child() -> Result<()> {
         Some(spawned_id.as_str())
     );
 
+    let session_start_inputs =
+        read_hook_log(test.codex_home_path(), "session_start_hook_log.jsonl")?;
+    assert_eq!(session_start_inputs.len(), 1);
+    assert_eq!(session_start_inputs[0]["source"].as_str(), Some("startup"));
+    assert_ne!(
+        session_start_inputs[0]["session_id"].as_str(),
+        Some(spawned_id.as_str())
+    );
+
     Ok(())
 }