diff --git a/codex-rs/codex-api/src/requests/chat.rs b/codex-rs/codex-api/src/requests/chat.rs
index 60f450ca0..30d8b478d 100644
--- a/codex-rs/codex-api/src/requests/chat.rs
+++ b/codex-rs/codex-api/src/requests/chat.rs
@@ -393,10 +393,6 @@ mod tests {
             .build(&provider())
             .expect("request");
 
-        assert_eq!(
-            req.headers.get("conversation_id"),
-            Some(&HeaderValue::from_static("conv-1"))
-        );
         assert_eq!(
             req.headers.get("session_id"),
             Some(&HeaderValue::from_static("conv-1"))
diff --git a/codex-rs/codex-api/src/requests/headers.rs b/codex-rs/codex-api/src/requests/headers.rs
index 4d8a17d18..bdc7bba4f 100644
--- a/codex-rs/codex-api/src/requests/headers.rs
+++ b/codex-rs/codex-api/src/requests/headers.rs
@@ -5,7 +5,6 @@ use http::HeaderValue;
 pub(crate) fn build_conversation_headers(conversation_id: Option<String>) -> HeaderMap {
     let mut headers = HeaderMap::new();
     if let Some(id) = conversation_id {
-        insert_header(&mut headers, "conversation_id", &id);
         insert_header(&mut headers, "session_id", &id);
     }
     headers
diff --git a/codex-rs/codex-api/src/requests/responses.rs b/codex-rs/codex-api/src/requests/responses.rs
index a18a147ab..65f7dc024 100644
--- a/codex-rs/codex-api/src/requests/responses.rs
+++ b/codex-rs/codex-api/src/requests/responses.rs
@@ -249,10 +249,6 @@ mod tests {
             .collect();
         assert_eq!(ids, vec![Some("m1".to_string()), None]);
 
-        assert_eq!(
-            request.headers.get("conversation_id"),
-            Some(&HeaderValue::from_static("conv-1"))
-        );
         assert_eq!(
             request.headers.get("session_id"),
             Some(&HeaderValue::from_static("conv-1"))
diff --git a/codex-rs/core/tests/suite/client.rs b/codex-rs/core/tests/suite/client.rs
index 06846c46a..13cb2fcd3 100644
--- a/codex-rs/core/tests/suite/client.rs
+++ b/codex-rs/core/tests/suite/client.rs
@@ -344,7 +344,7 @@ async fn includes_conversation_id_and_model_headers_in_request() {
     );
     let NewThread {
         thread: codex,
-        thread_id: conversation_id,
+        thread_id: session_id,
         session_configured: _,
         ..
     } = thread_manager
@@ -366,15 +366,13 @@ async fn includes_conversation_id_and_model_headers_in_request() {
 
     let request = resp_mock.single_request();
     assert_eq!(request.path(), "/v1/responses");
-    let request_conversation_id = request
-        .header("conversation_id")
-        .expect("conversation_id header");
+    let request_session_id = request.header("session_id").expect("session_id header");
     let request_authorization = request
         .header("authorization")
         .expect("authorization header");
     let request_originator = request.header("originator").expect("originator header");
 
-    assert_eq!(request_conversation_id, conversation_id.to_string());
+    assert_eq!(request_session_id, session_id.to_string());
     assert_eq!(request_originator, "codex_cli_rs");
     assert_eq!(request_authorization, "Bearer Test API Key");
 }
@@ -455,7 +453,7 @@ async fn chatgpt_auth_sends_correct_request() {
     );
     let NewThread {
         thread: codex,
-        thread_id: conversation_id,
+        thread_id,
         session_configured: _,
         ..
     } = thread_manager
@@ -477,9 +475,6 @@ async fn chatgpt_auth_sends_correct_request() {
 
     let request = resp_mock.single_request();
     assert_eq!(request.path(), "/api/codex/responses");
-    let request_conversation_id = request
-        .header("conversation_id")
-        .expect("conversation_id header");
     let request_authorization = request
         .header("authorization")
         .expect("authorization header");
@@ -489,7 +484,9 @@ async fn chatgpt_auth_sends_correct_request() {
         .expect("chatgpt-account-id header");
     let request_body = request.body_json();
 
-    assert_eq!(request_conversation_id, conversation_id.to_string());
+    let session_id = request.header("session_id").expect("session_id header");
+    assert_eq!(session_id, thread_id.to_string());
+
     assert_eq!(request_originator, "codex_cli_rs");
     assert_eq!(request_authorization, "Bearer Access Token");
     assert_eq!(request_chatgpt_account_id, "account_id");