diff --git a/codex-rs/core/src/client.rs b/codex-rs/core/src/client.rs
index 620645d8f6f..78dfee0b3c8 100644
--- a/codex-rs/core/src/client.rs
+++ b/codex-rs/core/src/client.rs
@@ -507,9 +507,6 @@ impl ModelClient {
         };
 
         let mut extra_headers = ApiHeaderMap::new();
-        if let Ok(header_value) = HeaderValue::from_str(&self.state.installation_id) {
-            extra_headers.insert(X_CODEX_INSTALLATION_ID_HEADER, header_value);
-        }
         extra_headers.extend(build_responses_headers(
             self.state.beta_features_header.as_deref(),
             /*turn_state*/ None,
@@ -637,6 +634,11 @@ impl ModelClient {
 
     fn build_responses_identity_headers(&self) -> ApiHeaderMap {
         let mut extra_headers = self.build_subagent_headers();
+        if !self.state.installation_id.is_empty()
+            && let Ok(val) = HeaderValue::from_str(&self.state.installation_id)
+        {
+            extra_headers.insert(X_CODEX_INSTALLATION_ID_HEADER, val);
+        }
         if let Some(parent_thread_id) = parent_thread_id_header_value(&self.state.session_source)
             && let Ok(val) = HeaderValue::from_str(&parent_thread_id)
         {
diff --git a/codex-rs/core/src/client_tests.rs b/codex-rs/core/src/client_tests.rs
index b9d9172c839..a57d3667bfe 100644
--- a/codex-rs/core/src/client_tests.rs
+++ b/codex-rs/core/src/client_tests.rs
@@ -61,13 +61,20 @@ use tracing_subscriber::registry::LookupSpan;
 use tracing_subscriber::util::SubscriberInitExt;
 
 fn test_model_client(session_source: SessionSource) -> ModelClient {
+    test_model_client_with_installation_id(session_source, "11111111-1111-4111-8111-111111111111")
+}
+
+fn test_model_client_with_installation_id(
+    session_source: SessionSource,
+    installation_id: &str,
+) -> ModelClient {
     let provider = create_oss_provider_with_base_url("https://example.com/v1", WireApi::Responses);
     let thread_id = ThreadId::new();
     ModelClient::new(
         /*auth_manager*/ None,
         thread_id.into(),
         thread_id,
-        /*installation_id*/ "11111111-1111-4111-8111-111111111111".to_string(),
+        /*installation_id*/ installation_id.to_string(),
         provider,
         session_source,
         /*model_verbosity*/ None,
@@ -269,6 +276,23 @@ fn build_subagent_headers_sets_internal_memory_consolidation_label() {
     assert_eq!(value, Some("memory_consolidation"));
 }
 
+#[test]
+fn build_responses_identity_headers_includes_installation_id_when_present() {
+    let client = test_model_client(SessionSource::Exec);
+    let headers = client.build_responses_identity_headers();
+    let value = headers
+        .get(X_CODEX_INSTALLATION_ID_HEADER)
+        .and_then(|value| value.to_str().ok());
+    assert_eq!(value, Some("11111111-1111-4111-8111-111111111111"));
+}
+
+#[test]
+fn build_responses_identity_headers_omits_installation_id_when_empty() {
+    let client = test_model_client_with_installation_id(SessionSource::Exec, "");
+    let headers = client.build_responses_identity_headers();
+    assert_eq!(headers.get(X_CODEX_INSTALLATION_ID_HEADER), None);
+}
+
 #[test]
 fn build_ws_client_metadata_includes_window_lineage_and_turn_metadata() {
     let parent_thread_id = ThreadId::new();
@@ -549,6 +573,17 @@ async fn websocket_handshake_includes_attestation_for_chatgpt_codex_responses()
     assert_eq!(attestation_calls.load(Ordering::Relaxed), 1);
 }
 
+#[tokio::test]
+async fn websocket_handshake_omits_installation_id_when_empty() {
+    let model_client = test_model_client_with_installation_id(SessionSource::Exec, "");
+
+    let headers = model_client
+        .build_websocket_headers(/*turn_state*/ None, /*turn_metadata_header*/ None)
+        .await;
+
+    assert_eq!(headers.get(X_CODEX_INSTALLATION_ID_HEADER), None);
+}
+
 #[tokio::test]
 async fn non_chatgpt_codex_endpoints_omit_attestation_generation() {
     let (model_client, attestation_calls) =
diff --git a/codex-rs/core/tests/responses_headers.rs b/codex-rs/core/tests/responses_headers.rs
index 6f0429e6449..465103b9cb4 100644
--- a/codex-rs/core/tests/responses_headers.rs
+++ b/codex-rs/core/tests/responses_headers.rs
@@ -152,6 +152,10 @@ async fn responses_stream_includes_subagent_header_on_review() {
         request.header("x-codex-window-id").as_deref(),
         Some(expected_window_id.as_str())
     );
+    assert_eq!(
+        request.header("x-codex-installation-id").as_deref(),
+        Some(TEST_INSTALLATION_ID)
+    );
     assert_eq!(request.header("x-codex-parent-thread-id"), None);
     assert_eq!(
         request.body_json()["client_metadata"]["x-codex-installation-id"].as_str(),
diff --git a/codex-rs/core/tests/suite/client_websockets.rs b/codex-rs/core/tests/suite/client_websockets.rs
index 09d0093f990..031261b8515 100755
--- a/codex-rs/core/tests/suite/client_websockets.rs
+++ b/codex-rs/core/tests/suite/client_websockets.rs
@@ -148,6 +148,10 @@ async fn responses_websocket_streams_request() {
         handshake.header(USER_AGENT_HEADER),
         Some(codex_login::default_client::get_codex_user_agent())
     );
+    assert_eq!(
+        handshake.header("x-codex-installation-id"),
+        Some(TEST_INSTALLATION_ID.to_string())
+    );
     assert_eq!(
         body["client_metadata"]["x-codex-installation-id"].as_str(),
         Some(TEST_INSTALLATION_ID)
diff --git a/codex-rs/core/tests/suite/compact_remote.rs b/codex-rs/core/tests/suite/compact_remote.rs
index 1c5b80f35f9..99f13313049 100644
--- a/codex-rs/core/tests/suite/compact_remote.rs
+++ b/codex-rs/core/tests/suite/compact_remote.rs
@@ -368,6 +368,13 @@ async fn remote_compact_replaces_history_for_followups() -> Result<()> {
         compact_request.header("thread-id").as_deref(),
         Some(thread_id.as_str())
     );
+    assert!(
+        compact_request
+            .header("x-codex-installation-id")
+            .as_deref()
+            .is_some_and(|value| !value.is_empty()),
+        "compact request should include a non-empty installation id header"
+    );
     let compact_metadata: Value = serde_json::from_str(
         &compact_request
             .header("x-codex-turn-metadata")