diff --git a/codex-rs/core/src/client.rs b/codex-rs/core/src/client.rs
index dd7b9f400..02d231c96 100644
--- a/codex-rs/core/src/client.rs
+++ b/codex-rs/core/src/client.rs
@@ -802,10 +802,16 @@ impl ModelClient {
             service_tier,
             prompt_cache_key,
             text,
-            client_metadata: Some(HashMap::from([(
-                X_CODEX_INSTALLATION_ID_HEADER.to_string(),
-                self.state.installation_id.clone(),
-            )])),
+            client_metadata: Some(HashMap::from([
+                (
+                    X_CODEX_INSTALLATION_ID_HEADER.to_string(),
+                    self.state.installation_id.clone(),
+                ),
+                (
+                    X_CODEX_WINDOW_ID_HEADER.to_string(),
+                    self.current_window_id(),
+                ),
+            ])),
         };
         Ok(request)
     }
diff --git a/codex-rs/core/tests/responses_headers.rs b/codex-rs/core/tests/responses_headers.rs
index 75649fa4b..1ba93aec1 100644
--- a/codex-rs/core/tests/responses_headers.rs
+++ b/codex-rs/core/tests/responses_headers.rs
@@ -158,6 +158,10 @@ async fn responses_stream_includes_subagent_header_on_review() {
         request.body_json()["client_metadata"]["x-codex-installation-id"].as_str(),
         Some(TEST_INSTALLATION_ID)
     );
+    assert_eq!(
+        request.body_json()["client_metadata"]["x-codex-window-id"].as_str(),
+        Some(expected_window_id.as_str())
+    );
     assert_eq!(request.header("x-codex-sandbox"), None);
 }
 
diff --git a/codex-rs/core/tests/suite/compact_remote.rs b/codex-rs/core/tests/suite/compact_remote.rs
index 2200c7b77..12fd570c5 100644
--- a/codex-rs/core/tests/suite/compact_remote.rs
+++ b/codex-rs/core/tests/suite/compact_remote.rs
@@ -883,6 +883,10 @@ async fn remote_compact_v2_reuses_compaction_trigger_for_followups() -> Result<(
         compact_metadata["window_id"].as_str(),
         compact_request.header("x-codex-window-id").as_deref()
     );
+    assert_eq!(
+        compact_request.body_json()["client_metadata"]["x-codex-window-id"].as_str(),
+        compact_metadata["window_id"].as_str()
+    );
     assert_eq!(
         compact_metadata["compaction"],
         json!({
diff --git a/codex-rs/core/tests/suite/snapshots/all__suite__compact_remote__remote_manual_compact_api_auth_prompt_cache_key_request_diff.snap b/codex-rs/core/tests/suite/snapshots/all__suite__compact_remote__remote_manual_compact_api_auth_prompt_cache_key_request_diff.snap
index 750250f86..89796cb91 100644
--- a/codex-rs/core/tests/suite/snapshots/all__suite__compact_remote__remote_manual_compact_api_auth_prompt_cache_key_request_diff.snap
+++ b/codex-rs/core/tests/suite/snapshots/all__suite__compact_remote__remote_manual_compact_api_auth_prompt_cache_key_request_diff.snap
@@ -7,7 +7,8 @@ Scenario: After five varied API-key-auth turns, remote manual compaction omits s
 --- Last Normal /responses Request
 +++ Remote /responses/compact Request
 -  "client_metadata": {
--    "x-codex-installation-id": "<UUID>"
+-    "x-codex-installation-id": "<UUID>",
+-    "x-codex-window-id": "<UUID>:0"
 -  },
 -  "include": [
 -    "reasoning.encrypted_content"
diff --git a/codex-rs/core/tests/suite/snapshots/all__suite__compact_remote__remote_manual_compact_chatgpt_auth_service_tier_prompt_cache_key_request_diff.snap b/codex-rs/core/tests/suite/snapshots/all__suite__compact_remote__remote_manual_compact_chatgpt_auth_service_tier_prompt_cache_key_request_diff.snap
index e28ddd30d..ab6c9d1b3 100644
--- a/codex-rs/core/tests/suite/snapshots/all__suite__compact_remote__remote_manual_compact_chatgpt_auth_service_tier_prompt_cache_key_request_diff.snap
+++ b/codex-rs/core/tests/suite/snapshots/all__suite__compact_remote__remote_manual_compact_chatgpt_auth_service_tier_prompt_cache_key_request_diff.snap
@@ -7,7 +7,8 @@ Scenario: After five varied ChatGPT-auth turns, remote manual compaction reuses
 --- Last Normal /responses Request
 +++ Remote /responses/compact Request
 -  "client_metadata": {
--    "x-codex-installation-id": "<UUID>"
+-    "x-codex-installation-id": "<UUID>",
+-    "x-codex-window-id": "<UUID>:0"
 -  },
 -  "include": [
 -    "reasoning.encrypted_content"