diff --git a/packages/optimization/src/ldai_optimizer/ld_api_client.py b/packages/optimization/src/ldai_optimizer/ld_api_client.py
index 6dbd580a..447bb32a 100644
--- a/packages/optimization/src/ldai_optimizer/ld_api_client.py
+++ b/packages/optimization/src/ldai_optimizer/ld_api_client.py
@@ -198,7 +198,10 @@ def __repr__(self) -> str:
         return f"LDApiClient(base_url={self._base_url!r})"
 
     def _auth_headers(self) -> Dict[str, str]:
-        return {"Authorization": self._api_key}
+        return {
+            "Authorization": self._api_key,
+            "LD-API-Version": "20240415",
+        }
 
     def _request(
         self,
diff --git a/packages/optimization/tests/test_ld_api_client.py b/packages/optimization/tests/test_ld_api_client.py
index e2c11cb8..88c6104d 100644
--- a/packages/optimization/tests/test_ld_api_client.py
+++ b/packages/optimization/tests/test_ld_api_client.py
@@ -168,6 +168,14 @@ def test_authorization_header_always_sent(self):
             req: urllib.request.Request = mock_open.call_args[0][0]
             assert req.get_header("Authorization") == "my-api-key"
 
+    def test_default_api_version_header_is_v4(self):
+        client = LDApiClient("test-key")
+        with patch("urllib.request.urlopen", return_value=_mock_urlopen({})) as mock_open:
+            client._request("GET", "/path")
+            req: urllib.request.Request = mock_open.call_args[0][0]
+            # urllib capitalizes header names; the wire value is still LD-API-Version.
+            assert req.get_header("Ld-api-version") == "20240415"
+
     def test_raises_ld_api_error_on_http_error(self):
         client = LDApiClient("test-key")
         http_error = urllib.error.HTTPError(