diff --git a/src/ml4t/data/providers/fxmacrodata.py b/src/ml4t/data/providers/fxmacrodata.py
index 918948a..e5460eb 100644
--- a/src/ml4t/data/providers/fxmacrodata.py
+++ b/src/ml4t/data/providers/fxmacrodata.py
@@ -593,14 +593,17 @@ def fetch_endpoint(
 
     def _request_json(self, path: str, params: dict[str, Any]) -> Any:
         clean_params = {key: value for key, value in params.items() if value is not None}
-        if self.api_key:
-            clean_params["api_key"] = self.api_key
+        # Send the key as a header rather than a query parameter. FXMacroData
+        # accepts both, but documents the header as the preferred transport for
+        # server-side clients: a key in the query string is recorded verbatim by
+        # proxies, CDNs and server access logs along the whole request path.
+        headers = {"X-API-Key": self.api_key} if self.api_key else None
 
         url = f"{self.base_url}/{path.lstrip('/')}"
         self._acquire_rate_limit()
 
         try:
-            response = self.session.get(url, params=clean_params)
+            response = self.session.get(url, params=clean_params, headers=headers)
         except httpx.RequestError as err:
             raise NetworkError(provider=self.name, message=f"Request failed: {err}") from err
 
diff --git a/tests/test_fxmacrodata_provider.py b/tests/test_fxmacrodata_provider.py
index 3666377..cd0a619 100644
--- a/tests/test_fxmacrodata_provider.py
+++ b/tests/test_fxmacrodata_provider.py
@@ -117,12 +117,39 @@ def test_fetch_announcements_preserves_key_fields_and_metadata(self, provider):
 
         url = get.call_args.args[0]
         params = get.call_args.kwargs["params"]
+        headers = get.call_args.kwargs["headers"]
         assert url == "https://api.fxmacrodata.com/v1/announcements/usd/inflation"
-        assert params["api_key"] == "test_key"
+        assert headers["X-API-Key"] == "test_key"
+        assert "api_key" not in params
         assert params["start_date"] == "2025-01-01"
         assert params["limit"] == 1
         assert "end_date" not in params
 
+    def test_api_key_is_never_sent_as_a_query_parameter(self, provider):
+        response = _response(payload=ANNOUNCEMENT_PAYLOAD)
+
+        with patch.object(provider.session, "get", return_value=response) as get:
+            provider.fetch_announcements("USD", "inflation", limit=1)
+
+        params = get.call_args.kwargs["params"]
+        url = get.call_args.args[0]
+        assert "test_key" not in url
+        assert not any(value == "test_key" for value in params.values())
+
+    def test_no_auth_header_is_sent_without_a_key(self):
+        # clear=True matches the init tests: without it an ambient
+        # FXMACRODATA_API_KEY/FXMD_API_KEY in the developer's environment would
+        # be picked up and the assertion would pass for the wrong reason.
+        with patch.dict("os.environ", {}, clear=True):
+            provider = FXMacroDataProvider(rate_limit=(1000, 1.0))
+        try:
+            response = _response(payload=ANNOUNCEMENT_PAYLOAD)
+            with patch.object(provider.session, "get", return_value=response) as get:
+                provider.fetch_announcements("USD", "inflation", limit=1)
+            assert get.call_args.kwargs["headers"] is None
+        finally:
+            provider.close()
+
     def test_fetch_catalogue_reshapes_indicator_mapping(self, provider):
         payload = {
             "gdp": {