diff --git a/infrastructure/abri/abri_client.py b/infrastructure/abri/abri_client.py
index 7c9c018a9..ebcd65f64 100644
--- a/infrastructure/abri/abri_client.py
+++ b/infrastructure/abri/abri_client.py
@@ -142,7 +142,11 @@ class AbriClient:
 
     def _post(self, envelope: bytes) -> bytes:
         try:
-            response = self._session.post(self._config.endpoint_url, data=envelope)
+            response = self._session.post(
+                self._config.endpoint_url,
+                data=envelope,
+                headers={"Content-Type": "application/xml"},
+            )
             response.raise_for_status()
         except requests.RequestException as error:
             raise AbriTransportError(str(error)) from error
diff --git a/tests/infrastructure/abri/test_abri_client.py b/tests/infrastructure/abri/test_abri_client.py
index fff0a3865..13bac577b 100644
--- a/tests/infrastructure/abri/test_abri_client.py
+++ b/tests/infrastructure/abri/test_abri_client.py
@@ -129,6 +129,22 @@ def test_log_job_sends_the_recorded_logjob_envelope_to_the_relay_endpoint(
     )
 
 
+def test_log_job_declares_the_xml_body_content_type_the_relay_requires(
+    client: AbriClient, mock_session: MagicMock
+) -> None:
+    # Arrange: the relay 400s an XML body posted without an XML content type.
+    mock_session.post.return_value.content = _load_fixture(
+        "abri_relay_logjob_success_response.xml"
+    )
+
+    # Act
+    client.log_job(_spec_example_request())
+
+    # Assert
+    headers: dict[str, str] = mock_session.post.call_args.kwargs["headers"]
+    assert headers["Content-Type"] == "application/xml"
+
+
 # --- abandon_job: rejection (canceljob's own error-details shape) ---