fix(messages): state no headers for a native failure without wire headers

A failure the native path raises without an HTTP error response, such as an unreadable 200 body or a header value the client refuses to send, carries no wire headers, and Python logs none for it. The rebuilt response added a content-length from the error text

litellm/rust_bridge/host/routes/messages.py

@@ -186,6 +186,8 @@ def _upstream_failure(
         headers: Final = _UPSTREAM_HEADERS.validate_python(getattr(error, "headers", None))
     except ValidationError:
         return None
+    if not headers:
+        return status, text, headers
     # Python's `MaskedHTTPStatusError` rebuilds the response from its decoded body without the
     # wire's content-encoding and content-length, so httpx states a length only for a non-empty body.
     rebuilt: Final = httpx.Response(

tests/unit/rust_bridge/host/routes/test_messages.py

@@ -243,6 +243,19 @@ def test_upstream_failure_headers_match_the_python_clients_rebuilt_response(
     assert dict(_get_response_headers(mapped) or {}) == expected
 
 
+@pytest.mark.parametrize("status", [200, 500])
+def test_upstream_failure_without_wire_headers_states_no_headers(status: int) -> None:
+    from litellm.litellm_core_utils.exception_mapping_utils import _get_response_headers
+
+    upstream: Final = ValueError(status, "not a messages response")
+    upstream.headers = []  # pyright: ignore[reportAttributeAccessIssue]  # set by the native host
+
+    mapped: Final = messages_host.map_failure(upstream, {"model": _ADAPTIVE_OPUS}, "anthropic")
+
+    assert isinstance(mapped, Exception)
+    assert not dict(_get_response_headers(mapped) or {})
+
+
 def test_a_late_rejection_tells_the_logger_the_resolved_call_first() -> None:
     import datetime