fix(proxy): drop non-ASCII forwarded header values before calling the LLM provider

litellm/proxy/litellm_pre_call_utils.py

@@ -703,6 +703,29 @@ def clean_headers(
     return clean_headers
 
 
+def _is_transmittable_header(name: object, value: object) -> bool:
+    """
+    Whether a header name/value pair can be forwarded over HTTP.
+
+    httpx (and Python's http.client) encode header values with a single-byte
+    codec, so a non-ASCII value such as CJK text raises UnicodeEncodeError deep
+    in the request stack and surfaces to the client as a 500. See
+    https://github.com/BerriAI/litellm/issues/34633
+    """
+    if not isinstance(name, str):
+        return False
+    if isinstance(value, bytes):
+        return True
+    if not isinstance(value, str):
+        return False
+    try:
+        name.encode("ascii")
+        value.encode("ascii")
+    except UnicodeEncodeError:
+        return False
+    return True
+
+
 class LiteLLMProxyRequestSetup:
     @staticmethod
     def _get_timeout_from_request(headers: dict) -> Optional[float]:
@@ -875,7 +898,18 @@ class LiteLLMProxyRequestSetup:
                 else:
                     returned_headers["x-litellm-{}".format(k)] = str(v)
 
-        return returned_headers
+        transmittable_headers = {
+            name: value for name, value in returned_headers.items() if _is_transmittable_header(name, value)
+        }
+        dropped_headers = returned_headers.keys() - transmittable_headers.keys()
+        if dropped_headers:
+            verbose_logger.warning(
+                "Skipping non-ASCII request header(s) %s when forwarding to the LLM provider; "
+                "HTTP header values must be ASCII-encodable (see issue #34633)",
+                sorted(dropped_headers),
+            )
+
+        return transmittable_headers
 
     @staticmethod
     def add_headers_to_llm_call_by_model_group(data: dict, headers: dict, user_api_key_dict: UserAPIKeyAuth) -> dict:

tests/test_litellm/proxy/test_litellm_pre_call_utils.py

@@ -27,6 +27,7 @@ from litellm.proxy.litellm_pre_call_utils import (
     add_litellm_data_to_request,
     check_if_token_is_service_account,
     clean_headers,
+    _is_transmittable_header,
 )
 from litellm.types.utils import CredentialItem
 
@@ -5402,6 +5403,52 @@ async def test_overwrite_user_with_key_hash_stamps_master_key_alias(monkeypatch)
     assert updated_data["user"] == LITELLM_PROXY_MASTER_KEY_ALIAS
 
 
+@pytest.mark.parametrize(
+    "name, value, expected",
+    [
+        ("x-project", "acme", True),
+        ("x-project", b"\xe4\xb8\xad", True),
+        ("x-project", "中文", False),
+        ("x-project", "café", False),
+        ("x-中文", "acme", False),
+        ("x-count", 5, False),
+    ],
+)
+def test_is_transmittable_header(name, value, expected):
+    assert _is_transmittable_header(name, value) is expected
+
+
+def test_add_headers_to_llm_call_drops_non_ascii_header_values():
+    """
+    Regression for https://github.com/BerriAI/litellm/issues/34633
+
+    A forwarded header whose value contains non-ASCII characters must not reach
+    httpx/http.client (which encode header values with a single-byte codec and
+    raise UnicodeEncodeError -> 500). ASCII headers alongside it must survive.
+    """
+    import httpx
+
+    non_ascii_wire_value = "中文".encode("utf-8")
+    headers = Headers(
+        raw=[
+            (b"x-keep", b"plain-ascii"),
+            (b"x-non-ascii", non_ascii_wire_value),
+            (b"anthropic-beta", b"prompt-caching-2024-07-31"),
+        ]
+    )
+    user_api_key_dict = UserAPIKeyAuth(api_key="sk-test")
+
+    result = LiteLLMProxyRequestSetup.add_headers_to_llm_call(
+        headers, user_api_key_dict
+    )
+
+    assert result == {
+        "x-keep": "plain-ascii",
+        "anthropic-beta": "prompt-caching-2024-07-31",
+    }
+    httpx.Request("POST", "http://localhost:4000", headers=result)
+
+
 @pytest.mark.asyncio
 async def test_overwrite_user_with_key_hash_rejects_alias_without_marker(monkeypatch):
     from litellm.constants import LITELLM_PROXY_MASTER_KEY_ALIAS