fix(passthrough): stop forwarding client Accept-Encoding upstream

Co-Authored-By: Devin AI <158243242+devin-ai-integration[bot]@users.noreply.github.com>

litellm/passthrough/utils.py

@@ -18,12 +18,26 @@ _PASS_THROUGH_PROTECTED_HEADERS: Final[frozenset] = frozenset(
         "x-goog-api-key",
         "host",
         "content-length",
+        "accept-encoding",
     }
 )
 
 # Header name prefix used to block AWS SigV4 signing headers from being overridden.
 _PASS_THROUGH_PROTECTED_HEADER_PREFIXES: Final[tuple] = ("x-amz-",)
 
+# Client headers that must never reach the upstream provider. `accept-encoding` is
+# dropped so httpx negotiates a content coding it actually has a decoder for: an
+# encoding it cannot decode (e.g. brotli, absent the optional `brotli` package)
+# leaves the body compressed while get_response_headers strips Content-Encoding,
+# handing the client bytes it has no way to read.
+_NON_FORWARDED_REQUEST_HEADERS: Final[frozenset[str]] = frozenset(
+    {
+        "content-length",
+        "host",
+        "accept-encoding",
+    }
+)
+
 
 class BasePassthroughUtils:
     @staticmethod
@@ -66,17 +80,14 @@ class BasePassthroughUtils:
         e.g., 'x-pass-anthropic-beta: value' becomes 'anthropic-beta: value'
         """
         if forward_headers is True:
-            # Header We Should NOT forward
-            request_headers.pop("content-length", None)
-            request_headers.pop("host", None)
-
             custom_header_names: Final = {header_name.lower() for header_name in headers}
-            for header_name in list(request_headers.keys()):
-                if header_name.lower() in custom_header_names:
-                    request_headers.pop(header_name, None)
-
-            # Combine request headers with custom headers
-            headers = {**request_headers, **headers}
+            forwardable_headers: Final = {
+                header_name: header_value
+                for header_name, header_value in request_headers.items()
+                if header_name.lower() not in _NON_FORWARDED_REQUEST_HEADERS
+                and header_name.lower() not in custom_header_names
+            }
+            headers = {**forwardable_headers, **headers}
 
         # Process x-pass- prefixed headers (strip prefix and forward)
         # Credential and protocol-level headers are excluded from this mechanism.

tests/test_litellm/passthrough/test_passthrough_utils.py

@@ -0,0 +1,57 @@
+import httpx
+from httpx._decoders import SUPPORTED_DECODERS
+
+from litellm.passthrough.utils import BasePassthroughUtils
+
+ANTHROPIC_MESSAGES_URL = "https://api.anthropic.com/v1/messages"
+
+
+def _build_upstream_request(forwarded_headers: dict) -> httpx.Request:
+    with httpx.Client(headers={"user-agent": "litellm/test"}) as client:
+        return client.build_request("POST", ANTHROPIC_MESSAGES_URL, headers=forwarded_headers)
+
+
+def test_client_accept_encoding_is_not_forwarded_upstream():
+    headers = BasePassthroughUtils.forward_headers_from_request(
+        request_headers={
+            "accept-encoding": "br",
+            "anthropic-version": "2023-06-01",
+            "host": "localhost:4000",
+            "content-length": "123",
+        },
+        headers={"x-api-key": "sk-anthropic"},
+        forward_headers=True,
+    )
+
+    assert "accept-encoding" not in {name.lower() for name in headers}
+    assert headers["anthropic-version"] == "2023-06-01"
+    assert "host" not in headers
+    assert "content-length" not in headers
+
+
+def test_client_accept_encoding_is_not_forwarded_via_x_pass_prefix():
+    headers = BasePassthroughUtils.forward_headers_from_request(
+        request_headers={"x-pass-accept-encoding": "br"},
+        headers={},
+        forward_headers=False,
+    )
+
+    assert "accept-encoding" not in headers
+
+
+def test_upstream_request_only_advertises_decodable_encodings():
+    """A content coding httpx cannot decode would reach the client still compressed,
+    with Content-Encoding stripped by get_response_headers (LIT-5613)."""
+    forwarded_headers = BasePassthroughUtils.forward_headers_from_request(
+        request_headers={"accept-encoding": "br, zstd, exotic"},
+        headers={"x-api-key": "sk-anthropic"},
+        forward_headers=True,
+    )
+
+    advertised = {
+        value.strip().lower()
+        for value in _build_upstream_request(forwarded_headers).headers["accept-encoding"].split(",")
+    }
+
+    assert advertised
+    assert advertised <= set(SUPPORTED_DECODERS)