From d0be6eee8a34cb2a9c62a63a5aca71bdbd18f5db Mon Sep 17 00:00:00 2001
From: mateo-berri <277851410+mateo-berri@users.noreply.github.com>
Date: Sat, 15 Aug 2026 15:22:06 -0700
Subject: [PATCH 1/2] fix(passthrough): stop forwarding client Accept-Encoding
 upstream

---
 litellm/passthrough/utils.py                  |  3 +++
 .../test_vertex_passthrough_load_balancing.py | 25 +++++++++++++++++++
 2 files changed, 28 insertions(+)

diff --git a/litellm/passthrough/utils.py b/litellm/passthrough/utils.py
index e419322dca6..1572913f46e 100644
--- a/litellm/passthrough/utils.py
+++ b/litellm/passthrough/utils.py
@@ -69,6 +69,9 @@ class BasePassthroughUtils:
             # Header We Should NOT forward
             request_headers.pop("content-length", None)
             request_headers.pop("host", None)
+            # accept-encoding must stay client-negotiated: forwarding e.g. "br" when
+            # the brotli package is absent relays undecodable bytes to the caller
+            request_headers.pop("accept-encoding", None)
 
             custom_header_names: Final = {header_name.lower() for header_name in headers}
             for header_name in list(request_headers.keys()):
diff --git a/tests/test_litellm/proxy/pass_through_endpoints/test_vertex_passthrough_load_balancing.py b/tests/test_litellm/proxy/pass_through_endpoints/test_vertex_passthrough_load_balancing.py
index aaf1dad4910..02464ad5caa 100644
--- a/tests/test_litellm/proxy/pass_through_endpoints/test_vertex_passthrough_load_balancing.py
+++ b/tests/test_litellm/proxy/pass_through_endpoints/test_vertex_passthrough_load_balancing.py
@@ -568,6 +568,31 @@ def test_forward_headers_custom_wins_case_insensitive_over_request_authorization
     assert result["x-request-id"] == "req-123"
 
 
+def test_forward_headers_never_forwards_client_accept_encoding():
+    """
+    The client's Accept-Encoding must not reach the upstream provider: the proxy's
+    HTTP client decodes the upstream body and advertises only encodings it can
+    decode. Forwarding e.g. "br" on an install without the brotli package makes
+    the proxy relay raw compressed bytes with the content-encoding header stripped
+    (garbled JSON for /v1/models and count_tokens through the Anthropic passthrough).
+    """
+    from litellm.passthrough.utils import BasePassthroughUtils
+
+    request_headers = {
+        "accept-encoding": "gzip, deflate, br, zstd",
+        "x-request-id": "req-123",
+    }
+
+    result = BasePassthroughUtils.forward_headers_from_request(
+        request_headers=request_headers,
+        headers={},
+        forward_headers=True,
+    )
+
+    assert "accept-encoding" not in result
+    assert result["x-request-id"] == "req-123"
+
+
 @pytest.mark.asyncio
 async def test_vertex_passthrough_custom_model_name_replaced_in_url():
     """

From 90493a217f06b21cc31ee1683ebe746139cb256e Mon Sep 17 00:00:00 2001
From: mateo-berri <277851410+mateo-berri@users.noreply.github.com>
Date: Sat, 15 Aug 2026 15:34:28 -0700
Subject: [PATCH 2/2] fix(passthrough): protect accept-encoding from x-pass-
 forwarding

---
 litellm/passthrough/utils.py                                     | 1 +
 .../test_vertex_passthrough_load_balancing.py                    | 1 +
 2 files changed, 2 insertions(+)

diff --git a/litellm/passthrough/utils.py b/litellm/passthrough/utils.py
index 1572913f46e..df39b8fad48 100644
--- a/litellm/passthrough/utils.py
+++ b/litellm/passthrough/utils.py
@@ -18,6 +18,7 @@ _PASS_THROUGH_PROTECTED_HEADERS: Final[frozenset] = frozenset(
         "x-goog-api-key",
         "host",
         "content-length",
+        "accept-encoding",
     }
 )
 
diff --git a/tests/test_litellm/proxy/pass_through_endpoints/test_vertex_passthrough_load_balancing.py b/tests/test_litellm/proxy/pass_through_endpoints/test_vertex_passthrough_load_balancing.py
index 02464ad5caa..8e973fc3771 100644
--- a/tests/test_litellm/proxy/pass_through_endpoints/test_vertex_passthrough_load_balancing.py
+++ b/tests/test_litellm/proxy/pass_through_endpoints/test_vertex_passthrough_load_balancing.py
@@ -580,6 +580,7 @@ def test_forward_headers_never_forwards_client_accept_encoding():
 
     request_headers = {
         "accept-encoding": "gzip, deflate, br, zstd",
+        "x-pass-accept-encoding": "br",
         "x-request-id": "req-123",
     }