diff --git a/tests/test_litellm/passthrough/test_passthrough_utils.py b/tests/test_litellm/passthrough/test_passthrough_utils.py
deleted file mode 100644
index 55df39d6235..00000000000
--- a/tests/test_litellm/passthrough/test_passthrough_utils.py
+++ /dev/null
@@ -1,68 +0,0 @@
-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_configured_custom_accept_encoding_is_dropped():
-    headers = BasePassthroughUtils.forward_headers_from_request(
-        request_headers={},
-        headers={"x-api-key": "sk-anthropic", "Accept-Encoding": "br"},
-        forward_headers=False,
-    )
-
-    assert "accept-encoding" not in {name.lower() for name in headers}
-    assert headers["x-api-key"] == "sk-anthropic"
-
-
-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)
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..08376c4c7b4 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
@@ -1,5 +1,6 @@
 from unittest.mock import AsyncMock, MagicMock, patch
 
+import httpx
 import pytest
 
 from litellm.proxy.pass_through_endpoints.llm_passthrough_endpoints import (
@@ -538,6 +539,81 @@ def test_forward_headers_from_request_protected_headers_not_overwritten():
     assert "Anthropic-Beta" not in result
 
 
+def test_forward_headers_drops_client_accept_encoding():
+    """
+    A client's Accept-Encoding must not reach the vendor API: the proxy can only
+    decode what httpx supports, and an undecodable body reaches clients compressed
+    with Content-Encoding stripped (LIT-5613).
+    """
+    from litellm.passthrough.utils import BasePassthroughUtils
+
+    result = 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 result}
+    assert result["anthropic-version"] == "2023-06-01"
+    assert "host" not in result
+    assert "content-length" not in result
+
+
+def test_forward_headers_drops_accept_encoding_from_x_pass_prefix_and_custom_headers():
+    """
+    Neither the x-pass- mechanism nor endpoint-configured custom headers may
+    reintroduce an Accept-Encoding the proxy cannot decode.
+    """
+    from litellm.passthrough.utils import BasePassthroughUtils
+
+    via_prefix = BasePassthroughUtils.forward_headers_from_request(
+        request_headers={"x-pass-accept-encoding": "br"},
+        headers={},
+        forward_headers=False,
+    )
+    via_custom_headers = BasePassthroughUtils.forward_headers_from_request(
+        request_headers={},
+        headers={"x-api-key": "sk-anthropic", "Accept-Encoding": "br"},
+        forward_headers=False,
+    )
+
+    assert "accept-encoding" not in via_prefix
+    assert "accept-encoding" not in {name.lower() for name in via_custom_headers}
+    assert via_custom_headers["x-api-key"] == "sk-anthropic"
+
+
+def test_forwarded_headers_only_advertise_encodings_httpx_can_decode():
+    """
+    The outbound request must negotiate a content coding httpx has a decoder for,
+    otherwise the vendor replies with a body the proxy passes through unreadable.
+    """
+    from httpx._decoders import SUPPORTED_DECODERS
+
+    from litellm.passthrough.utils import BasePassthroughUtils
+
+    forwarded_headers = BasePassthroughUtils.forward_headers_from_request(
+        request_headers={"accept-encoding": "br, zstd, exotic"},
+        headers={"x-api-key": "sk-anthropic"},
+        forward_headers=True,
+    )
+    with httpx.Client(headers={"user-agent": "litellm/test"}) as client:
+        upstream_request = client.build_request(
+            "POST",
+            "https://api.anthropic.com/v1/messages",
+            headers=forwarded_headers,
+        )
+
+    advertised = {value.strip().lower() for value in upstream_request.headers["accept-encoding"].split(",")}
+
+    assert advertised
+    assert advertised <= set(SUPPORTED_DECODERS)
+
+
 def test_forward_headers_custom_wins_case_insensitive_over_request_authorization():
     """
     When forwarding request headers, provider-signed/custom headers must win