fix(mcp): skip JWT injection when per-user mcp_auth_header is set

MCPClient._get_auth_headers() applies extra_headers AFTER writing
Authorization from auth_value, so an injected JWT silently overwrites
the user's per-server OAuth token. Guard the JWT signer with
'not mcp_auth_header' so per-user OAuth (and any dict-form per-user
auth) takes precedence, mirroring the existing static_headers guard.

Adds a regression test that the signer's inject helper is not called
when mcp_auth_header is supplied.

litellm/proxy/_experimental/mcp_server/mcp_server_manager.py

@@ -1436,10 +1436,13 @@ class MCPServerManager:
 
             # MCPJWTSigner: inject signed JWT for tools/list (list path skips pre_call_hook).
             # Skip entirely when the signer is not configured (avoid an unnecessary
-            # dict copy on every list call) and when the server has its own
-            # static Authorization header — admin-configured static auth must
-            # take precedence per-server so the signer doesn't silently
-            # overwrite e.g. an upstream API key.
+            # dict copy on every list call), when the server has its own static
+            # Authorization header, or when a per-user mcp_auth_header has already
+            # been resolved — admin-configured static auth and per-user OAuth must
+            # take precedence so the signer doesn't silently overwrite e.g. an
+            # upstream API key or a user's OAuth token (MCPClient._get_auth_headers
+            # applies extra_headers after writing Authorization from auth_value, so
+            # an injected JWT would otherwise clobber the per-user token).
             if user_api_key_auth is not None and not server.spec_path:
                 from litellm.proxy.guardrails.guardrail_hooks.mcp_jwt_signer.mcp_jwt_signer import (
                     get_mcp_jwt_signer,
@@ -1452,7 +1455,11 @@ class MCPServerManager:
                     for k in static_headers.keys()
                 )
 
-                if get_mcp_jwt_signer() is not None and not has_static_authorization:
+                if (
+                    get_mcp_jwt_signer() is not None
+                    and not has_static_authorization
+                    and not mcp_auth_header
+                ):
                     extra_headers = await inject_mcp_jwt_headers_for_upstream(
                         user_api_key_dict=user_api_key_auth,
                         extra_headers=extra_headers,

tests/test_litellm/proxy/_experimental/mcp_server/test_mcp_server_manager.py

@@ -1805,6 +1805,53 @@ class TestMCPServerManager:
         assert len(tools_unprefixed) == 1
         assert tools_unprefixed[0].name == "send_email"
 
+    @pytest.mark.asyncio
+    async def test_get_tools_from_server_jwt_skipped_when_mcp_auth_header_set(self):
+        """When a per-user mcp_auth_header is resolved, JWT injection must be skipped.
+
+        MCPClient._get_auth_headers() applies extra_headers AFTER writing
+        Authorization from auth_value, so an injected JWT would clobber the
+        user's per-server OAuth token. Regression test for that interaction.
+        """
+        from litellm.proxy._types import UserAPIKeyAuth
+
+        manager = MCPServerManager()
+        server = MCPServer(
+            server_id="zapier",
+            name="zapier",
+            transport=MCPTransport.http,
+        )
+
+        manager._create_mcp_client = AsyncMock(return_value=object())
+        manager._fetch_tools_with_timeout = AsyncMock(return_value=[])
+
+        user_auth = UserAPIKeyAuth(api_key="sk-test", user_id="alice")
+
+        with (
+            patch(
+                "litellm.proxy.guardrails.guardrail_hooks.mcp_jwt_signer.mcp_jwt_signer.get_mcp_jwt_signer",
+                return_value=MagicMock(),
+            ),
+            patch(
+                "litellm.proxy.guardrails.guardrail_hooks.mcp_jwt_signer.mcp_jwt_signer.inject_mcp_jwt_headers_for_upstream",
+                new=AsyncMock(return_value={"Authorization": "Bearer signed-jwt"}),
+            ) as mock_inject,
+        ):
+            # Case A: mcp_auth_header present -> JWT must NOT be injected
+            await manager._get_tools_from_server(
+                server,
+                mcp_auth_header="oauth-user-token",
+                user_api_key_auth=user_auth,
+            )
+            mock_inject.assert_not_called()
+
+            # Case B: no mcp_auth_header -> JWT injection runs as before
+            await manager._get_tools_from_server(
+                server,
+                user_api_key_auth=user_auth,
+            )
+            mock_inject.assert_awaited_once()
+
     def test_create_prefixed_tools_updates_mapping_for_both_forms(self):
         """_create_prefixed_tools should populate mapping for prefixed and original names even when not adding prefix in output."""
         manager = MCPServerManager()