fix(mcp): never validate a supplied header on the tools/list BYOK path

The pre-listing resolver ran the tool-call byok_auth_required check even
when the caller already supplied x-mcp-auth, and it ran outside the
per-server error boundary, so a single deprecated-header caller dropped
the server from the aggregate list. Listing now returns a supplied header
unchanged and falls back to the stored credential without raising

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

litellm/proxy/_experimental/mcp_server/mcp_server_manager.py

@@ -1286,6 +1286,21 @@ async def _resolve_byok_mcp_auth_header(
     return mcp_auth_header
 
 
+async def _byok_listing_auth_header(
+    mcp_server: MCPServer,
+    user_api_key_auth: UserAPIKeyAuth | None,
+    mcp_auth_header: str | dict[str, str] | None,
+) -> str | dict[str, str] | None:
+    """The credential a tools/list may use: a supplied header forwards unchanged, and a missing one
+    falls back to the stored credential without the tool-call path's byok_auth_required raise."""
+    if not mcp_server.is_byok or mcp_auth_header is not None:
+        return mcp_auth_header
+
+    from litellm.proxy._experimental.mcp_server.operations import _get_byok_credential
+
+    return await _get_byok_credential(mcp_server, user_api_key_auth)
+
+
 def _client_forwarded_authorization_headers(
     mcp_server: MCPServer,
     oauth2_headers: dict[str, str] | None,
@@ -4401,11 +4416,7 @@ class MCPServerManager:
         verbose_logger.info("_get_tools_from_server for %s...", server.name)
 
         client = None
-        resolved_mcp_auth_header: Final = (
-            mcp_auth_header
-            if not server.is_byok or isinstance(mcp_auth_header, dict)
-            else await _resolve_byok_mcp_auth_header(server, user_api_key_auth, mcp_auth_header)
-        )
+        resolved_mcp_auth_header: Final = await _byok_listing_auth_header(server, user_api_key_auth, mcp_auth_header)
         listed_caller: Final = ListedToolsCaller(
             user_api_key_auth=user_api_key_auth,
             mcp_auth_header=resolved_mcp_auth_header,

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

@@ -7068,6 +7068,28 @@ class TestMCPServerManager:
         listed = manager.get_listed_tool(server, "turn", call_side)
         assert listed is not None and listed.description == "stored cred catalog"
 
+    @pytest.mark.asyncio
+    async def test_byok_supplied_header_lists_without_credential_validation(self):
+        manager = MCPServerManager()
+        server = MCPServer(
+            server_id="byok-catalog",
+            name="byok_catalog",
+            transport=MCPTransport.http,
+            url="http://byok-catalog",
+            is_byok=True,
+        )
+        manager._create_mcp_client = AsyncMock(return_value=AsyncMock())
+        manager._fetch_tools_with_timeout = AsyncMock(return_value=[MCPTool(name="turn", description="t", inputSchema={})])
+
+        with patch("litellm.proxy.proxy_server.prisma_client", None):
+            await manager._get_tools_from_server(
+                server=server,
+                mcp_auth_header="Bearer hdr",
+                user_api_key_auth=UserAPIKeyAuth(api_key="sk-litellm"),
+            )
+
+        assert manager._create_mcp_client.await_args.kwargs["mcp_auth_header"] == "Bearer hdr"
+
     @pytest.mark.parametrize(
         ("signer", "static_headers", "shared"),
         [