fix(mcp): address bugbot user_fields + BYOK conflict and header dedup

- Reject combining is_byok=True with non-empty user_fields at server
  create/update time: both share the (user_id, server_id) credential row
  and the store paths refuse cross-type overwrites, so the combination
  trapped users in an unresolvable 401 loop.
- Make the managed MCP dispatch path apply user-field headers with the
  same case-insensitive precedence as the OpenAPI/local path, so a
  static_header named 'Authorization' is no longer left alongside a
  user-field header named 'authorization' on the outbound request.

litellm/proxy/_experimental/mcp_server/mcp_server_manager.py

@@ -2827,7 +2827,16 @@ class MCPServerManager:
         if user_field_headers:
             if extra_headers is None:
                 extra_headers = {}
-            extra_headers.update(user_field_headers)
+            # Match the OpenAPI/local path's case-insensitive precedence: a
+            # user-field header replaces any existing header (e.g. from
+            # static_headers) whose name matches case-insensitively, instead
+            # of leaving two differently-cased duplicates on the wire.
+            existing_lower_names = {k.lower(): k for k in extra_headers}
+            for header_name, value in user_field_headers.items():
+                collision = existing_lower_names.get(header_name.lower())
+                if collision is not None and collision != header_name:
+                    del extra_headers[collision]
+                extra_headers[header_name] = value
 
         if hook_extra_headers:
             if extra_headers is None:

litellm/proxy/management_endpoints/mcp_management_endpoints.py

@@ -216,6 +216,33 @@ if MCP_AVAILABLE:
     def validate_and_normalize_mcp_server_payload(payload: Any) -> None:
         _base_validate_and_normalize_mcp_server_payload(payload)
         _validate_mcp_server_name_fields(payload)
+        _validate_mcp_user_fields_byok_exclusive(payload)
+
+    def _validate_mcp_user_fields_byok_exclusive(payload: Any) -> None:
+        """Reject server configurations that combine is_byok with user_fields.
+
+        Both credential types share the same (user_id, server_id) row in
+        LiteLLM_MCPUserCredentials, and the store paths refuse to overwrite
+        the other type — so a user can save BYOK or user-fields for a given
+        server, never both. Allowing this combination at admin time would
+        trap end-users in an unresolvable state: every tool call would 401
+        on whichever check the user has not (and cannot) satisfy.
+        """
+        if not getattr(payload, "is_byok", False):
+            return
+        user_fields = getattr(payload, "user_fields", None) or []
+        if user_fields:
+            raise HTTPException(
+                status_code=status.HTTP_400_BAD_REQUEST,
+                detail={
+                    "error": (
+                        "MCP servers cannot enable both is_byok and user_fields. "
+                        "Use user_fields if the server needs multiple per-user "
+                        "values; use is_byok only for a single legacy BYOK "
+                        "credential."
+                    )
+                },
+            )
 
     _VALID_MCP_REQUIRED_FIELDS: frozenset = frozenset(NewMCPServerRequest.model_fields)