PRODUCTION SCENARIO
Your logistics assistant aggregates 240 tools from five MCP-backed services. To save context, the team added the regex tool search tool and set defer_loading to true on every entry in the tools array. Every request now fails with a 400 invalid_request_error.
What change makes the requests valid again?
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Answer: Remove defer_loading from the tool search tool itself so at least one tool loads up front
A request in which every tool is deferred is rejected, because the tool search tool itself has to stay loaded for Claude to have anything to search the catalog with. Keeping the three to five most used tools non-deferred as well lets Claude call them without a search step first.