What
Every tool Claude can use comes with a schema, a formal description of what inputs the tool accepts. Before sending these schemas to the API, Claude Code cleans them up. That cleanup now also looks at anyOf and oneOf lists. These are parts of a schema that list alternative shapes an input may take.
- Alternatives whose description passes a certain check are removed from the list.
- If no alternatives remain, the whole
anyOforoneOfentry is removed.
This extra cleanup runs only in some cases, not every time.
Why
The tool definitions sent to the API, including those from MCP servers (external tool providers connected to Claude Code), can now be smaller than the original definitions. If a tool you added has inputs with several allowed shapes, some of those shapes may no longer reach the model.
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It is not clear which descriptions trigger the removal, or when this cleanup runs.