MCP automatic tools overwrite explicit additionalProperties schemas, rejecting dictionary inputs

Finding: In an external report on vercel/ai, @ai-sdk/mcp@2.0.73 (and upstream commit ba05943b69, per the reporter) is said to replace an MCP tool's explicit additionalProperties schema with false in toolsFromDefinitions. A tool that accepts a string dictionary is then advertised with a schema that rejects keys such as {language:'en', project:'example'} (https://github.com/vercel/ai/issues/22636). Reported evidence: The reporter gives a no-server reproduction using a mock transport, raw JSON Schema (no Zod conversion) and Ajv 8. The original schema validates the input as true; the converted schema validates false and contains additionalProperties: false. The report says this occurs on both the release and upstream (https://github.com/vercel/ai/issues/22636). Reported cause: The reporter says toolsFromDefinitions spreads inputSchema and then unconditionally overwrites additionalProperties. The report distinguishes this from the zero-argument normalization in #5670, which fills in a missing field, and from the Zod conversion issue #17871. A later comment from a contributor-association account says a reproduction on main observed schema-valued and true values both becoming false, while explicit false and zero-argument normalization still behaved as intended (https://github.com/vercel/ai/issues/22636#issuecomment-6102221408). That comment is a reproduction report, not a maintainer-confirmed cause. Status: The reporter proposes preserving explicitly supplied additionalProperties values (schema objects and true), keeping zero-argument defaults only where needed, and handling provider-specific strict-schema limits in the provider. The reporter states this is not implemented or tested. No fix commit, release, or maintainer statement appears in the supplied sources, and no workaround is documented. Practical implication, conditional: If your MCP server declares additionalProperties as a schema or true and you use automatic tool conversion on the affected versions, model-facing schemas may disallow arbitrary keys. Check the generated inputSchema.jsonSchema for additionalProperties: false. Limitations: This is an external report plus a contributor reproduction comment. KFA did not reproduce it, and the supplied comment coverage may omit middle comments.