typescript-sdk/.changeset/zod-jsonschema-fallback.md

@modelcontextprotocol/core-internal	@modelcontextprotocol/server	@modelcontextprotocol/client
patch	patch	patch

Fix runtime crash on tools/list when a tool's inputSchema comes from zod 4.0–4.1. The SDK requires ~standard.jsonSchema (StandardJSONSchemaV1, added in zod 4.2.0); previously a missing jsonSchema crashed at undefined[io]. standardSchemaToJsonSchema now detects zod 4 schemas lacking jsonSchema and falls back to the SDK-bundled z.toJSONSchema(), emitting a one-time console warning. zod 3 schemas (which the bundled zod 4 converter cannot introspect) and non-zod schema libraries without jsonSchema get a clear error pointing to fromJsonSchema(). The workspace zod catalog is also bumped to ^4.2.0.