fix: default to JSON Schema draft-2020-12 for Zod v3 schemas

[limehawk]

Summary

Fixes #745

The Zod v3 branch of toJsonSchemaCompat doesn't pass a target option to zod-to-json-schema, causing it to default to draft-07. This breaks compatibility with Claude's API (and other modern MCP clients) which require strict JSON Schema draft-2020-12 compliance.

Users see errors like:

API Error: 400 {"type":"error","error":{"type":"invalid_request_error","message":"tools.31.custom.input_schema: JSON schema is invalid. It must match JSON Schema draft 2020-12"}}

Changes

• Add target: 'jsonSchema2020-12' to the zodToJsonSchema call for Zod v3 schemas in packages/core/src/util/zodJsonSchemaCompat.ts

Testing

Verified that MCP tools now work with Claude Code without the "JSON schema is invalid" error after applying this fix as a patch to node_modules.

[changeset-bot]

🦋 Changeset detected

Latest commit: 74cb8ce

The changes in this PR will be included in the next version bump.

Not sure what this means? Click here to learn what changesets are.

Click here if you're a maintainer who wants to add another changeset to this PR

[pkg-pr-new]

Open in StackBlitz

@modelcontextprotocol/client

npm i https://pkg.pr.new/modelcontextprotocol/typescript-sdk/@modelcontextprotocol/client@1432

@modelcontextprotocol/server

npm i https://pkg.pr.new/modelcontextprotocol/typescript-sdk/@modelcontextprotocol/server@1432

@modelcontextprotocol/express

npm i https://pkg.pr.new/modelcontextprotocol/typescript-sdk/@modelcontextprotocol/express@1432

@modelcontextprotocol/hono

npm i https://pkg.pr.new/modelcontextprotocol/typescript-sdk/@modelcontextprotocol/hono@1432

@modelcontextprotocol/node

npm i https://pkg.pr.new/modelcontextprotocol/typescript-sdk/@modelcontextprotocol/node@1432

commit: 74cb8ce

[limehawk]

Note: This is a workaround, not a comprehensive fix.

As mentioned in #745, the proper solution would be to upgrade to Zod v4's native toJSONSchema({ target: "draft-2020-12" }) which has first-class support for draft-2020-12.

This PR works by passing an unrecognized target value ('jsonSchema2020-12') to zod-to-json-schema, which causes it to omit the $schema field entirely. Without the $schema: "http://json-schema.org/draft-07/schema#" declaration, Claude's API accepts the schema since the actual syntax is compatible.

This is a stopgap to unblock users hitting the 400 error today. A proper migration to Zod v4's native JSON Schema generation would be the better long-term fix.