[BUG] TS2589: Type instantiation is excessively deep and possibly infinite

[lumirlumir]

Describe the bug

Hello — thanks to the team for their work on this!

After upgrading @modelcontextprotocol/sdk from 1.22.0 to 1.23.0, I started getting a TS2589: Type instantiation is excessively deep and possibly infinite. error.

It worked in 1.22.0, but 1.23.0 reports this error.

---

• PR: refactor: fix CI, use zod v4, and latest method in mcp eslint/rewrite#332
• CI: https://github.com/eslint/rewrite/actions/runs/19728654491/job/56524795666?pr=332

The PR and GitHub Actions CI show the failure:

At first I thought the error was caused by our use of the deprecated tools method, but it also occurs with the new registerTool method.

To Reproduce

Steps to reproduce the behavior:

1. Clone the PR: refactor: fix CI, use zod v4, and latest method in mcp eslint/rewrite#332
2. Run npm install from the project root.
3. Run npm run build -w packages/mcp from the project root.

Expected behavior

No TS2589: Type instantiation is excessively deep and possibly infinite. error should occur.

Logs

Full logs can be found here:

• PR: refactor: fix CI, use zod v4, and latest method in mcp eslint/rewrite#332
• CI: https://github.com/eslint/rewrite/actions/runs/19728654491/job/56524795666?pr=332

Additional context

This appears to be a regression introduced when adding Zod v4 support in 1.23.0.

[felixweinberger]

@mattzcarey @KKonstantinov would either of you be able to take a look? Seems like a potential issue with the zod upgrade

[felixweinberger]

Hi @lumirlumir thanks for reporting this - can you try whether it works with this branch?

# install the mcp sdk version from the linked PR
npm install https://pkg.pr.new/modelcontextprotocol/typescript-sdk/@modelcontextprotocol/sdk@1181

# Then build
npm run build -w packages/mcp

[lumirlumir]

Thanks for your work on this issue!

But, unfortunately, I've pushed a new commit (eslint/rewrite@a3d66e9) to the reproduction PR that I metioned above, and it still shows the same error message:

https://github.com/eslint/rewrite/actions/runs/19741871181/job/56567495873?pr=332