MCP Inspector fails with `keyValidator._parse is not a function`on tools/list

[hariprasadkc]

Describe the bug
When using MCP Inspector to connect to an MCP server, the proxy fails during tools/list calls with the error: keyValidator._parse is not a function. /initialize succeeds though.

To Reproduce
Steps to reproduce the behavior:

1. npx frontmcp create my-app
2. npm run dev
3. npm run inspect
4. Connect then List tools

Expected behavior
List of available tools defined to be returned.

Additional context
Seems to be a zod version incompatibility issue with @modelcontextprotocol/sdk": "^1.22.0 and fixed in 1.23.0.
Same setup was working last week.

[frontegg-david]

@hariprasadkc

Thanks for the detailed report and the repro steps 🙏

This does look like a Zod / MCP SDK compatibility issue around tools/list — especially since /initialize works and the same setup was working last week.

To help us reproduce this exactly, could you please share the following details?

🔍 Environment Details Needed

1. Package manager

   • Are you using npm, yarn, or pnpm?

2. frontmcp version

   • From your package.json (or npm ls frontmcp / yarn why frontmcp).

3. zod version

   • From package.json or npm ls zod.

4. Tool setup style

   • Are you using the OpenAPIAdapter from @frontmcp/adapters,
     or a regular tool setup?

   • If regular:

     • Are your tools defined using the @Tool decorator/annotation, or
     • Using the tool function builder API?

---

📌 Note About Zod Compatibility

The currently supported and working Zod major version is v3, due to limitations in the TypeScript implementation of @modelcontextprotocol/sdk.

We are addressing this in the next release, where we will:

• Remove the dependency on the TypeScript @modelcontextprotocol/sdk
• Ship our own built-in transports that track the latest MCP protocol updates
• Keep important dependencies like Zod fully up-to-date and compatible

---

Once we have your environment details, we’ll confirm the failing version combo and provide a workaround or include a fix in the upcoming release.

[hariprasadkc]

Please find the environment details below,

1. Package manager - npm
2. frontmcp@0.3.1
3. zod@3.25.76
4. We're using the @Tool decorator to define tools.

Thanks for the update on the roadmap - looking forward to the release.

[frontegg-david]

@hariprasadkc

Following up on the latest findings: @modelcontextprotocol/sdk was updated 3 days ago to use Zod v4, which includes breaking changes. This was released as a minor version, so new installs automatically pull it in as the latest version.

As a workaround, you can pin the dependency to Zod v3 by adding:

"@modelcontextprotocol/sdk": "1.22.0"

to your dependencies to force version 1.22.0.

I’ll publish a new version of frontmcp that works with @modelcontextprotocol/sdk@1.23.0 as soon as possible.

Thank you.

References:
https://github.com/modelcontextprotocol/typescript-sdk/releases/tag/1.23.0
modelcontextprotocol/typescript-sdk#1040

[frontegg-david]

Hi @hariprasadkc 👋

Thanks again for the thorough report and for providing the environment details — they helped a lot.

We've just released frontmcp v0.4.1, which fully restores compatibility with MCP Inspector.
This version pins @modelcontextprotocol/sdk to 1.22.0, ensuring Zod v3 is used and preventing the keyValidator._parse is not a function error during tools/list.

✅ Fix included in: frontmcp@0.4.1
🔧 No action needed on your side other than updating to the latest version.

Please give it a try and let me know if everything works smoothly on your end.
If all looks good, I’ll go ahead and close the issue.

[hariprasadkc]

Thank you so much for the quick turnaround @frontegg-david
I can confirm that the latest version works 🙌
Please close the issue.