Zod 4 supported

[tangye1234]

Is your feature request related to a problem? Please describe.
Currently, server.tool() uses a zod3 raw shape type for its input schema. Zod 4 is now currently release, and our input schema validation is all written in zod 4.

There isn't a compatible conversion between zod 3/4 up to now, so we are stuck here.

Describe the solution you'd like
I recommend to use a custom validation schema as an overload for this method, like vercel/ai-sdk.

Describe alternatives you've considered
We would also like to see the method supports v4/v3 both if the type can be inferred.

[pattobrien]

Relevant issue: #164

[tangye1234]

I use this type to adopt any json schema library:

#164 (comment)

[timonbimon]

We're also blocked by this (all our schema validation is zod 4)

[ydennisy]

+1 here

[pattobrien]

FWIW, we have full Standard Schema support over at FastMCP; it's a good alternative while this issue is prioritized.

[acaumartin-cameleon]

+1

[nicolasembleton]

I've upgraded the library to support Zod V4 and have been using it successfully in my CLI > MCP DXT building library library. If you need compatibility, you can install via NPM registry:

pnpm add @alcyone-labs/modelcontextprotocol-sdk
# or
npm install @alcyone-labs/modelcontextprotocol-sdk

Since it's a re-published package, you will have to import from the NPM package as follows, for example:

import { McpServer, ResourceTemplate } from "@alcyone-labs/modelcontextprotocol-sdk/server/mcp";

The repo is available here: https://github.com/Alcyone-Labs/mcp-typescript-sdk

The update focuses on upgrading to V4 + getting all tests to pass. Happy to create a PR if needed, but this would potentially break anyone not using V4. I haven't tested it well with V3.

EDIT: I published the package on NPM to make it more deterministic and allow me to keep updating the package from upstream, and have a better sense of whether anyone's using the fork or not (via weekly downloads). The repo is the same, the NPM package is available here:
https://www.npmjs.com/package/@alcyone-labs/modelcontextprotocol-sdk

[wottpal]

amazing! would be personally fine with a breaking v4 release/pr