Fix Zod v4 schema description extraction

[felixweinberger]

Zod v4 stores descriptions in z.globalRegistry, not in ._zod.def.description as the original implementation assumed. Both v3 and v4 expose a .description getter, so this simplifies getSchemaDescription() to just use that.

Motivation and Context

Users registering prompts with described arguments were not seeing the descriptions in the output:

server.prompt("test", { arg: z.string().describe("My description") }, ...)
// Expected: argument has description: "My description"
// Actual: description was missing

Root cause: The original v4 implementation looked for descriptions in ._zod.def.description, but this property doesn't exist in Zod v4.

See: https://github.com/colinhacks/zod/blob/main/packages/zod/src/v4/classic/schemas.ts shows the .description getter reads from the global registry:

Object.defineProperty(inst, "description", {
    get() {
      return core.globalRegistry.get(inst)?.description;
    },
});

The https://zod.dev/metadata confirm this architecture:

"The .describe() method is a shorthand for registering a schema in z.globalRegistry"

So the correct way to access descriptions is via the .description getter (which internally reads from the registry), not ._zod.def.description (which doesn't exist).

How Has This Been Tested?

• Added regression test in new test/issues/ folder (following Python SDK pattern)
• Test runs for both Zod v3 and v4 via zodTestMatrix
• All existing tests pass

Breaking Changes

None - this is a bug fix.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update

Checklist

• I have read the https://modelcontextprotocol.io
• My code follows the repository's style guidelines
• New and existing tests pass locally
• I have added appropriate error handling
• I have added or updated documentation as needed

Additional context

Fixes #1277

[pkg-pr-new]

Open in StackBlitz

npm i https://pkg.pr.new/modelcontextprotocol/typescript-sdk/@modelcontextprotocol/sdk@1296

commit: 03a6d09

[felixweinberger]

The ?? v3Schema._def?.description is unnecessary because ZodType on v3 already has this getter: https://github.com/colinhacks/zod/blob/main/packages/zod/src/v3/types.ts#L164-L166

get description(): string | undefined {
  return this._def.description;
}

[felixweinberger]

In v4 .describe() stores in globalRegistry with the .description getter reading from it directly via v4Schema.description rather than on _def so we don't need this.

https://github.com/colinhacks/zod/blob/main/packages/zod/src/v4/classic/schemas.ts#L223-L230

[felixweinberger]

@dclark27 @colinhacks @mattzcarey in case I'm missing any edge cases you were thinking of here with this helper - but it looks to me like branching on zod version might have been unnecessary here?

[mattzcarey]

lgtm