"Type instantiation is excessively deep and possibly infinite" typescript error when importing ToolCallback type

[matv-stripe]

Describe the bug
Hi! I am working on a wrapper tool around server.tool that allows you to define a tool with custom options called registerPaidTool.

I am trying to have registerPaidTool have a similar function signature to server.tool where the paramsSchema type defines the callback function:

export async function registerPaidTool<Args extends ZodRawShape>(
 mcpServer: McpServer,
 toolName: string,
 toolDescription: string,
 paramsSchema: Args,
 // @ts-ignore
 paidCallback: ToolCallback<Args>,
 options: PaidToolOptions
) {

However, the line paidCallback: ToolCallback<Args> results in "Type instantiation is excessively deep and possibly infinite.ts(2589)" (hence the @ts-ignore). I have also noticed this slows down the Typescript server in my IDE.

Full code here:
https://github.com/stripe/agent-toolkit/blob/main/typescript/src/modelcontextprotocol/register-paid-tool.ts#L22

To Reproduce
Steps to reproduce the behavior:

1. Clone https://github.com/stripe/agent-toolkit/blob/main/typescript/src/modelcontextprotocol/register-paid-tool.ts#L22
2. Delete the // @ts-ignore above paidCallback
3. In the typescript/ folder, run pnpm run build

Expected behavior
I would expect this error not to show up.

I am curious if anyone has run into this problem - am I doing something incorrect here, or is ToolCallback<Args> inherently too complex for the TS compiler since it has multiple zod schemas under the hood?

Thank you!

[quuu]

I just started running into this issue too -- sometimes shows up in my editor, sometimes fails my tests

but most recently very re-proable

[culli]

I was running into this too and lucked my way into the combination of @modelcontextprotocol/sdk@1.12.1 and zod@3.25.49 working. There is probably an incompatibility with zod somewhere in there. Hope this helps you too!

I'm experiencing the same issue, and updating the version did not resolve it. However, I came across this discussion in the Zod repository, which suggests that the problem can occur when multiple versions of Zod are installed. Ensuring that only a single version is present resolved the issue in my case.

[seawatts]

I'm getting this same issue. I've tried all the combinations of zod and the sdk. Still no luck. This grinds my IDE (Cursor) to a halt. I can't save any files it just hangs.

[wottpal]

same

[eduardoborges]

hey guys, i getting same problem, but im using this combination:

"dependencies": {
 "@modelcontextprotocol/sdk": "1.17.1",
 "agents": "^0.0.109",
 "zod": "^3.25.67"
}

i hope help.

[ycjcl868]

same, this issue was introduced starting from version >=1.16.0.

[colinhacks]

Without knowing much about this particular issue, this "Excessively deep" issue is likely fixed in Zod 4.1.6: colinhacks/zod#5222.

If you are using Zod 3.x, you can fix this issue by updating your tsconfig to use a modern module/moduleResolution setting instead of the legacy "node": https://x.com/colinhacks/status/1966261915115782654

To the maintainers: if you haven't already, I recommend checking out the Library Authors guide for best practices around peer dependencies and accepting user-defined schemas: https://zod.dev/library-authors

[Floriferous]

@colinhacks this library is currently not compatible with Zod 4 however right? Or is there a way to make it work right now?

#869 seems to be coming along though.

[oleduc]

Using v1.15.0 with zod v3.24.1 worked for.