> ## Documentation Index
> Fetch the complete documentation index at: https://colin-feat-client-cache-mode.mintlify.site/llms.txt
> Use this file to discover all available pages before exploring further.

# AssertionSource

```ts theme={null}
type AssertionSource = string | (() => string | Promise<string>);
```

Defined in: [client/auth.ts:746](https://github.com/PrefectHQ/fastmcp-ts/blob/8976c37f79efa6b87809b884f29a1f235a8c4ad1/src/client/auth.ts#L746)

A workload assertion: a signed JWT string, or an (async) factory for one.