> ## Documentation Index
> Fetch the complete documentation index at: https://fastmcp-ts.docs.prefect.io/llms.txt
> Use this file to discover all available pages before exploring further.

# CacheKeyFn

```ts theme={null}
type CacheKeyFn = (ctx) => string;
```

Defined in: [server/middleware.ts:146](https://github.com/PrefectHQ/fastmcp-ts/blob/e4b6038f642b529f601463a576229747afebe9a4/src/server/middleware.ts#L146)

Custom cache key function. Receives the full middleware context so callers can
incorporate auth identity or any other dimension into the key.

**The default key already partitions by auth identity — you do not need a `keyFn`
for auth safety.** The default is
`"method:<auth-partition>:JSON(params)"`, where the auth partition is `anon` for a
request with no bearer token and the SHA-256 hash of the bearer token for an
authenticated request. So a result computed under one identity is never served under
another (anonymous and authenticated included, both directions), and auth-filtered
list results (tools, resources, prompts) stay per identity.

**A custom `keyFn` REPLACES the default partitioning entirely — you then own it.**
The key you return is used verbatim; the auth partition is not merged in. If your
cached values depend on the caller identity, include an identity dimension yourself.
Use the same dimension the default uses — the bearer token, HASHED. Never place the
raw token in a cache key:

```ts theme={null}
import { createHash } from 'node:crypto'

new CachingMiddleware(60_000, (ctx) => {
  const token = ctx.mcpContext.auth?.token
  const id = token ? createHash('sha256').update(token).digest('hex') : 'anon'
  return `${ctx.method}:${id}:${JSON.stringify(ctx.request)}`
})
```

`clientId` is a coarser alternative (`ctx.mcpContext.auth?.clientId`), but two tokens
may share a `clientId` yet differ in scope, so a `clientId` key can merge identities
that must stay apart. Prefer the hashed token.

## Parameters

### ctx

[`MiddlewareContext`](/api/server/interfaces/MiddlewareContext)

## Returns

`string`