---
url: 'https://adk.nht.io/api/@nhtio/adk/type-aliases/TokenEstimatorFn.md'
description: >-
  A custom token-count function registered for a non-built-in {@link
  TokenEncodingId} via [`registerTokenEstimator`](https://adk.nht.io/api/@nhtio/adk/functions/registerTokenEstimator).
---

# Type Alias: TokenEstimatorFn

```ts
type TokenEstimatorFn = (value: string) => number;
```

Defined in: [src/lib/classes/tokenizable.ts:112](https://github.com/NHTIO/ADK/blob/v1.20260713.1/src/src/lib/classes/tokenizable.ts#L112)

A custom token-count function registered for a non-built-in [TokenEncodingId](TokenEncodingId.md) via
[registerTokenEstimator](../functions/registerTokenEstimator.md).

## Parameters

| Parameter | Type     | Description                                                                                                                                                       |
| --------- | -------- | ----------------------------------------------------------------------------------------------------------------------------------------------------------------- |
| `value`   | `string` | The already-RESOLVED text to count (the same string `countFor` measures for the built-in backends — i.e. `render(ctx)`'s output, never the raw evaluator or ctx). |

## Returns

`number`

The estimated token count for `value`.

## Remarks

Synchronous, matching the built-in estimation backends: every backend behind the
[TokenEncoding](../common/variables/TokenEncoding.md) switch (`js-tiktoken`, `@lenml/tokenizer-gemini`, `llama-tokenizer-js`, the
Claude heuristic) resolves synchronously, and [Tokenizable.estimateTokens](../common/classes/Tokenizable.md#estimatetokens) itself returns a
plain `number` — not a `Promise<number>`. A custom estimator therefore must not be async either, so
a registered encoding stays a drop-in peer of the built-ins (same call shape, no `await` threaded
through `estimateTokens` for some encodings and not others). Wrap an inherently-async tokenizer in a
synchronous cache (measure ahead of time / memoize a warmed instance) before registering it.