eyaltoledano/claude-task-master · error

estimateTokens must be a function

Error message

estimateTokens must be a function

What it means

When supplied, the estimateTokens option must be a callable used to estimate token counts for streamed items. The constructor throws if the value is truthy but not a function. Passing a non-function value indicates a wiring mistake, so fail-fast is preferred over runtime crashes mid-stream.

Source

Thrown at src/utils/stream-parser.js:68

		this.validate();
	}

	validate() {
		if (!this.jsonPaths || !Array.isArray(this.jsonPaths)) {
			throw new Error('jsonPaths is required and must be an array');
		}
		if (this.jsonPaths.length === 0) {
			throw new Error('jsonPaths array cannot be empty');
		}
		if (this.maxBufferSize <= 0) {
			throw new Error('maxBufferSize must be positive');
		}
		if (this.expectedTotal < 0) {
			throw new Error('expectedTotal cannot be negative');
		}
		if (this.estimateTokens && typeof this.estimateTokens !== 'function') {
			throw new Error('estimateTokens must be a function');
		}
		if (this.onProgress && typeof this.onProgress !== 'function') {
			throw new Error('onProgress must be a function');
		}
		if (this.onError && typeof this.onError !== 'function') {
			throw new Error('onError must be a function');
		}
		if (
			this.fallbackItemExtractor &&
			typeof this.fallbackItemExtractor !== 'function'
		) {
			throw new Error('fallbackItemExtractor must be a function');
		}
		if (this.itemValidator && typeof this.itemValidator !== 'function') {
			throw new Error('itemValidator must be a function');
		}
	}

View on GitHub (pinned to c0c98d367c)

Solutions

1. Pass an actual function: estimateTokens: (item) => JSON.stringify(item).length / 4.
2. If the value comes from config, look it up from a function registry instead of the raw config value.
3. Remove the option entirely if token estimation is not needed.

Example fix

// before
new StreamParser({ jsonPaths: ['$.items'], estimateTokens: 'estimateTokens' })
// after
const estimateTokens = (item) => Math.ceil(JSON.stringify(item).length / 4);
new StreamParser({ jsonPaths: ['$.items'], estimateTokens })

if (options.estimateTokens != null && typeof options.estimateTokens !== 'function') {
  throw new TypeError('estimateTokens must be a function');
}

function isCallable(v) {
  return typeof v === 'function';
}

try {
  return new StreamParser(options);
} catch (err) {
  if (err.message === 'estimateTokens must be a function') {
    const { estimateTokens, ...rest } = options;
    return new StreamParser(rest); // drop invalid callback
  }
  throw err;
}

When it happens

Trigger: new StreamParser({ jsonPaths: [...], estimateTokens: true }), estimateTokens: 'someTokenCounter' (passing a name instead of a reference), or a misconfigured DI container returning undefined-wrapped values.

Common situations: Passing the string name of a function instead of the function; importing a default export that is an object wrapping the counter; YAML/JSON config where only strings are representable.

Understand the failure class

Background: "Wrong argument type", "must be a string", "expected Array or Prism::Scope": TypeError and ArgumentError when a library receives a value of the wrong type — this error's family across 28 libraries.

Related errors

• MISSING_CONFIGURATION
• INVALID_INPUT
• Unknown provider '${providerName}' for API key resolution.
• Parameter validation failed: ${errors.join('; ')}
• ${this.name} Model ID is required

AI-assisted analysis of eyaltoledano/claude-task-master@c0c98d367c (2026-08-29). Data as JSON: /api/errors/2b9fac0315ef98e2. Report an issue: GitHub.