eyaltoledano/claude-task-master · error

itemValidator must be a function

Error message

itemValidator must be a function

What it means

The itemValidator option must be a callable used to filter/validate parsed items (the library defaults to a validator requiring a non-empty title string). The constructor throws if itemValidator is truthy but not a function.

Source

Thrown at src/utils/stream-parser.js:83

			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');
		}
	}

	static defaultItemValidator(item) {
		return (
			item && item.title && typeof item.title === 'string' && item.title.trim()
		);
	}
}

/**
 * Manages progress tracking and metadata
 */
class ProgressTracker {
	constructor(config) {
		this.onProgress = config.onProgress;
		this.onError = config.onError;
		this.estimateTokens = config.estimateTokens;

View on GitHub (pinned to c0c98d367c)

Solutions

1. Pass a predicate function: itemValidator: (item) => typeof item?.id === 'string'.
2. If using a schema, wrap it: itemValidator: (item) => schema.safeParse(item).success.
3. Remove the option to use the built-in defaultItemValidator.

Example fix

// before
new StreamParser({ jsonPaths: ['$.items'], itemValidator: itemSchema })
// after
new StreamParser({ jsonPaths: ['$.items'], itemValidator: (item) => itemSchema.safeParse(item).success })

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

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

try {
  return new StreamParser(options);
} catch (err) {
  if (err.message === 'itemValidator must be a function') {
    return new StreamParser({ ...options, itemValidator: (item) => Boolean(item && item.title) });
  }
  throw err;
}

When it happens

Trigger: new StreamParser({ jsonPaths: ['$.items'], itemValidator: true }) or itemValidator set to a schema object/Zod schema instead of a predicate function.

Common situations: Passing a Zod/Joi schema object expecting the parser to integrate it natively; passing a validator name string; copy-paste from a library whose validator option accepts schemas.

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

• onProgress must be a function
• onError must be a function
• fallbackItemExtractor must be a function
• MISSING_CONFIGURATION
• INVALID_INPUT

AI-assisted analysis of eyaltoledano/claude-task-master@c0c98d367c (2026-08-29). Data as JSON: /api/errors/63d5a003902fe59f. Report an issue: GitHub.