eyaltoledano/claude-task-master · error

Prompt must be a non-empty string

Error message

Prompt must be a non-empty string

What it means

validatePrompt() is the base provider's input guard for completion calls. It throws this error when the prompt is falsy or not a string — i.e. null, undefined, a number, or an empty string was passed where a prompt is required.

Source

Thrown at packages/tm-core/src/modules/ai/interfaces/ai-provider.interface.ts:416

			stream: false,
			topP: 1.0,
			frequencyPenalty: 0.0,
			presencePenalty: 0.0,
			timeout: 30000,
			retries: 3,
			...this.config.defaultOptions,
			...userOptions
		};
	}

	/**
	 * Validate prompt input
	 * @param prompt - Prompt to validate
	 * @throws Error if prompt is invalid
	 */
	protected validatePrompt(prompt: string): void {
		if (!prompt || typeof prompt !== 'string') {
			throw new Error('Prompt must be a non-empty string');
		}

		if (prompt.trim().length === 0) {
			throw new Error('Prompt cannot be empty or only whitespace');
		}
	}
}

View on GitHub (pinned to c0c98d367c)

Solutions

1. Ensure the prompt is a non-empty string before calling generateCompletion(prompt, options)
2. Stringify/serialize structured messages yourself before passing (e.g. messages.map(m => m.content).join('\n'))
3. Coalesce defaults: const prompt = userPrompt ?? fallbackPrompt

Example fix

// before
await provider.generateCompletion(messages); // array, not string
// after
const prompt = messages.map(m => m.content).join('\n');
await provider.generateCompletion(prompt);

if (typeof prompt !== 'string' || prompt.length === 0) throw new Error('generateCompletion requires a non-empty string prompt');

function isUsablePrompt(p: unknown): p is string {
  return typeof p === 'string' && p.length > 0;
}

try {
  return await provider.generateCompletion(prompt, options);
} catch (err) {
  if (err.message === 'Prompt must be a non-empty string') {
    throw new Error('Prompt building failed — check upstream variables');
  }
  throw err;
}

When it happens

Trigger: generateCompletion(null as any); generateCompletion(undefined) when a variable failed to populate; passing an options object or message array instead of a string; calling with '' after a failed upstream string interpolation.

Common situations: Template variables left undefined in prompt-building code; API refactors that now pass message arrays to a method expecting a plain string; reading a prompt from config/DB that came back null.

Related errors

• Prompt cannot be empty or only whitespace
• Model "${model}" is not available for provider "${this.getNa
• AUTHENTICATION_ERROR
• VALIDATION_ERROR
• ${this.getName()} provider error: ${errorMessage}

AI-assisted analysis of eyaltoledano/claude-task-master@c0c98d367c (2026-08-29). Data as JSON: /api/errors/4c0f04ab761c1bbf. Report an issue: GitHub.