Prompt Validation

Prompt validation checks that prompt input contains the variables referenced by the prompt templates.

import { createChatPrompt, isLlmExeError } from "llm-exe";

const prompt = createChatPrompt<{ name: string }>("Hello {{name}}");

try {
  prompt.validate({});
} catch (error) {
  if (isLlmExeError(error, "prompt.missing_template_variable")) {
    console.log(error.context?.missingVariables);
  }
}

validate(input) does not render the prompt or call registered helpers. It only checks template references against the input object.

Strict Mode

Set validateInput: "strict" to validate before format() or formatAsync().

const prompt = createChatPrompt<{ name: string }>("Hello {{name}}", {
  validateInput: "strict",
});

prompt.format({});
// throws LlmExeError with code "prompt.missing_template_variable"

Use strict mode when a missing prompt variable should throw before calling the LLM.

Warn Mode

Set validateInput: "warn" to emit a Node warning and continue rendering.

const prompt = createChatPrompt<{ name: string }>("Hello {{name}}", {
  validateInput: "warn",
});

prompt.format({});

Warn mode uses process.emitWarning when available. In runtimes without process.emitWarning, rendering continues without a warning.

Default Behavior

By default, validateInput is false.

const prompt = createChatPrompt("Hello {{name}}");
prompt.format({});

This keeps the default prompt rendering behavior.

Missing Helpers

Validation also reports helper names that appear in templates but are not registered.

const prompt = createChatPrompt("{{unknownHelper input}}", {
  validateInput: "strict",
});

prompt.format({ input: "hello" });
// throws prompt.missing_template_variable

The error context contains:

{
  missingVariables: string[];
  missingHelpers: string[];
}

Prompt Existence

validate(input) validates template references. To check whether a prompt has messages, inspect messages directly:

const hasMessages = prompt.messages.length > 0;