A 5-line CLI that finds JSON schema errors in your LLM output before they crash your app

dev.to JavaScript2026-10-16 10:010 views

A 5-line CLI that finds JSON schema errors in your LLM output before they crash your app

LLMs are amazing at generating JSON, but they make mistakes. About 1 time in 20, they return JSON that crashes json.loads() with a syntax error. This breaks your application and creates a terrible user experience.

I built a simple zero-dependency Node CLI tool called "JSON-Schema Guard" that solves this problem. It extracts JSON from LLM output, validates it against a schema, and returns a repair prompt if validation fails.

The Problem

When you ask an LLM to generate JSON, you might get responses like:

{ "name": "John Doe", "age": 30, "email": "john@example.com" }

But sometimes you get:

{ "name": "John Doe", "age": 30, "email": "john@example.com", }

Notice the trailing comma? That's invalid JSON and will crash your application.

The Solution: JSON-Schema Guard

JSON-Schema Guard is a simple CLI tool that:

Extracts JSON from LLM output (even if there's extra text)
Validates it against your schema
Returns a repair prompt if validation fails

Here's how to use it:

# Install npm install -g json-schema-guard # Validate JSON against a schema json-schema-guard --schema user-schema.json --output "Here's the user data: {\"name\":\"John\",\"age\":30}" # If validation fails, it returns a repair prompt json-schema-guard --schema user-schema.json --output "Here's the user data: {\"name\":\"John\",\"age\":30,}" # Returns: "Fix the JSON syntax error: remove the trailing comma after '30'"

Why This Matters

Prevent crashes: Catch JSON errors before they break your application
Better UX: Get clear error messages instead of cryptic JSON parse errors
Zero dependencies: Works with any Node.js project
CI/CD ready: Exits with code 1 on validation errors for easy gating

Try It Yourself

JSON-Schema Guard is available on Gumroad as a pay-what-you-want download (minimum $1). It includes:

The CLI tool
A Node.js library for integration
Example schemas and usage patterns
Comprehensive documentation

Get JSON-Schema Guard →

What's Next?

I'm building more tools to help developers work with LLMs more reliably. Next up is a prompt comparison tool that helps you test and improve your prompts.

What JSON validation challenges are you facing with LLMs? Let me know in the comments!