Validate JSON payloads against a schema with detailed error messages (no AI)

What is this?

A modular schema checker that returns detailed error messages on validation failure.

Stop your workflows from breaking due to bad input. This subworkflow validates incoming JSON against a schema you define and returns clear, human readable errors before anything has the chance to break.

The most common use: a webhook receives data, you validate it, and either continue or return a 400 with exactly what was wrong. This is how production APIs handle input validation, and this template brings that same reliability to webhooks in n8n.

Works anywhere you need to check data: webhook payloads, form submissions, API requests, or LLM outputs.

Supports: type checks, required fields, enums, regex patterns, numeric ranges, array constraints, conditional logic (oneOf / anyOf / allOf), and rejection of unknown fields.

Why use this?

Bad input breaks workflows and is a nightmare to debug. This catches it upfront so you can error fast, fail safe, and return useful messages instead of breaking down.

It also makes a self documenting API. The error messages are so clear that the caller doesn't even need to check docs, they're given the fix right in the error.

How it works

Call this workflow via an Execute Sub-Workflow node with two params:

• requiredSchema - the JSON Schema defining what valid data looks like
• paramsToValidate - the actual JSON to check (for instance, $json.body)

requiredSchema must be an expression wrapped in {{ }} to be treated like an object.

Returns { valid: true } on success. On failure, returns valid: false with a validationError string and the full requiredSchema so you know exactly what went wrong and what was expected.

see the "Usage Example" in the template

Example error output

Validation failed (3 issues):
• name: Missing required field "name" - Customer full name
• email: "not-an-email" is not valid - expected: Contact email address
• plan: "premium" is not an allowed value. Must be one of: starter, pro, enterprise

Every error includes the field path, what went wrong, and the description from your schema.

Don't know JSON Schema?

The template includes a prompt template sticky note. Copy it into any LLM chat with an example of your data and it'll generate a ready-to-use schema for you.

Quick start

1. Add an Execute Sub-Workflow node pointing to this workflow
2. Set requiredSchema to ={{ your_schema_here }}
3. Set paramsToValidate to ={{ $json.body }}
4. Route on valid: true continues, false handles the error