API Response Contract Validator

Validate a real API response JSON against the response schema declared in an OpenAPI 3.x specification

Paste an OpenAPI 3.x document plus one actual API response payload, then point the validator at the path, method, and status code you want to check. The tool resolves the matching response schema and highlights missing fields, type mismatches, enum problems, and undocumented fields.

How to use it:

  • OpenAPI Spec: paste YAML or JSON spec content
  • Response JSON: paste the actual runtime response payload
  • Path / Method / Status Code: identify the operation and response branch to validate
  • Spec Format: leave auto unless you know the spec format already
  • Disallow Additional Properties: raise warnings for payload fields that are not documented in the schema

Example Results

1 examples

Check whether a production response still matches the documented user schema

Validate a real JSON response against the OpenAPI contract and surface missing or extra fields before the frontend breaks.

Contract validation issues
View input parameters
{ "openApiSpec": "openapi: 3.0.3\npaths:\n /users/{id}:\n get:\n responses:\n \"200\":\n description: ok\n content:\n application/json:\n schema:\n type: object\n required: [id, name]\n properties:\n id: { type: integer }\n name: { type: string }\n active: { type: boolean }", "responseJson": "{\"id\":\"1\",\"active\":\"yes\"}", "path": "/users/42", "method": "get", "statusCode": "200", "specFormat": "yaml", "disallowAdditionalProperties": true }

Key Facts

Category
Developer & Web
Input Types
textarea, text, select, checkbox
Output Type
html
Sample Coverage
4
API Ready
Yes

Overview

The API Response Contract Validator is a developer tool that checks real JSON payloads against OpenAPI 3.x specifications. By providing your OpenAPI document, an actual API response, and the specific endpoint details, you can instantly identify missing fields, type mismatches, enum violations, and undocumented properties to ensure strict contract adherence.

When to Use

  • •When verifying that a backend API implementation matches the documented OpenAPI specification before deploying to production.
  • •When debugging frontend integration issues caused by unexpected or missing fields in an API response.
  • •When auditing legacy APIs to identify undocumented fields being returned in production payloads.

How It Works

  • •Paste your OpenAPI 3.x specification (in YAML or JSON format) and the actual JSON response payload you want to validate.
  • •Specify the endpoint path, HTTP method, and status code to locate the exact response schema within your OpenAPI document.
  • •Optionally enable the 'Disallow Additional Properties' setting to flag any fields in the payload that are not explicitly defined in the schema.
  • •The tool parses the specification and outputs an HTML report highlighting schema violations, type mismatches, and missing required fields.

Use Cases

Validating a newly developed API endpoint against its design-first OpenAPI contract.
Troubleshooting a broken frontend application by checking if the backend response violates the expected schema.
Writing consumer-driven contract tests and needing a quick manual validation of a specific endpoint's payload.

Examples

1. Validating a GET User Response

Backend Developer
Background
A developer just implemented a new /users/{id} endpoint and wants to ensure the JSON response matches the agreed-upon OpenAPI contract.
Problem
Need to verify that the API returns all required fields with the correct data types without writing a custom test script.
How to Use
Paste the OpenAPI YAML and the JSON response {"id":"1","active":"yes"}. Set the path to /users/42, method to GET, and status code to 200.
Example Config
Disallow Additional Properties: true
Outcome
The tool highlights that 'id' is a string instead of an integer, 'name' is missing, and 'active' is a string instead of a boolean.

2. Detecting Undocumented Fields in Production

QA Engineer
Background
During testing, a QA engineer notices the API is returning extra data that isn't documented in the API specification.
Problem
Identify exactly which fields in the production payload are missing from the OpenAPI schema.
How to Use
Paste the production JSON response and the OpenAPI spec. Enter the endpoint details and check 'Disallow Additional Properties'.
Example Config
Method: POST, Status Code: 201, Disallow Additional Properties: true
Outcome
The validator flags the extra fields in the JSON payload, allowing the team to either update the OpenAPI spec or remove the data leak from the backend.

Try with Samples

json
Postman Collections - API Testing
Comprehensive Postman collection examples including API testing, automation scripts, environment variables, mock servers, and advanced testing patterns for REST APIs
title token api
json
AWS EventBridge Samples
AWS EventBridge examples including event buses, rules, targets, schema registry, custom events, and cross-account event routing for serverless event-driven architecture
preferred input family json
json
Apache Pulsar Samples
Apache Pulsar examples including pub/sub, readers, consumers with different subscription types, schema management, and tiered storage for enterprise messaging
preferred input family json
json
OpenAPI/Swagger Samples
Comprehensive API documentation examples using OpenAPI 3.0 and Swagger specifications for RESTful services
preferred input family json
json

Related Hubs

Json Validate Tools
Explore 7 json tools for validate workflows and compare closely related utilities quickly.
Json Convert Tools
Explore 100 json tools for convert workflows and compare closely related utilities quickly.
Json Tools
Explore 45 json tools for utility workflows and compare closely related utilities quickly.
Json Generate Tools
Explore 33 json tools for generate workflows and compare closely related utilities quickly.

FAQ

Does this tool support OpenAPI 2.0 (Swagger)?

No, this validator specifically requires OpenAPI 3.x specifications to resolve response schemas accurately.

Can I validate YAML and JSON specifications?

Yes, the tool accepts both YAML and JSON formats. You can set the format manually or leave it on 'Auto' for automatic detection.

What does 'Disallow Additional Properties' do?

It enforces strict validation by throwing warnings if the response JSON contains fields that are not documented in the OpenAPI schema.

How do I format the path input?

Enter the actual request path, such as /users/123. The tool will match it against the templated paths in your OpenAPI specification.

Can I validate error responses?

Yes, simply enter the corresponding error status code (like 400 or 404) to validate the payload against that specific error schema.

API Documentation

Request Endpoint

POST /en/api/tools/api-response-contract-validator

Request Parameters

Parameter Name Type Required Description
openApiSpec textarea Yes -
responseJson textarea Yes -
path text Yes -
method select No -
statusCode text Yes -
specFormat select No -
disallowAdditionalProperties checkbox No -

Response Format

{
  "result": "
Processed HTML content
",
  "error": "Error message (optional)",
  "message": "Notification message (optional)",
  "metadata": {
    "key": "value"
  }
}
HTML: HTML

AI MCP Documentation

Add this tool to your MCP server configuration: 

{
  "mcpServers": {
    "elysiatools-api-response-contract-validator": {
      "name": "api-response-contract-validator",
      "description": "Validate a real API response JSON against the response schema declared in an OpenAPI 3.x specification",
      "baseUrl": "https://elysiatools.com/mcp/sse?toolId=api-response-contract-validator",
      "command": "",
      "args": [],
      "env": {},
      "isActive": true,
      "type": "sse"
    }
  }
}

You can chain multiple tools, e.g.: `https://elysiatools.com/mcp/sse?toolId=png-to-webp,jpg-to-webp,gif-to-webp`, max 20 tools.

If you encounter any issues, please contact us at [email protected]