Real Time
Override Request Body
Test different responses
Works for REST & GraphQL APIs
Change user token during testing
Trusted by 300,000+ developers from Fortune 500 companies.
Modify Request Body Without Changing Code
Instantly intercept and update fetch or XHR request payloads from your browser. Simulate edge cases, test error flows, and speed up development, no backend changes needed.
Test Error Scenarios Without Backend Changes Learn more -> 
Supports APIs triggered as XHR or fetch
Requestly handles daily dev tasks like bug replication, API mocking, and testing. Learn more -> 
GraphQL Support
It supports GraphQL APIs. Select GraphQL as the resource type, filter by operationName, and modify the query or variables using static JSON or custom JavaScript. Learn more ->
Match URLs using Regex Learn more -> 
Collaboration & 1-Click Sharing
Promote consistency and accelerate development by sharing Requestly rules within your team. 
More features for your testing needs
These HTTP Rules designed for all your local development and testing needs. Modify Responses Intercept, Modify & Mock API Responses in Real-Time Replace string Replace part of URL like hostname, query values. Redirect URL Redirect any URL to a new destination. Modify Headers Modify HTTP request & response headers. Query params Add or Remove Query Parameters Insert scripts Inject Custom JS & CSS on any website(s). Delay network Simulate delay in response of API calls. Modify request body Modify body of website or application POST requests.
Frequently Asked Questions
What does the Modify Request Body rule do in Requestly?
The Modify Request Body rule lets you intercept outgoing API requests and change the request payload before it reaches the server. This helps you test different request scenarios without changing your frontend code or backend setup.
What request formats can I modify using this rule?
You can modify JSON, XML, form data, plain text, and other text-based request payloads. This makes it easy to test APIs that accept different request formats.
Is Requestly open-source
Feel free to check out the GitHub repo
Is the Modify Request Body rule available in the free plan?
Yes, the Modify Request Body rule is available in Requestly’s free plan for local API testing and debugging.
Do I need a paid plan to create multiple API response modification rules?
No, you can create and use multiple response modification rules on the free plan. Paid plans mainly unlock collaboration, syncing, and team workflows.
Is Requestly free for students?
Yes! Requestly is free for students, enjoy access to powerful tools and features with no cost.
My Modify Request Body rule is not working. What should I check?
First, make sure the rule is enabled, the URL matching condition is correct, and the outgoing request matches your configured endpoint. Also verify that Requestly interception is active.
Why are my request body changes not reaching the server?
The request may not be matching your rule, or the payload format may be different from what you expected. Inspect the actual request in browser DevTools and verify the request body structure.
Why Aren’t My Requestly Rules Working?
How do I change a JSON field in an API request using Requestly?
Create a Modify Request Body rule, add the API endpoint in the source condition, select the JSON field you want to update, enter the new value, save the rule, and trigger the API call again. Matching requests will now send the modified payload.
How do I test different user roles or permissions without changing frontend code?
Intercept the API request that sends user data, modify fields like role, account type, or permissions, and resend the request. This lets you test different backend scenarios instantly.
How do I test invalid or edge-case payloads?
Use the Modify Request Body rule to send empty values, invalid formats, large payloads, null fields, or missing parameters. This helps you test server-side validation and error handling.