How to modify the Response Body of HTTP requests?

Modern web development often demands flexibility when testing and debugging APIs. Whether you’re simulating server responses, injecting mock data, or bypassing backend limitations, the ability to modify the HTTP response body (resp.body) in real time is a powerful capability.

In this guide, you’ll learn how to use the Requestly Chrome extension to override response bodies—quickly, safely, and without altering the server.

What is the HTTP Response Body?

The HTTP response body, commonly referred to in code as resp.body is the part of an HTTP response that contains the actual data sent by the server to the client. This data can be in various formats like:

• JSON (APIs)
• HTML (web pages)
• XML
• Plain text
• Binary files (images, PDFs, etc.)

When a client (like a browser or mobile app) makes a request to a server, it receives an HTTP response that includes:

• Status line (for example, 200 OK, 404 Not Found)
• Headers (metadata like Content-Type, Content-Length)
• Body – the actual payload/data being requested

Here’s a simplified example of an HTTP response with a body:

css

HTTP/1.1 200 OK  

Content-Type: application/json  

Content-Length: 58  



{ "userId": 123, "name": "Alice", "role": "admin" }

In this example, the JSON string is the response body, which the frontend code often reads via response.body, response.text(), or response.json() depending on the language or framework.

Why Modify HTTP Response Bodies?

Modifying the HTTP response body is a valuable technique for developers and testers to control or simulate how an application behaves, without changing the backend or API source. Here are some common reasons for doing so:

• Testing edge cases: You can simulate rare scenarios such as null values, missing fields, or unexpected data formats to ensure the frontend handles them gracefully.
• Frontend-backend decoupling: Frontend developers can continue building and testing UI components even if the backend is incomplete, unavailable, or unstable.
• Bypassing backend limitations: Override static or restricted responses to test different application states, such as user roles, permissions, or feature flags.
• Debugging UI issues: Inject specific mock data into the response body to reproduce bugs or UI behavior for easier debugging.
• Simulating offline or error states: Test how the app responds to partial, delayed, or malformed responses, which is critical for improving user experience during failures.
• Previewing A/B variants: Serve alternate versions of data to the frontend to test layout changes or logic differences without changing backend configurations.

Tools to Modify HTTP Responses in Real Time

Several tools allow developers and testers to intercept and modify HTTP responses as they flow between the server and client. These are particularly useful for debugging, testing edge cases, or simulating different backend behaviors. Here are some commonly used options:

1. Requestly

Requestly is a lightweight Chrome extension designed to intercept, modify, and redirect HTTP(S) requests and responses directly within the browser. It’s built for developers, testers, and QA engineers who need real-time control over frontend behavior without touching backend code.

Key Features

• Modify HTTP response body and headers without writing scripts
• Create rules to rewrite URLs, block requests, or simulate delays
• GUI-based editor for modifying JSON, HTML, or plain-text responses
• Workspace sharing for teams and version control
• Cloud sync and CLI support for automation
• Works seamlessly with BrowserStack, Cypress, Playwright, and more

Pros:

• No coding required
• Browser-native and beginner-friendly
• Ideal for quick testing and frontend development
• Free plan available with robust features

2. Chrome DevTools

Built into every Chromium-based browser, Chrome DevTools allows you to inspect, debug, and occasionally manipulate frontend behavior. While powerful for diagnostics, it’s not built for persistent or rule-based response modification.

Key Features

• Inspect and modify DOM, cookies, and local storage
• Network tab to analyze HTTP requests/responses
• Live editing of frontend code and JS debugging
• Snippets and overrides for local development

Pros:

• Built-in and always available
• Great for debugging and inspection
• No additional installation required

Cons:

• Limited support for modifying actual response bodies
• Not rule-based or automatable
• Edits are temporary and not repeatable

3. Postman Interceptor

Postman is a leading API testing platform, and its Interceptor extension lets you capture and inspect browser traffic. It’s more focused on sending and validating requests rather than modifying live responses.

Key Features

• API request building and testing
• Captures browser cookies and requests
• Response validation and scripting
• Automation and collection runners for CI/CD pipelines

Pros:

• Excellent for API testing and development
• Interceptor helps bridge the browser and Postman

Cons:

• No built-in ability to modify live HTTP response bodies
• Heavier than needed for a simple response overrides

4. Fiddler

Fiddler is a powerful proxy tool that sits between your system and the network, allowing full control over HTTP and HTTPS traffic. It’s suited for in-depth traffic manipulation and system-wide use.

Key Features

• Modify, replay, and inspect all HTTP/S traffic
• Supports response mocking, rewriting, and scripting
• System-wide interception (browser, apps, services)
• Plugin support for automation

Pros:

• Powerful and highly configurable
• Suitable for enterprise-level debugging
• Supports non-browser environments

Cons:

• Complex UI with a learning curve
• Requires installation and setup
• Overkill for simple frontend testing

5. Charles Proxy

Charles Proxy is a cross-platform web debugging tool that allows you to view and alter network traffic. Commonly used in mobile and desktop app testing, it offers SSL proxying and response rewriting.

Key Features

• Modify requests and responses with rewrite rules
• Throttle bandwidth or simulate slow networks
• SSL proxying and session recording
• iOS and Android support via proxy configuration

Pros:

• Reliable and widely used
• Supports mobile device testing
• Good for latency simulation

Cons:

• Paid software with limited free use
• UI feels dated
• Less intuitive than browser-based tools like Requestly

Step-by-Step: Modify Response Body using BrowserStack Requestly

With BrowserStack Requestly, you can intercept and modify HTTP responses directly in your browser, without touching backend code or writing scripts.

Follow these steps to override a response body using BrowserStack Requestly:

Step 1: Install the BrowserStack Requestly Extension

• Open Google Chrome (or any Chromium-based browser).
• Visit the BrowserStack Requestly extension page.
• Click Add to Chrome and complete the installation.

Step 2: Open the Target Website or Web App

• Navigate to the page that triggers the HTTP request you want to intercept.
• Make sure the target API call occurs when the page loads or an action is triggered (for example, form submission, button click).

Step 3: Create a Modify Response Rule

• Click the Requestly icon in the browser toolbar.
• Choose New Rule > Modify Response.
• Configure your rule:
  • URL Contains: Enter part or all of the API endpoint URL you want to match.
  • Response Body: Paste the new response body (for example, JSON, HTML, or plain text).
  • Optional: Change the status code, headers, or content type.

Step 4: Save and Enable the Rule

• Click Save and make sure the rule toggle is turned on.
• You can name the rule (for example, “Mock user data” or “Simulate 500 error”) for easy identification.

Step 5: Reload the Page to Trigger the Rule

• Refresh the website.
• When the matching request is made, the browser will receive your modified response body instead of the real server response.

Step 6: Inspect the Result

• Open DevTools > Network tab to confirm the modified response.
• Verify that the frontend behaves correctly with the mocked data.

Tip: You can create multiple Modify Response rules to simulate different user roles, error messages, or data states without backend changes—making frontend testing faster and more reliable.

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Best Practices when Overriding HTTP Response Bodies

Overriding the HTTP response body can greatly speed up frontend development and testing—but doing it incorrectly can lead to false positives, missed bugs, or unnecessary confusion. Here are key best practices to follow when using tools like BrowserStack Requestly:

1. Be Precise with URL Matchers: Use specific and unique URL patterns (like exact paths or query parameters) in your rules to avoid unintentionally overriding unrelated API calls.
2. Maintain Valid Syntax: Ensure your overridden response body is well-formed. For example, if you’re injecting JSON, validate it using a linter or online tool to prevent runtime errors on the frontend.
3. Mirror the Real API Structure: Try to keep your mock data structure as close as possible to the actual backend response. This ensures that your frontend behaves realistically and avoids overfitting to ideal conditions.
4. Use Rule Naming Conventions: Label your rules clearly (for example, MockUser-AdminRole, Simulate404-ProductsAPI) so you can quickly identify and switch between different test scenarios.
5. Toggle Rules On/Off Cleanly: Disable rules after each test to avoid affecting unrelated sessions or future browser behavior. Keep your rule list tidy and active only when needed.
6. Test with Both Success and Failure Cases: Simulate both valid and invalid responses—like empty arrays, missing fields, or 500 errors—to ensure the frontend gracefully handles all states.
7. Document Your Mock Scenarios: If working in a team, document what each mock or override does. Shared understanding helps avoid duplicated efforts and aligns testing workflows.
8. Avoid Use in Production Environments: Never keep response override extensions active during production use. They can mask real issues or expose sensitive testing logic.
9. Version Control Your Rules (if possible): BrowserStack Requestly supports shared workspaces—use them to maintain versioned rules, especially in collaborative QA or CI workflows.

Conclusion

Modifying the HTTP response body is a powerful technique that enables faster, more flexible frontend testing without relying on backend changes or infrastructure setups.

Whether you’re simulating edge cases, testing UI behavior, or debugging real-time issues, having the ability to override resp.body gives developers and QA teams precise control over what the frontend receives.

Among the various tools available, BrowserStack Requestly stands out as the most user-friendly and browser-native solution. It allows you to intercept, edit, and test network traffic directly from your browser—no coding required, no backend dependencies, and no environment bottlenecks.

By following best practices and leveraging tools like Requestly, teams can streamline testing workflows, improve coverage, and ship more reliable web applications faster.

Try BrowserStack Requestly

FAQs about Modifying resp.body in HTTP

1. What does resp.body mean?

resp.body refers to the content (typically JSON, HTML, or plain text) returned by an HTTP response. Modifying it means changing the actual payload sent from the server before it’s rendered or processed by the frontend.

2. Can I modify resp.body directly in the browser?

Yes, with tools like BrowserStack Requestly, you can intercept HTTP responses and override the response body in real time—no need to alter backend servers or use mock APIs.

3. What types of response bodies can be modified?

You can modify:

• JSON (e.g., API responses)
• HTML (e.g., page fragments)
• Plain text
• XML

Binary content, like images or PDFs, is not editable via response override tools.

4. Is it possible to simulate error responses like 404 or 500?

Yes. With tools like Requestly, you can change both the response body and the HTTP status code to simulate server errors or alternate conditions.

5. How is this different from using mock APIs?

Mock APIs simulate endpoints externally, often requiring setup. Modifying resp.body happens inline, giving you faster, rule-based control directly in the browser—ideal for isolated, quick testing.