How to Debug API Calls?

APIs often fail due to unexpected inputs, backend errors, or integration mismatches. Debugging helps identify the root cause by inspecting the request and response flow between client and server.

This article explains how to debug API calls directly in the browser or desktop environment.

What Is API Debugging?

API debugging means checking how an API behaves when it receives a request and sends back a response. It helps you understand why a request failed, returned the wrong data, or behaved differently than expected.

For example, say your frontend form submits a user’s email and password to a login API, but the response shows “unauthorized” even though the credentials are correct. Debugging helps you inspect the request and response to determine if the issue is a missing header, an invalid token, or a backend error.

You use API debugging to:

• Confirm that the request is reaching the correct endpoint with valid parameters.
• Check whether the response contains the expected data or an error message
• Identify problems with authentication or missing headers.
• Detect logic or validation errors in the backend.
• Understand how external APIs behave with different inputs.

Common Scenarios Where Debugging Is Crucial

These are situations where debugging gives you the visibility needed to track down and fix API issues that can break features or block integrations.

• Authentication errors: The API returns a 401 or 403 error even though the correct credentials or tokens are provided. This could be due to missing headers, expired tokens, or incorrect scopes.
• Environment mismatch: A request works in staging but fails in production. Debugging helps compare base URLs, headers, or payloads that might differ across environments.
• Broken responses: The API responds, but key fields are missing, renamed, or have the wrong structure. This can lead to UI crashes or data misrepresentation.
• Data not saved: A POST or PUT request appears successful (like 200 OK), but the backend doesn’t reflect any changes. Debugging helps confirm whether the payload is complete and correctly formatted.
• Slow responses: The API takes too long to respond or times out entirely. You may need to simulate load or delays to reproduce and investigate bottlenecks.
• Missing parameters: A required header, query parameter, or value is missing, which leads to validation errors or incorrect processing by the server.
• CORS failures: The browser blocks API calls due to missing or misconfigured CORS headers on the server. Debugging helps verify how the server responds to preflight or actual requests.
• Third-party failures: A payment gateway, analytics service, or third-party API suddenly breaks, and debugging is needed to isolate whether the issue is with your request or their system.

Core Concepts You Need to Know Before Debugging

Before debugging an API call, it’s essential to know how requests and responses are structured and how each part affects the system’s behavior.

• HTTP methods: APIs use methods like GET, POST, PUT, and DELETE to perform specific actions. Each method has rules, and using the wrong one can result in errors or unexpected changes.
• Status codes: These are three-digit codes that indicate the result of a request. Codes in the 2xx range mean success, 4xx signals client errors, and 5xx points to server-side failures.
• Request vs. response: The request contains the data sent to the API, while the response contains what the server returns. A mismatch between the two can lead to errors or missing data.

• Headers: These include metadata such as content type, authorization tokens, and custom values. APIs may reject requests if required headers are missing or incorrect.

• Query parameters and body: Some APIs expect data in the query string, others in the request body. Errors can occur if parameters are misplaced, incomplete, or incorrectly formatted.
• CORS and origin policy: Browsers block requests to APIs from different origins unless the server includes specific CORS headers. A missing or misconfigured CORS setup results in blocked requests.
• Timeouts and retries: APIs may terminate requests that take too long or may have retry logic that affects how calls behave under load or network delays.
• Authentication: Most APIs require keys, tokens, or session identifiers. Requests without valid credentials are typically rejected with 401 or 403 responses.

Step-by-Step Debugging with Requestly

Requestly is a debugging and testing tool that lets you directly intercept, modify, redirect, or mock HTTP requests and responses in the browser or desktop app. It applies rules targeting specific requests based on URL, method, or headers, allowing developers to test APIs without changing the application code.

Here’s how to use Requestly Modify Request Body to debug API calls.

Step 1: Install the Requestly extension or desktop app

Install Requestly from the Chrome Web Store or download the desktop app. Once installed, open the interface and go to the Rules section.

Step 2: Create a new rule

Click on “Create Rule” and select “Modify Request Body”. This rule type allows you to intercept requests and change their payload before sending them to the server.

Step 3: Define the source condition

Specify when this rule should apply. Use match types like Contains, Equals, Regex Match, or Wildcard Match to target requests based on the URL, hostname, or path.

Step 4: Refine with source filters (optional)

Click the filter icon next to the source field to narrow the rule further. You can filter based on the URL resource type or the request method, such as POST or PUT.

Step 5: Configure how to modify the request body

Choose either:

• Static modification to replace the body with a fixed JSON or text value
• Programmatic modification to write JavaScript that updates the body dynamically

Step 6: Enter the modification content

Based on the selected method, either paste the new body or write a script to adjust the existing one. You can modify fields, add or remove values, or simulate edge cases.

Step 7: Save and activate the rule

Save the rule and make sure it’s enabled. Then trigger the API call from your application or browser.

Step 8: Observe and debug the response

Check how the backend or client responds to the modified request. This helps you test different inputs, reproduce bugs, or validate how the system handles unexpected data.

Use Cases for Debugging with Requestly

Requestly gives you complete control to test different inputs, reproduce bugs, and simulate edge cases or failures.

Here are some of the most common use cases:

• Testing form submissions: During development, you might need to test how your application handles different types of form inputs. Instead of filling out the form manually every time, you can intercept the request and modify the submitted data to test various input combinations, error cases, and validation rules.
• Debugging API requests: If an API call fails or returns incorrect data, you can modify the request payload to narrow down the issue. This helps you check if the problem is caused by the frontend, the request structure, or the backend logic.
• Simulating security scenarios: To test how your system handles malicious input, you can inject payloads that simulate security threats like SQL injection or cross-site scripting. This helps verify whether your backend validation is working correctly.
• Triggering network failures or delays: You can create rules that delay responses or return specific status codes. This helps you see how your application behaves when the API is slow, unreachable, or returning unexpected errors.
• Switching between environments: You can reroute requests from production to staging or from one API version to another. This is useful when testing changes in a different environment without changing code or configuration files.

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Conclusion

Debugging API calls is an essential part of building stable, reliable applications. It helps you understand how systems communicate, identify broken requests or responses, and resolve issues before they impact users.

Requestly simplifies this by giving you control over HTTP traffic without changing the application code. You can modify payloads, simulate errors, inject test data, or switch environments directly from the browser or desktop app.

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