Free Developer Tools for API Testing and Debugging

API debugging tools for developers

Modern API development involves a lot of moving parts: authentication tokens, JSON payloads, HTTP response codes, file uploads, and network requests. MockFlow developer tools are designed to remove friction from these workflows. Whether you are a frontend developer building UI before backend APIs are ready, a QA engineer verifying API behavior, or a backend developer debugging integrations, these tools help you inspect, validate, and understand API data without leaving your browser.

These tools work well alongside MockFlow mock APIs, which let you create fake API endpoints with realistic responses for frontend testing and prototyping. Use the JSON Formatter to build clean mock response bodies, the JWT Decoder to inspect auth tokens your mock endpoints return, and the HTTP Status Code Explorer to choose the right error codes for your mock responses.

Tools included

Show Your IP

Check your public IP address instantly. Useful for configuring API allowlists, setting up firewall rules, verifying VPN connections, and debugging geo-based routing.

Base64 to PDF

Paste a Base64-encoded string and instantly preview or download the PDF. Useful for debugging API responses that return PDF documents as Base64 payloads.

PDF to Base64

Upload a PDF and get the Base64 string in seconds. Useful for building API request payloads for document-management and e-signature APIs.

JWT Decoder

Decode JWT headers and payloads locally in your browser. Inspect token claims like exp, iat, sub, aud, and iss without uploading data to any server.

JSON Formatter & Validator

Format, validate, pretty-print, and minify JSON directly in your browser. Useful for inspecting API responses, building mock payloads, and fixing malformed JSON.

HTTP Status Code Explorer

Explore HTTP status codes, meanings, categories, and API testing examples. Search by code number or name to understand when to use 200, 201, 400, 401, 403, 404, 429, 500, and more.

Privacy-first browser tools

Developer tools should not require you to upload sensitive data to a third-party server. MockFlow tools are designed with privacy as a priority. JSON formatting, JWT decoding, PDF-to-Base64 encoding, and Base64-to-PDF decoding all run entirely in your browser using client-side JavaScript. Your data never leaves your device for these operations. You can use these tools offline and they will still work correctly.

How these tools work with mock APIs

MockFlow developer tools are designed to complement the mock API builder. When you create a mock endpoint, you often need to design realistic JSON responses, choose the right HTTP status codes for success and error states, and verify that authentication tokens your mock API returns are structured correctly.

• Designing responses: Use the JSON Formatter to build and validate the JSON body before pasting it into your mock endpoint.
• Choosing status codes: Use the HTTP Status Code Explorer to pick the right status code for each response profile in your mock API.
• Debugging auth flows: Use the JWT Decoder to inspect the tokens your mock API returns and verify claims are correct.
• Testing file payloads: Use the PDF tools to create and verify Base64-encoded PDF payloads in your mock API responses.

Open MockFlow to create your first mock API →

Related guides

• How to Mock API Responses Without Building a Backend
• HTTP Status Codes Every Frontend Developer Should Test
• How to Test API Error Responses Like a Real QA Engineer
• Frontend Development Without a Backend: A Modern Workflow Guide

Frequently asked questions

Are MockFlow developer tools free?

Yes. All MockFlow developer tools are completely free to use. There is no account required, no usage limits, and no paywalls for any of the tools.

Do these tools upload my data to a server?

Most tools run entirely in your browser and never send your data to a server. JSON formatting, JWT decoding, PDF-to-Base64 conversion, and Base64-to-PDF conversion all run locally in your browser. The Show Your IP tool makes a server request to detect your public IP address, but no data you enter is uploaded.

Can I use these tools for API testing?

Yes. These tools are designed specifically for API testing workflows. JSON Formatter helps you inspect and validate API payloads, JWT Decoder helps you debug authentication tokens, HTTP Status Code Explorer helps you understand response codes, and the Base64 and PDF tools help you work with file payloads in API requests and responses.

Can I format JSON for mock API responses?

Yes. The JSON Formatter tool lets you paste any JSON string and instantly format, validate, minify, and copy it. This is useful for building mock API responses in MockFlow or inspecting API payloads during debugging.

Can I decode JWT tokens safely?

Yes. The JWT Decoder runs entirely in your browser — your token is never sent to a server. It decodes the header and payload from any JWT token so you can inspect claims like exp, iat, sub, aud, and iss. Note: decoding is not the same as verifying the signature. The JWT Decoder shows the token contents but does not verify that the signature is valid.

Can I convert PDF files to Base64 for API payloads?

Yes. The PDF to Base64 tool lets you upload any PDF file and instantly get the Base64-encoded string. The conversion runs locally in your browser — your file is never uploaded to a server. You can use the output directly in API request bodies, JSON configs, or test fixtures.

Who are these tools for?

These tools are designed for frontend developers, QA engineers, backend developers, and DevOps engineers who work with APIs. They are especially useful when debugging API responses, testing file upload workflows, decoding authentication tokens, and understanding HTTP response codes.