Free Developer Tools for API Testing and Debugging
Browser-based utilities for inspecting data, formatting API responses, decoding tokens, converting timestamps, generating UUIDs, encoding URLs, converting files, checking IP addresses, and debugging API workflows. All tools are free and most run entirely in your browser.
Show Your IP
View and copy your public IP address for API testing, allowlists, webhook debugging, and firewall rules.
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Base64 to PDF
Paste a Base64 PDF string, preview it in the browser, and download the converted PDF.
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PDF to Base64
Upload or drag a PDF and copy its Base64 encoding for APIs, fixtures, and integration tests.
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JWT Decoder
Decode JWT access tokens locally in your browser and inspect headers, payloads, and expiration.
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JSON Formatter & Validator
Format, validate, minify, copy, and clean JSON for mock API responses and debugging.
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HTTP Status Code Explorer
Search common HTTP status codes and understand when to use each response.
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Unix Timestamp Converter
Convert epoch seconds and milliseconds to UTC and local time, or turn a date into a Unix timestamp.
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UUID Generator
Generate UUID v4 and time-ordered UUID v7 values for fixtures, database rows, and mock API ids.
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URL Encoder / Decoder
Percent-encode or decode URLs and query values for webhooks, OAuth callbacks, and API requests.
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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.
- Unix Timestamp Converter
- Convert epoch seconds and milliseconds to UTC, local time, and ISO 8601, or turn a calendar date into a Unix timestamp for API fields and JWT claims.
- UUID Generator
- Generate UUID v4 and time-ordered UUID v7 values for database rows, test fixtures, and mock API record ids.
- URL Encoder / Decoder
- Percent-encode query values and full URLs, or decode an encoded callback, webhook, or OAuth redirect URL.
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, timestamp conversion, UUID generation, URL encoding, 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.
- Filling realistic fields: Use the UUID Generator and Unix Timestamp Converter when mock records need ids and created-at values.
- Building request URLs: Use the URL Encoder when a mock or webhook callback includes query parameters.
Related guides
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, Unix timestamp conversion, UUID generation, URL encoding, 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, the Unix Timestamp Converter reads epoch fields, the UUID Generator builds fixture ids, the URL Encoder prepares query strings, HTTP Status Code Explorer helps you understand response codes, and the Base64 and PDF tools help you work with file payloads.
- 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.