Tools for API Developers

BY TOOLS.FUN  ·  MARCH 28, 2026  ·  5 min read

API developers spend their days building, testing, and documenting HTTP services. These Tools.Fun utilities cover the recurring browser-based tasks — decode a JWT, construct a cURL command, format a JSON payload, test a WebSocket — without leaving your browser or spinning up Postman for a quick check.

cURL Converter

Convert raw cURL commands into structured request breakdowns, or build cURL commands from individual parts (method, headers, body). Paste your curl one-liner into runbooks, README files, and API documentation so consumers can reproduce requests without guessing the syntax.

JSON Formatter & Validator

Format, validate, and inspect JSON request and response bodies. Instantly spots structural issues — missing commas, unmatched brackets, trailing commas — before they reach your integration tests or client consumers. Essential when reviewing API contracts and OpenAPI schemas.

Base64 Encoder / Decoder

Decode JWT payloads (header and claims are Base64url-encoded) to inspect iss, sub, aud, exp, and custom claims. Encode API credentials for HTTP Basic Auth headers (Authorization: Basic base64(user:pass)). Decode binary blobs embedded in JSON responses.

WebSocket Tester

Connect to any WebSocket endpoint and send/receive frames directly in the browser. Test subscription-based APIs, real-time event streams, chat backends, and push notification services without writing a single line of client code.

URL Encoder / Decoder

Encode query string parameters with special characters, decode percent-encoded URIs from redirect chains and OAuth callbacks, and construct properly escaped API endpoint URLs. Essential for building query strings containing JSON, arrays, or special characters.

RegExp Tester

Build and test the regex patterns used in API route matching, input validation, and response parsing. Test your Express/FastAPI route patterns, validate field sanitization regex, and extract values from API response strings before embedding patterns in code.

Timestamp Converter

Convert Unix epoch timestamps from API responses, webhook payloads, and OAuth tokens (iat, exp, nbf claims) to human-readable dates. Generate epoch values for constructing time-range request parameters without manual arithmetic.

MD5 / Hash Generator

Generate HMAC-SHA256 and other hash values to verify API request signatures — common in webhook verification (Stripe, GitHub, Shopify all use HMAC-SHA256 payload signatures). Test your signature implementation against a known-good value before deploying.

Code Diff Tool

Compare two versions of an API response, OpenAPI spec, or request body side by side. Identify exactly what changed between API versions during contract testing and backwards-compatibility review.

AES Encryptor

Test AES-256 encrypted payloads for APIs that use payload encryption (common in financial services and health APIs). Verify that your encryption and decryption implementations produce consistent output before integration testing.

RSA Encryptor

Test RSA key operations for APIs that use asymmetric encryption for request signing or payload encryption. Verify key pairs and padding schemes before writing production crypto code.

JSON to YAML Converter

Convert JSON OpenAPI/Swagger specs to YAML format (most tooling prefers YAML), or translate JSON API configs to YAML for use in API gateways like Kong, AWS API Gateway, and Traefik.

Character Counter

Verify that API field values stay within documented length constraints before sending requests. Useful when testing edge cases near VARCHAR or string-length validation boundaries.

Password Generator

Generate strong random API keys, webhook secrets, client secrets, and signing keys for development and staging environments. Use entropy-maximising character sets to simulate realistic secret values in test suites.

Duplicate Line Counter

Detect duplicate values in API response arrays — IDs, slugs, email addresses — when spot-checking pagination, deduplication logic, or data integrity in bulk API responses.