API Tester & Request Builder Online — Free REST Client Tool
API Request Builder
Build and test HTTP API requests with headers, body, auth, and response visualization.
A powerful, lightweight, browser-based HTTP client for building, inspecting, and testing REST API endpoints without installing heavy desktop software. Configure all standard HTTP methods (GET, POST, PUT, PATCH, DELETE, HEAD, OPTIONS), custom request headers, query parameters, multipart form data, and authentication schemes including Bearer tokens, Basic auth, and API keys. Inspect color-coded responses, response headers, cookie attributes, latency timings, and status codes. Import directly from cURL commands and generate code snippets across 14+ programming languages. Stateless and private with zero telemetry storage.
Keywords: api tester, rest client, http request builder, postman alternative, api request tool, http client online, rest api tester, api endpoint tester, free postman, api testing tool, http method tester, request builder online
Tags: api, http, rest, request, postman, client, fetch, endpoint
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How to API Request Builder Online
Enter your API endpoint URL in the URL bar at the top.
Select the HTTP method (GET, POST, PUT, PATCH, DELETE, HEAD, OPTIONS) from the dropdown.
Add query parameters, headers, and request body using the tabs below the URL bar.
Configure authentication — Bearer token, Basic auth, or API Key — in the Auth tab.
Click Send or press Cmd+Enter to execute the request and view the formatted response on the right.
API Request Builder Features
Support for all HTTP methods: GET, POST, PUT, PATCH, DELETE, HEAD, and OPTIONS.
Key-value editors for headers and query parameters with enable/disable toggle per row.
Multiple request body types: JSON, Form Data, URL-encoded, and raw text.
Built-in authentication support: Bearer token, HTTP Basic auth, and API key (header or query).
Response visualization with syntax-highlighted JSON, status code badge, timing, and size.
Response headers and cookie viewer with parsed Set-Cookie attributes.
Copy response body or copy request as cURL with one click.
Import requests from cURL commands pasted from the clipboard.
Request history storing your last 20 requests for one-click replay.
Server-side proxy for CORS bypass — send requests to any API without browser restrictions.
AI-powered response explanations, error analysis, and request improvement suggestions.
Keyboard shortcuts for fast workflow: Cmd+Enter to send, Cmd+Shift+C to copy, Cmd+Shift+H for history.
In-Depth Technical Guides
All GuidesAutomating API Contract Testing: Converting OpenAPI 3.1 to Postman Collections
Transform OpenAPI (Swagger) specifications into executable Postman Collections (v2.1). Automate contract testing, generate mock servers, and streamline API integration.
Converting cURL Commands to Clean Node.js, Python, and Go HTTP Clients
Transform raw cURL commands and browser network exports into production-ready JavaScript Fetch, Axios, Python Requests/HTTPX, and Go net/http code.
Debugging CORS Errors: The Complete Guide to Preflight OPTIONS & Headers
A production debugging guide to fixing missing origin headers, failed preflight OPTIONS requests, credentialed CORS mismatches, and reverse-proxy proxy pass drops.
Debugging JWT Errors: Fixing Signature Verification, Expiration & JWKS Failures
A comprehensive developer debugging guide to resolving JSON Web Token (JWT) errors: signature mismatches, expired tokens, alg: none attacks, clock skew, and JWKS key rotation.
GraphQL Schema to TypeScript: Automated Type Generation, Operations & Codegen Guide
Generate end-to-end type safety across your GraphQL stack. Learn how to convert GraphQL SDL schemas and documents into typed queries, mutations, and resolver signatures.
Handling HTTP 429 Too Many Requests: Rate Limiting Headers, Exponential Backoff & Jitter
Diagnose and handle HTTP 429 status codes. Master IETF standard RateLimit and Retry-After headers, and implement production-grade exponential backoff with full jitter in Node.js, Python, and Go.
HAR File Analysis: How to Diagnose Slow API Endpoints & Network Bottlenecks
A practical guide to analyzing HTTP Archive (HAR) files, decoding network timing waterfalls, identifying high TTFB, and debugging slow REST and GraphQL APIs.
HTTP Cache-Control & CDN Caching: The Production Guide to stale-while-revalidate, ETags & Edge Invalidation
Master HTTP caching headers, understand browser vs CDN shared cache directives (s-maxage, stale-while-revalidate, immutable), and eliminate stale content and cache poisoning bugs.
ISO 8601, Unix Timestamps & Timezones: Zero-Discrepancy Time in Distributed Systems
Master epoch timestamps, millisecond vs microsecond precision, RFC 3339 / ISO 8601 formatting, UTC storage rules, and prevent subtle timezone bugs across databases and APIs.
Mastering SQL Execution Plans: Reading EXPLAIN ANALYZE, Index Scans & Query Optimization
Diagnose slow database queries by decoding EXPLAIN (ANALYZE, BUFFERS) output trees, optimizing composite indexes, understanding bitmap heap scans, and eliminating sequential scans.
Server-Sent Events (SSE) vs WebSockets: Streaming Architecture for LLM & Real-Time APIs
Master unidirectional SSE vs bidirectional WebSockets for modern applications. Learn HTTP/2 multiplexing, automatic reconnection, Nginx proxy buffering gotchas, and token-by-token LLM streaming.
URL Encoding & Query Strings: Mastering RFC 3986, encodeURIComponent & URLSearchParams
Demystify percent-encoding rules, avoid double-encoding pitfalls, master + vs %20 space representations, and serialize complex nested objects and arrays for REST query strings.
Webhook Security & HMAC Signature Verification: Production Implementation Guide
Master webhook security: implement HMAC-SHA256 signature verification, prevent replay attacks with timestamp headers, avoid raw-body parser bugs, and secure asynchronous API callbacks.
Related Standards & RFC Specifications
All StandardsAPI Rate Limiting is a traffic management strategy that controls the rate of incoming client requests to protect server infrastructure from overload and abuse.
CORS is an HTTP-header based security mechanism enforced by web browsers to restrict cross-origin network requests from accessing sensitive server resources.
cURL is a command-line tool and library (libcurl) for transferring data with URLs across HTTP, HTTPS, FTP, and dozens of network protocols.
An ETag is an HTTP response header providing a content-based validator that allows web clients and CDNs to make efficient conditional HTTP requests (304 Not Modified).
gRPC is a high-performance open-source universal RPC framework developed by Google that leverages HTTP/2 transport and Protocol Buffers for schema-driven communication.
OAuth 2.0 (RFC 6749) is the industry-standard authorization protocol that enables third-party applications to obtain scoped access to HTTP resources on behalf of a user.
Server-Sent Events (SSE) is an HTTP-based standard that allows servers to stream real-time text events unidirectionally over a single persistent connection.
Transport Layer Security (TLS) is the standard cryptographic protocol that encrypts and authenticates internet communications across HTTPS, WebSockets, and APIs.
A webhook is an event-driven HTTP callback mechanism that sends real-time automated payloads between web systems when specific triggers occur.
WebSocket is a persistent, bidirectional, full-duplex communication protocol operating over a single TCP connection, initiated via an HTTP/1.1 Upgrade handshake.
Explore Full AI Model Pricing Directory
Compare per-token rates, prompt caching discounts, and context windows across leading LLMs (GPT-4o, Claude 3.5 Sonnet, Gemini 2.5 Flash, DeepSeek, and more) in our verified catalog.
Pre-built Automation Pipelines
Chain API Request Builder with other utilities in a multi-step visual workflow.
Base64 Decode → JSON Format
Decode a Base64 string and pretty-print the JSON inside it.
CSV → JSON → YAML
Convert CSV data to JSON, then to YAML format.
JSON Format → TypeScript Schema
Format JSON and generate TypeScript/Zod schema from it.
Frequently Asked Questions
- Is this a replacement for Postman?
- It is a lightweight alternative for quick API testing directly in your browser. For complex workflows, collections, environments, and team collaboration, dedicated API clients like Postman or Insomnia are still recommended. DevFlow API Request Builder is ideal for fast, one-off requests during development.
- Are my API requests private?
- Requests are proxied through our server to bypass browser CORS restrictions. We do not log or store your request data, URLs, headers, request bodies, or response bodies. The proxy is stateless — your data is never persisted.
- Why does it use a server proxy instead of sending requests directly from the browser?
- Browsers enforce CORS (Cross-Origin Resource Sharing) security policy which blocks most cross-origin API requests. Without a proxy, most API calls would fail with a CORS error. Our server-side proxy forwards your request to the target URL, returns the response, and never stores any data.
- What is the maximum response size?
- Responses up to 5 MB are supported. Larger responses will be automatically truncated with a warning. The response body display is also limited to 500 KB for browser performance, but the full size is shown.
- Can I import cURL commands?
- Yes. Click the 'Import cURL' button to read a cURL command from your clipboard and automatically populate all request fields including URL, method, headers, and request body.
- Does it support file uploads?
- File uploads via multipart form-data are not supported in the initial version. You can send JSON, URL-encoded, form data (key-value pairs), and raw text bodies.
- How does request history work?
- The last 20 requests you send are saved to your browser's localStorage. Click the History button to open the history panel and click any entry to reload its full configuration. History is stored locally and never sent to any server.
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