HTTPLens
Protocol Reference

HTTP & API Diagnostic Reference

Understand HTTP errors, compare status codes, debug browser CORS issues, and reference HTTP headers with practical, source-backed guides.

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Find the right guide

Jump directly to actionable troubleshooting guides based on what you are debugging.

HTTP Errors

I received an HTTP error

Diagnose 4xx client errors and 5xx server errors with protocol-accurate causes and fixes.

Status Comparisons

I need to compare status codes

Understand subtle distinctions between similar HTTP status codes to choose the right diagnosis.

CORS & Security

My API works in Postman but fails in the browser

Learn why browser Same-Origin Policy enforcement creates preflight and cross-origin failures.

HTTP Headers

I need to understand an HTTP header

Reference HTTP request and response header syntax, content negotiation, and security behavior.

Documentation

Explore the documentation

Structured reference libraries covering HTTP status codes, comparisons, CORS, and headers.

HTTP Errors

Full reference library for 4xx client and 5xx server error codes with root causes and verification steps.

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Compare Status Codes

Side-by-side protocol comparisons to prevent misdiagnosing related or overlapping response codes.

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CORS Guides

In-depth guides for cross-origin resource sharing, preflight requests, credentials, and header rules.

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HTTP Headers

Comprehensive reference for request and response headers, content negotiation, and caching directives.

Reference headers →
Core Guides

Foundational technical reference guides for common HTTP errors, headers, and security mechanisms.

  • [ 401 ]http-errors

    HTTP 401 Unauthorized

    Understand HTTP 401 Unauthorized, why it signals missing or invalid credentials, how to inspect WWW-Authenticate, and how to fix it.

  • [ 404 ]http-errors

    HTTP 404 Not Found

    Understand HTTP 404 Not Found, why the server cannot find the requested resource, how to check routing, and how to fix it.

  • [ Compare ]compare

    HTTP 502 Bad Gateway vs 504 Gateway Timeout

    Understand the difference between HTTP 502 Bad Gateway and 504 Gateway Timeout, invalid responses vs timeouts, and how to debug proxy failures.

  • [ CORS ]cors

    What Is CORS? A Practical HTTP Guide

    Understand Cross-Origin Resource Sharing (CORS), how browser Same-Origin Policy works, what CORS headers do, and how requests are handled.

Methodology

How HTTPLens approaches debugging

A diagnostic reference built on protocol specifications and reproducible verification.

Protocol-focused reference guides

Explanations verified against IETF RFC specifications and official IANA registries.

Clear distinction between observed behavior and inference

Diagnostic paths separate facts established by a response from system inferences.

Source and verification details on technical articles

Every guide cites relevant RFC anchors with technical verification dates.