How to Fix HTTP 505 Version Not Supported Error? [SOLVED]
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What is HTTP 505 Version Not Supported? The HTTP 505 error, known as “HTTP Version Not Supported,” occurs when the web server cannot handle the request or the server doesn’t …
HTTP Error Status Code Reference — 35+ 4xx & 5xx Fixes Explained (2026)
itsourcecode.com hosts 35+ documented explanations and fixes for HTTP error status codes — both 4xx client errors (you sent something wrong) and 5xx server errors (the server failed). Whether you’re hitting 403 Forbidden, 404 Not Found, 429 Too Many Requests, 500 Internal Server Error, or 504 Gateway Timeout, this hub explains what each code means, who’s at fault (client vs server), and how to fix it. Browse by status code category below.
What is an HTTP status code?
An HTTP status code is a three-digit number a web server returns to tell a client (browser, mobile app, API consumer) the result of an HTTP request. The first digit categorizes the response:
- 1xx Informational — request received, continuing
- 2xx Success — request completed (200 OK, 201 Created, 204 No Content)
- 3xx Redirection — further action needed (301 Moved, 302 Found, 304 Not Modified)
- 4xx Client Error — the request was bad (the client’s fault)
- 5xx Server Error — the server failed to fulfill a valid request (the server’s fault)
The codes below are the 4xx and 5xx ones — the ones you have to fix.
How to debug any HTTP error in 4 steps
- Identify the responsible party. 4xx = your fault (fix the request). 5xx = the server’s fault (fix the server, or wait if it’s not yours). 401/403/429 specifically are about permissions/limits.
- Check the response body. Most APIs return useful detail in the JSON/HTML body alongside the status code. The body often tells you exactly which field is invalid, which permission is missing, or which limit was exceeded.
- Check headers. 429 returns
Retry-After. 503 may returnRetry-After. 301/302 redirects come withLocation. Inspecting headers (curl-I, browser DevTools Network tab) gives you context. - Check the right log. 5xx errors are logged server-side — check application logs, web server logs (nginx/Apache), or reverse proxy logs (Cloudflare). 4xx errors usually aren’t (unless your server logs them).
Featured HTTP error fixes by category
🔴 4xx Client Errors — your request is wrong
Authentication / Authorization (401, 402, 403, 407)
Resource Not Available (404, 405, 406, 408, 410)
Request Content / Format (411, 413, 414, 415, 416, 417, 418, 421, 422)
Concurrency / Rate Limiting (423, 425, 426, 428, 429, 431)
Legal / Censorship (451)
🔥 5xx Server Errors — the server failed
Generic Server Failures (500, 501, 502, 503, 504, 505)
Resource / Storage (507, 508, 509, 510, 511)
📑 Status code reference articles
For the most-trafficked codes (404, 500, 502, 503, 504), browse the full archive below. Each post explains the error in plain English, identifies common causes, and walks through fixes for both developers and end-users.
Related error categories
HTTP Errors is one of 10 hubs in our Python & JavaScript error reference cluster — 980+ documented fixes total. If your error is in your code rather than at the HTTP layer, jump to the right hub below:
- TypeError Reference — 220+ Python & JS TypeError fixes
- ModuleNotFoundError Reference — 198+ Python import errors
- AttributeError Reference — 173+ “object has no attribute X” fixes
- ValueError Reference — 100+ pandas/NumPy/sklearn fixes
- ImportError Reference — 67+ “cannot import name X from Y” fixes
- NameError Reference — 49+ Python “name X is not defined” fixes
- RuntimeError Reference — 49+ PyTorch/CUDA/async runtime errors
- SyntaxError Reference — 48+ Python & JavaScript syntax errors
- ReferenceError Reference — 34+ JavaScript “is not defined” fixes
- Python Tutorial — beginner-to-intermediate Python lessons
About this HTTP error reference
Built since 2015 by PIES Information Technology Solutions, Binalbagan, Negros Occidental, Philippines. Every status code explanation traces to real production incidents. Used by 12,000+ developers monthly.
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HTTP 451: Unavailable For Legal Reasons|Explanation & Solutions
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HTTP Error 431: How to fix 431 Request Header Fields Too Large?
In this article, we will walk you through the process of resolving the HTTP error 431, which returns the status code: “Request Header Fields Too Large”. HTTP errors are designed …
What is HTTP 429 Too Many Requests Error and How to Fix it?
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HTTP Status Code 428 Precondition Required
What is HTTP Status Code 428? The HTTP Status Code 428, or “Precondition Required”, is a status code that signifies the server needs the client to meet certain conditions before …
How to fix HTTP Status Code 426 Upgrade Required Error?
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425 HTTP Status Code: How to Fix the HTTP 425 Too Early Error?
What is the 425 HTTP Status Code? The 425 HTTP status code, also known as “Too Early“, is a client-side error that signifies that while a secure connection has been …
How to fix HTTP Status Code 424 Failed Dependency?
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Frequently Asked Questions
What is the difference between 4xx and 5xx errors?
4xx means YOUR request was wrong — you need to fix the request (wrong URL, missing auth, bad payload, too many requests). 5xx means the server received your request but failed to fulfill it — the server admin needs to fix the server (crash, config, dependency down). As an API consumer hitting 5xx, your options are: retry with backoff, contact the API provider, or wait.
What does 403 Forbidden actually mean?
The server understood your request but is refusing to fulfill it because you do not have permission for THIS specific resource. 403 is different from 401 (you are not authenticated at all) and 404 (resource does not exist). 403 says: "you exist, the resource exists, but you cannot have it." Common causes: file permissions, CDN blocking your country/IP, missing roles/scopes, Cloudflare bot challenge.
What does 429 Too Many Requests mean?
You hit a rate limit. The server is telling you to slow down. Look at the Retry-After response header — it tells you how many seconds to wait before retrying. Best practice: implement exponential backoff (wait 1s, 2s, 4s, 8s...) with jitter. For APIs, also check if there is a X-RateLimit-Reset header that tells you when the bucket refills.
What does 502 Bad Gateway mean?
A reverse proxy or gateway (nginx, Cloudflare, AWS ALB) tried to pass your request to an upstream server, and the upstream server returned an invalid response (or no response). Common causes: the upstream is down/crashed, networking issue between proxy and upstream, upstream timeout, response too large. Check the upstream's health first.
What does 504 Gateway Timeout mean?
Similar to 502, but specifically: the upstream server did not respond in time. The reverse proxy gave up waiting. Common causes: long-running database query, blocked async operation, deadlock, infinite loop, network latency. Increase the proxy's read timeout temporarily to diagnose, but the real fix is making the upstream respond faster.
Should I retry on every 5xx?
Generally yes, with exponential backoff and a max attempt count. 500/502/503/504 are usually transient — the server might recover. But: (1) Only retry idempotent requests (GET, PUT, DELETE — not POST that creates duplicate resources). (2) Cap retries at 3-5. (3) Add jitter to avoid thundering-herd retries from many clients at once. (4) Do not retry on 501 (not implemented) — it will never work.
Why does my site keep returning 503?
503 Service Unavailable typically means: (1) your server is overwhelmed (out of CPU/memory), (2) your app is in maintenance mode, (3) your reverse proxy cannot connect to the app (app crashed, process pool exhausted), or (4) a rate limiter at the infrastructure level is shedding load. Check application monitoring plus server resources plus recent deploys.
How often is this HTTP error reference updated?
New posts are added when HTTP standards add codes (rare) or when major web platforms ship new error patterns. Existing posts are revised periodically. Last refreshed: May 2026.