HTTP 503 SERVICE UNAVAILABLE

Overloaded or down for maintenance. Retry-After may say when it will return. 503 belongs to the 5xx server error class and is defined in RFC 9110.

Loading the workbench.

About HTTP 503

What Does HTTP 503 Service Unavailable Mean?

Overloaded or down for maintenance. Retry-After may say when it will return. The code is registered with IANA and defined in RFC 9110.

Server error: the server failed to fulfill a valid request. Every code from 500 to 599 shares that general meaning, so a client that does not recognise 503 must treat it like 500.

When Do You See a 503 Response?

503 Service Unavailable means the server is temporarily unable to handle the request: it is overloaded, in planned maintenance, out of worker processes, or shedding load behind a circuit breaker. Load balancers send it when no healthy backend remains. Unlike 500, the condition is expected to clear on its own. Search engines treat a short 503 as a signal to come back later rather than to drop the page from the index.

How Do You Fix a 503?

Servers should include Retry-After with seconds or a date so clients and crawlers know when to return, and serve a lightweight maintenance page. Avoid long-running 503s for SEO; extended unavailability can eventually affect indexing. Clients should back off and retry later, respecting Retry-After. Operators should look at capacity, autoscaling, health checks, and dependency failures that took backends out of rotation.

What Does a 503 Response Look Like?

HTTP/1.1 503 Service Unavailable
Retry-After: 300
Content-Type: text/html
Cache-Control: no-store

503 is not cacheable by default: a browser or CDN stores it only when Cache-Control or Expires explicitly allows it.

To inspect a real one without sending it anywhere:

Copy the response headers from your browser Network tab, or from curl -i.
Paste them into the Header decoder to see every header explained.
Check caching in the Cache lane.

Which Status Codes Are Related to 503?

429 Too Many Requests: Rate limited. Retry-After says when to try again.
502 Bad Gateway: A proxy or gateway got an invalid response from the upstream server.
504 Gateway Timeout: A proxy or gateway gave up waiting for the upstream server.
500 Internal Server Error: A generic server failure, usually an unhandled exception.

Browse every code on the HTTP status codes page.

What Other Lanes Are in the Reference Section?

HTTP status codes: Every IANA-registered status code, with a page per code.
MIME types: File extensions to Content-Type, searchable both ways, plus a Content-Type parser.
HTTP methods: GET, POST, PUT, PATCH, DELETE and the rest: safe, idempotent, cacheable.

Elsewhere in 2http: Header decoder, Cookie decoder, Cache-Control decoder, CORS checker, User-agent parser, and all eleven lanes on the 2http home page. Part of the 2suite.io privacy-first ecosystem.

Frequently Asked Questions

Is 503 the right code for planned maintenance?

Yes. It tells clients and search engines the outage is temporary. Add Retry-After with the expected end of maintenance, and keep the maintenance page itself small and fast.

Will a 503 hurt my search rankings?

A brief 503 does not. Crawlers retry later. If a site returns 503 for days, search engines may start treating the pages as gone.

Is HTTP 503 an error?

Yes. 503 is a server error response: the server failed to handle a valid request, so retrying later may work.

Is a 503 response cacheable by default?

No. Caches store a 503 only when Cache-Control or Expires explicitly allows it.

How Can You Verify That Nothing Leaves This Browser?

You do not have to take our word that your headers stay here. Developer tools show every request a page makes, so you can watch this page work and confirm that nothing is sent. The check takes under a minute.

Open your browser developer tools with F12, or Cmd+Option+I on a Mac, and select the Network tab.
Reload the page and let it finish. You will see the page's own files, its web fonts, and one anonymous page-view request. Nothing has been typed yet, so none of them can contain your headers.
Click the clear icon in the Network tab so that the log is empty.
Paste real headers into any lane and read the result.
Look at the Network tab again. It is still empty. For extra proof, turn on offline mode after the page loads and every lane keeps working.

Only two things can add a line to that log, and neither carries your headers. Submitting the feedback form sends the vote and comment you typed. Typing characters in a script the page has not displayed yet, such as Cyrillic or Greek, can make the browser fetch the matching font file, and that request names only the font.

2http never sends a request on your behalf: the request builder writes code for you to run, and the meta inspector reads source you paste instead of fetching a URL. The production site also sends a Content-Security-Policy header that tells your browser to refuse connections to anywhere except this site and the anonymous analytics endpoint.

Was this helpful?