# AI API error 503: unavailable or overloaded endpoints

A 503 means the responding endpoint is unavailable for this request. Read its error details and the matching official report; it does not by itself confirm a global outage.

By IsAnythingDown. Reviewed 2026-10-06.

Canonical page: https://isanythingdown.com/guides/ai-api-error-503

## Quick answer

A 503 means the responding endpoint is unavailable for this request. Read its error details and the matching official report; it does not by itself confirm a global outage.

## Provider differences

OpenAI documents 503 overload responses, including server_is_overloaded. Honor Retry-After when supplied. Gemini identifies 503 UNAVAILABLE as a transient error for which bounded backoff can be appropriate.

Claude’s direct API documents 529 for overload. A 503 seen through a gateway should be attributed to the responding layer until you have further evidence.

- [OpenAI API error codes](https://developers.openai.com/api/docs/guides/error-codes)
- [Gemini API troubleshooting](https://ai.google.dev/gemini-api/docs/troubleshooting)
- [Claude API errors](https://platform.claude.com/docs/en/api/errors)

## Recommended application response

Use exponential backoff with jitter and an explicit attempt and time budget. Avoid sending the same failed request in a tight loop. Keep a request ID and sanitized response details for investigation.

If you consider another endpoint, verify its supported model, authentication, data handling and actual response. A normal status page is only one piece of evidence. A router’s report cannot establish every upstream model’s availability.

- [Model API reports](https://isanythingdown.com/categories/model-apis)
- [AI API error 529](https://isanythingdown.com/guides/ai-api-error-529)

## Related service pages

- [OpenAI API](https://isanythingdown.com/services/openai)
- [Claude](https://isanythingdown.com/services/claude)
- [Gemini API / AI Studio](https://isanythingdown.com/services/gemini-api)
- [OpenRouter](https://isanythingdown.com/services/openrouter)

Each entry shows its own collection coverage. A linked service is not necessarily automatically collected.

Editorial review date applies to this guide. Live report collection times appear on the service pages.