HTTP 509 Unassigned

What this status code means

This status code value is currently unassigned in the IANA registry and should not be used in production APIs.

Who should act first

Service, gateway, or upstream dependency is usually the first place to investigate.

What to check first

• Correlate request ID across edge, proxy, and app logs.
• Check upstream dependency health and timeouts.
• Review recent deploy or config changes.

How it differs from similar codes

• HTTP 508 Loop Detected — full guide
• HTTP 510 Not Extended — full guide

Reference and next step

Specification: RFC 9110.

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