Not Implemented

Server perspective

Use 501 when the server does not support the request method or required functionality anywhere on the server.

When to use

• •Return 501 when the method or capability is unimplemented across the server, not merely disallowed on one resource
• •Use it for truly unsupported functionality, not for temporary outages or ordinary application exceptions

How to respond

• •Confirm the method or feature is not implemented before returning 501; if the resource-specific method is known but disallowed, return 405 with Allow instead
• •Explain the unsupported capability in a safe machine-readable error body when possible

Headers to consider

• •No status-specific header is required; unlike 405, 501 does not require an Allow header
• •Set explicit cache controls if you do not want default cacheability to apply

Response body

• •A body is optional but helpful; identify the unsupported method or capability and any supported alternative if one exists

Server-side pitfalls

• •Do not return 501 for GET or HEAD, which compliant servers are expected to support
• •Do not use 501 when the method is understood but blocked on a particular resource

Examples