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Slot Unavailable

This is not a Problem type. slot_unavailable is a business outcome code, not an RFC 9457 Problem Details type. There is no https://usp-protocol.dev/errors/slot-unavailable URI to branch on.

The selected slot is no longer available.

The business processed the request successfully and is reporting a scheduling outcome, so it is carried with HTTP 200 OK:

  • REST: an entry in the response body's messages[] array.
  • MCP: an entry in result.structuredContent.messages[].

Severity is recoverable: the platform can query availability again and retry with a different slot.

Clients branch on messages[].code, not on a type URI. See the business outcome code registry in specification Section 9.4.2.

A separate ESP code shares this name

The Embedded Scheduling Protocol defines an esp.error frame code also named slot_unavailable, for a slot that became unavailable during an embedded flow. That is a postMessage frame, not an HTTP response. See specification Section 9.5.5.