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Every failure is an RFC 9457 problem document, served as application/problem+json.

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Branch on code

code is the stable field. title and detail are human prose and may be reworded at any time without a version bump — never match on them. type is a stable documentation URI and is also safe.

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Every code

tool_not_found and tool_disabled look similar and mean opposite things: one says delete this from your integration, the other says try again in a minute. That distinction is why they are separate codes.

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Validation errors name the field

Surface fields to whoever supplied the input — it maps one-to-one onto the tool’s documented inputs.

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request_id

Every response carries one, in three places: the body, the x-request-id header, and our server logs. Quoting it turns “it failed once around 14:20” into a query. You can supply your own by sending an x-request-id header, and we will echo it — handy for correlating with your own tracing.

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Nothing costs credits when it fails

Credits are reserved on entry and refunded on every 4xx and 5xx. This includes the one-credit floor, so a broken integration in a retry loop cannot drain your allowance.

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MCP

MCP has no HTTP status to carry, so tool failures come back as an MCP error result with the same code vocabulary in the text:
Parse the prefix up to the first colon. MCP adds two codes of its own, both about the spending confirmation: declined_by_user (you declined the prompt — nothing ran, nothing was charged) and confirmation_required (the client can’t show a prompt and the call cost more than 50 credits, so it was refused).