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MCP clients must distinguish admission to the server, validity of the MCP request and the outcome of a tool call. A successful HTTP exchange does not by itself mean a tool completed successfully. The Arbol MCP integration still has the rollout prerequisites described in its overview. This guide explains the protocol and client recovery model; it does not confirm production activation.

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Three failure boundaries

MCP defines tool execution failures separately from JSON-RPC protocol errors so a caller can understand and act on the tool outcome. Check isError before treating content as success. When a tool declares structured output, validate it against the advertised output schema; text is not a substitute for that contract. See the MCP tool specification.

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Downstream API refusals

Arbol MCP calls the public API using delegated authority. An API validation failure, revision conflict or missing permission does not become permission to try another organization, account or credential. The API remains the authority for its resource and operation. Its generated contract defines the available input, output and error data. Do not parse an internal exception name or English message to infer a business outcome. Retain a request reference when a response provides one and report it with the tool name and approximate time. Do not share bearer tokens, patient data, tool arguments or raw upstream responses as troubleshooting material.

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Retry only with the operation’s guarantee

A tool call can fail after an API mutation has committed. A fresh MCP request id does not make replay safe and is not a domain idempotency key. Reuse only the idempotency key declared by the underlying operation, with the same semantic input; otherwise inspect current state before repeating the tool call. For an authorization failure, correct the delegated credentials or permissions. For a validation failure, correct the input. For a revision conflict, read and reconcile the resource. Temporary dependency failures may be retried with bounded backoff only when the operation is safe to repeat. See retries and idempotency.