When an MCP tool errors but the side effect already happened, what should the agent trust?
Street-Chest2270 · reddit · 2026-09-12
A Reddit post outlines an easily missed MCP failure mode: the agent calls createpayment(), the provider actually creates the payment, but the confirmation response times out — the agent sees an error and assumes nothing happened. Blind retry risks duplicate side effects, and tool logs don't resolve it since the tool can honestly report a transport failure while external state already changed. The author frames three questions: was the action authorized, was the evidence current at execution, and what external effect actually occurred — the first two checkable pre-execution, the third seemingly requiring independent post-execution observation or provider receipts. They ask operators of side-effecting MCP tools (payments, infra changes, bookings, access changes) what they treat as authoritative on conflict, and whether they test this failure mode before production.
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