
Where Does Exactly-Once Live? Model, Harness, and Tool-Contract Effects on Duplicate Side Effects in LLM Agents
The technique replaces blind retry or model-level caution when a tool write times out or errors: a small harness guard that attaches an idempotency key to every write, letting the service deduplicate redeliveries. Cost is modest engineering (a key-generation and propagation guard, plus tool contracts that accept keys) with no model, GPU, or serving overhead; where contracts cannot accept keys, only waiting with a known in-flight bound partially substitutes. It fails when tool APIs do not support idempotency keys, when faults occur outside the service boundary the guard covers, and agents still misreport success (90% of duplicated episodes were reported as successful), so ledger-level auditing remains necessary.








































