Repeats, retries, or replays a side-effecting tool action without deduplication or idempotency safeguards, causing duplicate or inconsistent effects. Unlike Tool Loop, the problem is unsafe repeated side effects, not just wasted calls. Unlike Unsafe Tool Call, the first call may be safe while repetition makes it unsafe.
The timeout left the first call's fate unknown, but the model retries as if it definitely failed — and the customer is charged twice.
User:Send the $1,200 refund-replacement charge to the customer's card.Tool:charge_card(amount=1200) → ERROR: gateway timeoutAssistant:The charge timed out — retrying.Tool:charge_card(amount=1200) → success[… the first charge had also committed; the customer is charged $2,400 …]
The record that the email already went out was summarized away, so the agent re-executes a completed side effect.
[… session compacted; the summary notes "working on launch announcement" but not that it was already sent …]Assistant:Next step is to send the launch announcement to the customer list.Tool:send_bulk_email(list=customers) → sent to 18,400 recipients[… every customer receives the announcement a second time …]
After the ambiguous outcome, the model checks whether the action committed before doing anything that could duplicate it.
User:Send the $1,200 refund-replacement charge to the customer's card.Tool:charge_card(amount=1200) → ERROR: gateway timeoutAssistant:The outcome is ambiguous, so I'm checking the transaction log before any retry.Tool:list_recent_charges() → shows the $1,200 charge committed 20s agoAssistant:The charge went through despite the timeout — no retry needed.
The model has no reliable representation of which tools are side-effecting and which are read-only. Tool descriptions rarely encode this distinction, so retrying a payment call looks the same to the model as retrying a search.
Ambiguous outcomes default to retrying. After a timeout or unclear response, the model cannot tell whether the action committed, and retry is the dominant pattern for transient errors in training data, applied without deduplication keys or a success check first.
Knowing an action already ran depends on reading it from the transcript. In long sessions that evidence sits mid-context where retrieval is weakest, or is dropped entirely by truncation and summarization, so the model re-executes work it has already done (Liu et al., 2023, "Lost in the Middle: How Language Models Use Long Contexts").
Agent frameworks replay trajectories after crashes, restarts, or context compaction without a durable record of which side effects already committed, re-issuing completed actions mechanically.
Categories of checks that can identify the issue. These are strategies, not specific implementations.
Normalize side-effecting calls by tool and arguments and scan the action log for repeats. A second charge_card with the same amount minutes after a gateway timeout is the signature of this mode, detectable mechanically before the customer notices the double charge.
Diff the effects recorded in the external system — charges, sent emails, created tickets — against the task's intended once-only actions. Duplicates that left no trace in the transcript, such as a replay after context compaction, surface as two committed effects for one instruction.
Script ambiguous-outcome scenarios where a side-effecting call times out but actually committed, then score whether the system checks the transaction state before retrying. A system that retries blind fails every variant where the first call went through.
High-level reliability strategies that reduce how often this failure occurs.
Have the scaffold attach a deduplication key to every side-effecting call — derived from the task and intent, not generated fresh per attempt — so a retry after a timeout lands on the same key and the API suppresses the duplicate. This moves exactly-once semantics out of the model, which cannot tell a committed timeout from a failed one, into the boundary that can.
Record committed side effects in a durable ledger outside the transcript, and have the agent consult it before executing — a record that survives truncation, summarization, and restarts. The re-sent launch announcement happened because "already sent" lived only in tokens that compaction discarded; a ledger entry doesn't compact away.
Mark which tools are side-effecting in their descriptions, and instruct that an ambiguous outcome on one — a timeout, an unclear response — means verify commit status first, never retry blind. Retry-on-error is the trained default for transient failures; the instruction carves out the cases where that default double-charges a customer.