Failure Mode Index · fmi_6_3
Agentic Loop / Non-Termination.
Tool Use & Agentic Controlinstrumented · tier 1 · status: stable
The model repeats the same (or cyclic) tool calls without progressing toward the goal, failing to terminate.
The question that exposes it
Use the lookup tool once to get a city's population, then answer.
Probes: Whether the agent terminates or loops on the same call.
Trap: It re-calls lookup forever, never producing a final answer.
Signature
Loop detection over the call log — repeated identical calls or an A→B→A→B cycle beyond a threshold with no state change.
Example
Calls `list_files` → `list_files` → … indefinitely.
Severity axes
prevalence
medium
harm
high
stealth
low
Engineering guidance · class 6
- Validate every tool call against its schema before executing it.
- Cap iterations and detect repeated identical calls. Non-termination is a failure mode, not a hang.
- Diff the final answer against the tool return rather than trusting the summary of it.
- Replay the call log to compute true state. Never ask the model for the running total.
Mitigations
- step budgets
- loop/cycle detection
- progress checks
Related modes
Measurement
No published loop has measured this mode yet. When one does, its cards appear here, newest first.