ModelCensusopen-source ai reliability harness
Failure Mode Index · fmi_6_5

Ledger Reconciliation Failure.

Tool Use & Agentic Controlinstrumented · tier 1 · status: stable

Across a sequence of state-changing tool calls, the model's running account of state diverges from what the calls actually produced.

The question that exposes it

Opening balance 100. +50, −20, +30, −15. What's the final balance?

Probes: Whether running state matches a replay of the transactions.

Trap: The model's stated balance drifts from what the ledger actually implies.

Signature

Replay the tool call/return log to compute true final state; failure = the model's stated final state ≠ replayed state.

Example

After three transfers the model reports a balance the ledger does not support.

Severity axes
prevalence
medium
harm
high
stealth
high
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
  • external state tracking
  • reconcile before reporting
Measurement

One tab per model, newest first. Each card is that model against this mode under every condition on a single day. The mitigations above are the claim; the residual on each card is what actually survived the intervention.

fmi_6_5_v1 · 5 repeats · served 2026-09-10

loop 2026-09-10 · x-ai/grok-4.6 as served · closed weights · released 2026-01 · set s1.4-live · cases v1.7 · conditions v1.0 · ground truth as of 2026-08-09

ConditionFailure rate95% intervalnTrapAnswerable
No context
bare · The model alone, with no supplied context.
0%0%–20%n=40 · eff 150% (n=25)0% (n=15)
Irrelevant context
null · Length-matched irrelevant context. The control.
0%0%–20%n=40 · eff 150% (n=25)0% (n=15)
General context
recipe_a · Broad retrieval — the first passage the source returns, unreviewed.
0%0%–20%n=40 · eff 150% (n=25)0% (n=15)
Curated context
recipe_b · Hand-checked retrieval — the same passage plus the provenance chain behind it.
0%0%–20%n=40 · eff 150% (n=25)0% (n=15)
Presence effect · no context → irrelevant context
0.0pp

intervals overlap — not separated

What a document being there did, before any of its content. Should be small.

Content effect · irrelevant → best real context
0.0pp

intervals overlap — not separated

What the information itself did. This is the publishable claim.

Residual — what survives

Unchanged by context: 0% with none, 0% with the strongest supplied source. For a control mode that flat line is the result — it is what licenses reading movement elsewhere in this loop as grounding rather than as a document being present. Nothing survives at the best condition on the cases measured here.

Mitigations from the index — the claim
  • external state tracking
  • reconcile before reporting

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