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PreprintsResearch ArticleNot peer reviewed

Mining Failure Modes from Agent Trajectories at Scale

Version 1 · 21 October 2026 · 10.xxxx/ctm.pp.2026.0007

Agent developers inspect failed runs by hand, which does not scale beyond a few dozen trajectories. We propose a clustering approach that groups failures by the step at which the trajectory diverges from a successful reference run, and label the clusters with short natural-language descriptions. On 18,000 trajectories from three agent frameworks, the method recovers 83% of failure categories identified by manual analysis and surfaces four categories that were previously unreported.

Abstract PDF 2,065 6901 cited

PublishingResearch ArticlePeer reviewed

Statistical Power in Agent Benchmarks: How Many Runs Are Enough?

Hana Kovács, Kofi Mensah, Carlos Medina

Version 2 · 3 reviewer reports · 14 August 2026 · 10.xxxx/ctm.pb.2026.0008

Most agent benchmarks report one run per task, yet agents are stochastic. We analyse run-to-run variance on four public agent benchmarks and derive the number of repeated runs needed to detect a given difference between two systems at conventional power. For differences smaller than five points, single-run comparisons have less than 40% power on all four benchmarks. We provide a sequential testing procedure that stops early when the evidence is clear, and show that it halves the cost of repeated evaluation while keeping the false-positive rate below 5%.

Abstract PDF 2,740 1,1033 cited

PublishingResearch ArticlePeer reviewed

Refusal Calibration in Instruction-Tuned Language Models: When should a model decline, and how well do current models know?

Nadia Haddad, Hana Kovács

Version 2 · 3 reviewer reports · 9 July 2026 · 10.xxxx/ctm.pb.2026.0007

Instruction-tuned models decline requests for many reasons, but little is known about how well their refusals track the actual risk of a request. We build a labelled set of 6,400 requests spanning benign, borderline and harmful categories and measure calibration of refusal decisions across eight open models. Refusal rates correlate only weakly with annotated risk for borderline requests, and over-refusal is concentrated in a handful of phrasing patterns rather than topics. A light-weight calibration step that uses the model's own confidence reduces over-refusal by 31% at no measurable cost to harmful-request refusal. We release the labelled set and the annotation guidelines.

Abstract PDF 3,180 1,2604 cited