04 — testing & measurement

False negative

Also written FN, miss

A real attack the control failed to flag. False negatives and false positives trade off against each other: any threshold that reduces one increases the other, so a control is only describable by both numbers together, on a stated corpus. The useful question is never "how many did it miss" but "how many did it miss, at what false-positive rate, on which set".

Related terms

Recall Testing & measurement
Of all the real attacks in the corpus, the share the control caught. It is the number vendors quote, and on its own it means very little: recall is…
False positive Testing & measurement
Benign activity flagged as malicious. In an agent security control this is the failure mode that actually kills deployments: a tool that blocks legitimate…

Testing & measurement

The words that decide whether a published detection number is evidence or decoration. Most vendor numbers in this space are quoted without any of them; see how we test for how these are applied in practice.

False negative is term 6 of 15 in this part of the glossary.

Held-out set Tune/test split Overfitting Baseline validation False positiveFalse negativePrecision Recall Hard negative Detection vs prevention Model refusal Marginal value Red teaming Adversarial mutation Variance

This page is one entry from the agentic AI security glossary, which defines 60 terms in the same style. Where a term belongs to a published taxonomy or standard — the HackAgent attack families, the OWASP lists, AMTSO's guidelines, MITRE ATLAS, ISO/IEC 42001, the NIST AI RMF, the EU AI Act — the authoritative wording is the source document's, not ours, and specifics should be verified there. "Lethal trifecta" is Simon Willison's term. OWASP® is a trademark of the OWASP Foundation; ATT&CK® and ATLAS™ are trademarks of The MITRE Corporation. Naming a framework here is description, not a claim of certification or endorsement.