Severity is a code decision, not a confidence score

Most AI security products surface model confidence as severity. Nexus does not. When TCSF_PRECISION_MODE=1 (the default), High and Critical are blocked unless proof tier permits them.

Proof tiers

• LLM hypothesis — never High/Critical
• Tool probe, unconfirmed — demoted in precision mode
• exploit_verified — permitted with response proof
• Grader-confirmed — CVE-Bench pass, sqlmap confirmed

Where it lives in code

nexus/validators/evidence_gate.py      # severity demotion at ingest
tcsf/exploit/impact_validator.py       # High/Critical proof check
tcsf/exploit/confirmation_loop.py      # plan → execute → reflect

The exploit confirmation loop records exploit_verified evidence nodes. Unverified chains stay hypothesis or medium. Analyst FP rejections feed nexus_memory.db and trajectory memory down-ranks repeats.

We run a 12-case Web80 golden subset as regression: 100% critical precision, zero false criticals. That gate holds under precision, aggressive, and omega power modes.