The safety net
Most of a coach's job is nudging load up and down. A small part of it is recognising the handful of signals where training stops being the question. Those are hard-coded, they come before any plan, and they don't negotiate with motivation.
Chest pain, disproportionate breathlessness, fainting or near-fainting. Not "finish the interval and see". The session is over and the next step is medical, not athletic. No readiness score outranks this.
Confusion, collapse or staggering in the heat is treated as heat stroke until proven otherwise. Cool first, transport second: cold-water immersion where possible, targeting a core temperature below 39 °C, while emergency services are called.
Sharp or burning pain, or anything at 6/10 or above, ends the session rather than getting run through. Pain that alters gait, wakes you at night, or worsens day over day gets escalated instead of managed with ibuprofen and optimism.
Relative Energy Deficiency in Sport — the pattern where training load keeps climbing while fuelling doesn't. Watched through weight trend, fuelling logs, sleep quality, resting HR drift and stalled performance, because it degrades slowly and is easy to miss.
This is a training assistant, not a clinician. It has no diagnostic authority: its job at these thresholds is to stop the session and say see a doctor — clearly, early, and without being talked out of it. Medical judgement always belongs to a medical professional.