diff --git a/CONTRACT.md b/CONTRACT.md
index 78624c5..986d4f0 100644
--- a/CONTRACT.md
+++ b/CONTRACT.md
@@ -3,6 +3,28 @@
 Hand-maintained for Phase 6; the `## Inputs` table is generated from
 `src/commands/index.js` (see Phase 9 generator). Keep ≤ 80 lines.
 
+## Unit convention — approved exception to the canonical-unit rule
+
+EVOLV's canonical units (`CLAUDE.md`, `generalFunctions/CONTRACT.md`)
+are Pa / m³/s / W / K. **reactor diverges deliberately** — it follows
+the ASM (Activated Sludge Model) kinetics literature convention:
+
+- Concentrations: `mg/L` (= g/m³), `mmol/L` for alkalinity.
+- Flow internally: `m³/d` (engine integrator runs in days; see
+  `baseEngine.js` line 40 — `timeStep` config field is seconds, but the
+  internal time base is days).
+- Temperature: `°C`.
+- KLa: `1/h` per the schema; multiplied by the seconds-input `timeStep`
+  inside `_calcOTR` — readers verifying the math should account for the
+  day-internal time base.
+
+Unit conversion at the parent/child boundary happens via
+`MeasurementContainer.UnitPolicy` and the `convert` utility. Other
+nodes (rotatingMachine, pumpingStation, …) honour canonical units;
+reactor is the only ASM-modelled node and pays the small cost of
+domain-textbook units to stay aligned with every published reactor
+reference.
+
 ## Inputs (msg.topic on Port 0)
 
 | Canonical | Aliases (deprecated) | Payload | Effect |