diff --git a/skills/objectstack-data/SKILL.md b/skills/objectstack-data/SKILL.md
index 182d366fcc..bfe1c3037c 100644
--- a/skills/objectstack-data/SKILL.md
+++ b/skills/objectstack-data/SKILL.md
@@ -282,12 +282,13 @@ export const Invoice = ObjectSchema.create({
 - Use `requiredWhen` for conditional requiredness; the ObjectQL validator
   enforces it on submit. The `conditionalRequired` alias was REMOVED in
   protocol 17 — emitting it is a parse error.
-- **Choose by intent — invariant or transition gate.** A fact that must hold for
-  *every stored record* is an invariant: express it in `validations[]`. A
-  condition on a *transition* ("required once the record reaches `paid`") is
-  `requiredWhen` / field bounds, which let already-stored rows through. A
-  transition gate written as an invariant bricks existing data; an invariant
-  written as a transition gate never enforces itself.
+- **Choose by intent — invariant or transition gate.** A fact true of *every
+  stored record* is an invariant: a `validations[]` `script` rule. A row that
+  violates it is refused on any edit until repaired. A *transition* condition
+  ("required once the record reaches `paid`") is `requiredWhen` / field bounds,
+  which judge the write, not the stored row. "Required when X" reads like an
+  invariant and is not one; an invariant written as a gate never enforces
+  itself.
 - For inline `master_detail` grids, predicates are evaluated row-by-row against
   the child row's `record`, so line-item rules should live on child fields.
 - For complex predicates, load **objectstack-formula** and emit CEL via
diff --git a/skills/objectstack-data/rules/validation.md b/skills/objectstack-data/rules/validation.md
index 180e6c0f8a..930d790dd7 100644
--- a/skills/objectstack-data/rules/validation.md
+++ b/skills/objectstack-data/rules/validation.md
@@ -36,6 +36,15 @@ the **failure** condition — validation **fails** when it evaluates to `true`.
 
 ## Script Validation
 
+**Tool choice — this is the invariant tool.** `script` and `cross_field`
+are judged against the *merged* record on every write, with no pre-state
+exemption, so a row that already violates the rule is refused on any edit until
+a repairing write lands (frozen, not bricked). A condition that must hold only
+from a transition onward is not this tool: use `requiredWhen` or a field bound
+(`min` / `max` / `minLength` / `maxLength`), which judge the write, not the
+stored row (see **Choose by intent — invariant or transition gate** in
+`SKILL.md`).
+
 ```typescript
 import { P } from '@objectstack/spec';