skills: three skills say a bare field ref in a predicate "evaluates to null" and silently hides or never fires; main fails loud #30893

domain:skills#2 · os-tesla · session_01L9qibz5RLVEYDFco93jZ7u

Filing gate: ① residue found in contract review of PR #30875 (card #30857, record 6680689068) on a shipped surface (skills/**). ⛔ Not a claim. Suggested grade: p3.
Dedupe: issue search "bare predicate", "evaluates to null", skills predicate null hides (open and closed): #27587 (closed) is about JavaScript-syntax predicates, not this symptom. New.
Reader: domain:skills. Tier H (skills/**).

Business test

An agent that hits a broken predicate on a live stack is told by the skill to expect silence: "the button never shows" or "the validation never fires". What main actually does is louder, and in the opposite direction for actions: an object validation rule that cannot be evaluated rejects the write, and an action's visible predicate that faults shows the control and reports the fault. An agent debugging "every save is refused with a message naming my rule" or "this button shows for everyone" gets no help from the skill. It may even be steered the wrong way, for example into hunting a hidden button that is in fact visible. The fix still recommends the same prevention (os validate, record.<field>); only the symptom description changes.

What is wrong

Three passages describe the pre-#4649 / pre-objectui#6443 behaviour:

• skills/objectstack-platform/references/operations.md, Verify your work (L30–32): "a bare field ref in a predicate (done instead of record.done) evaluates to null and silently hides an action/validation on every record".
• skills/objectstack-data/SKILL.md (≈L450–453): a bare field ref in requiredWhen / readonlyWhen / visibleWhen, a validation rule, a formula, or an RLS/sharing predicate "evaluates to null and never fires".
• skills/objectstack-ui/SKILL.md (≈L269–271): a bare field ref in an action visible / disabled or a field visibleWhen "evaluates to null and hides the control on every record (the "button never shows" trap)".

What main does (to be re-read per site by the dev):

• Object validation predicates: ruleScope binds only record / previous, so a bare done faults (Unknown variable). Since Script validation rules are silently skipped when their predicate fails to evaluate — fail-open is the wrong direction for a validation #4649 the rule validator is fail-CLOSED: an error-severity rule that cannot be evaluated rejects the write with a message naming the rule and the key, and a warning / info rule is logged (packages/objectql/src/validation/rule-validator.ts, docblock "Fail-CLOSED for unevaluable predicates"). The field-level predicates moved under ADR-0137 D2 (same docblock); the dev reads that section for requiredWhen / readonlyWhen / option visibleWhen.
• Action / field visible in the console: evaluateVisibility (ui/packages/app-shell/src/providers/ExpressionProvider.tsx) fails OPEN on a fault, so the control is shown and the fault is reported (objectui#6443). Only navigation entries are withheld on a fault (objectui#12700).
• The "evaluates to null" wording traces to the Formula guardrail: cel-js arithmetic silently returns null (double × int + bare identifiers) #1928-era comment on firstUndeclaredReference in packages/formula/src/cel-engine.ts. It describes the historical class, not today's runtime.
• skills/objectstack-formula/SKILL.md (Verify) already says the engine throws. It is right and needs no change.

Expected

Each of the three passages states the symptom per site as main has it (validation: write rejected, naming the rule; action / field visible: shown, fault reported; and the field-level and RLS/sharing sites as their code reads), and keeps the prevention advice (os validate / os build, record.<field>). Growth is paid by deletion in the same file (ratchet at headroom 0). One PR for the three files. Eval cases only where a skill's eval convention asks for one.