diff --git a/.changeset/24720-validation-rule-parent.md b/.changeset/24720-validation-rule-parent.md
new file mode 100644
index 00000000000..2efe9027f54
--- /dev/null
+++ b/.changeset/24720-validation-rule-parent.md
@@ -0,0 +1,22 @@
+---
+"@objectstack/lint": patch
+---
+
+fix(lint): `os validate`, `os build` and the object save door warn on a `validations[]` rule whose predicate reads `parent` (#24720)
+
+Clause-②: no
+
+A `validations[]` rule's predicates — a `script` or `cross_field` rule's `condition`, a `conditional` rule's `when`, and the same slots on the rules nested in its `then` / `otherwise` — are evaluated by the server against `record` and `previous` only. The master-detail header is bound as `parent` for a field's own `readonlyWhen` and `requiredWhen` alone, on an object with exactly one `master_detail`. A rule that read `parent`, such as `parent.status == 'paid'`, passed `os validate` silently, because `parent` is a root the platform declares. At runtime the predicate faulted (`Unknown variable: parent`) and every write the rule judged was refused as unevaluable, the seed's own rows at boot included.
+
+The build's expression rule (`validateStackExpressions`) now reports such a predicate at `warning`, located where the rule's other findings are (`object 'NAME' · validation 'RULE'`, `… when`, or the nested rule's label). The message names the slot, the rule and the root. The object save door runs the same pass and lists the same finding among its advisories: the same rule id (`expression-invalid`), location and message.
+
+**Nothing new is refused in 18.x.** A warning fails neither `os build` nor `os validate`, and a publish-mode object save that carries such a rule is still admitted. No ADR-0087 conversion can rewrite a predicate that reads `parent`, so under the post-GA rule for such narrowings only this warning ships in 18.x. The refusal, the same finding at `error`, is planned for 19.0. It will also cover a rule marked `active: false`, a rule whose `events` a stack never fires, and a `conditional` branch whose `when` is never true, because the verdict judges the rule as written.
+
+**Remedy.** To lock or require a field on the header's state, write the predicate on that field's own `readonlyWhen` or `requiredWhen`, where `parent` is bound. For any other check, read a column the detail object declares, and denormalise the header value you need onto the detail.
+
+**Unchanged.**
+
+- The runtime is unchanged: a `validations[]` rule still binds `record` and `previous` and no `parent`. If the runtime comes to bind `parent` there, this verdict is withdrawn in that same change.
+- `parent` on a field's own `readonlyWhen` / `requiredWhen` is still accepted with no finding, as before, and so is a `record` column spelled like the root (`record.parent_code`).
+- No example or package object in this repository authors `parent` in a `validations[]` rule.
+- No public export or signature moves.
diff --git a/packages/lint/src/validate-expressions.ts b/packages/lint/src/validate-expressions.ts
index 2f0c605228c..eff83c96b2e 100644
--- a/packages/lint/src/validate-expressions.ts
+++ b/packages/lint/src/validate-expressions.ts
@@ -552,6 +552,10 @@ interface RulePredicate {
   raw: unknown;
   /** 0 for the rule itself; 1 inside its `then` / `otherwise`, and so on down. */
   depth: number;
+  /** The key the predicate is written under on its own rule. */
+  slot: 'condition' | 'when';
+  /** The name of the rule that carries it (the nested rule's own, below depth 0). */
+  ruleName: string;
 }
 
 /**
@@ -572,9 +576,9 @@ function rulePredicates(rule: AnyRec, path: string, depth = 0): RulePredicate[]
   // author who wrote both `condition` and a rejected alias had their canonical
   // predicate short-circuited away and the alias validated instead (#5017).
   const main = rule.condition;
-  if (main != null) out.push({ label: `validation rule ${here}`, raw: main, depth });
+  if (main != null) out.push({ label: `validation rule ${here}`, raw: main, depth, slot: 'condition', ruleName: name });
   if (rule.when != null) {
-    out.push({ label: `validation rule ${here} when-predicate`, raw: rule.when, depth });
+    out.push({ label: `validation rule ${here} when-predicate`, raw: rule.when, depth, slot: 'when', ruleName: name });
   }
   for (const branch of ['then', 'otherwise'] as const) {
     const nested = rule[branch];
@@ -1710,6 +1714,73 @@ function optionVisibleWhenUserMemberIssue(
   };
 }
 
+/**
+ * [#24720] The root verdict for a `validations[]` rule's own predicates — a
+ * `script` / `cross_field` rule's `condition`, a `conditional` rule's `when`,
+ * and the same two slots on every rule nested in its `then` / `otherwise`: a
+ * predicate that reads `parent` is reported, at `warning` (18.x).
+ *
+ * ## Why it is reported
+ *
+ * ObjectQL evaluates every one of those predicates in `ruleScope(record,
+ * previous)` (`rule-validator.ts`: `checkPredicate`, `checkConditional`), which
+ * binds `record` and `previous` and no master-detail header. The header is
+ * bound as `parent` for a field's own `readonlyWhen` / `requiredWhen` only
+ * ({@link FIELD_RULE_BOUND_ROOTS}), on an object with exactly one
+ * `master_detail`. So the predicate faults (`Unknown variable: parent`) on
+ * every write it judges, and an unevaluable rule fails closed: the write is
+ * refused, the seed's own rows at boot included. Nothing else named it,
+ * because `parent` is in `SCOPE_ROOTS`, which the strict env declares, so the
+ * bare-reference check stays silent. The field-rule slots one level up accept
+ * the same text, which is what makes it the shape an author copies.
+ *
+ * ## Why a warning, and when it becomes a refusal
+ *
+ * A rule the runtime cannot evaluate must not pass a door (ADR-0049), so the
+ * end state is `error`. But refusing it narrows what `os build` and the object
+ * save door accept (a rule marked `active: false`, or a `conditional` branch
+ * whose `when` is never true, builds and writes today), and no ADR-0087
+ * conversion can rewrite a predicate that reads `parent`. Under the standing
+ * post-GA ruling (甲), a narrowing no conversion carries ships only its loud
+ * half in 18.x: this verdict reports at `warning`, which neither fails
+ * `os build` nor refuses a save (the door lists it among its advisories). The
+ * refusal — the same finding at `error` — is planned for 19.0.
+ *
+ * ## Why `parent` alone
+ *
+ * This is the one root the record's own field rules bind and its validation
+ * rules do not: the credible copy, the one the runtime's own diagnostics name.
+ * Whether the other platform-wide roots the rule evaluator binds no more of
+ * (the acting user's among them) earn the same verdict is not decided here,
+ * because neither the runtime nor its callers were measured for them.
+ *
+ * ⛔ This verdict is withdrawn in the same change that binds `parent` for a
+ * `validations[]` rule, never before.
+ *
+ * `null` = nothing to report: the source does not parse (the syntax pass owns
+ * that), or it reads no `parent`.
+ */
+function validationRuleParentIssue(
+  objectName: string | undefined,
+  ruleName: string,
+  slot: 'condition' | 'when',
+  source: string,
+): string | null {
+  if (!readsParentRoot(source)) return null;
+  const owner = objectName ? `'${objectName}'` : 'this object';
+  return (
+    `\`${slot}\` of validation rule '${ruleName}' reads \`parent\`, but the server evaluates a ` +
+    `\`validations[]\` rule against \`record\` and \`previous\` only, so \`parent\` is unbound there. ` +
+    `The rule can never be evaluated, and the server REFUSES every write it judges — a rule that ` +
+    `cannot be evaluated fails closed, seed rows included. This is a warning today; ` +
+    `\`os build\` and the object save door will refuse it from 19.0. The master-detail header is bound as ` +
+    `\`parent\` only for a field's own \`readonlyWhen\` and \`requiredWhen\`, on an object with exactly ` +
+    `one \`master_detail\`: to lock or require a field on the header's state, write the predicate on ` +
+    `that field's \`readonlyWhen\` or \`requiredWhen\` instead. For any other check, read a column ` +
+    `${owner} declares (denormalise the header value you need onto ${owner}).`
+  );
+}
+
 /**
  * [#20078] The master object of an object with exactly ONE `master_detail`
  * relationship — the record the field level binds as `parent` — or `undefined`
@@ -2848,6 +2919,21 @@ export function runStackExpressionPasses(stack: AnyRec, options: StackExpression
         if (p.depth === 0) continue;
         check(`object '${objectName}' · ${p.label}`, p.raw, objectName, 'record', undefined, true);
       }
+      // [#24720] The rule's root verdict, over every predicate it carries,
+      // located where `check()` above located the same predicate. `parent` is
+      // a `SCOPE_ROOTS` member, so `check()` never reports a bare reference to
+      // it and the two verdicts stay disjoint; see
+      // {@link validationRuleParentIssue}.
+      for (const p of predicates) {
+        const source = celSourceOf(p.raw);
+        const message = source ? validationRuleParentIssue(objectName, p.ruleName, p.slot, source) : null;
+        if (!message) continue;
+        const at = p.depth > 0
+          ? `object '${objectName}' · ${p.label}`
+          : p.slot === 'when' ? `${where} when` : where;
+        // `warning` in 18.x, `error` from 19.0: see the docblock's ruling.
+        issues.push({ where: at, message, source: source!, severity: 'warning' });
+      }
       // #4763 — null-guard gate over every predicate the rule carries, nested
       // `then`/`otherwise` branches included.
       for (const p of predicates) {
diff --git a/packages/lint/src/validate-expressions.validation-rule-parent.test.ts b/packages/lint/src/validate-expressions.validation-rule-parent.test.ts
new file mode 100644
index 00000000000..1031c85ae69
--- /dev/null
+++ b/packages/lint/src/validate-expressions.validation-rule-parent.test.ts
@@ -0,0 +1,205 @@
+// Copyright (c) 2026 ObjectStack. Licensed under the Apache-2.0 license.
+
+/**
+ * #24720 — a `validations[]` rule's predicate that reads `parent` is reported
+ * at authoring, at `os validate` / `os build` and at the object save door, at
+ * `warning` in 18.x (the standing post-GA ruling 甲: a narrowing no ADR-0087
+ * conversion carries ships its loud half first; the refusal comes in 19.0).
+ *
+ * ## The state this closes
+ *
+ * ObjectQL evaluates every `validations[]` predicate — a `script` /
+ * `cross_field` rule's `condition` (`checkPredicate`), a `conditional` rule's
+ * `when` (`checkConditional`), and the rules nested in its `then` /
+ * `otherwise` — in `ruleScope(record, previous)`: `record` and `previous`, and
+ * no master-detail header. The header is bound as `parent` for a field's own
+ * `readonlyWhen` / `requiredWhen` only. `parent` is a `SCOPE_ROOTS` member, so
+ * the bare-reference check was silent on it, and a rule reading
+ * `parent.status` passed every door — then faulted (`Unknown variable:
+ * parent`) on every write it judged, which the server refuses as unevaluable,
+ * the seed's own rows included.
+ *
+ * ## The verdict
+ *
+ * One root verdict over every predicate the rule carries, located where
+ * `check()` locates the same predicate, reporting `parent` at warning, with the
+ * prescription that is true for it: the field-rule slots where `parent` IS
+ * bound, or a denormalised column. The runtime is unchanged.
+ */
+import { describe, expect, it } from 'vitest';
+import { EXPRESSION_INVALID, runAuthoringRules } from './authoring-rules.js';
+import { runRuntimeAuthoringRules } from './runtime-gate.js';
+import { validateStackExpressions } from './validate-expressions.js';
+
+const PARENT = "parent.status == 'paid'";
+
+const header = {
+  name: 'fx_header',
+  label: 'Header',
+  sharingModel: 'private',
+  fields: { name: { type: 'text', label: 'Name' }, status: { type: 'text', label: 'Status' } },
+};
+
+/** A detail with exactly ONE `master_detail` — the shape on which the field-rule slots bind `parent`. */
+const line = (
+  validations: unknown[],
+  extraFields: Record<string, unknown> = {},
+  masters: number = 1,
+) => ({
+  name: 'fx_line',
+  label: 'Line',
+  sharingModel: 'private',
+  fields: {
+    ...(masters >= 1 ? { hdr: { type: 'master_detail', label: 'Header', reference: 'fx_header' } } : {}),
+    ...(masters >= 2 ? { hdr2: { type: 'master_detail', label: 'Header 2', reference: 'fx_header' } } : {}),
+    amount: { type: 'number', label: 'Amount' },
+    parent_code: { type: 'text', label: 'Parent code' },
+    ...extraFields,
+  },
+  validations,
+});
+
+const script = (condition: string, name = 'paid_lock') => ({ name, type: 'script', condition, message: 'x' });
+
+const stackOf = (...objects: unknown[]) => ({ objects });
+
+const expressionFindings = <T extends { rule: string }>(fs: readonly T[]): T[] =>
+  fs.filter((f) => f.rule === EXPRESSION_INVALID);
+
+const commandFindings = (command: 'validate' | 'build', ...objects: unknown[]) => {
+  const stack = stackOf(...objects);
+  return expressionFindings(runAuthoringRules(command, { normalized: stack, parsed: stack }));
+};
+
+const gateObject = (item: unknown, context: unknown[]) =>
+  runRuntimeAuthoringRules({ type: 'object', item, context: { objects: context } });
+
+const dump = (r: unknown) => JSON.stringify(r, null, 2);
+
+const WHERE = "object 'fx_line' · validation 'paid_lock'";
+
+describe('#24720 — a `validations[]` predicate reading `parent` is reported at authoring', () => {
+  it('⭐ LIT — a `script` rule\'s `condition` reading `parent` is reported at warning, located at the rule', () => {
+    const issues = validateStackExpressions(stackOf(header, line([script(PARENT)])));
+
+    expect(issues, dump(issues)).toHaveLength(1);
+    expect(issues[0]).toMatchObject({ where: WHERE, severity: 'warning', source: PARENT });
+    expect(issues[0]!.message).toContain("validation rule 'paid_lock' reads `parent`");
+  });
+
+  it('⭐ the prescription names the field-rule slots where `parent` IS bound, and the denormalise remedy', () => {
+    const [issue] = validateStackExpressions(stackOf(header, line([script(PARENT)])));
+
+    expect(issue!.message).toContain('`readonlyWhen`');
+    expect(issue!.message).toContain('`requiredWhen`');
+    expect(issue!.message).toContain("denormalise the header value you need onto 'fx_line'");
+  });
+
+  it('⭐ the `os validate` and `os build` commands both give the finding, at warning: nothing they gate on', () => {
+    for (const command of ['validate', 'build'] as const) {
+      const stack = stackOf(header, line([script(PARENT)]));
+      const all = runAuthoringRules(command, { normalized: stack, parsed: stack });
+      const found = expressionFindings(all);
+      expect(found, `${command}: ${dump(found)}`).toHaveLength(1);
+      expect(found[0]).toMatchObject({ severity: 'warning', where: WHERE });
+      // The 18.x half of the ruling: no `error` anywhere, so the command does not fail.
+      expect(all.filter((f) => f.severity === 'error'), `${command}: ${dump(all)}`).toEqual([]);
+    }
+  });
+
+  it('⭐ a `cross_field` rule\'s `condition` is judged the same way', () => {
+    const rule = { name: 'paid_lock', type: 'cross_field', fields: ['amount'], condition: PARENT, message: 'x' };
+    const issues = validateStackExpressions(stackOf(header, line([rule])));
+
+    expect(issues, dump(issues)).toHaveLength(1);
+    expect(issues[0]).toMatchObject({ where: WHERE, severity: 'warning' });
+  });
+
+  it('⭐ a `conditional` rule\'s `when`, and the rules in its `then` / `otherwise`, are judged at their own locations', () => {
+    const rule = {
+      type: 'conditional',
+      name: 'gate',
+      when: PARENT,
+      message: 'x',
+      then: { type: 'script', name: 'inner', condition: "parent.status != 'void'", message: 'y' },
+      otherwise: { type: 'script', name: 'other', condition: 'record.amount != null && record.amount > 0', message: 'z' },
+    };
+    const issues = validateStackExpressions(stackOf(header, line([rule])));
+
+    expect(issues.map((i) => i.where), dump(issues)).toEqual([
+      "object 'fx_line' · validation 'gate' when",
+      "object 'fx_line' · validation rule 'gate' then → 'inner'",
+    ]);
+    for (const i of issues) {
+      expect(i.severity).toBe('warning');
+      expect(i.message).toContain('reads `parent`');
+    }
+  });
+
+  it('gives one finding per predicate however often it reads `parent`', () => {
+    const issues = validateStackExpressions(
+      stackOf(header, line([script(`${PARENT} && parent.name != ''`)])),
+    );
+    expect(issues, dump(issues)).toHaveLength(1);
+  });
+
+  it('refuses it whatever the object\'s master count: no validation rule binds `parent` on any object', () => {
+    for (const masters of [0, 2]) {
+      const issues = validateStackExpressions(stackOf(header, line([script(PARENT)], {}, masters)));
+      const here = issues.filter((i) => i.where === WHERE);
+      expect(here, `${masters} masters: ${dump(issues)}`).toHaveLength(1);
+      expect(here[0]!.severity).toBe('warning');
+    }
+  });
+
+  it('⭐ CONTROL — `parent` on the field\'s own `readonlyWhen` / `requiredWhen` is still accepted', () => {
+    for (const key of ['readonlyWhen', 'requiredWhen']) {
+      const issues = validateStackExpressions(
+        stackOf(header, line([], { note: { type: 'text', label: 'Note', [key]: PARENT } })),
+      );
+      expect(issues, `${key}: ${dump(issues)}`).toEqual([]);
+    }
+  });
+
+  it('⭐ CONTROL — `record` / `previous` reads, a column spelled like the root and the string `parent` pass', () => {
+    for (const condition of [
+      'record.amount != null && record.amount > 100',
+      'previous != null && previous.amount != null && previous.amount > 100',
+      "record.parent_code == 'a'",
+      "record.parent_code == 'parent'",
+    ]) {
+      const issues = validateStackExpressions(stackOf(header, line([script(condition)])));
+      expect(issues, `${condition}: ${dump(issues)}`).toEqual([]);
+    }
+  });
+});
+
+describe('#24720 — the object door gives the build\'s verdict', () => {
+  it('⭐ LIT — the measured body is ADMITTED at the door with the finding as an advisory, located at the rule', () => {
+    const result = gateObject(line([script(PARENT)]), [header]);
+
+    expect(result.rulesRun).toContain('validateStackExpressions');
+    expect(result.errors, dump(result)).toEqual([]);
+    const advs = expressionFindings(result.advisories);
+    expect(advs, dump(result)).toHaveLength(1);
+    expect(advs[0]).toMatchObject({ severity: 'warning', where: WHERE, path: WHERE });
+    expect(advs[0]!.message).toContain("validation rule 'paid_lock' reads `parent`");
+  });
+
+  it('⭐ PARITY — the door finding IS the build finding', () => {
+    const atBuild = commandFindings('build', header, line([script(PARENT)]));
+    const atDoor = expressionFindings(gateObject(line([script(PARENT)]), [header]).advisories);
+
+    expect(atBuild, dump(atBuild)).toHaveLength(1);
+    expect(atDoor, dump(atDoor)).toEqual(atBuild);
+  });
+
+  it('⭐ CONTROL — `parent` on a field\'s own `readonlyWhen` publishes clean, and the build agrees', () => {
+    const body = line([], { note: { type: 'text', label: 'Note', readonlyWhen: PARENT } });
+    const result = gateObject(body, [header]);
+
+    expect(expressionFindings(result.errors), dump(result)).toEqual([]);
+    expect(expressionFindings(result.advisories), dump(result)).toEqual([]);
+    expect(commandFindings('build', header, body)).toEqual([]);
+  });
+});