@@ -0,0 +1,16 @@
+---
+'@objectstack/lint': patch
+---
+
+fix(lint): a validation rule's `expression-invalid` finding carries the slot's key path (`objects[0].validations[1].condition`) beside its `where` text
+
+Clause-②: no
+Metadata-protocol: none
+
+An `expression-invalid` finding on an object validation rule's predicate used to be located by its `where` prose alone (`object 'invoice' · validation rule 'amount_rule'`), which is also what its `path` carried, so a reader keyed on paths could not address it. Its `path` is now the judged slot's key path in the spelling a flow edge's findings use, so a draft's `_diagnostics[].path` reports it item-relative (`validations.1.condition`, `validations.2.when`, `validations.2.then.condition`) and the publish refusal's `issues[].path` reads `objects.invoice.validations[1].condition`.
+
+- `where`, the message and the verdict are unchanged: nothing is accepted or refused differently.
+- Every finding of the validation-rule pass moves: the expression check, the `parent` root verdict and the null-guard gate, on a rule's `condition` and `when` and on a `conditional` rule's nested `then` / `otherwise` branches.
+- The object and the rule are indexed in the lists as authored (a `null` member before them does not renumber them).
+- An object in a map-form `objects` collection keeps `where` as its `path`, as before.
+- `os lint` prints the key path after `at` for these findings, where it printed the `where` text a second time.
@@ -66,6 +66,16 @@ const WHERE: Readonly<Record<Slot, string>> = {
   'validation condition': "object 'fx_probe' · validation 'probe_rule'",
 };
 
+/**
+ * The door finding's `path`: the `where` prose on the two field slots, and on
+ * the validation rule (#27643) the judged slot's key path, its top-level index
+ * re-spelled by name at the door.
+ */
+const DOOR_PATH: Readonly<Record<Slot, string>> = {
+  ...WHERE,
+  'validation condition': 'objects.fx_probe.validations[0].condition',
+};
+
 const SLOTS = Object.keys(WHERE) as Slot[];
 
 /** Every member spelling that names `f`; the dot row is the one that was already refused. */
@@ -103,7 +113,7 @@ describe('an undeclared field is refused in every member spelling, at both doors
         expect(door.rulesRun).toContain('validateStackExpressions');
         const errs = expressionFindings(door.errors);
         expect(errs, dump(door)).toHaveLength(1);
-        expect(errs[0]).toMatchObject({ severity: 'error', where: WHERE[slot], path: WHERE[slot] });
+        expect(errs[0]).toMatchObject({ severity: 'error', where: WHERE[slot], path: DOOR_PATH[slot] });
         expect(errs[0]!.message).toContain('unknown field `zz_typo`');
       });
     }
 
@@ -111,6 +111,15 @@ const buildFindings = (body: unknown) => {
 
 const dump = (r: unknown) => JSON.stringify(r, null, 2);
 
+/**
+ * [#27643] The build's findings as the door spells them: a validation rule's
+ * finding carries the judged slot's key path, and the door re-spells its
+ * top-level index by name (`objects[0]` → `objects.fx_rule`); every other key
+ * is the build's own.
+ */
+const asAtDoor = <T extends { path: string }>(fs: readonly T[]): T[] =>
+  fs.map((f) => ({ ...f, path: f.path.replace(/^objects\[0\]/, 'objects.fx_rule') }));
+
 describe('#22032 pass 1 — the object door gives the build\'s validation-rule verdict', () => {
   it('needs no registry change: `validateStackExpressions` is already on the object door', () => {
     expect(runtimeAuthoringRulesFor('object').map((r) => r.name)).toContain('validateStackExpressions');
@@ -123,7 +132,7 @@ describe('#22032 pass 1 — the object door gives the build\'s validation-rule v
     const errs = expressionFindings(result.errors);
     const where = "object 'fx_rule' · validation 'amount_root'";
     expect(errs, dump(result)).toHaveLength(1);
-    expect(errs[0]).toMatchObject({ severity: 'error', where, path: where });
+    expect(errs[0]).toMatchObject({ severity: 'error', where, path: 'objects.fx_rule.validations[0].condition' });
     expect(errs[0]!.message).toContain('`sqrt` is not a callable name here');
   });
 
@@ -171,7 +180,7 @@ describe('#22032 pass 1 — the object door gives the build\'s validation-rule v
 
       // Non-vacuous: the build refuses each of them.
       expect(atBuild.length, dump(rule)).toBeGreaterThan(0);
-      expect(atDoor, dump(rule)).toEqual(atBuild);
+      expect(atDoor, dump(rule)).toEqual(asAtDoor(atBuild));
     }
   });
 
@@ -265,7 +274,11 @@ describe('#22042 — a `conditional` rule\'s nested predicates meet the same ver
     const atBuild = buildFindings(fxRule([NESTED_REFUSED]));
 
     expect(atBuild.map((f) => f.where), dump(atBuild)).toEqual([THEN_WHERE, OTHERWISE_WHERE]);
-    for (const f of atBuild) expect(f).toMatchObject({ severity: 'error', path: f.where });
+    expect(atBuild.map((f) => f.path), dump(atBuild)).toEqual([
+      'objects[0].validations[0].then.condition',
+      'objects[0].validations[0].otherwise.condition',
+    ]);
+    for (const f of atBuild) expect(f).toMatchObject({ severity: 'error' });
     expect(atBuild[0]!.message).toContain('`sqrt` is not a callable name here');
     expect(atBuild[1]!.message).toContain('bare reference `amont`');
   });
@@ -276,7 +289,7 @@ describe('#22042 — a `conditional` rule\'s nested predicates meet the same ver
     expect(result.rulesRun).toContain('validateStackExpressions');
     const atDoor = expressionFindings(result.errors);
     expect(atDoor.map((f) => f.where), dump(result)).toEqual([THEN_WHERE, OTHERWISE_WHERE]);
-    expect(atDoor).toEqual(buildFindings(fxRule([NESTED_REFUSED])));
+    expect(atDoor).toEqual(asAtDoor(buildFindings(fxRule([NESTED_REFUSED]))));
   });
 
   it('⭐ LIT — two levels down: a nested `conditional`\'s `when` and the rule below it are judged', () => {
@@ -288,7 +301,11 @@ describe('#22042 — a `conditional` rule\'s nested predicates meet the same ver
     ]);
     expect(atBuild[0]!.message).toContain('bare reference `amount`');
     expect(atBuild[1]!.message).toContain('`sqrt` is not a callable name here');
-    expect(expressionFindings(gateObject(fxRule([TWO_LEVEL])).errors)).toEqual(atBuild);
+    expect(atBuild.map((f) => f.path), dump(atBuild)).toEqual([
+      'objects[0].validations[0].then.when',
+      'objects[0].validations[0].then.then.condition',
+    ]);
+    expect(expressionFindings(gateObject(fxRule([TWO_LEVEL])).errors)).toEqual(asAtDoor(atBuild));
   });
 
   it('⭐ CONTROL — valid nested predicates publish clean, two levels down and in `otherwise`', () => {
@@ -326,7 +343,7 @@ describe('#22042 — a `conditional` rule\'s nested predicates meet the same ver
     const topWhen = buildFindings(fxRef([{ ...HYDRATION, when: MULTI_HOP, then: NESTED_VALID.otherwise, otherwise: undefined }]));
     expect(topWhen.map((f) => f.where), dump(topWhen)).toEqual(["object 'fx_rule' · validation 'outer' when"]);
     expect(topWhen[0]!.message).toContain('ONE hop');
-    expect(expressionFindings(gateObject(fxRef([HYDRATION])).errors)).toEqual(atBuild);
+    expect(expressionFindings(gateObject(fxRef([HYDRATION])).errors)).toEqual(asAtDoor(atBuild));
   });
 
   it('a stored sibling\'s nested fault is not this write\'s to answer for (the differential)', () => {
@@ -364,7 +381,8 @@ describe('#24468 — the object door refuses a rule hop to a field the target do
     const errs = expressionFindings(result.errors);
     expect(errs, dump(result)).toHaveLength(1);
     const where = "object 'fx_order' · validation 'north_only'";
-    expect(errs[0]).toMatchObject({ severity: 'error', where, path: where });
+    // [#27643] The second rule's `condition`, by key path; `where` stays the prose.
+    expect(errs[0]).toMatchObject({ severity: 'error', where, path: 'objects.fx_order.validations[1].condition' });
     expect(errs[0]!.message).toContain('unknown field `region` on `fx_account`');
     expect(errs[0]!.message).toContain('the rule has no verdict');
   });
@@ -428,7 +446,7 @@ describe('#24512 — the object door judges a `conditional` rule\'s `when` hop a
     const errs = expressionFindings(result.errors);
     expect(errs, dump(result)).toHaveLength(1);
     const where = "object 'fx_order' · validation 'gate' when";
-    expect(errs[0]).toMatchObject({ rule: EXPRESSION_INVALID, severity: 'error', where, path: where });
+    expect(errs[0]).toMatchObject({ rule: EXPRESSION_INVALID, severity: 'error', where, path: 'objects.fx_order.validations[0].when' });
     expect(errs[0]!.message).toContain('unknown field `region` on `fx_account`');
     expect(errs[0]!.message).toContain('the rule has no verdict');
   });
 
@@ -165,7 +165,10 @@ export interface ExprIssue {
    * [#26858] The judged slot's key path in the stack, in the spelling the other
    * rules' findings carry (`flows[0].edges[1].condition`), beside the human
    * `where`. Set only where the slot has one: a top-level flow edge's
-   * `condition`. Absent, the finding is located by `where` alone.
+   * `condition`, and (#27643) an object validation rule's `condition` / `when`
+   * (`objects[0].validations[1].condition`), a nested branch's included
+   * (`objects[0].validations[1].then.condition`). Absent, the finding is
+   * located by `where` alone.
    */
   path?: string;
 }
@@ -573,14 +576,19 @@ interface RulePredicate {
   slot: 'condition' | 'when';
   /** The name of the rule that carries it (the nested rule's own, below depth 0). */
   ruleName: string;
+  /**
+   * [#27643] Its key path inside the top-level rule (`condition`, `when`,
+   * `then.condition`, `otherwise.then.when`), for a finding's `path`.
+   */
+  key: string;
 }
 
 /**
  * Every predicate a validation rule carries, including the ones nested inside a
  * `conditional` rule's `then` / `otherwise` — the trap hides there just as
  * happily as at the top level.
  */
-function rulePredicates(rule: AnyRec, path: string, depth = 0): RulePredicate[] {
+function rulePredicates(rule: AnyRec, path: string, depth = 0, keyPrefix = ''): RulePredicate[] {
   const out: RulePredicate[] = [];
   const name = typeof rule.name === 'string' ? rule.name : '?';
   const here = path ? `${path} → '${name}'` : `'${name}'`;
@@ -593,14 +601,18 @@ 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, slot: 'condition', ruleName: name });
+  if (main != null) {
+    out.push({ label: `validation rule ${here}`, raw: main, depth, slot: 'condition', ruleName: name, key: `${keyPrefix}condition` });
+  }
   if (rule.when != null) {
-    out.push({ label: `validation rule ${here} when-predicate`, raw: rule.when, depth, slot: 'when', ruleName: name });
+    out.push({
+      label: `validation rule ${here} when-predicate`, raw: rule.when, depth, slot: 'when', ruleName: name, key: `${keyPrefix}when`,
+    });
   }
   for (const branch of ['then', 'otherwise'] as const) {
     const nested = rule[branch];
     if (nested && typeof nested === 'object' && !Array.isArray(nested)) {
-      out.push(...rulePredicates(nested as AnyRec, `${here} ${branch}`, depth + 1));
+      out.push(...rulePredicates(nested as AnyRec, `${here} ${branch}`, depth + 1, `${keyPrefix}${branch}.`));
     }
   }
   return out;
@@ -2955,6 +2967,11 @@ export function runStackExpressionPasses(stack: AnyRec, options: StackExpression
   // ── Object validation-rule + formula predicates ────────────────────
   for (const obj of objects) {
     const objectName = typeof obj.name === 'string' ? obj.name : undefined;
+    // [#27643] The object's index in the authored list, for a validation-rule
+    // finding's key path (`recordsOf` hands an array's own members back). A
+    // map-form `objects` has no index to spell, so its findings keep `where`
+    // alone, as a map-form `flows` collection's do (#26858).
+    const objectIndex = Array.isArray(stack.objects) ? stack.objects.indexOf(obj) : -1;
     // `validations` is the key `ObjectSchema` declares; `validationRules` is a
     // rejected alias of it (#5017) — see the `## Scope` table above.
     const validations = obj.validations;
@@ -2963,6 +2980,18 @@ export function runStackExpressionPasses(stack: AnyRec, options: StackExpression
     // position, so the door's findings and their order are the build's.
     for (const rule of recordsOf(validations)) {
       const where = `object '${objectName}' · validation '${(rule.name as string) ?? '?'}'`;
+      // [#27643] Every finding below also carries the judged slot's key path
+      // (`objects[0].validations[1].condition`), indexed over the lists the
+      // author wrote, in the spelling a flow edge's findings use (#26858), so
+      // a draft's `_diagnostics` reports it item-relative
+      // (`validations.1.condition`). `where` stays the human locator.
+      const ruleIndex = objectIndex >= 0 && Array.isArray(obj.validations) ? obj.validations.indexOf(rule) : -1;
+      const rulePath = ruleIndex >= 0 ? `objects[${objectIndex}].validations[${ruleIndex}]` : undefined;
+      const atSlot = (key: string, first: number): void => {
+        if (!rulePath) return;
+        const path = `${rulePath}.${key}`;
+        for (let k = first; k < issues.length; k++) issues[k] = { ...issues[k]!, path };
+      };
       // The declared predicate key is `condition` (see `rulePredicates`).
       // Validation predicates are `record`-scoped — no field flattening — so
       // bare refs are flagged (#1928).
@@ -2973,8 +3002,12 @@ export function runStackExpressionPasses(stack: AnyRec, options: StackExpression
       // `traversalHydration` is passed on those two slots here and nested below
       // (#22042), and on a formula field's `expression` (`judgeFieldFormula`,
       // #24197), and NOWHERE else.
+      let first = issues.length;
       check(where, rule.condition, objectName, 'record', undefined, true);
+      atSlot('condition', first);
+      first = issues.length;
       check(`${where} when`, (rule as AnyRec).when, objectName, 'record', undefined, true);
+      atSlot('when', first);
       const predicates = rulePredicates(rule, '');
       // [#22042] The same verdict one level down, and every level below it: a
       // `conditional` rule's `then` / `otherwise` is a rule the evaluator runs
@@ -2989,7 +3022,9 @@ export function runStackExpressionPasses(stack: AnyRec, options: StackExpression
       // meet the traversal checks.
       for (const p of predicates) {
         if (p.depth === 0) continue;
+        first = issues.length;
         check(`object '${objectName}' · ${p.label}`, p.raw, objectName, 'record', undefined, true);
+        atSlot(p.key, first);
       }
       // [#24720] The rule's root verdict, over every predicate it carries,
       // located where `check()` above located the same predicate. `parent` is
@@ -3004,12 +3039,16 @@ export function runStackExpressionPasses(stack: AnyRec, options: StackExpression
           ? `object '${objectName}' · ${p.label}`
           : p.slot === 'when' ? `${where} when` : where;
         // `error`: a rule the runtime cannot evaluate must not pass a door; see the docblock.
+        first = issues.length;
         issues.push({ where: at, message, source: source!, severity: 'error' });
+        atSlot(p.key, first);
       }
       // #4763 — null-guard gate over every predicate the rule carries, nested
       // `then`/`otherwise` branches included.
       for (const p of predicates) {
+        first = issues.length;
         checkNullGuards(`object '${objectName}' · ${p.label}`, p.label, p.raw, objectName);
+        atSlot(p.key, first);
       }
     }
     // Field-level formulas (computed fields) reference the same object.
 
@@ -196,7 +196,8 @@ describe('#24720 — the object door gives the build\'s verdict', () => {
     expect(result.rulesRun).toContain('validateStackExpressions');
     const errs = expressionFindings(result.errors);
     expect(errs, dump(result)).toHaveLength(1);
-    expect(errs[0]).toMatchObject({ severity: 'error', where: WHERE, path: WHERE });
+    // [#27643] Located at the judged slot's key path; `where` stays the prose.
+    expect(errs[0]).toMatchObject({ severity: 'error', where: WHERE, path: 'objects.fx_line.validations[0].condition' });
     expect(errs[0]!.message).toContain("validation rule 'paid_lock' reads `parent`");
     // No longer an advisory: the finding moved, it was not duplicated.
     expect(expressionFindings(result.advisories), dump(result)).toEqual([]);
@@ -207,7 +208,12 @@ describe('#24720 — the object door gives the build\'s verdict', () => {
     const atDoor = expressionFindings(gateObject(line([script(PARENT)]), [header]).errors);
 
     expect(atBuild, dump(atBuild)).toHaveLength(1);
-    expect(atDoor, dump(atDoor)).toEqual(atBuild);
+    // [#27643] Equal once the door's by-name spelling of the top-level index
+    // is applied to the build's positional key path (`objects[1]`, after the header).
+    expect(atBuild[0]!.path).toBe('objects[1].validations[0].condition');
+    expect(atDoor, dump(atDoor)).toEqual(
+      atBuild.map((f) => ({ ...f, path: f.path.replace(/^objects\[1\]/, 'objects.fx_line') })),
+    );
   });
 
   it('⭐ CONTROL — `parent` on a field\'s own `readonlyWhen` publishes clean, and the build agrees', () => {