@@ -0,0 +1,9 @@
+---
+"@objectstack/lint": patch
+---
+
+`os validate` and `os build` now check an `autonumber` field's `format` when its `autonumberFormat` is an empty string (#19772).
+
+An empty `autonumberFormat` counts as not set: the platform numbers records with the field's `format` instead, and with `{0000}` when neither is set. The build-time check stopped at the empty `autonumberFormat` and looked no further, so a `format` naming a field the object does not have — `{ type: 'autonumber', autonumberFormat: '', format: '{nope}{000}' }` — passed `os validate` and `os build`, and then every record create failed with `Cannot generate autonumber … referenced field(s) [nope] are empty on the record`.
+
+The check now reads the format the platform numbers records with. That field now fails the build with the same `autonumber-references-unknown-field` error it gets when `format` is written alone, and publishing the object at runtime is refused with the same error. The optional-field, self-reference and unrecognised-token checks follow the same format. A field that sets a non-empty `autonumberFormat`, sets only `format`, or sets neither is checked exactly as before.
@@ -1,6 +1,7 @@
 // Copyright (c) 2025 ObjectStack. Licensed under the Apache-2.0 license.
 
 import { describe, it, expect } from 'vitest';
+import { DEFAULT_AUTONUMBER_FORMAT, resolveAutonumberFormat } from '@objectstack/spec/data';
 import {
   lintAutonumberFormats,
   AUTONUMBER_UNKNOWN_FIELD,
@@ -127,3 +128,67 @@ describe('lintAutonumberFormats', () => {
     expect(lintAutonumberFormats(stack)).toEqual([]);
   });
 });
+
+/**
+ * Which format the lint checks. The engine and the SQL driver both mint through
+ * `resolveAutonumberFormat`: canonical `autonumberFormat`, then the `format`
+ * shorthand, then the contract default — and a key holding anything but a
+ * NON-EMPTY string counts as undeclared. A lint that answers "which format?"
+ * any other way checks a format the runtime never renders, so it can pass a
+ * field whose every create then throws.
+ */
+describe('lintAutonumberFormats — checks the format the runtime mints with', () => {
+  const stackOf = (field: Record<string, unknown>) => ({
+    objects: [
+      {
+        name: 'task',
+        fields: [{ name: 'plan_no', type: 'text', required: true }, { name: 'task_no', type: 'autonumber', ...field }],
+      },
+    ],
+  });
+
+  it('an EMPTY autonumberFormat is undeclared: the `format` shorthand is linted, as the runtime mints it', () => {
+    const out = lintAutonumberFormats(stackOf({ autonumberFormat: '', format: '{nope}{000}' }));
+    expect(out).toHaveLength(1);
+    expect(out[0].rule).toBe(AUTONUMBER_UNKNOWN_FIELD);
+    expect(out[0].severity).toBe('error');
+    expect(out[0].where).toContain('"{nope}{000}"');
+  });
+
+  it('a non-empty autonumberFormat wins over the `format` shorthand', () => {
+    const out = lintAutonumberFormats(stackOf({ autonumberFormat: '{gone}{000}', format: '{plan_no}{000}' }));
+    expect(out).toHaveLength(1);
+    expect(out[0].rule).toBe(AUTONUMBER_UNKNOWN_FIELD);
+    expect(out[0].message).toContain('{gone}');
+  });
+
+  it('the `format` shorthand is linted when autonumberFormat is absent', () => {
+    const out = lintAutonumberFormats(stackOf({ format: '{nope}{000}' }));
+    expect(out).toHaveLength(1);
+    expect(out[0].rule).toBe(AUTONUMBER_UNKNOWN_FIELD);
+  });
+
+  it('with neither key declared, the contract default is linted — and draws no finding', () => {
+    const declaredDefault = lintAutonumberFormats(stackOf({ autonumberFormat: DEFAULT_AUTONUMBER_FORMAT }));
+    for (const undeclared of [{}, { autonumberFormat: '', format: '' }]) {
+      expect(lintAutonumberFormats(stackOf(undeclared))).toEqual(declaredDefault);
+    }
+    expect(declaredDefault).toEqual([]);
+  });
+
+  it('for every spelling of the two keys, the findings equal those for the resolved format', () => {
+    const values: unknown[] = [undefined, '', 7, 'A-{0000}', '{nope}{000}'];
+    const diverged: string[] = [];
+    for (const autonumberFormat of values) {
+      for (const format of values) {
+        const field: Record<string, unknown> = {};
+        if (autonumberFormat !== undefined) field.autonumberFormat = autonumberFormat;
+        if (format !== undefined) field.format = format;
+        const asWritten = lintAutonumberFormats(stackOf(field));
+        const asMinted = lintAutonumberFormats(stackOf({ autonumberFormat: resolveAutonumberFormat(field) }));
+        if (JSON.stringify(asWritten) !== JSON.stringify(asMinted)) diverged.push(JSON.stringify(field));
+      }
+    }
+    expect(diverged).toEqual([]);
+  });
+});
@@ -19,9 +19,17 @@
  *
  * A self-reference (`{self}` on the autonumber field itself) is always an
  * ERROR — the value does not exist yet when the format renders.
+ *
+ * The format checked is the one the runtime mints with, answered by
+ * `resolveAutonumberFormat` — the resolver the engine and the SQL driver call —
+ * never by a precedence rule kept here. A local copy already disagreed with it
+ * once: it took `autonumberFormat: ''` as declared, checked nothing, and passed
+ * a `format` shorthand the runtime then rendered and threw on. A field that
+ * declares neither key is checked against the contract default, which is what
+ * it mints with.
  */
 
-import { parseAutonumberFormat, referencedFields } from '@objectstack/spec/data';
+import { parseAutonumberFormat, referencedFields, resolveAutonumberFormat } from '@objectstack/spec/data';
 import { recordsOf } from './object-graph.js';
 
 export interface AutonumberLintFinding {
@@ -57,10 +65,7 @@ export function lintAutonumberFormats(stack: AnyRec): AutonumberLintFinding[] {
     for (const f of fields) {
       if (f.type !== 'autonumber') continue;
       const name = typeof f.name === 'string' ? f.name : '(unnamed field)';
-      const fmt = typeof f.autonumberFormat === 'string'
-        ? f.autonumberFormat
-        : (typeof f.format === 'string' ? f.format : '');
-      if (!fmt) continue;
+      const fmt = resolveAutonumberFormat(f);
       const tokens = parseAutonumberFormat(fmt);
       const refs = referencedFields(tokens);
       const where = `object '${objectName}' · field '${name}' (autonumber "${fmt}")`;