diff --git a/.changeset/autonumber-format-defallback-driver-sql.md b/.changeset/autonumber-format-defallback-driver-sql.md
new file mode 100644
index 0000000000..1f23613771
--- /dev/null
+++ b/.changeset/autonumber-format-defallback-driver-sql.md
@@ -0,0 +1,45 @@
+---
+"@objectstack/driver-sql": patch
+---
+
+refactor(driver-sql): read the autonumber default from the contract instead of a hardcoded fallback (#7263)
+
+Execution half 3/3 of the maintainer's route-3 ruling on #6555. `{0000}` is now a
+declared contract default (`DEFAULT_AUTONUMBER_FORMAT`, landed with
+`resolveAutonumberFormat` in `@objectstack/spec/data`), so this driver stops
+writing the default down for itself.
+
+Two sites in `sql-driver.ts` — `initObjects` and the external-object
+registration path — each spelled the same four lines by hand:
+
+```ts
+const rawFmt = (typeof field.autonumberFormat === 'string' && field.autonumberFormat)
+  ? field.autonumberFormat
+  : (typeof field.format === 'string' && field.format ? field.format : '');
+const fmt = rawFmt || '{0000}';
+```
+
+Both are now `const fmt = resolveAutonumberFormat(field);`. That is the whole
+change: one symbol added to an import this file already had, no new dependency,
+and the `#1603` comment about honouring both spellings retired to the resolver's
+own docstring, which carries it.
+
+**Behaviour-neutral, by construction and by measurement.** `resolveAutonumberFormat`'s
+precedence — canonical `autonumberFormat`, then the `format` shorthand, then the
+declared default, with anything that is not a **non-empty string** counting as
+undeclared — was deliberately taken from these very lines, including their
+truthiness rule (not the engine's `??`). A differential check over 484 field
+documents, spanning both spellings across 22 value shapes (absent key,
+`undefined`, `null`, `''`, non-empty strings, numbers, booleans, `NaN`, arrays,
+objects, a boxed `String`, `Symbol`, function, `BigInt`), found the old
+expressions and the resolver returning the identical string in every case —
+`format: ''`, `autonumberFormat: ''` and the non-string values included, not just
+the happy path.
+
+Compatibility note, per the ruling: choosing {0000} keeps stored driver-sql data
+undisturbed; engine-fallback deployments flip from bare 1 to 0001 for newly
+issued numbers. Counter continuity itself is unaffected (#6468 pinned it).
+
+The engine half of the same ruling is #7262; #6555 stays open until it lands, so
+a format-less field still renders `0001` on SQL and a bare `1` on the engine's
+in-memory fallback until then. This half moves neither.
diff --git a/packages/drivers/driver-sql/src/sql-driver-autonumber-suffix.test.ts b/packages/drivers/driver-sql/src/sql-driver-autonumber-suffix.test.ts
index f14e440a9a..b859a8f547 100644
--- a/packages/drivers/driver-sql/src/sql-driver-autonumber-suffix.test.ts
+++ b/packages/drivers/driver-sql/src/sql-driver-autonumber-suffix.test.ts
@@ -165,11 +165,12 @@ describe('SqlDriver autonumber seeding — the counter is located by the declare
       // lexicographic one — so the counter continues at 11.
       //
       // The RENDERING of a format-less field is a separate, pre-existing matter
-      // this fix does not touch: this driver substitutes `{0000}` for a missing
-      // format (see `initObjects`), so 11 renders `0011` here while the engine's
-      // fallback emits the bare `11`. That divergence is in the render default,
-      // not in the seeding parse #6468 is about, so the cross-side parity test
-      // uses explicitly-formatted fields.
+      // this fix does not touch: a format-less field resolves to the contract
+      // default `{0000}` (`resolveAutonumberFormat`, #6555), so 11 renders
+      // `0011` here — while the engine's fallback still emits the bare `11`
+      // until #7262 lands the other half. That divergence is in the render
+      // default, not in the seeding parse #6468 is about, so the cross-side
+      // parity test uses explicitly-formatted fields.
       await initRec();
       await seedRows(['1', '2', '10']);
 
diff --git a/packages/drivers/driver-sql/src/sql-driver.ts b/packages/drivers/driver-sql/src/sql-driver.ts
index a72ef68f07..543f3cc91a 100644
--- a/packages/drivers/driver-sql/src/sql-driver.ts
+++ b/packages/drivers/driver-sql/src/sql-driver.ts
@@ -8,7 +8,7 @@
  */
 
 import type { DriverOptions, FilterCondition, SchemaMode } from '@objectstack/spec/data';
-import { parseAutonumberFormat, renderAutonumber, readAutonumberCounter, missingFieldValues, isTenancyDisabled, type AutonumberToken } from '@objectstack/spec/data';
+import { parseAutonumberFormat, renderAutonumber, resolveAutonumberFormat, readAutonumberCounter, missingFieldValues, isTenancyDisabled, type AutonumberToken } from '@objectstack/spec/data';
 // The DECLARED aggregate vocabulary (#5907). Read from the spec so this driver's
 // "the protocol has no such function" refusal cannot drift from what
 // `AggregationNodeSchema.function` actually admits.
@@ -5353,10 +5353,7 @@ export class SqlDriver implements IDataDriver {
         if (type === 'datetime') datetimeCols.push(name);
         if (type === 'time') timeCols.push(name);
         if (type === 'auto_number' || type === 'autonumber') {
-          const rawFmt = (typeof field.autonumberFormat === 'string' && field.autonumberFormat)
-            ? field.autonumberFormat
-            : (typeof field.format === 'string' && field.format ? field.format : '');
-          const fmt = rawFmt || '{0000}';
+          const fmt = resolveAutonumberFormat(field);
           autoNumberCols.push({ name, format: fmt, tokens: parseAutonumberFormat(fmt), tenantField });
         }
       }
@@ -5433,12 +5430,7 @@ export class SqlDriver implements IDataDriver {
             (this.timeFields[tableName] ??= new Set()).add(name);
           }
           if (type === 'auto_number' || type === 'autonumber') {
-            // Honor either the spec-canonical `autonumberFormat` or the
-            // shorthand `format` (both appear in metadata) — see #1603.
-            const rawFmt = (typeof field.autonumberFormat === 'string' && field.autonumberFormat)
-              ? field.autonumberFormat
-              : (typeof field.format === 'string' && field.format ? field.format : '');
-            const fmt = rawFmt || '{0000}';
+            const fmt = resolveAutonumberFormat(field);
             // Tokenize once: the renderer resolves date tokens (`{YYYYMMDD}`),
             // field interpolation (`{island_zone}`) and the sequence slot at
             // fill time. The counter scopes to whatever renders before the slot.