@@ -0,0 +1,12 @@
+---
+"@objectstack/driver-sql": minor
+"@objectstack/driver-turso": patch
+---
+
+fix(driver-sql): the local SQLite `Field.json` storage backfill no longer turns a deeply nested array or object into a string on the next schema sync (#19912)
+
+The backfill that converges legacy json cells on their JSON-encoded form (it came with the change that made the SQLite write path JSON-encode every json value; the issue number that change cites no longer resolves, and its live record is the `SqlDriver.backfillCanonicalJsonEncoding` doc block and `sql-driver-12380-json-roundtrip.test.ts`) ran one `UPDATE … set col = json_quote(col)` over every TEXT cell SQLite's `json_valid()` rejects. `json_valid()` answers 0 for JSON nested more than 1000 levels deep (SQLite's JSON depth limit in every build this repository bundles), while the driver reads such a cell with `JSON.parse` without trouble. So a deep array written correctly through the driver was quoted into a JSON string by the next `syncSchema` / `initObjects`, and read back as a string from then on — silently, with no error.
+
+SQL now only pre-selects the candidate cells, a page at a time. The driver's own codec decides each one: a cell `JSON.parse` reads is left exactly as stored; a cell it cannot read is a legacy plain string and is rewritten to `JSON.stringify` of that string, byte-for-byte what the old statement wrote for it wherever the engine reads the stored bytes back verbatim. A cell the engine does not read back verbatim is left as stored and keeps reading as it did, where the old statement rewrote it: on `SqliteWasmDriver`, a legacy text with a leading U+FEFF or an embedded NUL (sql.js drops both when it reads the text); on any engine, text holding invalid UTF-8. Each rewrite is a compare-and-set on the text it was decided from, so a value written concurrently is never overwritten, and a re-run over a converged table still writes nothing. This covers every local SQLite face that inherits the backfill: `SqlDriver` on every client it treats as SQLite (`better-sqlite3`, `sqlite3` and its alias `sqlite`), `SqliteWasmDriver`, and `TursoDriver` in local mode.
+
+The decision rule is exported from `@objectstack/driver-sql` as `recoverUnencodedJsonText(stored)`, and `@objectstack/driver-turso`'s remote codec-residue backfill now imports it instead of carrying its own copy, so the local and remote backfills apply one rule. That is a new public export on `@objectstack/driver-sql`'s root entry, and the reason this package takes `minor`: the function returns `null` for text `JSON.parse` accepts and `JSON.stringify(stored)` for text it rejects, and it is exported so that `SqlDriver.backfillCanonicalJsonEncoding` and the remote backfill's `recoverResidueCell` decide each cell by one shared rule rather than by two copies that could drift apart. The remote backfill's behaviour is unchanged.
@@ -14,6 +14,10 @@ export { resolveSqliteAbsentFileTarget } from './sql-driver.js';
 // stops an embedder from re-deriving the seam (or, worse, putting the text back
 // on the wire by spreading the error, which the symbol key exists to prevent).
 export { withheldFilterDiagnosticOf } from './sql-driver.js';
+// [#19912] The rule that decides which json TEXT cell a storage backfill
+// rewrites. Exported because the Turso remote backfill applies the same rule to
+// its own residue, and a second copy there is how the two faces would drift.
+export { recoverUnencodedJsonText } from './unencoded-json-text.js';
 // [#11991] The #11756 emission-scope refusal. Exported because a host that
 // renders its own diagnostics (Studio, `os migrate plan`, an installer) needs
 // the structured `client` / `supportedClients` and the stable `code` — the
 
@@ -0,0 +1,303 @@
+// Copyright (c) 2026 ObjectStack. Licensed under the Apache-2.0 license.
+
+/**
+ * [#19912] The local `Field.json` storage backfill
+ * (`SqlDriver.backfillCanonicalJsonEncoding`, #12380) lets the driver's own
+ * codec decide which cell to rewrite. SQL only pre-filters.
+ *
+ * The defect: the backfill was one `UPDATE … set col = json_quote(col) where
+ * typeof(col) = 'text' and json_valid(col) = 0`. SQLite's `json_valid()`
+ * answers 0 for JSON nested past its depth limit (1000 levels in the bundled
+ * better-sqlite3), while `JSON.parse` — what `formatOutput` reads every json
+ * TEXT cell with — reads it. So a deep array the current write door stored
+ * correctly was quoted into a JSON string on the next `syncSchema`, and read
+ * back as a string from then on. Nothing failed; the cell was simply wrong.
+ *
+ * What is pinned:
+ *
+ * 1. The card's reproduction: `bare`, a 1001-level array and `[1,2]` → only
+ *    `bare` is quoted, and the array still reads as an array after a SECOND
+ *    backfill.
+ * 2. Idempotence: a re-run over a converged table (withheld cell included)
+ *    changes zero rows.
+ * 3. Preservation: legacy bare text that is not JSON is still converted to
+ *    exactly the bytes the old `json_quote()` statement wrote.
+ * 4. The rule is the read codec: a cell is rewritten exactly when the driver
+ *    reads it back as its own text.
+ * 5. Compare-and-set: a value written between the page read and the write is
+ *    not overwritten.
+ * 6. Paging: a column with more candidates than one page converges in one pass.
+ *
+ * The same round-trip through `TursoDriver`'s local face lives in
+ * `driver-turso/src/turso-local-json-backfill-depth-limit.test.ts`.
+ */
+
+import { describe, it, expect, afterEach, vi } from 'vitest';
+import { mkdtempSync, rmSync } from 'node:fs';
+import { createRequire } from 'node:module';
+import { tmpdir } from 'node:os';
+import { join } from 'node:path';
+import { SqlDriver } from './sql-driver.js';
+import { recoverUnencodedJsonText } from './unencoded-json-text.js';
+
+/**
+ * A second, raw connection — another process on the same file. Loaded through
+ * `createRequire` because the package carries no `@types/better-sqlite3`; the
+ * two members used are typed here.
+ */
+const Database = createRequire(import.meta.url)('better-sqlite3') as new (file: string) => {
+  prepare(sql: string): { run(...args: unknown[]): unknown };
+  close(): void;
+};
+
+const T = 'json_depth_19912';
+const FIELDS = { label: { type: 'text' }, val: { type: 'json' } };
+const SCHEMA = { name: T, fields: FIELDS };
+
+/** An array nested `levels` deep: `[]` is 1, `[[]]` is 2. */
+function deepArray(levels: number): unknown {
+  let v: unknown = [];
+  for (let i = 1; i < levels; i++) v = [v];
+  return v;
+}
+
+function deepObject(levels: number): unknown {
+  let v: unknown = {};
+  for (let i = 1; i < levels; i++) v = { a: v };
+  return v;
+}
+
+function arrayDepth(v: unknown): number {
+  let d = 0;
+  while (Array.isArray(v)) {
+    d++;
+    v = v[0];
+  }
+  return d;
+}
+
+const open: SqlDriver[] = [];
+const dirs: string[] = [];
+
+async function makeDriver(filename = ':memory:'): Promise<SqlDriver> {
+  const d = new SqlDriver({ client: 'better-sqlite3', connection: { filename }, useNullAsDefault: true });
+  (d as any).logger = { warn: vi.fn(), info: vi.fn(), error: vi.fn(), debug: vi.fn() };
+  open.push(d);
+  await d.connect();
+  return d;
+}
+
+afterEach(async () => {
+  for (const d of open.splice(0)) await d.disconnect();
+  for (const dir of dirs.splice(0)) rmSync(dir, { recursive: true, force: true });
+});
+
+/** A cell the way a pre-#12380 door (or any raw writer) left it: bound as-is. */
+async function plantRaw(d: SqlDriver, id: string, val: unknown): Promise<void> {
+  await d.execute(`insert into "${T}" ("id", "label", "val") values (?, ?, ?)`, [id, id, val as any]);
+}
+
+async function disk(d: SqlDriver, id: string): Promise<{ t: string; v: unknown }> {
+  const rows = (await d.execute(`select typeof("val") as t, "val" as v from "${T}" where "id" = ?`, [id])) as any[];
+  return rows[0];
+}
+
+async function readAll(d: SqlDriver): Promise<Map<string, unknown>> {
+  const rows = (await d.find(T, {})) as Array<Record<string, unknown>>;
+  return new Map(rows.map((r) => [r.id as string, r.val]));
+}
+
+async function totalChanges(d: SqlDriver): Promise<number> {
+  const rows = (await d.execute('select total_changes() as n')) as any[];
+  return Number(rows[0].n);
+}
+
+describe('[#19912] the local json backfill leaves JSON nested past SQLite\'s depth limit as stored', () => {
+  it('the card\'s reproduction: only `bare` is quoted; the 1001-level array reads as an array after a SECOND backfill', async () => {
+    const d = await makeDriver();
+    await d.syncSchema(T, SCHEMA); // creates the table: no backfill
+    await plantRaw(d, 'bare', 'bare'); // the pre-#12380 form of the string 'bare'
+    await d.create(T, { id: 'deep', label: 'deep', val: deepArray(1001) }, { bypassTenantAudit: true });
+    await d.create(T, { id: 'pair', label: 'pair', val: [1, 2] }, { bypassTenantAudit: true });
+
+    const deepText = JSON.stringify(deepArray(1001));
+    // The current door stored the array encoded — and the pre-filter selects it,
+    // so it is the codec, not SQL, that must keep it.
+    expect(await disk(d, 'deep')).toEqual({ t: 'text', v: deepText });
+    const valid = (await d.execute(`select json_valid("val") as ok from "${T}" where "id" = 'deep'`)) as any[];
+    expect(valid[0].ok).toBe(0);
+    expect(arrayDepth((await readAll(d)).get('deep'))).toBe(1001);
+    const pairBefore = await disk(d, 'pair');
+
+    await d.syncSchema(T, SCHEMA); // first backfill
+    expect(await disk(d, 'bare')).toEqual({ t: 'text', v: '"bare"' });
+    expect(await disk(d, 'deep')).toEqual({ t: 'text', v: deepText });
+    expect(await disk(d, 'pair')).toEqual(pairBefore);
+
+    await d.syncSchema(T, SCHEMA); // second backfill
+    const read = await readAll(d);
+    expect(Array.isArray(read.get('deep'))).toBe(true);
+    expect(arrayDepth(read.get('deep'))).toBe(1001);
+    expect(read.get('bare')).toBe('bare');
+    expect(read.get('pair')).toStrictEqual([1, 2]);
+    expect(await disk(d, 'deep')).toEqual({ t: 'text', v: deepText });
+  });
+
+  it('is idempotent: a re-run over a converged table changes zero rows, withheld cell included', async () => {
+    const d = await makeDriver();
+    await d.syncSchema(T, SCHEMA);
+    await plantRaw(d, 'bare', 'bare');
+    await d.create(T, { id: 'deep', label: 'deep', val: deepArray(1001) }, { bypassTenantAudit: true });
+    await d.syncSchema(T, SCHEMA); // converges `bare`, withholds `deep`
+
+    const before = await totalChanges(d);
+    await d.syncSchema(T, SCHEMA);
+    await d.syncSchema(T, SCHEMA);
+    expect(await totalChanges(d)).toBe(before);
+  });
+
+  it('preservation: legacy bare text that is not JSON is converted to exactly the bytes json_quote() wrote', async () => {
+    const d = await makeDriver();
+    await d.syncSchema(T, SCHEMA);
+    const LEGACY = [
+      'America/New_York',
+      '',
+      '{bad json',
+      '[1,2',
+      'quote " and backslash \\ and slash /',
+      'line\nbreak\ttab\rreturn',
+      'a\u0000b', // an embedded NUL: better-sqlite3 stores and reads it verbatim
+      '\u0001\u001f\u007f',
+      '  ',
+      'leading BOM',
+      'emoji 😀 and 中文',
+    ];
+    for (const [i, s] of LEGACY.entries()) await plantRaw(d, `l${i}`, s);
+    for (const [i, s] of LEGACY.entries()) expect(await disk(d, `l${i}`), `l${i} planted`).toEqual({ t: 'text', v: s });
+
+    await d.syncSchema(T, SCHEMA);
+    const read = await readAll(d);
+    for (const [i, s] of LEGACY.entries()) {
+      // What the retired statement wrote, computed by SQLite itself.
+      const quoted = ((await d.execute('select json_quote(?) as q', [s])) as any[])[0].q;
+      expect(await disk(d, `l${i}`), `l${i}`).toEqual({ t: 'text', v: quoted });
+      expect(quoted, `l${i}: json_quote and JSON.stringify agree`).toBe(JSON.stringify(s));
+      expect(read.get(`l${i}`), `l${i} reads as the string it is`).toBe(s);
+    }
+  });
+
+  it('the rule is the read codec: a cell is rewritten exactly when the driver reads it back as its own text', async () => {
+    const d = await makeDriver();
+    await d.syncSchema(T, SCHEMA);
+    const TEXTS = [
+      JSON.stringify(deepArray(1000)),
+      JSON.stringify(deepArray(1001)),
+      JSON.stringify(deepArray(2001)),
+      JSON.stringify(deepObject(1001)),
+      '{"a":1}',
+      '"already quoted"',
+      ' [1] ',
+      '1e400',
+      '"\\ud800"',
+      'bare',
+      '',
+      '{bad json',
+      '[1,]',
+    ];
+    for (const [i, s] of TEXTS.entries()) await plantRaw(d, `t${i}`, s);
+    const readBefore = await readAll(d);
+
+    await d.syncSchema(T, SCHEMA);
+    for (const [i, s] of TEXTS.entries()) {
+      const readsAsItself = readBefore.get(`t${i}`) === s;
+      const rule = recoverUnencodedJsonText(s);
+      expect(rule !== null, `t${i}: the rule rewrites iff the read answers the text itself`).toBe(readsAsItself);
+      expect((await disk(d, `t${i}`)).v, `t${i} on disk`).toBe(rule ?? s);
+    }
+    // …and no read moved.
+    const readAfter = await readAll(d);
+    for (const [i] of TEXTS.entries()) {
+      expect(readAfter.get(`t${i}`), `t${i} reads the same`).toStrictEqual(readBefore.get(`t${i}`));
+    }
+  });
+
+  it('compare-and-set: a value written between the page read and the write is not overwritten', async () => {
+    const dir = mkdtempSync(join(tmpdir(), 'os-19912-'));
+    dirs.push(dir);
+    const file = join(dir, 'cas.db');
+    const d = await makeDriver(file);
+    await d.syncSchema(T, SCHEMA);
+    await plantRaw(d, 'raced', 'bare');
+    await plantRaw(d, 'calm', 'also bare');
+
+    // A second connection — another process on the same file — writes the deep
+    // array into `raced` right after the backfill has read its candidates.
+    const other = new Database(file);
+    const deepText = JSON.stringify(deepArray(1001));
+    const knex = (d as any).knex;
+    let injected = 0;
+    const onResponse = (_res: unknown, q: { sql?: string }) => {
+      if (injected === 0 && /json_valid/.test(q.sql ?? '') && /^select/i.test(q.sql ?? '')) {
+        other.prepare(`update "${T}" set "val" = ? where "id" = 'raced'`).run(deepText);
+        injected++;
+      }
+    };
+    knex.on('query-response', onResponse);
+    try {
+      await d.syncSchema(T, SCHEMA);
+    } finally {
+      knex.removeListener('query-response', onResponse);
+      other.close();
+    }
+
+    expect(injected).toBe(1);
+    // The concurrent write survives: the backfill decided on 'bare', and the
+    // cell no longer holds 'bare'.
+    expect(await disk(d, 'raced')).toEqual({ t: 'text', v: deepText });
+    expect(arrayDepth((await readAll(d)).get('raced'))).toBe(1001);
+    // The row nobody raced converged as usual.
+    expect(await disk(d, 'calm')).toEqual({ t: 'text', v: '"also bare"' });
+  });
+
+  it('pages: a column with more candidates than one page converges in one pass, withheld cells on the page edges', async () => {
+    const d = await makeDriver();
+    await d.syncSchema(T, SCHEMA);
+    const knex = (d as any).knex;
+    const deepText = JSON.stringify(deepArray(1001));
+    const DEEP_AT = new Set([0, 499, 500, 999, 1099]);
+    const rows = Array.from({ length: 1100 }, (_, i) => ({
+      id: `r${String(i).padStart(4, '0')}`,
+      label: 'x',
+      val: DEEP_AT.has(i) ? deepText : `legacy ${i}`,
+    }));
+    await knex.batchInsert(T, rows, 200);
+
+    await d.syncSchema(T, SCHEMA);
+
+    const left = (await d.execute(
+      `select "id", "val" from "${T}" where json_valid("val") = 0 order by "id"`,
+    )) as Array<{ id: string; val: string }>;
+    expect(left.map((r) => r.id)).toEqual([...DEEP_AT].map((i) => `r${String(i).padStart(4, '0')}`));
+    for (const r of left) expect(r.val).toBe(deepText);
+    const converted = (await d.execute(
+      `select count(*) as n from "${T}" where "val" = json_quote('legacy ' || (substr("id", 2) + 0))`,
+    )) as any[];
+    expect(Number(converted[0].n)).toBe(1100 - DEEP_AT.size);
+
+    const info = ((d as any).logger.info as ReturnType<typeof vi.fn>).mock.calls.map((c) => c[1]);
+    expect(info).toContainEqual({ rowsConverted: 1100 - DEEP_AT.size });
+    expect(info).toContainEqual({ rowsWithheld: DEEP_AT.size });
+  });
+});
+
+describe('[#19912] recoverUnencodedJsonText — the shared rule', () => {
+  it('leaves text that parses; rewrites text that does not as its JSON string', () => {
+    expect(recoverUnencodedJsonText(JSON.stringify(deepArray(1001)))).toBeNull();
+    expect(recoverUnencodedJsonText('{"a":1}')).toBeNull();
+    expect(recoverUnencodedJsonText('123')).toBeNull();
+    expect(recoverUnencodedJsonText('"x"')).toBeNull();
+    expect(recoverUnencodedJsonText('bare')).toBe('"bare"');
+    expect(recoverUnencodedJsonText('')).toBe('""');
+    expect(recoverUnencodedJsonText('a\u0000b')).toBe('"a\\u0000b"');
+  });
+});