@@ -0,0 +1,42 @@
+---
+'@objectstack/driver-turso': minor
+---
+
+fix(driver-turso)!: a local or replica `TursoDriver` on a remote url, or a replica off a local file, is refused at construction
+
+Clause-②: no (narrowing)
+
+A remote `url` beside `syncUrl` was classified as an embedded replica, and the local SQLite engine that every replica read and write goes through was handed `:memory:`. Writes succeeded and read back, then vanished on restart, and none of them reached the remote. `@libsql/client` builds no embedded replica for a remote url: it routes `libsql://` / `https://` / `http://` to its HTTP client and `wss://` / `ws://` to its WebSocket client, neither of which reads `syncUrl`. A forced `mode: 'replica'` or `mode: 'local'` beside a remote url was handed the same `:memory:` engine, and so was a replica on `:memory:`. Measured before the change, with `create`, `find`, then a fresh driver on the same config:
+
+```
+libsql:// + syncUrl (sync.onConnect: false)  -> 1 row back, 0 rows after restart
+libsql:// + mode: 'replica' or mode: 'local' -> 1 row back, 0 rows after restart
+:memory: + syncUrl + a supplied client       -> 1 row back, 0 rows after restart
+file: + syncUrl (unchanged)                  -> 1 row back, 1 row after restart
+```
+
+With the driver building its own client and the default `sync.onConnect`, two of these did fail at `connect()`, but on a libsql error that did not say why: `libsql://` + `syncUrl` with `SYNC_NOT_SUPPORTED`, and `:memory:` + `syncUrl` with `URL_INVALID`.
+
+**BREAKING** accept-set narrowing on a published driver option, shipped as `minor` under the repo's launch-window convention for breaking changes (`scripts/check-changeset-no-major.mjs`). **The constructor now refuses configurations it accepted before**, at `new TursoDriver()`, ahead of the Knex base and of any client, with the ADR-0112 envelope `code: 'VALIDATION_ERROR'`, `status: 400`. A remote url here means one of the lowercase schemes `TursoDriver.detectMode` classifies as remote: `libsql://`, `https://`, `http://`, `wss://`, `ws://`. Refused:
+
+- a remote url beside `syncUrl`;
+- a remote url under a forced `mode: 'replica'` or `mode: 'local'`;
+- a replica on a url `@libsql/client` reads as in-memory (`:memory:`, or `file::memory:` with or without a query string), beside `syncUrl` or under a forced `mode: 'replica'`. `@libsql/client` refuses such an embedded replica itself. For `:memory:` and a bare `file::memory:` the local engine was a private in-memory database. With a query string it was a file literally named after the url's path (for example `:memory:?cache=shared`) in the working directory, which no sync reaches;
+- under a forced `mode: 'replica'` only, any `url` that is not a local `file:` path, such as a bare path or an uppercase scheme.
+
+The remote-url refusal names the scheme it met. Neither refusal echoes the url, which may carry a token. Both loaders (`@objectstack/runtime`'s host factory and the datasource factory) reach this refusal through the same constructor, so a datasource declaring one of these configurations now fails by name when its loader builds the driver.
+
+**What stays accepted**, pinned by preservation tests: a `file:` url with `syncUrl` (the embedded replica), a `file:` or `:memory:` local database, a remote url on its own or with `mode: 'remote'`. `TursoDriver.detectMode()` still classifies a remote url beside `syncUrl` as `'replica'`: the refusal sits in the constructor, not in a re-classification.
+
+**Not refused, unchanged here:** a url with no `mode` that is none of `file:`, `:memory:` or a lowercase remote scheme, such as an uppercase `LIBSQL://` or a bare path like `./data/app.db`, still auto-detects `'local'` and still runs on `:memory:`, with or without `syncUrl`. So does the same url under a forced `mode: 'local'`. That fall-through is tracked as #19976.
+
+**What an affected author does.** Each refusal names its ways out. For a remote url in a local or replica mode:
+
+- to use the remote database, drop `syncUrl` (and `sync`) and any forced `mode`; the remote url alone sends every read and write to it;
+- for an embedded replica, point `url` at a local file and keep the remote in `syncUrl`: `url: 'file:./data/replica.db', syncUrl: 'libsql://my-db.turso.io'`.
+
+For a replica off a local file, point `url` at a local `file:` path beside `syncUrl`. A throwaway in-memory database instead drops `syncUrl` (and `sync`) and any forced `mode: 'replica'`, and keeps `url: ':memory:'`.
+
+Blast radius, measured on this tree: no example, template, published skill, hand-written doc or factory default declares a remote url beside `syncUrl`, and the host boot path (`OS_DATABASE_URL`) passes no `syncUrl`. Outside this package's own tests, the in-repo configurations carrying the pair are test fixtures that never construct the real driver: loader fixtures that exercise the config builder or a capturing constructor, stored-row redaction fixtures and a schema-parse fixture. Whether any out-of-repo deployment declares it is NOT measured and is not claimed to be zero.
+
+<!-- adr-0087: not-required (no-migration-prescription) An accept-set narrowing performed at the driver constructor: no key, spec symbol, Zod schema, object definition or stored representation is added, removed or renamed — `TursoDriverConfig.url`, `syncUrl` and `mode` keep their names and types, and both `TursoConfigSchema` copies are untouched. What moves is which CONFIGURATIONS `new TursoDriver()` accepts, so `objectstack migrate meta` has nothing to visit and there is no tombstone to mint. Each refusal names its ways out, and which one an author wants (a remote database, an embedded replica on a local file, or a plain local database) is authoring intent no ledger line can decide. -->
@@ -130,6 +130,25 @@ Transport mode is automatically detected from the URL:
 | `libsql://...` | `remote` | @libsql/client only |
 | `https://...` | `remote` | @libsql/client only |
 
+An embedded replica is a local **file** kept in sync with a remote. The local and
+replica modes run every read and write through the local SQLite engine, which
+cannot open a remote url, and a replica needs a file for the sync to land in.
+The constructor therefore refuses (`VALIDATION_ERROR` / 400):
+
+- a remote url (the lowercase `libsql://`, `https://`, `http://`, `wss://`,
+  `ws://` that auto-detection matches) beside `syncUrl`, or under a forced
+  `mode: 'local'` / `'replica'`;
+- a replica on an in-memory url (`:memory:`, `file::memory:`). That covers a
+  replica auto-detected from a `:memory:` or `file:` url beside `syncUrl`, and
+  one forced with `mode: 'replica'`;
+- under a forced `mode: 'replica'`, any `url` that is not a local `file:` path.
+
+In each case the engine would otherwise run on a private in-memory database
+whose writes read back and then vanish on restart, and `@libsql/client` builds
+no embedded replica for a remote url anyway. For a remote database, drop
+`syncUrl` and any forced `mode`. For an embedded replica, use
+`url: 'file:./data/replica.db'` beside `syncUrl`.
+
 You can also force a specific mode:
 
 ```typescript
@@ -216,7 +235,7 @@ interface TursoDriverConfig {
    */
   concurrency?: number;
 
-  /** Remote sync URL for embedded replica mode (libsql:// or https://) */
+  /** Remote sync URL for embedded replica mode (libsql:// or https://); the replica is the local file: named by `url` */
   syncUrl?: string;
 
   /** Sync configuration (requires syncUrl) */
 
@@ -0,0 +1,44 @@
+// Copyright (c) 2026 ObjectStack. Licensed under the Apache-2.0 license.
+
+/**
+ * A fresh local FILE for an embedded-replica fixture.
+ *
+ * An embedded replica is a local file kept in sync with the remote named in
+ * `syncUrl`, and `TursoDriver` refuses a replica on anything else at
+ * construction (`:memory:`, a remote url). In those cases the local engine
+ * would have run on a private in-memory database that no sync ever reaches.
+ * A fixture that exercises the replica face therefore needs a real path. It
+ * needs a NEW one per driver, so each test starts from an empty database,
+ * exactly as the `:memory:` fixtures these replace did.
+ *
+ * ```ts
+ * const files = replicaFiles();
+ * afterAll(() => files.removeAll());
+ * new TursoDriver({ url: files.next(), syncUrl, client, sync: { onConnect: false } });
+ * ```
+ */
+
+import { mkdtempSync, rmSync } from 'node:fs';
+import { tmpdir } from 'node:os';
+import { join } from 'node:path';
+
+export interface ReplicaFiles {
+  /** A `file:` url naming a database file in a directory of its own; the file does not exist yet. */
+  next(): string;
+  /** Delete every directory `next()` created. Idempotent. */
+  removeAll(): void;
+}
+
+export function replicaFiles(): ReplicaFiles {
+  const dirs: string[] = [];
+  return {
+    next() {
+      const dir = mkdtempSync(join(tmpdir(), 'turso-replica-'));
+      dirs.push(dir);
+      return `file:${join(dir, 'replica.db')}`;
+    },
+    removeAll() {
+      while (dirs.length > 0) rmSync(dirs.pop()!, { recursive: true, force: true });
+    },
+  };
+}
@@ -0,0 +1,220 @@
+// Copyright (c) 2026 ObjectStack. Licensed under the Apache-2.0 license.
+
+/**
+ * A local or replica `TursoDriver` with nothing durable behind its engine is
+ * refused at construction. Before this, it silently ran on a private
+ * `:memory:` database.
+ *
+ * # What was measured (the reading this refusal stands on)
+ *
+ * The local and replica arms run every read and write through the inherited
+ * Knex + better-sqlite3 engine, and `TursoDriver.toKnexConfig` could hand that
+ * engine only a `file:` path or `:memory:`. Everything else went to its last
+ * arm, which was `:memory:`. On `main` @ `2c1011b01b`, `initObjects`, then
+ * `create`, then `find`, then a fresh driver on the same config:
+ *
+ * ```
+ * libsql:// + syncUrl + client stub, sync.onConnect false -> replica, knex :memory:,
+ *     find 1 row, the client's own database held NO tables, 0 rows after restart
+ * libsql:// / https:// / wss:// + syncUrl, real client, onConnect false -> same, 0 after restart
+ * libsql:// + syncUrl, real client, default sync -> connect() rejects SYNC_NOT_SUPPORTED
+ * libsql:// + mode 'replica' (no syncUrl)           -> 1 row, 0 after restart
+ * libsql:// + mode 'local'                          -> 1 row, 0 after restart
+ * :memory: + syncUrl, real client, default sync     -> connect() rejects URL_INVALID
+ * :memory: + syncUrl + client stub, onConnect false -> 1 row, stub holds no tables, 0 after restart
+ * file: + syncUrl + client stub (CONTROL)           -> knex on the file, 1 row, 1 after restart
+ * ```
+ *
+ * `@libsql/client@0.17.4` builds no embedded replica for a remote url:
+ * `lib-esm/node.js` routes http/https to its HTTP client and ws/wss to its
+ * WebSocket client, `syncUrl` is read only in `lib-esm/sqlite3.js` (a `syncUrl`
+ * grep over `http.js` / `ws.js` returns zero, while `authToken` returns six in
+ * each), and both remote clients' `sync()` throw `SYNC_NOT_SUPPORTED`. The
+ * sqlite3 client refuses an in-memory replica itself: "Embedded replica must use
+ * file for local db".
+ *
+ * # What this file pins
+ *
+ * - The card's own reproduction (remote url, `syncUrl`, a client stub,
+ *   `sync.onConnect: false`) is refused at construction, as the ADR-0112
+ *   envelope (`code` + `status`). The stub is never touched. Each remote scheme
+ *   the classifier knows is covered.
+ * - Each other arm, as measured: a forced `mode: 'replica'` or `mode: 'local'`
+ *   beside a remote url, and a replica on `:memory:` / `file::memory:` / a
+ *   url that is not `file:`.
+ * - The message names the way out and never echoes the url, which may carry a
+ *   live `?authToken=`.
+ * - PRESERVATION: the `file:` replica still constructs, connects, writes, and
+ *   keeps its rows across a restart. So do the local, in-memory and remote
+ *   faces. `detectMode` still classifies the refused pair `replica`: the
+ *   refusal is in the constructor, not a re-classification.
+ *
+ * # Reverse verification: direction predicted before it was run
+ *
+ * With the constructor's `localEngineDefect` call removed, every refusal case
+ * goes RED (the constructor returns a driver on `:memory:`, with no envelope to
+ * read) and every preservation case stays GREEN. Measured; see the PR.
+ */
+
+import { afterAll, describe, expect, it } from 'vitest';
+import { createTursoDriver } from './index.js';
+import { TursoDriver, type TursoDriverConfig } from './turso-driver.js';
+import { makeLibsqlSqliteStub } from './libsql-sqlite-stub.testkit.js';
+import { replicaFiles } from './replica-file.testkit.js';
+
+type Refusal = Error & { code?: string; status?: number };
+
+/** The error `build` threw, or `null` when it returned. */
+function refusalOf(build: () => unknown): Refusal | null {
+  try {
+    build();
+    return null;
+  } catch (error) {
+    return error as Refusal;
+  }
+}
+
+function expectEnvelope(refusal: Refusal | null): asserts refusal is Refusal {
+  expect(refusal, 'expected the constructor to refuse, and it returned a driver').not.toBeNull();
+  expect(refusal!.code).toBe('VALIDATION_ERROR');
+  expect(refusal!.status).toBe(400);
+}
+
+const PRIMARY = 'libsql://primary.example.turso.io';
+const NOTE = { name: 'note', fields: { title: { type: 'string' } } };
+const files = replicaFiles();
+afterAll(() => files.removeAll());
+
+describe('a remote url beside syncUrl: refused, never a replica on :memory:', () => {
+  it("the card's reproduction: libsql:// + syncUrl + a client stub + sync.onConnect false", () => {
+    const stub = makeLibsqlSqliteStub();
+    const tablesInStub = () =>
+      stub.raw.prepare(`select count(*) as c from sqlite_master where type='table'`).all()[0].c;
+
+    const refusal = refusalOf(
+      () =>
+        new TursoDriver({
+          url: 'libsql://r.turso.io',
+          syncUrl: 'libsql://r.turso.io',
+          client: stub as never,
+          sync: { onConnect: false },
+        }),
+    );
+
+    expectEnvelope(refusal);
+    // Both ways out, by the subjects they name: the key to drop for a remote
+    // database, the url spelling for a replica.
+    expect(refusal.message).toContain('`syncUrl`');
+    expect(refusal.message).toContain("url: 'file:");
+    // Refused before any client work: the stub is untouched.
+    expect(tablesInStub()).toBe(0);
+    stub.close();
+  });
+
+  it.each(['libsql://', 'https://', 'http://', 'wss://', 'ws://'])(
+    '%s + syncUrl is refused, naming the scheme it met',
+    (scheme) => {
+      const refusal = refusalOf(() => new TursoDriver({ url: `${scheme}db.example.turso.io`, syncUrl: PRIMARY }));
+
+      expectEnvelope(refusal);
+      expect(refusal.message).toContain(`\`${scheme}\``);
+      expect(refusal.message).toContain('`syncUrl`');
+    },
+  );
+
+  it('the factory door meets the same refusal: createTursoDriver is not a way around it', () => {
+    expectEnvelope(refusalOf(() => createTursoDriver({ url: 'libsql://r.turso.io', syncUrl: PRIMARY })));
+  });
+
+  it('a `timeout` beside the pair does not change which refusal fires: this one owns the pair', () => {
+    const refusal = refusalOf(() => new TursoDriver({ url: 'wss://r.turso.io', syncUrl: PRIMARY, timeout: 30_000 }));
+
+    expectEnvelope(refusal);
+    expect(refusal.message).toContain('`syncUrl`');
+    expect(refusal.message).not.toContain('TursoDriverConfig.timeout');
+  });
+
+  it('never echoes the url: a live `?authToken=` in it stays out of the boot log', () => {
+    const refusal = refusalOf(
+      () => new TursoDriver({ url: 'libsql://r.turso.io?authToken=SECRET-TOKEN-VALUE', syncUrl: PRIMARY }),
+    );
+
+    expectEnvelope(refusal);
+    expect(refusal.message).not.toContain('SECRET-TOKEN-VALUE');
+    expect(refusal.message).not.toContain('r.turso.io');
+  });
+});
+
+describe('a forced local or replica mode beside a remote url: refused', () => {
+  it.each<[string, TursoDriverConfig]>([
+    ["mode 'replica', no syncUrl", { url: 'libsql://r.turso.io', mode: 'replica' }],
+    ["mode 'replica' + syncUrl", { url: 'libsql://r.turso.io', syncUrl: PRIMARY, mode: 'replica' }],
+    ["mode 'local'", { url: 'libsql://r.turso.io', mode: 'local' }],
+    ["mode 'local', https://", { url: 'https://r.turso.io', mode: 'local' }],
+  ])('%s', (_label, config) => {
+    const refusal = refusalOf(() => new TursoDriver(config));
+
+    expectEnvelope(refusal);
+    // The way out for a remote database names the key that forced the mode,
+    // and the value that would have been right.
+    expect(refusal.message).toContain(`\`mode: '${config.mode}'\``);
+    expect(refusal.message).toContain("mode: 'remote'");
+  });
+});
+
+describe('a replica that is not a local file: refused', () => {
+  it.each<[string, TursoDriverConfig]>([
+    [':memory: + syncUrl', { url: ':memory:', syncUrl: PRIMARY }],
+    [':memory: + syncUrl + a supplied client', { url: ':memory:', syncUrl: PRIMARY, client: {} as never, sync: { onConnect: false } }],
+    ['file::memory: + syncUrl', { url: 'file::memory:', syncUrl: PRIMARY }],
+    ["file::memory:?cache=shared + mode 'replica'", { url: 'file::memory:?cache=shared', mode: 'replica' }],
+    [":memory: + mode 'replica'", { url: ':memory:', mode: 'replica' }],
+    ["a bare path + mode 'replica'", { url: './data/replica.db', mode: 'replica' }],
+  ])('%s', (_label, config) => {
+    const refusal = refusalOf(() => new TursoDriver(config));
+
+    expectEnvelope(refusal);
+    expect(refusal.message).toContain('embedded replica');
+    expect(refusal.message).toContain("url: 'file:");
+  });
+});
+
+describe('PRESERVATION: every configuration with a durable (or declared-ephemeral) engine still constructs', () => {
+  it('the file: replica constructs, connects, writes, and keeps its rows across a restart', async () => {
+    const url = files.next();
+    const stub = makeLibsqlSqliteStub();
+    const make = () => new TursoDriver({ url, syncUrl: PRIMARY, client: stub as never, sync: { onConnect: false } });
+
+    const first = make();
+    expect(first.transportMode).toBe('replica');
+    await first.connect();
+    await first.initObjects([NOTE as never]);
+    await first.create('note', { id: 'n1', title: 'kept' });
+    expect(await first.find('note', {})).toHaveLength(1);
+    await first.disconnect();
+
+    const second = make();
+    await second.connect();
+    await second.initObjects([NOTE as never]);
+    expect((await second.find('note', {})).map((r: { title?: unknown }) => r.title)).toEqual(['kept']);
+    await second.disconnect();
+    stub.close();
+  });
+
+  it.each<[string, TursoDriverConfig, string]>([
+    ['file: local', { url: 'file:./data/app.db' }, 'local'],
+    [':memory: local (ephemeral by declaration)', { url: ':memory:' }, 'local'],
+    ["file: + mode 'local'", { url: 'file:./data/app.db', mode: 'local' }, 'local'],
+    ["file: + mode 'replica' + syncUrl", { url: 'file:./data/replica.db', syncUrl: PRIMARY, mode: 'replica' }, 'replica'],
+    ['libsql:// remote', { url: 'libsql://r.turso.io', authToken: 't' }, 'remote'],
+    ["libsql:// + mode 'remote'", { url: 'libsql://r.turso.io', mode: 'remote' }, 'remote'],
+  ])('%s', (_label, config, mode) => {
+    // Knex opens its connection lazily, so constructing never touches the file.
+    expect(new TursoDriver(config).transportMode).toBe(mode);
+  });
+
+  it('detectMode still classifies the refused pair `replica`: the refusal is in the constructor, not a re-classification', () => {
+    expect(TursoDriver.detectMode({ url: 'libsql://r.turso.io', syncUrl: PRIMARY })).toBe('replica');
+    expect(TursoDriver.detectMode({ url: ':memory:', syncUrl: PRIMARY })).toBe('replica');
+  });
+});