@@ -0,0 +1,30 @@
+---
+'@objectstack/spec': minor
+'@objectstack/driver-turso': minor
+---
+
+fix(spec,driver-turso)!: a turso config that forces `mode: 'replica'` with no `syncUrl` is refused where it is written and when the driver is built, instead of running as a plain local database that never syncs
+
+Clause-②: yes (narrowing) — the accept set of the `turso` `datasource.config` contract narrows by one combination. No key is added, removed or renamed, and no exported symbol moves.
+
+An embedded replica is a local file kept in sync with the remote named in `syncUrl`. A config that forced `mode: 'replica'` on a `file:` url with no `syncUrl` (or an empty one) was accepted by `@objectstack/spec`'s `TursoConfigSchema`, by the published mirror in `@objectstack/driver-turso`, and by `new TursoDriver()`. Measured on the built driver before this change, with and without `sync`: it constructed with `transportMode` `'replica'`, `isSyncEnabled()` answered `false`, no sync interval started, the sync call did nothing, and every read and write went to the local file. A datasource declared as a replica ran as a plain local database that never replicated, with no error and no warning.
+
+**BREAKING** accept-set narrowing on a published schema and a published constructor, shipped as `minor` under the repo's launch-window convention for breaking changes (`scripts/check-changeset-no-major.mjs`). Refused now, at both doors together, with one message whose prescription names both ways out:
+
+- **at authoring**, as one `custom` issue on `mode` (`config.mode` on a datasource): `DatasourceSchema`, `validateDriverConfig`, `defineStack` / `os validate`, and a save or test connection through the datasource admin service;
+- **at construction**, `VALIDATION_ERROR` / 400 from `new TursoDriver()` (and `createTursoDriver()`), before any client or database is opened.
+
+The message is the same text at both doors, and a test holds the constructor's copy equal to the schema's issue byte for byte. The sibling refusals keep their order. A forced replica on a remote url, an in-memory url or a bare path still meets its `url` refusal first. One with `sync` and no `syncUrl` still meets the `sync` refusal first; the schema now reports the `mode` issue beside it. The driver mirror declares no `mode` key and strips an authored one, so it cannot see a forced mode: this refusal reaches it only as byte-identical text, and the spec contract and the constructor are the two doors that judge it.
+
+### Migration: FROM → TO
+
+| You wrote | Write instead |
+| --- | --- |
+| `url: 'file:./data/replica.db', mode: 'replica'` (no `syncUrl`, or `syncUrl: ''`) | an embedded replica: keep the `file:` url and name the remote, `syncUrl: 'libsql://my-db.turso.io'` |
+| the same | a plain local database: drop `mode` (`url: 'file:./data/app.db'` alone) |
+
+A datasource row stored in this shape is not re-parsed when it loads, so it now fails when the driver is built. `factory.create` throws the refusal. The connection service records the datasource as `failed-degraded` with the message, and a test connection answers `ok: false` ("Failed to build driver: …"). Under ADR-0062 D5, the boot fails fast when objects bind to that datasource or are routed to it, or when it is boot-critical, unless `OS_ALLOW_DRIVER_CONNECT_FAILURE` is set. Otherwise it is left unconnected with a warning. Before this change the same row booted and ran as a local database. The way out is the table above.
+
+Blast radius, measured on this tree: no example, template, published skill or hand-written doc authors the shape, and no host default or environment variable sets `mode` (a turso `mode` reaches the driver only from an authored `datasource.config`). Whether any out-of-repo deployment declares such a config is NOT measured and is not claimed to be zero.
+
+<!-- adr-0087: registered turso-config-forced-replica-without-sync-url-refused -->
@@ -28,6 +28,9 @@
  *  - [#20200] where the constructor refuses on `syncUrl` or `sync`, its message
  *    is the spec contract's issue message, byte for byte: those two texts are
  *    copies in `../turso-driver.ts`, and this is the pin that holds them equal.
+ *    [#20437] The same holds for a forced `mode: 'replica'` with no `syncUrl`,
+ *    refused on `mode` — the third copy. That row used to be accepted
+ *    everywhere, as a replica that never synced.
  *
  * ⚠️ The mirror declares no `mode`, so zod strips an authored one before its
  * refinement runs: rows that FORCE a mode are judged by the constructor and the
@@ -74,7 +77,9 @@ interface Row {
   /** The constructor's verdict on this config. */
   ctor: 'accept' | 'refuse';
   /** The key both schemas refuse on, or `undefined` when they accept. */
-  refusedOn?: 'url' | 'syncUrl' | 'timeoutMs' | 'sync';
+  refusedOn?: 'url' | 'syncUrl' | 'timeoutMs' | 'sync' | 'mode';
+  /** How many issues the spec contract raises on a refused row; 1 unless stated. */
+  issues?: number;
   /** The constructor accepts it and ignores a key: refused at authoring only. None today (#20200). */
   inert?: true;
 }
@@ -97,7 +102,7 @@ const ROWS: Row[] = [
   { name: 'a url behind whitespace (the loaders trim it)', config: { url: `  ${FILE}` }, ctor: 'accept' },
   { name: 'a remote url behind whitespace', config: { url: ` ${REMOTE}` }, ctor: 'accept' },
   { name: 'an empty syncUrl (unset)', config: { url: REMOTE, syncUrl: '' }, ctor: 'accept' },
-  { name: "file: under a forced mode: 'replica'", config: { url: FILE, mode: 'replica' }, ctor: 'accept' },
+  { name: "file: + syncUrl under a forced mode: 'replica'", config: { url: FILE, mode: 'replica', syncUrl: REMOTE, sync: { onConnect: false } }, ctor: 'accept' },
   { name: "file: + syncUrl under a forced mode: 'local'", config: { url: FILE, mode: 'local', syncUrl: REMOTE, sync: { onConnect: false } }, ctor: 'accept' },
   { name: "libsql:// under a forced mode: 'remote'", config: { url: REMOTE, mode: 'remote' }, ctor: 'accept' },
   { name: "file: under a forced mode: 'remote'", config: { url: FILE, mode: 'remote' }, ctor: 'accept' },
@@ -147,13 +152,24 @@ const ROWS: Row[] = [
   { name: 'sync with no syncUrl', config: { url: FILE, sync: { intervalSeconds: 60 } }, ctor: 'refuse', refusedOn: 'sync' },
   { name: 'sync with no syncUrl on a remote url', config: { url: REMOTE, sync: { intervalSeconds: 60 } }, ctor: 'refuse', refusedOn: 'sync' },
   { name: "sync with no syncUrl under a forced mode: 'remote'", config: { url: REMOTE, mode: 'remote', sync: { onConnect: true } }, ctor: 'refuse', refusedOn: 'sync' },
-  { name: "sync with no syncUrl under a forced mode: 'replica'", config: { url: FILE, mode: 'replica', sync: { intervalSeconds: 60 } }, ctor: 'refuse', refusedOn: 'sync' },
+  // The spec contract raises BOTH issues here (`sync`, then `mode`); the
+  // constructor throws one, the `sync` refusal, which is the spec's first.
+  { name: "sync with no syncUrl under a forced mode: 'replica'", config: { url: FILE, mode: 'replica', sync: { intervalSeconds: 60 } }, ctor: 'refuse', refusedOn: 'sync', issues: 2 },
   { name: 'sync beside an empty syncUrl (unset)', config: { url: FILE, syncUrl: '', sync: { intervalSeconds: 60 } }, ctor: 'refuse', refusedOn: 'sync' },
+
+  // ── a forced replica with no remote to replicate from: refused on `mode` (#20437) ──
+  { name: "file: under a forced mode: 'replica'", config: { url: FILE, mode: 'replica' }, ctor: 'refuse', refusedOn: 'mode' },
+  { name: "an uppercase FILE: url under a forced mode: 'replica'", config: { url: `FILE:${DIR}/upper-replica.db`, mode: 'replica' }, ctor: 'refuse', refusedOn: 'mode' },
+  { name: "file: + an empty syncUrl (unset) under a forced mode: 'replica'", config: { url: FILE, mode: 'replica', syncUrl: '' }, ctor: 'refuse', refusedOn: 'mode' },
+  { name: "file: + timeoutMs under a forced mode: 'replica'", config: { url: FILE, mode: 'replica', timeoutMs: 5000 }, ctor: 'refuse', refusedOn: 'mode' },
 ];
 
-/** The rows the constructor refuses on a sync key: its message is a copy of the spec's (#20200). */
+/**
+ * The rows the constructor refuses on a sync key, or on a forced replica with
+ * no `syncUrl`: its message is a copy of the spec's (#20200, #20437).
+ */
 const SYNC_KEY_REFUSALS = ROWS.filter(
-  (r) => r.ctor === 'refuse' && (r.refusedOn === 'syncUrl' || r.refusedOn === 'sync'),
+  (r) => r.ctor === 'refuse' && (r.refusedOn === 'syncUrl' || r.refusedOn === 'sync' || r.refusedOn === 'mode'),
 );
 
 /**
@@ -209,7 +225,8 @@ describe('turso config: the constructor, the spec contract and this mirror agree
     expect(ROWS.filter((r) => r.refusedOn === 'timeoutMs').length).toBeGreaterThanOrEqual(3);
     expect(ROWS.filter((r) => r.refusedOn === 'syncUrl').length).toBeGreaterThanOrEqual(3);
     expect(ROWS.filter((r) => r.refusedOn === 'sync').length).toBeGreaterThanOrEqual(5);
-    expect(SYNC_KEY_REFUSALS.length).toBeGreaterThanOrEqual(8);
+    expect(ROWS.filter((r) => r.refusedOn === 'mode').length).toBeGreaterThanOrEqual(4);
+    expect(SYNC_KEY_REFUSALS.length).toBeGreaterThanOrEqual(12);
     // [#20200] Exactly zero, not a floor: every key the constructor used to
     // build and ignore is refused at construction now (see the header).
     expect(ROWS.filter((r) => r.inert).length).toBe(0);
@@ -229,7 +246,7 @@ describe('turso config: the constructor, the spec contract and this mirror agree
       const verdict = schemaVerdict(SpecTursoConfigSchema, row.config);
       expect(verdict.refusedOn, verdict.message).toBe(row.refusedOn);
       if (row.refusedOn) {
-        expect(verdict.count, verdict.message).toBe(1);
+        expect(verdict.count, verdict.message).toBe(row.issues ?? 1);
         expect(verdict.code).toBe('custom');
       }
     });
@@ -241,11 +258,12 @@ describe('turso config: the constructor, the spec contract and this mirror agree
     });
   });
 
-  // [#20200] The two sync refusals are copies of the spec contract's texts in
-  // `../turso-driver.ts` (the spec keeps them module-local); this is the pin
-  // that holds each copy equal to the schema's issue, byte for byte.
+  // [#20200] The two sync refusals, and [#20437] the forced-replica refusal,
+  // are copies of the spec contract's texts in `../turso-driver.ts` (the spec
+  // keeps them module-local); this is the pin that holds each copy equal to the
+  // schema's issue, byte for byte.
   describe.each(SYNC_KEY_REFUSALS)('$name', (row) => {
-    it("the constructor's message is the spec contract's, byte for byte (#20200)", () => {
+    it("the constructor's message is the spec contract's, byte for byte (#20200, #20437)", () => {
       const spec = schemaVerdict(SpecTursoConfigSchema, row.config);
       expect(spec.refusedOn).toBe(row.refusedOn);
       expect(spec.message).toBeTypeOf('string');
 
@@ -155,7 +155,7 @@ interface TursoTransportKeys {
 
 /** One refusal: the key it sits on and its message. */
 interface TursoTransportIssue {
-  path: 'url' | 'syncUrl' | 'timeoutMs';
+  path: 'url' | 'syncUrl' | 'timeoutMs' | 'mode';
   message: string;
 }
 
@@ -239,6 +239,24 @@ function tursoTransportIssues(cfg: TursoTransportKeys): TursoTransportIssue[] {
           + `${drop} for a plain in-memory local database.`,
       }];
     }
+    if (mode === 'replica' && !hasSyncUrl) {
+      // #20437. Only a FORCED replica reaches here: with no `mode`, a replica is
+      // selected by `syncUrl` alone. The url is a `file:` url (every other one
+      // met a refusal above), so the url is fine and the MODE is what cannot be
+      // honoured — the issue sits on `mode`, as the `sync` refusal sits on `sync`.
+      // Unreachable through this mirror, which strips `mode` (see above); kept
+      // byte-identical to the spec contract's arm.
+      return [{
+        path: 'mode',
+        message:
+          "`mode: 'replica'` makes this datasource an embedded replica, a local file kept in sync with "
+          + 'the remote named in `syncUrl`, but no `syncUrl` is set: nothing would ever sync, so it would '
+          + 'run as a plain local database that never replicates — the turso driver refuses this '
+          + 'configuration when it starts. For an embedded replica, name the remote in `syncUrl` beside '
+          + "the local file: `url: 'file:./data/replica.db'` with `syncUrl` set to the `libsql://` or "
+          + "`https://` Turso endpoint. For a plain local database, drop `mode: 'replica'`.",
+      }];
+    }
     return [];
   }