@@ -0,0 +1,19 @@
+---
+'@objectstack/service-datasource': patch
+---
+
+fix(service-datasource)!: `POST /api/v1/datasources/:name/test` refuses a name that no saved datasource has with `404 RESOURCE_NOT_FOUND`, as the other by-name doors do, instead of answering `200 { ok: false }`
+
+Clause-②: no (narrowing) — `POST /api/v1/datasources/:name/test` and the in-process `ExternalDatasourceService.testConnection(name)` refuse a name that resolves no saved datasource with `404 RESOURCE_NOT_FOUND`, which they answered `200 { ok: false }` before. Nothing is widened.
+Metadata-protocol: none
+
+<!-- adr-0087: not-required (no-migration-prescription) No key of any schema is added, removed, renamed or re-shaped, and no export or type changes: the Test door's request and its { ok, latencyMs, tableCount, error } answer keep their shape, and a name that resolves no saved datasource is now refused instead of answered. A failed probe of a datasource that does not exist had no meaning to keep, so there is nothing for a migration to rewrite. -->
+
+**BREAKING** accept-set narrowing at the saved-datasource Test door (`@objectstack/service-datasource`), served at `POST /api/v1/datasources/:name/test`.
+
+- **Before.** Testing a saved datasource by a name that no saved datasource has (a typo, a removed datasource, a name from another deployment) was answered `200` with `{ ok: false, error: "[ObjectQL] Datasource '…' has no registered driver to introspect." }`. That is the answer a saved datasource that is down gets, so an admin tool or script could not tell "this datasource is down" from "this datasource does not exist". `GET`, `PATCH` and `DELETE /api/v1/datasources/:name` already answered the same name `404`.
+- **Now.** That request is refused `404` with `error.code` `RESOURCE_NOT_FOUND` and the message `GET /api/v1/datasources/:name` gives for the same name. Nothing is probed. The name is resolved through the same stored-record read the sibling by-name doors make. In process, `ExternalDatasourceService.testConnection(name)` throws that `404` error instead of returning `{ ok: false }`.
+- **Unchanged.** A saved datasource that fails its probe still answers `200 { ok: false, error }`, and one that is up still answers `200 { ok: true, … }`. The platform's primary datasource (`default`) is still tested through its live driver, whether or not a datasource record is stored for it. Every other outcome of `testConnection` is still returned, never thrown.
+- **Who sees it.** A caller holding the platform-settings capability who tests a name that no saved datasource has.
+
+**The remedy.** None needed: pick the datasource from `GET /api/v1/datasources`, or check the name. A script that read `ok: false` as "does not exist" now reads `404`.
@@ -67,13 +67,17 @@ type Table = { string(n: string): { primary(): void } };
  * the same records (`metadata.get('datasource', name)`, as `plugin.ts` wires
  * it), so a datasource's Sync answer and its listed `schemaMode` are one fact.
  * `stream` is a FEDERATED datasource on a driver that cannot introspect, the
- * subject of the engine-reason pin below.
+ * subject of the engine-reason pin below. `ghost` is saved but never
+ * connected, the no-live-driver subject of the Test pin below: [#27510] it is
+ * a stored record because Test answers a name with no record `404`
+ * (`saved-datasource-test-unknown-name.test.ts`).
  */
 const RECORDS = [
   { name: 'default', driver: 'sqlite', origin: 'code', config: {} },
   { name: 'warehouse', driver: 'sqlite', origin: 'runtime', config: {} },
   { name: 'scratch', driver: 'memory', origin: 'runtime', config: {} },
   { name: 'stream', driver: 'memory', origin: 'runtime', schemaMode: 'external', external: {}, config: {} },
+  { name: 'ghost', driver: 'sqlite', origin: 'runtime', config: {} },
 ];
 
 /**
@@ -248,7 +252,8 @@ describe('a driver with no introspectSchema', () => {
           throw new Error('introspect must not be reached');
         },
         liveDriver: () => ({ checkHealth }),
-        getDatasource: async () => undefined,
+        // [#27510] A saved datasource: Test answers a name with no record 404.
+        getDatasource: async (name) => ({ name }),
         getObject: async () => undefined,
         listObjects: async () => [],
       });
 
@@ -126,7 +126,7 @@ async function boot() {
     return { status: res.status, body: (await res.json()) as any };
   };
 
-  return { call, connection, build };
+  return { call, connection, build, records };
 }
 
 const listed = async (call: Awaited<ReturnType<typeof boot>>['call'], name: string) =>
@@ -192,7 +192,10 @@ describe('Test on a datasource whose connect failed', () => {
 
 describe('what does not move', () => {
   it('a datasource with no verdict and no driver keeps the driver-missing sentence', async () => {
-    const { call } = await boot();
+    const { call, records } = await boot();
+    // Saved, never connected. [#27510] It is a stored record because Test
+    // answers a name with no record 404 (`saved-datasource-test-unknown-name.test.ts`).
+    records.set('ghost', { name: 'ghost', driver: 'postgres', origin: 'runtime', config: UNREACHABLE });
 
     const test = await call('POST', '/ghost/test');
     expect(test.status).toBe(200);
@@ -234,7 +237,8 @@ describe('only a failed verdict is answered', () => {
       },
       liveDriver: () => undefined,
       connectionState: () => ({ availability, ...(reason ? { reason } : {}) }),
-      getDatasource: async () => undefined,
+      // [#27510] A saved datasource: Test answers a name with no record 404.
+      getDatasource: async (name) => ({ name }),
       getObject: async () => undefined,
       listObjects: async () => [],
     });
 
@@ -0,0 +1,182 @@
+// Copyright (c) 2026 ObjectStack. Licensed under the Apache-2.0 license.
+
+/**
+ * `POST /datasources/:name/test` on a name that resolves no saved datasource.
+ *
+ * Every other by-name door answers an unknown name `404 RESOURCE_NOT_FOUND`
+ * (`datasource-unknown-name.test.ts` holds the four `datasource-admin` ones).
+ * Test answered it `200 { ok: false }` with the engine's "has no registered
+ * driver to introspect" sentence, which is the answer it gives a saved
+ * datasource that is down. An admin tool or script testing a datasource by
+ * name could not tell "this datasource is down" from "this datasource does not
+ * exist".
+ *
+ * The triage ruling these pins hold: resolve the saved name with the lookup
+ * the sibling routes use and answer `RESOURCE_NOT_FOUND`; keep `{ ok: false }`
+ * for a datasource that exists and fails its probe. Two controls ride along:
+ * a saved datasource that is up still answers `ok: true`, and the platform's
+ * primary datasource, which may have no stored record, is still tested through
+ * its live driver.
+ *
+ * The composition is the real one, end to end: a real `ObjectQL` engine, the
+ * real connection service and driver factory, the real admin service, the real
+ * `ExternalDatasourceServicePlugin` wiring, and the real admin routes over a
+ * Hono server. Both services read the same stored records, as the two plugins
+ * do (`metadata.get('datasource', name)`).
+ */
+
+import { afterEach, describe, expect, it } from 'vitest';
+import { ObjectQL } from '@objectstack/objectql';
+import { HonoHttpServer } from '@objectstack/plugin-hono-server';
+import { DatasourceConnectionService, DEFAULT_DATASOURCE_NAME } from '../datasource-connection-service.js';
+import { createDefaultDatasourceDriverFactory } from '../default-datasource-driver-factory.js';
+import { ExternalDatasourceServicePlugin } from '../plugin.js';
+import { ExternalDatasourceService } from '../external-datasource-service.js';
+import { DatasourceAdminService, type StoredDatasource } from '../datasource-admin-service.js';
+import { registerDatasourceAdminRoutes } from '../admin-routes.js';
+import { ENTITLED_CREDENTIAL, createSessionAuthService, createGrantsEngine } from './entitled-caller.fixture.js';
+
+const teardowns: Array<() => Promise<void>> = [];
+afterEach(async () => {
+  for (const t of teardowns.splice(0)) await t();
+});
+
+/**
+ * The saved datasources. `warehouse` is connected; `offline` is saved but has
+ * no live driver (nothing connected it), so its probe fails. The primary
+ * datasource has NO record here: it is connected `asDefault`, as
+ * `DefaultDatasourcePlugin.init()` does, and nothing stores it.
+ */
+const RECORDS: StoredDatasource[] = [
+  { name: 'warehouse', driver: 'memory', origin: 'runtime', config: {} },
+  { name: 'offline', driver: 'memory', origin: 'runtime', config: {} },
+];
+
+async function boot() {
+  const ql = new ObjectQL();
+  const connection = new DatasourceConnectionService({
+    factory: () => createDefaultDatasourceDriverFactory({}),
+    engine: () => ql as never,
+  });
+  teardowns.push(() => connection.disconnectAll());
+
+  await connection.connect(
+    { name: DEFAULT_DATASOURCE_NAME, driver: 'memory', config: {}, origin: 'code', bootCritical: true },
+    { asDefault: true, context: { origin: 'code', trigger: 'declared-auto' } },
+  );
+  await connection.connect(RECORDS[0], { context: { origin: 'runtime', trigger: 'runtime-admin' } });
+
+  const records = new Map(RECORDS.map((r) => [r.name, r]));
+  const admin = new DatasourceAdminService({
+    listDatasourceRecords: async () => [...records.values()],
+    getDatasourceRecord: async (n: string) => records.get(n),
+    connectionStates: () => connection.listConnectionStates(),
+  } as never);
+
+  const services = new Map<string, unknown>([
+    ['data', ql],
+    ['metadata', { get: async (type: string, n: string) => (type === 'datasource' ? records.get(n) : undefined) }],
+    ['datasource-connection', connection],
+  ]);
+  await new ExternalDatasourceServicePlugin().init({
+    getService: (n: string) => {
+      if (!services.has(n)) throw new Error(`no service: ${n}`);
+      return services.get(n);
+    },
+    registerService: (n: string, s: unknown) => void services.set(n, s),
+  } as never);
+  const external = services.get('external-datasource') as ExternalDatasourceService;
+
+  const server = new HonoHttpServer(0);
+  const authService = createSessionAuthService();
+  const grantsEngine = createGrantsEngine();
+  registerDatasourceAdminRoutes(
+    server,
+    {
+      getService: (n: string) => {
+        if (n === 'auth') return authService;
+        if (n === 'objectql' || n === 'data') return grantsEngine;
+        if (n === 'external-datasource') return external;
+        if (n === 'datasource-admin') return admin;
+        throw new Error(`no service: ${n}`);
+      },
+    } as never,
+    '/api/v1',
+  );
+  const app = server.getRawApp();
+  const call = async (method: 'GET' | 'POST', path: string) => {
+    const res = await app.fetch(
+      new Request(`http://local/api/v1/datasources${path}`, {
+        method,
+        headers: { authorization: ENTITLED_CREDENTIAL, 'content-type': 'application/json' },
+        ...(method === 'POST' ? { body: '{}' } : {}),
+      }),
+    );
+    return { status: res.status, body: (await res.json()) as any };
+  };
+
+  return { call, records };
+}
+
+describe('POST /datasources/:name/test on a name that resolves no saved datasource', () => {
+  it('answers 404 RESOURCE_NOT_FOUND naming the datasource, not 200 { ok: false }', async () => {
+    const { call } = await boot();
+
+    const test = await call('POST', '/no_such_ds/test');
+
+    expect({ status: test.status, success: test.body.success, code: test.body.error?.code }).toEqual({
+      status: 404,
+      success: false,
+      code: 'RESOURCE_NOT_FOUND',
+    });
+    expect(test.body.error.message).toContain('no_such_ds');
+    expect(test.body).not.toHaveProperty('data');
+  });
+
+  it('gives the answer GET /datasources/:name gives the same name: status, code and message', async () => {
+    const { call } = await boot();
+
+    const read = await call('GET', '/no_such_ds');
+    const test = await call('POST', '/no_such_ds/test');
+
+    expect(read.status).toBe(404);
+    expect({ status: test.status, code: test.body.error?.code, message: test.body.error?.message }).toEqual({
+      status: read.status,
+      code: read.body.error.code,
+      message: read.body.error.message,
+    });
+  });
+});
+
+describe('POST /datasources/:name/test on a datasource that exists', () => {
+  it('a saved datasource that fails its probe still answers 200 { ok: false, error }', async () => {
+    const { call } = await boot();
+
+    const test = await call('POST', '/offline/test');
+
+    expect(test.status).toBe(200);
+    expect(test.body.data.ok).toBe(false);
+    expect(typeof test.body.data.error).toBe('string');
+    expect(test.body.data.error).toContain('offline');
+  });
+
+  it('a saved datasource that is up still answers 200 { ok: true }', async () => {
+    const { call } = await boot();
+
+    const test = await call('POST', '/warehouse/test');
+
+    expect(test.status).toBe(200);
+    expect(test.body.data).toMatchObject({ ok: true });
+  });
+
+  it('the primary datasource with no stored record is still tested through its live driver', async () => {
+    const { call, records } = await boot();
+    expect(records.has(DEFAULT_DATASOURCE_NAME)).toBe(false);
+
+    const test = await call('POST', `/${DEFAULT_DATASOURCE_NAME}/test`);
+
+    expect(test.status).toBe(200);
+    expect(test.body.data).toMatchObject({ ok: true });
+    expect(typeof test.body.data.latencyMs).toBe('number');
+  });
+});
@@ -86,7 +86,7 @@ const SERVICE_ERROR_CODE: Record<ServiceName, ErrorCode> = {
  * `DATASOURCE_ADMIN_ERROR` or `EXTERNAL_DATASOURCE_ERROR`. A name that resolves
  * no stored datasource is not a refusal: every `datasource-admin` door that
  * takes `:name` answers it `404` `RESOURCE_NOT_FOUND`, the read and the writes
- * alike (see `badRequest`).
+ * alike, and so does `POST /datasources/:name/test` (see `badRequest`).
  *
  * Served by `datasource-admin`:
  *
@@ -641,7 +641,8 @@ export function registerDatasourceAdminRoutes(
    * unknown datasource reads the same at every by-name door, and a caller can
    * tell "no such datasource" from "your request was refused". The throwers
    * today are the admin service's `updateDatasource`, `removeDatasource` and
-   * `migrateCredential` (`datasourceNotFoundError`).
+   * `migrateCredential`, and the external service's `testConnection` (each
+   * through its `datasourceNotFoundError`).
    */
   const badRequest = (res: any, service: ServiceName, err: unknown) => {
     const envelope = err as { code?: unknown; status?: unknown } | null | undefined;
 
@@ -447,6 +447,21 @@ function importNameRefusedError(message: string): Error {
   return err;
 }
 
+/**
+ * [#27510] What the saved-datasource Test answers for a name that resolves no
+ * stored datasource: the `404` / `RESOURCE_NOT_FOUND` envelope (ADR-0112), in
+ * the words `GET /datasources/:name` uses, so the admin route relays it as the
+ * read sibling answers the same name. It mirrors the admin service's helper of
+ * the same name; `saved-datasource-test-unknown-name.test.ts` holds the two
+ * doors' answers equal.
+ */
+function datasourceNotFoundError(name: string): Error {
+  const err = new Error(`Datasource "${name}" does not exist.`) as Error & { code?: string; status?: number };
+  err.code = 'RESOURCE_NOT_FOUND';
+  err.status = 404;
+  return err;
+}
+
 /**
  * [#24772] The refusal every federation door answers for the platform's
  * primary datasource: listing its remote tables, drafting or importing one,
@@ -696,10 +711,24 @@ export class ExternalDatasourceService implements IExternalDatasourceService {
    * rebuild does, and replaces the verdict either way. A datasource whose
    * failed rebuild kept its old pool serving has a live driver, so it is
    * probed as before.
+   *
+   * [#27510] A name that resolves no saved datasource is not a datasource that
+   * failed its probe, so it is the one outcome that THROWS: the 404
+   * {@link datasourceNotFoundError}, which the route answers exactly as
+   * `GET /datasources/:name` answers the same name. It answered `ok: false`
+   * with the engine's "has no registered driver" sentence, the answer a saved
+   * datasource that is down gets, so a caller could not tell the two apart.
+   * The name is resolved through `getDatasource`, the stored-record read the
+   * sibling `:name` doors make (both plugins wire it to
+   * `metadata.get('datasource', name)`). The primary datasource is not looked
+   * up: it may have no stored record, and its live driver answers for it.
    */
   async testConnection(
     datasource: string,
   ): Promise<{ ok: boolean; latencyMs?: number; tableCount?: number; error?: string }> {
+    if (datasource !== DEFAULT_DATASOURCE_NAME && !(await this.config.getDatasource(datasource))) {
+      throw datasourceNotFoundError(datasource);
+    }
     const started = Date.now();
     try {
       const driver = this.config.liveDriver?.(datasource);