diff --git a/packages/services/service-automation/src/connector-definition-cluster.test.ts b/packages/services/service-automation/src/connector-definition-cluster.test.ts
new file mode 100644
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+++ b/packages/services/service-automation/src/connector-definition-cluster.test.ts
@@ -0,0 +1,219 @@
+// Copyright (c) 2026 ObjectStack. Licensed under the Apache-2.0 license.
+
+/**
+ * [#27977] Cross-replica reach of a connector DEFINITION published through the
+ * per-item publish door — measured over the topology of
+ * `connector-credential-cluster.test.ts`: two kernels ("replicas") over ONE
+ * shared database, each with its own materialized connectors, joined by a bus
+ * that delivers every publish to every subscriber (publisher included).
+ *
+ * Each replica runs the real write path — ObjectKernel, ObjectQLPlugin's
+ * metadata protocol, driver-sql on one better-sqlite3 file both replicas open
+ * — and is attached the way `MetadataClusterBridgePlugin` attaches it at
+ * `kernel:ready`: lane 2 (`attachMetadataMutationPubSub`, `metadata.mutated`)
+ * and lane 4 (`attachConnectorCredentialPubSub`). Lane 1 (`metadata.changed`)
+ * replays into a `MetadataManager`'s caches; the connector reconcile reads the
+ * ObjectQL registry and the protocol's execution view, never that manager, so
+ * attaching it could not change what is read here.
+ *
+ * Directions declared before running:
+ *  • A publishes a changed definition: A re-materializes it (its own
+ *    `metadata:reloaded`, announced by ObjectQLPlugin)               -> GREEN
+ *  • lane 2 carries the mutation to B: B's protocol replays it into B's
+ *    mutation listeners                                              -> GREEN
+ *  • B's live instance follows the new definition with no reload on B
+ *                                                                    -> RED
+ *    (the automation plugin's mutation listener takes `flow` only, and
+ *    `metadata:reloaded` is a kernel-local hook; pinned as `it.fails`, the
+ *    fix is #28002)
+ *  • control: a `metadata:reloaded` on B converges B from its OWN read of the
+ *    shared store, so the gap is the trigger and not the store or the read
+ *                                                                    -> GREEN
+ */
+import { mkdtempSync, rmSync } from 'node:fs';
+import { tmpdir } from 'node:os';
+import { join } from 'node:path';
+import { afterEach, describe, expect, it, vi } from 'vitest';
+import { ObjectKernel } from '@objectstack/core';
+import { ObjectQLPlugin, type ObjectQL } from '@objectstack/objectql';
+import { SqlDriver } from '@objectstack/driver-sql';
+import type { IPubSub, PubSubHandler } from '@objectstack/spec/contracts';
+import type { Connector, ConnectorProviderContext, ConnectorProviderFactory } from '@objectstack/spec/integration';
+
+import { AutomationServicePlugin } from './plugin.js';
+import type { AutomationEngine } from './engine.js';
+import { installDefaultOrganization } from './default-organization.testkit.js';
+
+const NAME = 'billing_27977';
+const V1 = 'https://v1.billing.example.test';
+const V2 = 'https://v2.billing.example.test';
+
+const flush = () => new Promise<void>((r) => setTimeout(r, 0));
+
+/** A provider-bound connector instance; `endpoint` is the definition change under test. */
+const definition = (endpoint: string) => ({
+    name: NAME,
+    label: 'Billing',
+    type: 'api',
+    provider: 'fake',
+    providerConfig: { endpoint },
+});
+
+/** The protocol surface this suite drives: the doors `PUT /meta/:type/:name` and `POST …/publish` call. */
+interface Protocol {
+    saveMetaItem(request: { type: string; name: string; item: unknown; mode?: 'draft' | 'publish' }): Promise<unknown>;
+    publishMetaItem(request: { type: string; name: string }): Promise<unknown>;
+    onMetadataMutation(listener: (evt: { type: string; name: string; state?: string }) => void): () => void;
+    attachMetadataMutationPubSub(pubsub: IPubSub, nodeId: string): () => void;
+}
+
+/** One replica's upstream: records each materialization's endpoint. */
+function fakeProvider() {
+    const endpoints: unknown[] = [];
+    const factory: ConnectorProviderFactory = (ctx: ConnectorProviderContext) => {
+        endpoints.push((ctx.providerConfig as Record<string, unknown> | undefined)?.endpoint);
+        return {
+            def: {
+                name: ctx.name,
+                label: ctx.label,
+                type: 'api',
+                authentication: { type: 'none' },
+                actions: [{ key: 'request', label: 'Request' }],
+            } as unknown as Connector,
+            handlers: { request: async () => ({ status: 200, ok: true, body: {} }) },
+        };
+    };
+    return { factory, endpoints, last: () => endpoints[endpoints.length - 1] };
+}
+
+/** The shared bus: every publish reaches every subscriber on the channel, publisher included, one hop later. */
+function sharedBus(): IPubSub {
+    const handlers = new Map<string, Set<PubSubHandler<any>>>();
+    return {
+        async publish(channel, payload) {
+            for (const h of [...(handlers.get(channel) ?? [])]) {
+                setTimeout(() => void h({ channel, payload, publishedAt: Date.now() }), 0);
+            }
+        },
+        subscribe(channel, handler) {
+            const set = handlers.get(channel) ?? new Set();
+            set.add(handler as PubSubHandler<any>);
+            handlers.set(channel, set);
+            return () => set.delete(handler as PubSubHandler<any>);
+        },
+        async close() {},
+    };
+}
+
+const kernels: ObjectKernel[] = [];
+const dirs: string[] = [];
+afterEach(async () => {
+    for (const k of kernels.splice(0)) {
+        if (k.getState() === 'running') await k.shutdown().catch(() => {});
+    }
+    for (const d of dirs.splice(0)) rmSync(d, { recursive: true, force: true });
+    vi.restoreAllMocks();
+});
+
+/** The ONE database file both replicas open. */
+function sharedDatabaseFile(): string {
+    const dir = mkdtempSync(join(tmpdir(), 'os-27977-'));
+    dirs.push(dir);
+    return join(dir, 'shared.sqlite');
+}
+
+/** One replica over the shared file, attached to `bus` the way the cluster bridge attaches it. */
+async function replica(file: string, nodeId: string, bus: IPubSub) {
+    const provider = fakeProvider();
+    let hooks: { trigger(event: string, payload?: unknown): Promise<void> } | undefined;
+    const kernel = new ObjectKernel({ logger: { level: 'fatal' }, gracefulShutdown: false } as never);
+    kernels.push(kernel);
+    await kernel.use(new ObjectQLPlugin());
+    await kernel.use(new AutomationServicePlugin({ suspendedRunStore: 'memory', credentialResolver: () => undefined }));
+    await kernel.use({
+        name: `test.harness.${nodeId}`,
+        type: 'standard' as const,
+        version: '1.0.0',
+        dependencies: ['com.objectstack.service-automation'],
+        async init(ctx: any) {
+            hooks = ctx;
+            ctx.getService('automation').registerConnectorProvider('fake', provider.factory);
+        },
+        async start() {},
+    } as never);
+    await kernel.bootstrap();
+
+    const ql = kernel.getService<ObjectQL>('objectql');
+    const driver = new SqlDriver({ client: 'better-sqlite3', connection: { filename: file }, useNullAsDefault: true });
+    await driver.connect();
+    ql.registerDriver(driver as never, true);
+    await ql.syncSchemas();
+    await installDefaultOrganization(ql);
+
+    const protocol = kernel.getService<Protocol>('protocol');
+    const automation = kernel.getService<AutomationEngine>('automation');
+    // What `MetadataClusterBridgePlugin`'s lanes 2 and 4 do at `kernel:ready` on a cross-process driver.
+    protocol.attachMetadataMutationPubSub(bus, nodeId);
+    automation.attachConnectorCredentialPubSub(bus, nodeId);
+
+    const heard: string[] = [];
+    protocol.onMetadataMutation((evt) => heard.push(`${evt.type}/${evt.name}`));
+
+    /** The per-item publish door: a draft save, then `POST /api/v1/meta/connector/:name/publish`. */
+    const publish = async (item: unknown) => {
+        await protocol.saveMetaItem({ type: 'connector', name: NAME, item, mode: 'draft' });
+        await protocol.publishMetaItem({ type: 'connector', name: NAME });
+    };
+    const reload = () => hooks!.trigger('metadata:reloaded', { changed: [`connector/${NAME}`] });
+    const live = () => automation.getRegisteredConnectors().includes(NAME);
+    return { provider, publish, reload, live, heard };
+}
+
+describe('[#27977] a connector definition published on one replica', () => {
+    it('re-materializes on the publishing replica, and lane 2 carries the mutation to the peer', async () => {
+        const file = sharedDatabaseFile();
+        const bus = sharedBus();
+        const a = await replica(file, 'node-a', bus);
+        const b = await replica(file, 'node-b', bus);
+
+        await a.publish(definition(V1));
+        expect(a.live()).toBe(true);
+        expect(a.provider.last()).toBe(V1);
+
+        await a.publish(definition(V2));
+        expect(a.provider.last()).toBe(V2);
+        await vi.waitFor(() => expect(b.heard).toContain(`connector/${NAME}`));
+    });
+
+    // Measured by #27977; the fix is #28002. Promote to a plain `it` when #28002 lands.
+    it.fails('[#27977] converges the peer with no reload on the peer — known gap, filed as #28002', async () => {
+        const file = sharedDatabaseFile();
+        const bus = sharedBus();
+        const a = await replica(file, 'node-a', bus);
+        const b = await replica(file, 'node-b', bus);
+
+        await a.publish(definition(V1));
+        await a.publish(definition(V2));
+        await vi.waitFor(() => expect(b.heard).toContain(`connector/${NAME}`));
+        await vi.waitFor(() => expect(b.provider.last()).toBe(V2), { timeout: 500 });
+    });
+
+    it('control: a reload on the peer converges it from its own read of the shared store', async () => {
+        const file = sharedDatabaseFile();
+        const bus = sharedBus();
+        const a = await replica(file, 'node-a', bus);
+        const b = await replica(file, 'node-b', bus);
+
+        await a.publish(definition(V1));
+        await a.publish(definition(V2));
+        await vi.waitFor(() => expect(b.heard).toContain(`connector/${NAME}`));
+        for (let i = 0; i < 5; i++) await flush();
+        // The measured state before the reload: the peer never materialized the instance.
+        expect(b.live()).toBe(false);
+        expect(b.provider.endpoints).toEqual([]);
+
+        await b.reload();
+        expect(b.live()).toBe(true);
+        expect(b.provider.last()).toBe(V2);
+    });
+});