@@ -0,0 +1,168 @@
+// Copyright (c) 2026 ObjectStack. Licensed under the Apache-2.0 license.
+
+/**
+ * `kernel:listening` warns about a busy port only when a port was busy.
+ *
+ * Port `0` is a legal request: `listen(0)` binds whatever port the kernel hands
+ * out, so the bound port can never equal the request and nothing was in use.
+ * The hook used to compare the two numbers and warn whenever they differed, so
+ * every kernel-assigned boot (`os serve --port 0`, and every CLI test that boots
+ * that way) printed a busy-port warning under the banner's Boot diagnostics,
+ * at the CLI's default kernel log level (`warn`) and at `info`.
+ *
+ * Both arms run the plugin's real `init()` / `start()`, the REAL adapter (a
+ * real socket is bound) and the REAL core logger at the levels the CLI runs it
+ * at. They read the records the hook emits by KIND and COUNT, never by wording:
+ *
+ *   - a request for `0` that binds port N: no record at the default level, and
+ *     at `info` exactly one record, the one that names N;
+ *   - a non-zero request whose port is HELD: one `warn` record that names both
+ *     the requested port and the port actually bound.
+ *
+ * The neighbour holding the port in the second arm is a plain TCP listener on
+ * the same wildcard address the adapter binds, so the adapter's own
+ * EADDRINUSE walk is what moves the port. Nothing here simulates the shift.
+ */
+
+import { createServer, type Server } from 'node:net';
+import { describe, expect, it, vi } from 'vitest';
+import { createLogger } from '@objectstack/core';
+import { HonoServerPlugin } from './hono-plugin';
+
+/** The CLI's default kernel log level (`os serve --log-level`'s documented default). */
+const CLI_DEFAULT_LEVEL = 'warn';
+
+const LEVEL_RANK: Record<string, number> = { debug: 0, info: 1, warn: 2, error: 3, fatal: 4 };
+
+interface LogRecord {
+    level: string;
+    msg: string;
+    [key: string]: unknown;
+}
+
+interface Booted {
+    /** The records the `kernel:listening` hook wrote, as the real logger serialised them. */
+    records: LogRecord[];
+    /** The port the adapter really bound, read from the transport itself. */
+    bound: number;
+}
+
+/**
+ * Boot the plugin through `init()` / `start()` under a silent context, then
+ * fire its `kernel:listening` hooks under a real core logger at `level`,
+ * capturing exactly what that logger writes while the hooks run.
+ */
+async function listenAt(port: number, level: 'warn' | 'info'): Promise<Booted> {
+    const plugin = new HonoServerPlugin({ port, cors: false });
+    const services = new Map<string, unknown>();
+    const listeningHooks: Array<() => unknown> = [];
+    const silent = { debug() {}, info() {}, warn() {}, error() {}, fatal() {} };
+    const ctx: any = {
+        logger: silent,
+        getKernel: () => ({ hasPlugin: () => false, getService: () => undefined }),
+        registerService: (name: string, service: unknown) => services.set(name, service),
+        hook: (event: string, fn: () => unknown) => {
+            if (event === 'kernel:listening') listeningHooks.push(fn);
+        },
+        getService: () => undefined,
+    };
+
+    await plugin.init(ctx);
+    await plugin.start(ctx);
+    expect(listeningHooks, 'the plugin registered no kernel:listening hook').toHaveLength(1);
+
+    const server = services.get('http.server') as { getPort(): number; close(): Promise<void> };
+    ctx.logger = createLogger({ level, format: 'json' });
+
+    const chunks: string[] = [];
+    const capture = ((chunk: unknown) => {
+        chunks.push(String(chunk));
+        return true;
+    }) as never;
+    const stdout = vi.spyOn(process.stdout, 'write').mockImplementation(capture);
+    const stderr = vi.spyOn(process.stderr, 'write').mockImplementation(capture);
+    try {
+        for (const fn of listeningHooks) await fn();
+    } finally {
+        stdout.mockRestore();
+        stderr.mockRestore();
+    }
+
+    const bound = server.getPort();
+    await server.close();
+
+    const records = chunks
+        .join('')
+        .split('\n')
+        .filter((line) => line.trim() !== '')
+        .map((line) => JSON.parse(line) as LogRecord);
+    return { records, bound };
+}
+
+/** Records at `warn` or above: what the default level shows. */
+const warnings = (records: LogRecord[]) =>
+    records.filter((record) => (LEVEL_RANK[record.level] ?? 0) >= LEVEL_RANK.warn);
+
+/** A whole-number mention of `port` in a record's message. */
+const names = (record: LogRecord, port: number) => new RegExp(`\\b${port}\\b`).test(record.msg);
+
+function holdPort(): Promise<{ port: number; release: () => Promise<void> }> {
+    return new Promise((resolveHold, rejectHold) => {
+        const holder: Server = createServer();
+        holder.on('error', rejectHold);
+        // No host: the same wildcard address the adapter's `serve()` binds.
+        holder.listen(0, () => {
+            const address = holder.address();
+            if (address === null || typeof address === 'string') {
+                rejectHold(new Error(`listen(0) produced no numeric address: ${String(address)}`));
+                return;
+            }
+            resolveHold({
+                port: address.port,
+                release: () => new Promise<void>((done) => holder.close(() => done())),
+            });
+        });
+    });
+}
+
+describe('kernel:listening names a busy port only when one was busy', () => {
+    it('a request for port 0 says nothing at the default level', async () => {
+        const { records, bound } = await listenAt(0, CLI_DEFAULT_LEVEL);
+
+        // The control: the kernel really assigned a port.
+        expect(bound, 'listen(0) bound no port').toBeGreaterThan(0);
+        expect(records, JSON.stringify(records, null, 2)).toEqual([]);
+    });
+
+    it('a request for port 0 prints one line at info, the one naming the bound port', async () => {
+        const { records, bound } = await listenAt(0, 'info');
+        const seen = JSON.stringify(records, null, 2);
+
+        expect(bound, 'listen(0) bound no port').toBeGreaterThan(0);
+        expect(warnings(records), seen).toEqual([]);
+        // The existing started line already names the bound port, so no second
+        // line is owed, and none may say anything about a port in use.
+        expect(records, seen).toHaveLength(1);
+        expect(records[0]!.level, seen).toBe('info');
+        expect(records[0]!.port, seen).toBe(bound);
+    });
+
+    it('a held non-zero port still warns, naming the requested and the bound port', async () => {
+        const neighbour = await holdPort();
+        try {
+            const { records, bound } = await listenAt(neighbour.port, CLI_DEFAULT_LEVEL);
+            const seen = JSON.stringify(records, null, 2);
+
+            // The control: the port really moved, or there is nothing to announce.
+            expect(bound, `the adapter bound the held port ${neighbour.port}${seen}`).not.toBe(neighbour.port);
+
+            const warned = warnings(records);
+            expect(warned, seen).toHaveLength(1);
+            expect(warned[0]!.level, seen).toBe('warn');
+            expect(names(warned[0]!, neighbour.port), `the warning does not name the requested port${seen}`).toBe(true);
+            expect(names(warned[0]!, bound), `the warning does not name the bound port${seen}`).toBe(true);
+        } finally {
+            await neighbour.release();
+        }
+    });
+});