diff --git a/package.json b/package.json
index fd1a319..ffc5cc7 100644
--- a/package.json
+++ b/package.json
@@ -52,7 +52,7 @@
     "generate:server-json": "tsx scripts/sync-release-metadata.ts",
     "publish:mcp-registry": "tsx scripts/publish-mcp-registry.ts",
     "lint": "eslint .",
-    "test": "node --test --test-concurrency=1 dist/test/coordinator-socket-error.integration.test.js dist/test/identity-store.test.js dist/test/identity-cert.test.js dist/test/broadcast-window.integration.test.js dist/test/identity-restart.integration.test.js dist/test/mesh-e2e.integration.test.js dist/test/ws-broadcast-window.integration.test.js dist/test/tls-transport.integration.test.js dist/test/peer-id-verification.integration.test.js dist/test/state-sync-convergence.test.js dist/test/downtime-replay.test.js dist/test/downtime-replay.integration.test.js dist/test/filestore.test.js",
+    "test": "node --test --test-concurrency=1 dist/test/coordinator-socket-error.integration.test.js dist/test/identity-store.test.js dist/test/identity-cert.test.js dist/test/broadcast-window.integration.test.js dist/test/identity-restart.integration.test.js dist/test/mesh-e2e.integration.test.js dist/test/ws-broadcast-window.integration.test.js dist/test/tls-transport.integration.test.js dist/test/peer-id-verification.integration.test.js dist/test/become-coordinator-actual-port.integration.test.js dist/test/state-sync-convergence.test.js dist/test/downtime-replay.test.js dist/test/downtime-replay.integration.test.js dist/test/filestore.test.js",
     "test:visibility": "node --test dist/test/visibility.integration.test.js",
     "test:delivery": "node dist/test/delivery-receipt.runner.js",
     "test:federation": "node dist/test/federation.integration.test.js",
diff --git a/src/core/tcp-transport.ts b/src/core/tcp-transport.ts
index 602ec01..d833d3e 100644
--- a/src/core/tcp-transport.ts
+++ b/src/core/tcp-transport.ts
@@ -314,12 +314,15 @@ export class TcpTransport implements MeshTransport {
       });
 
       server.listen(port, host, () => {
+        const addr = server.address();
+        const actualPort =
+          typeof addr === "object" && addr !== null ? addr.port : port;
         this._isCoordinator = true;
         this.coordinatorListeners.set(id, {
           server,
           policy: "full",
           host,
-          port,
+          port: actualPort,
           isDefault: true,
         });
         this.defaultListenerId = id;
diff --git a/src/core/tls-transport.ts b/src/core/tls-transport.ts
index 3822bd2..42388c8 100644
--- a/src/core/tls-transport.ts
+++ b/src/core/tls-transport.ts
@@ -405,12 +405,15 @@ export class TlsTransport {
       });
 
       server.listen(port, host, () => {
+        const addr = server.address();
+        const actualPort =
+          typeof addr === "object" && addr !== null ? addr.port : port;
         this._isCoordinator = true;
         this.coordinatorListeners.set(id, {
           server,
           policy: "full",
           host,
-          port,
+          port: actualPort,
           isDefault: true,
         });
         this.defaultListenerId = id;
diff --git a/src/test/become-coordinator-actual-port.integration.test.ts b/src/test/become-coordinator-actual-port.integration.test.ts
new file mode 100644
index 0000000..734524c
--- /dev/null
+++ b/src/test/become-coordinator-actual-port.integration.test.ts
@@ -0,0 +1,122 @@
+/**
+ * becomeCoordinator actual-port regression test (#42) — becomeCoordinator(host, 0) must report the OS-assigned port it actually bound, not the literal 0 it was called with, on every transport whose listener bookkeeping goes through listListeners().
+ *
+ * Run: node dist/test/become-coordinator-actual-port.integration.test.js [test-name] With no argument, every scenario runs in order.
+ */
+
+import * as net from "node:net";
+import * as tls from "node:tls";
+import * as assert from "node:assert/strict";
+import { TlsTransport } from "../core/tls-transport.js";
+import { TcpTransport } from "../core/tcp-transport.js";
+import { generateIdentity } from "../core/identity.js";
+import type { TransportEvents } from "../core/transport.js";
+
+function noopEvents(): TransportEvents {
+  return {
+    onMessage: () => undefined,
+    onPeerConnected: () => undefined,
+    onPeerDisconnected: () => undefined,
+    onIntroduction: () => undefined,
+    onConnectionRequest: () => undefined,
+    onPeerList: () => undefined,
+    onPeerJoined: () => undefined,
+    onBecomeCoordinator: () => undefined,
+  };
+}
+
+async function testTlsReportsActualPort(): Promise<void> {
+  const identity = generateIdentity();
+  const transport = new TlsTransport(noopEvents(), identity);
+
+  await transport.becomeCoordinator("127.0.0.1", 0);
+  const [listener] = transport.listListeners();
+  assert.ok(listener);
+  assert.notStrictEqual(listener.port, 0);
+
+  await new Promise<void>((resolve, reject) => {
+    const socket = tls.connect(
+      { host: "127.0.0.1", port: listener.port, rejectUnauthorized: false },
+      () => {
+        socket.destroy();
+        resolve();
+      },
+    );
+    socket.once("error", reject);
+  });
+  console.log(
+    `  ✓ TlsTransport reported and bound the same port (${listener.port})`,
+  );
+
+  await transport.shutdown();
+}
+
+async function testTcpReportsActualPort(): Promise<void> {
+  const transport = new TcpTransport(noopEvents());
+
+  await transport.becomeCoordinator("127.0.0.1", 0);
+  const [listener] = transport.listListeners();
+  assert.ok(listener);
+  assert.notStrictEqual(listener.port, 0);
+
+  await new Promise<void>((resolve, reject) => {
+    const socket = net.connect(
+      { host: "127.0.0.1", port: listener.port },
+      () => {
+        socket.destroy();
+        resolve();
+      },
+    );
+    socket.once("error", reject);
+  });
+  console.log(
+    `  ✓ TcpTransport reported and bound the same port (${listener.port})`,
+  );
+
+  await transport.shutdown();
+}
+
+// ---------------------------------------------------------------------------
+// Runner
+// ---------------------------------------------------------------------------
+
+const testName = process.argv[2];
+
+const tests: Record<string, () => Promise<void>> = {
+  "tls-reports-actual-port": testTlsReportsActualPort,
+  "tcp-reports-actual-port": testTcpReportsActualPort,
+};
+
+const selected =
+  testName === undefined
+    ? Object.entries(tests)
+    : Object.entries(tests).filter(([name]) => name === testName);
+if (selected.length === 0) {
+  console.error(`Unknown test: ${testName}`);
+  console.error(`Available: ${Object.keys(tests).join(", ")}`);
+  process.exit(1);
+}
+
+async function run(): Promise<void> {
+  for (const [name, fn] of selected) {
+    console.log(`Running ${name}:`);
+    await fn();
+  }
+
+  const maxWait = 2000;
+  const start = Date.now();
+  while (
+    ((
+      process as unknown as { _getActiveHandles?: () => unknown[] }
+    )._getActiveHandles?.()?.length ?? 0) > 0 &&
+    Date.now() - start < maxWait
+  ) {
+    await new Promise<void>((resolve) => setTimeout(resolve, 50));
+  }
+  process.exit(0);
+}
+
+run().catch((err: unknown) => {
+  console.error(`FAIL [${testName ?? "all"}]:`, err);
+  process.exit(1);
+});