diff --git a/packages/cli/src/server/portUtils.test.ts b/packages/cli/src/server/portUtils.test.ts
new file mode 100644
index 0000000000..8f64704dd5
--- /dev/null
+++ b/packages/cli/src/server/portUtils.test.ts
@@ -0,0 +1,62 @@
+import { afterEach, describe, expect, it } from "vitest";
+import { createServer, type Server } from "node:net";
+import { testPortOnAllHosts } from "./portUtils.js";
+
+// A genuinely-free port on every OS we care about. Picked from the high
+// IANA-ephemeral range with enough runway that random collisions are rare;
+// if a test still flakes on a loaded CI shard we bump `BASE`.
+const BASE = 45_000;
+
+const openServers: Server[] = [];
+
+function allocFreePort(): number {
+  // Each test picks its own port out of a counter to avoid cross-test reuse.
+  return BASE + Math.floor(Math.random() * 1_000);
+}
+
+afterEach(async () => {
+  await Promise.all(
+    openServers.splice(0).map(
+      (s) =>
+        new Promise<void>((resolve) => {
+          s.close(() => resolve());
+        }),
+    ),
+  );
+});
+
+describe("testPortOnAllHosts", () => {
+  it("returns true for a genuinely free port (regression: #309)", async () => {
+    // The bug: running the four probes in parallel caused the `0.0.0.0` bind
+    // to collide with the still-open `127.0.0.1` socket, so `every port is
+    // occupied` on Linux. This test binds NOTHING and asserts the scanner
+    // sees the port as free — which only works when the internal probes are
+    // sequential and each socket is fully closed before the next opens.
+    const port = allocFreePort();
+    const result = await testPortOnAllHosts(port);
+    expect(result).toBe(true);
+  });
+
+  it("returns false when the port is occupied on 0.0.0.0", async () => {
+    const port = allocFreePort();
+    const blocker = createServer();
+    openServers.push(blocker);
+    await new Promise<void>((resolve, reject) => {
+      blocker.once("error", reject);
+      blocker.listen({ port, host: "0.0.0.0" }, () => resolve());
+    });
+    const result = await testPortOnAllHosts(port);
+    expect(result).toBe(false);
+  });
+
+  it("releases each probe socket before starting the next (no wildcard-vs-loopback race)", async () => {
+    // Sanity-check the shape of the bug itself: two back-to-back calls for
+    // the same free port must both return true. If the first call left a
+    // socket lingering, the second would see EADDRINUSE.
+    const port = allocFreePort();
+    const first = await testPortOnAllHosts(port);
+    const second = await testPortOnAllHosts(port);
+    expect(first).toBe(true);
+    expect(second).toBe(true);
+  });
+});
diff --git a/packages/cli/src/server/portUtils.ts b/packages/cli/src/server/portUtils.ts
index d66a74e7dc..d681e12e55 100644
--- a/packages/cli/src/server/portUtils.ts
+++ b/packages/cli/src/server/portUtils.ts
@@ -50,14 +50,28 @@ function isPortAvailableOnHost(port: number, host: string): Promise<boolean> {
 }
 
 /**
- * Test a port across IPv4 and IPv6 interfaces in parallel. A port is only
- * unavailable if ANY host reports EADDRINUSE. This catches the devbox bug
- * where a port is free on localhost but occupied on 0.0.0.0 via SSH forwarding.
+ * Test a port across IPv4 and IPv6 interfaces. A port is only available if
+ * EVERY host binds and releases cleanly — that catches the devbox class of
+ * bug where a port is free on `127.0.0.1` but held on `0.0.0.0` via SSH
+ * forwarding.
+ *
+ * **Must be sequential, not Promise.all.** Binding `127.0.0.1` holds the
+ * socket open until `server.close()` resolves on the next event-loop tick.
+ * In parallel, the wildcard `0.0.0.0` / `::` tests race that still-open
+ * socket and return spurious `EADDRINUSE` — which makes every port in the
+ * scan range look occupied and the preview server refuse to start. Repro
+ * on Linux (Crostini on ChromeOS in the reporting environment, issue #309)
+ * is deterministic; on macOS/Windows the behaviour is less consistent but
+ * the race is there all the same. Serializing each bind past its close
+ * callback eliminates the window entirely.
  */
 export async function testPortOnAllHosts(port: number): Promise<boolean> {
   const hosts = ["127.0.0.1", "0.0.0.0", "::1", "::"];
-  const results = await Promise.all(hosts.map((h) => isPortAvailableOnHost(port, h)));
-  return results.every(Boolean);
+  for (const host of hosts) {
+    const available = await isPortAvailableOnHost(port, host);
+    if (!available) return false;
+  }
+  return true;
 }
 
 // ── Existing instance detection ────────────────────────────────────────────