diff --git a/CHANGELOG.md b/CHANGELOG.md
index d446e0438..c9e8a7158 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -8,6 +8,13 @@ Newest first. `Unreleased` is what is on `main` and not yet tagged.
 
 ## Unreleased
 
+### An empty supervisor PORT is unset, not an ephemeral bind
+
+`PORT=` left blank in compose or a `.env` used to reach `Bun.serve` as `NaN`, so the supervisor
+bound a random port while the published mapping still pointed at 4300. Empty now means the default
+4300; a non-numeric or out-of-range value refuses to start instead of binding port 30 from a typo
+like `30o0`.
+
 ### Coworkers are made in a wizard and managed in a dialog
 
 Creating a coworker is now a three-step wizard — who it is, who may see it, then where it runs,
diff --git a/supervisor/src/index.ts b/supervisor/src/index.ts
index 41fc2e08c..89357ccbc 100644
--- a/supervisor/src/index.ts
+++ b/supervisor/src/index.ts
@@ -12,6 +12,7 @@ import {
 } from "./docker";
 import { registerEntry } from "./identity";
 import { namesFor } from "./names";
+import { listenPort } from "./listen-port";
 
 /**
  * The container supervisor: the only thing here that holds the Docker socket.
@@ -43,7 +44,12 @@ import { namesFor } from "./names";
  * root on the host, so missing authentication is a deployment failure.
  */
 
-const port = Number.parseInt(process.env.PORT ?? "4300", 10);
+const resolvedPort = listenPort(process.env.PORT, 4300);
+if (!resolvedPort.ok) {
+  console.error(resolvedPort.reason);
+  process.exit(1);
+}
+const port = resolvedPort.port;
 const token = process.env.SUPERVISOR_TOKEN?.trim();
 if (!token) {
   console.error(
diff --git a/supervisor/src/listen-port.ts b/supervisor/src/listen-port.ts
new file mode 100644
index 000000000..2e0b2bedd
--- /dev/null
+++ b/supervisor/src/listen-port.ts
@@ -0,0 +1,29 @@
+/**
+ * Listen port for the container supervisor.
+ *
+ * An empty `PORT=` (compose blank, leftover `.env` line) is unset, not zero — the same empty-string
+ * trap #96/#114/#312 found for the server and computer. `??` only fires on undefined, so
+ * `Number.parseInt("", 10)` used to be `NaN` and `Bun.serve({ port: NaN })` bound an ephemeral port
+ * while compose still published 4300. Prefix typos (`30o0`) also used to start on 30 via parseInt.
+ */
+export function listenPort(
+  raw: string | undefined,
+  fallback: number,
+): { ok: true; port: number } | { ok: false; reason: string } {
+  const trimmed = raw?.trim();
+  if (!trimmed) return { ok: true, port: fallback };
+  if (!/^\d+$/.test(trimmed)) {
+    return {
+      ok: false,
+      reason: `PORT must be a whole number from 1 to 65535 (got ${JSON.stringify(raw)}).`,
+    };
+  }
+  const value = Number.parseInt(trimmed, 10);
+  if (value < 1 || value > 65535) {
+    return {
+      ok: false,
+      reason: `PORT must be a whole number from 1 to 65535 (got ${JSON.stringify(raw)}).`,
+    };
+  }
+  return { ok: true, port: value };
+}
diff --git a/supervisor/tests/listen-port.test.ts b/supervisor/tests/listen-port.test.ts
new file mode 100644
index 000000000..503f1c53c
--- /dev/null
+++ b/supervisor/tests/listen-port.test.ts
@@ -0,0 +1,28 @@
+import { describe, expect, test } from "bun:test";
+import { listenPort } from "../src/listen-port";
+
+/**
+ * Empty PORT must not become NaN / an ephemeral bind. Same empty-string trap as the API server.
+ */
+describe("supervisor listen port", () => {
+  test("unset and empty string fall back to 4300", () => {
+    expect(listenPort(undefined, 4300)).toEqual({ ok: true, port: 4300 });
+    expect(listenPort("", 4300)).toEqual({ ok: true, port: 4300 });
+    expect(listenPort("   ", 4300)).toEqual({ ok: true, port: 4300 });
+  });
+
+  test("a whole number in range is accepted", () => {
+    expect(listenPort("4300", 4300)).toEqual({ ok: true, port: 4300 });
+    expect(listenPort("4500", 4300)).toEqual({ ok: true, port: 4500 });
+    expect(listenPort("1", 4300)).toEqual({ ok: true, port: 1 });
+    expect(listenPort("65535", 4300)).toEqual({ ok: true, port: 65535 });
+  });
+
+  test("prefix typos and out-of-range values are refused", () => {
+    expect(listenPort("30o0", 4300).ok).toBe(false);
+    expect(listenPort("0", 4300).ok).toBe(false);
+    expect(listenPort("65536", 4300).ok).toBe(false);
+    expect(listenPort("-1", 4300).ok).toBe(false);
+    expect(listenPort("1.5", 4300).ok).toBe(false);
+  });
+});