diff --git a/CHANGELOG.md b/CHANGELOG.md
index 1c6169fda..65d99fecb 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -8,6 +8,11 @@ Newest first. `Unreleased` is what is on `main` and not yet tagged.
 
 ## Unreleased
 
+### A bad `COMPUTER_MEMORY_BYTES` refuses to start the supervisor, instead of capping a computer at 512 bytes
+
+`COMPUTER_MEMORY_BYTES=512m` used to parse as `512` via `parseInt`, which Docker accepts as a memory
+cap Chromium cannot live in. Empty `COMPUTER_MEMORY_BYTES=` is still unset (no cap). A value that is
+not a whole number of bytes now exits before any computer is created.
 ### An IPv6 address in `AGENT_ENDPOINT_ALLOWED_HOSTS` now matches however it is written
 
 The endpoint check compares the list against the address as the URL parser spells it, compressed
diff --git a/supervisor/src/computer-memory-bytes.ts b/supervisor/src/computer-memory-bytes.ts
new file mode 100644
index 000000000..8a1c60574
--- /dev/null
+++ b/supervisor/src/computer-memory-bytes.ts
@@ -0,0 +1,27 @@
+/**
+ * Optional Docker memory cap for each computer, from `COMPUTER_MEMORY_BYTES`.
+ *
+ * Unset or empty (compose blank, leftover `.env` line) means no cap. The same empty-string and
+ * prefix traps as `listenPort`: `Number.parseInt("", 10)` is `NaN` (silently dropped) and
+ * `Number.parseInt("512m", 10)` is `512` bytes, which Docker accepts and Chromium cannot live in.
+ */
+export function computerMemoryBytes(
+  raw: string | undefined,
+): { ok: true; bytes: number | undefined } | { ok: false; reason: string } {
+  const trimmed = raw?.trim();
+  if (!trimmed) return { ok: true, bytes: undefined };
+  if (!/^\d+$/.test(trimmed)) {
+    return {
+      ok: false,
+      reason: `COMPUTER_MEMORY_BYTES must be a whole number of bytes (got ${JSON.stringify(raw)}).`,
+    };
+  }
+  const value = Number.parseInt(trimmed, 10);
+  if (value < 1) {
+    return {
+      ok: false,
+      reason: `COMPUTER_MEMORY_BYTES must be a whole number of bytes (got ${JSON.stringify(raw)}).`,
+    };
+  }
+  return { ok: true, bytes: value };
+}
diff --git a/supervisor/src/index.ts b/supervisor/src/index.ts
index 89357ccbc..a875679a0 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 { computerMemoryBytes } from "./computer-memory-bytes";
 import { listenPort } from "./listen-port";
 
 /**
@@ -60,9 +61,12 @@ if (!token) {
 const image = process.env.COMPUTER_IMAGE ?? "openbot-agent-computer:latest";
 const network = process.env.COMPUTER_NETWORK;
 const runtime = process.env.COMPUTER_RUNTIME;
-const memoryBytes = process.env.COMPUTER_MEMORY_BYTES
-  ? Number.parseInt(process.env.COMPUTER_MEMORY_BYTES, 10)
-  : undefined;
+const resolvedMemory = computerMemoryBytes(process.env.COMPUTER_MEMORY_BYTES);
+if (!resolvedMemory.ok) {
+  console.error(resolvedMemory.reason);
+  process.exit(1);
+}
+const memoryBytes = resolvedMemory.bytes;
 const spireSocketVolume = process.env.SPIRE_AGENT_SOCKET_VOLUME;
 
 /**
diff --git a/supervisor/tests/computer-memory-bytes.test.ts b/supervisor/tests/computer-memory-bytes.test.ts
new file mode 100644
index 000000000..d5f696b55
--- /dev/null
+++ b/supervisor/tests/computer-memory-bytes.test.ts
@@ -0,0 +1,29 @@
+import { describe, expect, test } from "bun:test";
+import { computerMemoryBytes } from "../src/computer-memory-bytes";
+
+describe("supervisor computer memory bytes", () => {
+  test("unset and empty string mean no cap", () => {
+    expect(computerMemoryBytes(undefined)).toEqual({
+      ok: true,
+      bytes: undefined,
+    });
+    expect(computerMemoryBytes("")).toEqual({ ok: true, bytes: undefined });
+    expect(computerMemoryBytes("   ")).toEqual({ ok: true, bytes: undefined });
+  });
+
+  test("a whole number of bytes is accepted", () => {
+    expect(computerMemoryBytes("1073741824")).toEqual({
+      ok: true,
+      bytes: 1_073_741_824,
+    });
+    expect(computerMemoryBytes("1")).toEqual({ ok: true, bytes: 1 });
+  });
+
+  test("suffix units and non-digits are refused instead of parseInt prefixes", () => {
+    expect(computerMemoryBytes("512m").ok).toBe(false);
+    expect(computerMemoryBytes("1e9").ok).toBe(false);
+    expect(computerMemoryBytes("0").ok).toBe(false);
+    expect(computerMemoryBytes("-1").ok).toBe(false);
+    expect(computerMemoryBytes("1.5").ok).toBe(false);
+  });
+});