diff --git a/agent-computer/src/env.ts b/agent-computer/src/env.ts
new file mode 100644
index 00000000..2cd93262
--- /dev/null
+++ b/agent-computer/src/env.ts
@@ -0,0 +1,16 @@
+/**
+ * A positive number from the environment, or the fallback.
+ *
+ * `Number.parseInt(process.env.X ?? "default")` is not enough: an unset variable declared in a
+ * compose file arrives as an empty string rather than as absent, so `??` never fires and the parse
+ * yields `NaN`. Empty, absent, non-numeric and non-positive all mean "not set" and take the fallback.
+ *
+ * Its own module, free of the `playwright` import `profiles.ts` carries, so a test can reach it
+ * without loading a browser driver that is not installed where the tests run.
+ */
+export function numberFromEnv(name: string, fallback: number): number {
+  const raw = process.env[name]?.trim();
+  if (!raw) return fallback;
+  const value = Number(raw);
+  return Number.isFinite(value) && value > 0 ? value : fallback;
+}
diff --git a/agent-computer/src/index.ts b/agent-computer/src/index.ts
index 98a6c5f7..9441f7e0 100644
--- a/agent-computer/src/index.ts
+++ b/agent-computer/src/index.ts
@@ -12,7 +12,7 @@ import {
   TAKE_CONTROL_FIRST,
 } from "./control";
 import { identity } from "./identity";
-import { createProfiles, VIEWPORT } from "./profiles";
+import { createProfiles, numberFromEnv, VIEWPORT } from "./profiles";
 import {
   type InputMessage,
   type Screencast,
@@ -70,11 +70,8 @@ if (!COMPUTER_TOKEN) {
   process.exit(1);
 }
 
-const PORT = Number.parseInt(process.env.PORT ?? "4100", 10);
-const NAVIGATION_TIMEOUT_MS = Number.parseInt(
-  process.env.NAVIGATION_TIMEOUT_MS ?? "30000",
-  10,
-);
+const PORT = numberFromEnv("PORT", 4100);
+const NAVIGATION_TIMEOUT_MS = numberFromEnv("NAVIGATION_TIMEOUT_MS", 30000);
 
 /**
  * How long one action waits for its element.
@@ -83,10 +80,7 @@ const NAVIGATION_TIMEOUT_MS = Number.parseInt(
  * behaviour we want, but a ref that no longer resolves would otherwise hang for the full navigation
  * timeout before saying so, and the person is sitting watching a screen that is not changing.
  */
-const ACTION_TIMEOUT_MS = Number.parseInt(
-  process.env.ACTION_TIMEOUT_MS ?? "10000",
-  10,
-);
+const ACTION_TIMEOUT_MS = numberFromEnv("ACTION_TIMEOUT_MS", 10000);
 
 /**
  * How much page text a navigation hands back.
diff --git a/agent-computer/src/profiles.ts b/agent-computer/src/profiles.ts
index b0b2e714..420931ee 100644
--- a/agent-computer/src/profiles.ts
+++ b/agent-computer/src/profiles.ts
@@ -39,6 +39,11 @@ import { type BrowserContext, chromium, type Page } from "playwright";
 import { profileDirectoryFor } from "./bot-id";
 import { chooseEvictions, chooseIdle } from "./browser-eviction";
 import { egressFor, egressLabel } from "./egress";
+import { numberFromEnv } from "./env";
+
+// Re-exported so callers that already import it from here do not change, while the test imports it
+// from the playwright-free `./env` instead of pulling this module's browser driver in with it.
+export { numberFromEnv };
 
 /** The viewport, which is what a person's click coordinates are relative to. */
 export const VIEWPORT = { width: 1280, height: 800 };
@@ -144,21 +149,6 @@ async function closeAndWait(context: BrowserContext): Promise<void> {
   await new Promise((resolve) => setTimeout(resolve, CLOSE_SETTLE_MS));
 }
 
-/**
- * A number an operator set, or the default.
- *
- * `Number("")` is zero, and an unset variable in a compose file arrives as an empty string rather
- * than as absent. Read with `??` alone, an operator who had not set the cap would get a cap of zero
- * and every browser would be closed the moment it opened. Anything that is not a positive number
- * falls back, because "I typed this wrong" and "I did not set it" both mean the default.
- */
-function numberFromEnv(name: string, fallback: number): number {
-  const raw = process.env[name]?.trim();
-  if (!raw) return fallback;
-  const value = Number(raw);
-  return Number.isFinite(value) && value > 0 ? value : fallback;
-}
-
 /**
  * How many browsers one computer holds at once.
  *
diff --git a/agent-computer/tests/number-from-env.test.ts b/agent-computer/tests/number-from-env.test.ts
new file mode 100644
index 00000000..6233aa0f
--- /dev/null
+++ b/agent-computer/tests/number-from-env.test.ts
@@ -0,0 +1,38 @@
+import { afterEach, describe, expect, test } from "bun:test";
+import { numberFromEnv } from "../src/env";
+
+const NAME = "OPENBOT_TEST_NUMBER_FROM_ENV";
+
+afterEach(() => {
+  delete process.env[NAME];
+});
+
+describe("numberFromEnv", () => {
+  test("takes a positive number, trimming surrounding whitespace", () => {
+    process.env[NAME] = "  5000 ";
+    expect(numberFromEnv(NAME, 10000)).toBe(5000);
+  });
+
+  test("falls back when the variable is unset", () => {
+    expect(numberFromEnv(NAME, 10000)).toBe(10000);
+  });
+
+  test("falls back on the empty string a compose file passes for an unset variable", () => {
+    // The bug this guards: `Number.parseInt(process.env.X ?? "default")` sees "" here, not undefined,
+    // so `??` never fires and the parse is NaN. An empty value means "not set" and takes the fallback.
+    process.env[NAME] = "";
+    expect(numberFromEnv(NAME, 10000)).toBe(10000);
+  });
+
+  test("falls back on a non-numeric value", () => {
+    process.env[NAME] = "soon";
+    expect(numberFromEnv(NAME, 10000)).toBe(10000);
+  });
+
+  test("falls back on zero and negatives, so a bad timeout is never enforced", () => {
+    process.env[NAME] = "0";
+    expect(numberFromEnv(NAME, 10000)).toBe(10000);
+    process.env[NAME] = "-5";
+    expect(numberFromEnv(NAME, 10000)).toBe(10000);
+  });
+});