diff --git a/CHANGELOG.md b/CHANGELOG.md
index 6e2f952b..4d7d421f 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -8,6 +8,15 @@ Newest first. `Unreleased` is what is on `main` and not yet tagged.
 
 ## Unreleased
 
+### A fractional or out-of-range computer setting takes the fallback instead of breaking the boot
+
+`numberFromEnv` accepted anything `Number` called finite and positive, so `PORT=80.5` bound
+nothing usable, `PORT=99999` misbound at boot, a fractional timeout fired before any action could
+finish, and `COMPUTER_MAX_BROWSERS=2.5` reached eviction math as a fraction — each reading as a
+broken computer rather than a mistaken variable. Every reader is a port, a timeout, or a count,
+so only whole numbers on sight are values now and anything else takes the documented fallback;
+the port additionally keeps its 1–65535 range, the way the supervisor's own port parser already
+does. Zero semantics are unchanged: `COMPUTER_BROWSER_IDLE_MS=0` still keeps browsers resident.
 ### A malformed page size is refused instead of silently coerced
 
 `GET /channels` and `GET /api/admin/people` read `?limit=` with `Number.parseInt`, which
diff --git a/agent-computer/src/env.ts b/agent-computer/src/env.ts
index 96eaacf1..fbcbd82c 100644
--- a/agent-computer/src/env.ts
+++ b/agent-computer/src/env.ts
@@ -1,10 +1,21 @@
 /**
- * A positive number from the environment, or the fallback.
+ * A positive whole 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.
  *
+ * Whole numbers only: every reader is a port, a timeout in milliseconds, or a count of browsers,
+ * and none of them has a fractional answer. `Number("80.5")` is finite and greater than zero, so
+ * without this `PORT=80.5` bound nothing usable, `ACTION_TIMEOUT_MS=0.5` fired before any action
+ * could finish, and `COMPUTER_MAX_BROWSERS=2.5` reached eviction math as a fraction. Scientific
+ * notation and hex are not whole numbers on sight either, for the same reason `listenPort` on the
+ * supervisor refuses them rather than reading part of one.
+ *
+ * `min`/`max` bound the value where the setting has a range, which is the port: `PORT=99999`
+ * parsed fine and then misbound at boot, a deployment failure instead of the documented fallback.
+ * Timeouts and counts carry no range and stay unbounded above, like before.
+ *
  * `zeroSwitchesItOff` is for the one setting where zero is an answer rather than a mistake.
  * `COMPUTER_BROWSER_IDLE_MS=0` is documented as "keeps them resident", and `chooseIdle` reads a
  * timeout of zero as the sweep being switched off — but the value never reached it, because zero is
@@ -23,12 +34,18 @@
 export function numberFromEnv(
   name: string,
   fallback: number,
-  { zeroSwitchesItOff = false }: { zeroSwitchesItOff?: boolean } = {},
+  {
+    zeroSwitchesItOff = false,
+    min,
+    max,
+  }: { zeroSwitchesItOff?: boolean; min?: number; max?: number } = {},
 ): number {
   const raw = process.env[name]?.trim();
   if (!raw) return fallback;
-  const value = Number(raw);
-  if (!Number.isFinite(value)) return fallback;
+  if (!/^\d+$/.test(raw)) return fallback;
+  const value = Number.parseInt(raw, 10);
+  if (min !== undefined && value < min) return fallback;
+  if (max !== undefined && value > max) return fallback;
   return (zeroSwitchesItOff ? value >= 0 : value > 0) ? value : fallback;
 }
 
diff --git a/agent-computer/src/index.ts b/agent-computer/src/index.ts
index 16868122..16b66ddb 100644
--- a/agent-computer/src/index.ts
+++ b/agent-computer/src/index.ts
@@ -91,7 +91,11 @@ console.info(
   }),
 );
 
-const PORT = numberFromEnv("PORT", 4100);
+/*
+ * A whole port in range, or the default: a fraction never binds and an out-of-range one
+ * misbinds at boot, which is a deployment failure instead of the documented fallback.
+ */
+const PORT = numberFromEnv("PORT", 4100, { min: 1, max: 65535 });
 const NAVIGATION_TIMEOUT_MS = numberFromEnv("NAVIGATION_TIMEOUT_MS", 30000);
 
 /**
diff --git a/agent-computer/tests/number-from-env.test.ts b/agent-computer/tests/number-from-env.test.ts
index 00aa36da..f3109bff 100644
--- a/agent-computer/tests/number-from-env.test.ts
+++ b/agent-computer/tests/number-from-env.test.ts
@@ -35,6 +35,27 @@ describe("numberFromEnv", () => {
     process.env[NAME] = "-5";
     expect(numberFromEnv(NAME, 10000)).toBe(10000);
   });
+
+  /**
+   * Whole numbers only.
+   *
+   * Every reader is a port, a timeout in milliseconds, or a count of browsers, and none of them
+   * has a fractional answer. `Number("80.5")` is finite and greater than zero, so a fraction used
+   * to be returned verbatim: a fractional port misbound at boot and a fractional timeout fired
+   * before any action could finish, reading as a broken computer rather than a caller error.
+   */
+  test.each([["80.5"], ["0.5"], ["2.5"], ["30000.0"], ["1e3"], ["0x10"]])(
+    "falls back on %p, which is not a whole number on sight",
+    (raw) => {
+      process.env[NAME] = raw;
+      expect(numberFromEnv(NAME, 10000)).toBe(10000);
+    },
+  );
+
+  test("still takes a large whole number where the setting has no range", () => {
+    process.env[NAME] = "1800000";
+    expect(numberFromEnv(NAME, 10000)).toBe(1800000);
+  });
 });
 
 /**
@@ -80,4 +101,39 @@ describe("numberFromEnv where zero switches the setting off", () => {
     process.env[NAME] = "5000";
     expect(numberFromEnv(NAME, 10000, { zeroSwitchesItOff: true })).toBe(5000);
   });
+
+  test("still falls back on a fraction where zero switches the setting off", () => {
+    process.env[NAME] = "0.5";
+    expect(numberFromEnv(NAME, 10000, { zeroSwitchesItOff: true })).toBe(10000);
+  });
+});
+
+/**
+ * Settings with a range.
+ *
+ * A port parsed fine and then misbound at boot: `PORT=99999` was returned verbatim and the
+ * deployment failed instead of taking the documented fallback. The range rides on the same
+ * function so every integer setting keeps one rule, rather than the port growing its own parser
+ * that the next setting copies slightly wrong.
+ */
+describe("numberFromEnv with a range", () => {
+  const RANGE = { min: 1, max: 65535 };
+
+  test.each([
+    ["4300", 4300],
+    ["1", 1],
+    ["65535", 65535],
+    ["  4100  ", 4100],
+  ])("takes a whole port in range: %p", (raw, port) => {
+    process.env[NAME] = raw;
+    expect(numberFromEnv(NAME, 4100, RANGE)).toBe(port);
+  });
+
+  test.each([["0"], ["99999"], ["65536"], ["80.5"], ["-1"], ["soon"]])(
+    "falls back on %p, which is out of range or not a whole number",
+    (raw) => {
+      process.env[NAME] = raw;
+      expect(numberFromEnv(NAME, 4100, RANGE)).toBe(4100);
+    },
+  );
 });