diff --git a/README.md b/README.md
index 7e053a8..97767dc 100644
--- a/README.md
+++ b/README.md
@@ -273,10 +273,12 @@ A live single-line progress indicator runs on stderr during interactive sessions
 
 ## Requirements
 
-- macOS 27 or newer (Apple's `fm` CLI is preinstalled).
+- macOS 27.0 or newer (Apple's `fm` CLI is preinstalled there).
 - Node.js 20 or newer.
 - Apple Intelligence enabled on the device.
 
+Benchmark commands refuse to start on older macOS versions and report the detected version plus the latest supported macOS — see [docs/supported-platforms.md](docs/supported-platforms.md).
+
 `pcc` (Private Cloud Compute) availability depends on Apple's current eligibility. `fm-bench` shows it as skipped if `fm available --model pcc` reports unavailable.
 
 See [docs/methodology.md](docs/methodology.md) for benchmark methodology and metric references.
diff --git a/docs/releasing.md b/docs/releasing.md
index 071a9b7..058216f 100644
--- a/docs/releasing.md
+++ b/docs/releasing.md
@@ -17,7 +17,7 @@
 ## Option B — Local
 
 ```sh
-npm ci && npm test && npm run lint
+npm ci && npm test && npm run lint && npm run check:pack
 npm version patch   # or minor / major
 git push --follow-tags
 ```
@@ -37,6 +37,7 @@ gh release edit v0.6.0 --notes-file /tmp/notes.md --repo devinoldenburg/fm-bench
 
 ```sh
 npm run publish:dry-run
+npm run check:pack
 ```
 
 CI runs the same dry-run on every push to `main` and on pull requests.
\ No newline at end of file
diff --git a/docs/supported-platforms.md b/docs/supported-platforms.md
new file mode 100644
index 0000000..5eb1942
--- /dev/null
+++ b/docs/supported-platforms.md
@@ -0,0 +1,26 @@
+# Supported platforms
+
+`fm-bench` benchmarks Apple's `fm` command, so support follows the platforms where Apple ships that CLI.
+
+## Requirements
+
+- **macOS 27.0 or newer** — Apple's `fm` CLI is preinstalled starting with macOS 27. Older macOS releases do not ship `fm`, so `fm-bench` cannot run there.
+- **Node.js 20 or newer**.
+- **Apple Intelligence enabled** on the device.
+
+`pcc` (Private Cloud Compute) availability additionally depends on Apple's current eligibility. `fm-bench` reports it as skipped when `fm available --model pcc` reports unavailable.
+
+## Version enforcement
+
+Commands that launch `fm` benchmarks — the default `run` command and `models` — check the macOS version first and refuse to start on anything older than macOS 27:
+
+```text
+fm-bench: unsupported macOS: detected macOS 26.1, but fm-bench requires macOS 27 or newer (Apple's fm CLI is preinstalled there).
+Latest supported: macOS 27.0 or newer (fm is not available on older macOS releases).
+```
+
+The process exits with code `2`. This is deliberate: running on an unsupported macOS could never produce a valid benchmark, so the CLI fails fast with the exact version it found and the latest supported macOS version.
+
+Commands that only read local report files — `compare`, `history`, `validate`, `export`, and `legend` — are not gated and work anywhere Node.js runs.
+
+`fm-bench doctor` still runs on unsupported hosts so you can diagnose the environment: it prints the detected macOS version, an explicit `macOS support` line, and the latest supported version.
diff --git a/package.json b/package.json
index 25a0b4f..8c4d301 100644
--- a/package.json
+++ b/package.json
@@ -16,11 +16,12 @@
   "scripts": {
     "test": "node --test",
     "lint": "node --check bin/fm-bench.js && find src test -name '*.js' -print0 | xargs -0 -n1 node --check",
-    "prepack": "npm test && npm run lint",
+    "prepack": "npm test && npm run lint && npm run check:pack",
     "release:patch": "npm version patch && git push --follow-tags",
     "release:minor": "npm version minor && git push --follow-tags",
     "release:major": "npm version major && git push --follow-tags",
-    "publish:dry-run": "npm publish --dry-run --access public"
+    "publish:dry-run": "npm publish --dry-run --access public",
+    "check:pack": "node scripts/check-package.mjs"
   },
   "keywords": [
     "apple",
@@ -46,5 +47,8 @@
   "publishConfig": {
     "access": "public",
     "registry": "https://registry.npmjs.org/"
-  }
+  },
+  "os": [
+    "darwin"
+  ]
 }
diff --git a/scripts/check-package.mjs b/scripts/check-package.mjs
new file mode 100644
index 0000000..fe42462
--- /dev/null
+++ b/scripts/check-package.mjs
@@ -0,0 +1,49 @@
+#!/usr/bin/env node
+/**
+ * Verify the packed npm tarball includes every runtime file.
+ * Runtime imports outside bin/, src/, and package.json are refused so a
+ * missing "files" entry cannot ship a broken release.
+ */
+import { execFileSync } from 'node:child_process';
+import { readFileSync } from 'node:fs';
+import { join } from 'node:path';
+
+const root = join(import.meta.dirname, '..');
+const pkg = JSON.parse(readFileSync(join(root, 'package.json'), 'utf8'));
+
+// --ignore-scripts: this script runs from `prepack`, so a nested pack that
+// ran lifecycle scripts would recurse forever.
+const pack = JSON.parse(execFileSync('npm', ['pack', '--dry-run', '--json', '--ignore-scripts'], { cwd: root, encoding: 'utf8' }));
+const files = new Set(pack[0].files.map((file) => file.path));
+
+const missing = [];
+const check = (path, reason) => {
+  if (!files.has(path)) missing.push(`${path} (${reason})`);
+};
+
+const entry = pkg.bin['fm-bench'];
+check(entry, 'bin entry');
+
+const seen = new Set();
+const queue = [entry];
+const importRe = /from\s+['"](\.\.?\/[^'"]+)['"]/g;
+while (queue.length > 0) {
+  const current = queue.pop();
+  if (seen.has(current)) continue;
+  seen.add(current);
+  const source = readFileSync(join(root, current), 'utf8');
+  for (const match of source.matchAll(importRe)) {
+    const resolved = join(current, '..', match[1]).replace(/\\/g, '/');
+    queue.push(resolved);
+    check(resolved, `imported by ${current}`);
+  }
+}
+
+if (!files.has('package.json')) missing.push('package.json');
+
+if (missing.length > 0) {
+  console.error(`Packed tarball is missing runtime files:\n  ${missing.join('\n  ')}`);
+  process.exit(1);
+}
+
+console.log(`check:pack ok — ${seen.size} runtime module(s) all present in tarball (${files.size} files total).`);