diff --git a/cli/bin/cli.js b/cli/bin/cli.js
index 43c218ae9..f63e677f9 100755
--- a/cli/bin/cli.js
+++ b/cli/bin/cli.js
@@ -33,15 +33,30 @@ async function download() {
   const res = await fetch(URL, { redirect: 'follow' });
   if (!res.ok) return null;
   const buf = Buffer.from(await res.arrayBuffer());
+  // Fail closed, like the skill launcher and `impeccable install`: a sidecar
+  // that cannot be fetched, or that carries no hash, refuses the download
+  // instead of caching an unverified binary. Nothing is written until the
+  // hash matches, so a refusal leaves the cache dir untouched.
   const sum = await fetch(`${URL}.sha256`, { redirect: 'follow' }).then(r => (r.ok ? r.text() : ''), () => '');
-  const expected = sum.trim().split(/\s+/)[0];
-  if (expected && createHash('sha256').update(buf).digest('hex') !== expected) {
+  const expected = sum.trim().split(/\s+/)[0].toLowerCase();
+  if (!expected) {
+    throw new Error(
+      `cannot verify ${URL} against ${URL}.sha256 (sidecar unavailable or empty); `
+      + 'refusing the unverified download',
+    );
+  }
+  if (createHash('sha256').update(buf).digest('hex') !== expected) {
     throw new Error(`checksum mismatch downloading ${URL}`);
   }
   fs.mkdirSync(path.dirname(CACHED), { recursive: true });
   const tmp = `${CACHED}.part.${process.pid}`;
-  fs.writeFileSync(tmp, buf, { mode: 0o755 });
-  fs.renameSync(tmp, CACHED);
+  try {
+    fs.writeFileSync(tmp, buf, { mode: 0o755 });
+    fs.renameSync(tmp, CACHED);
+  } catch (err) {
+    try { fs.rmSync(tmp, { force: true }); } catch { /* best effort */ }
+    throw err;
+  }
   return CACHED;
 }
 async function locate() {
diff --git a/scripts/test-suites.mjs b/scripts/test-suites.mjs
index e96b2e377..acc483c78 100644
--- a/scripts/test-suites.mjs
+++ b/scripts/test-suites.mjs
@@ -63,6 +63,7 @@ export const SUITES = {
         timeoutMs: 180000,
         files: [
           'tests/ci-test-plan.test.mjs',
+          'tests/cli-shim.test.mjs',
           'tests/github-sheriff.test.mjs',
           'tests/hook-build.test.mjs',
           'tests/openai-plugin.test.mjs',
diff --git a/tests/cli-shim.test.mjs b/tests/cli-shim.test.mjs
new file mode 100644
index 000000000..b1277f1bc
--- /dev/null
+++ b/tests/cli-shim.test.mjs
@@ -0,0 +1,155 @@
+/**
+ * The npm shim's download path (`cli/bin/cli.js`).
+ *
+ * The shim is the last resort in the binary lookup, and it caches what it
+ * fetches, so it has to fail closed the same way the skill launcher
+ * (`skill/scripts/impeccable`) and `impeccable install`
+ * (`crates/skills/src/engine_binary.rs`) do: a `.sha256` sidecar that cannot
+ * be fetched, is empty, or disagrees with the payload refuses the download and
+ * leaves nothing in the cache dir.
+ *
+ * Every case runs the real shim against a throwaway HTTP server, with
+ * IMPECCABLE_HOME pointed at a temp dir so the user's cache is never touched.
+ */
+import assert from 'node:assert/strict';
+import { spawn } from 'node:child_process';
+import { createHash } from 'node:crypto';
+import fs from 'node:fs';
+import http from 'node:http';
+import os from 'node:os';
+import path from 'node:path';
+import { fileURLToPath } from 'node:url';
+import { after, before, describe, it } from 'node:test';
+
+const REPO = path.resolve(path.dirname(fileURLToPath(import.meta.url)), '..');
+const SHIM = path.join(REPO, 'cli', 'bin', 'cli.js');
+const VERSION = JSON.parse(fs.readFileSync(path.join(REPO, 'package.json'), 'utf8'))
+  .optionalDependencies['@impeccable/cli-darwin-arm64'];
+const TARGET = `${{ darwin: 'darwin', linux: 'linux', win32: 'windows' }[process.platform] || process.platform}`
+  + `-${{ arm64: 'arm64', x64: 'x64' }[process.arch] || process.arch}`;
+const ASSET = `impeccable-${TARGET}${process.platform === 'win32' ? '.exe' : ''}`;
+
+// A stand-in engine binary: a script that prints its argv so a successful
+// download is observable end to end.
+const PAYLOAD = Buffer.from('#!/bin/sh\necho "fake-engine $*"\n');
+const DIGEST = createHash('sha256').update(PAYLOAD).digest('hex');
+
+/** What the server answers for `<asset>.sha256` on the next request. */
+let sidecar = { status: 200, body: `${DIGEST}  ${ASSET}\n` };
+let server;
+let base;
+
+before(async () => {
+  server = http.createServer((req, res) => {
+    if (req.url.endsWith('.sha256')) {
+      if (sidecar.status !== 200) {
+        res.writeHead(sidecar.status);
+        res.end('');
+        return;
+      }
+      res.writeHead(200, { 'content-type': 'text/plain' });
+      res.end(sidecar.body);
+      return;
+    }
+    if (req.url.endsWith(ASSET)) {
+      res.writeHead(200, { 'content-type': 'application/octet-stream' });
+      res.end(PAYLOAD);
+      return;
+    }
+    res.writeHead(404);
+    res.end('');
+  });
+  await new Promise((resolve) => server.listen(0, '127.0.0.1', resolve));
+  base = `http://127.0.0.1:${server.address().port}`;
+});
+
+after(() => new Promise((resolve) => server.close(resolve)));
+
+/**
+ * Run the shim and collect its output. Async on purpose: the fixture server
+ * lives in this process, so a synchronous spawn would block the event loop
+ * and the child's fetch would never be answered.
+ */
+function run(args, env) {
+  return new Promise((resolve) => {
+    const child = spawn(process.execPath, [SHIM, ...args], { env });
+    let stdout = '';
+    let stderr = '';
+    child.stdout.on('data', (d) => { stdout += d; });
+    child.stderr.on('data', (d) => { stderr += d; });
+    child.on('close', (status) => resolve({ status, stdout, stderr }));
+  });
+}
+
+/** Run the shim with a fresh cache dir. Returns the result plus that dir. */
+async function runShim(args = ['engine-probe']) {
+  const home = fs.mkdtempSync(path.join(os.tmpdir(), 'impeccable-shim-'));
+  const env = { ...process.env, IMPECCABLE_HOME: home, IMPECCABLE_DOWNLOAD_BASE: base };
+  delete env.IMPECCABLE_BIN;
+  return { ...(await run(args, env)), home };
+}
+
+/** Everything under the cache dir, so a refusal can be shown to leave no trace. */
+function cacheEntries(home) {
+  const root = path.join(home, 'bin');
+  if (!fs.existsSync(root)) return [];
+  const out = [];
+  const walk = (dir) => {
+    for (const name of fs.readdirSync(dir)) {
+      const p = path.join(dir, name);
+      if (fs.statSync(p).isDirectory()) walk(p);
+      else out.push(path.relative(root, p));
+    }
+  };
+  walk(root);
+  return out;
+}
+
+describe('npm shim download verification', { skip: process.platform === 'win32' ? 'posix only' : false }, () => {
+  it('caches and runs a download whose sidecar matches', async () => {
+    sidecar = { status: 200, body: `${DIGEST}  ${ASSET}\n` };
+    const res = await runShim(['hello']);
+    assert.equal(res.status, 0, res.stderr);
+    assert.match(res.stdout, /fake-engine hello/);
+    assert.deepEqual(cacheEntries(res.home), [path.join(VERSION, 'impeccable')]);
+  });
+
+  it('refuses when the sidecar is missing', async () => {
+    sidecar = { status: 404, body: '' };
+    const res = await runShim();
+    assert.equal(res.status, 127);
+    assert.match(res.stderr, /sidecar unavailable or empty/);
+    assert.match(res.stderr, /refusing the unverified download/);
+    assert.deepEqual(cacheEntries(res.home), []);
+  });
+
+  it('refuses when the sidecar is empty', async () => {
+    sidecar = { status: 200, body: '   \n' };
+    const res = await runShim();
+    assert.equal(res.status, 127);
+    assert.match(res.stderr, /sidecar unavailable or empty/);
+    assert.match(res.stderr, /refusing the unverified download/);
+    assert.deepEqual(cacheEntries(res.home), []);
+  });
+
+  it('refuses when the sidecar hash does not match', async () => {
+    sidecar = { status: 200, body: `${'0'.repeat(64)}  ${ASSET}\n` };
+    const res = await runShim();
+    assert.equal(res.status, 127);
+    assert.match(res.stderr, /checksum mismatch downloading/);
+    assert.deepEqual(cacheEntries(res.home), []);
+  });
+
+  it('prefers IMPECCABLE_BIN and never downloads', async () => {
+    sidecar = { status: 404, body: '' };
+    const home = fs.mkdtempSync(path.join(os.tmpdir(), 'impeccable-shim-'));
+    const bin = path.join(home, 'preinstalled');
+    fs.writeFileSync(bin, '#!/bin/sh\necho "preinstalled $*"\n', { mode: 0o755 });
+    const res = await run(['hi'], {
+      ...process.env, IMPECCABLE_HOME: home, IMPECCABLE_DOWNLOAD_BASE: base, IMPECCABLE_BIN: bin,
+    });
+    assert.equal(res.status, 0, res.stderr);
+    assert.match(res.stdout, /preinstalled hi/);
+    assert.deepEqual(cacheEntries(home), []);
+  });
+});