diff --git a/npm/postinstall.js b/npm/postinstall.js
index 07bf79b..8c7a6b5 100644
--- a/npm/postinstall.js
+++ b/npm/postinstall.js
@@ -6,7 +6,7 @@
 const fs = require('fs');
 const path = require('path');
 const https = require('https');
-const zlib = require('zlib');
+const crypto = require('crypto');
 const { pipeline } = require('stream');
 const { promisify } = require('util');
 const { execFileSync } = require('child_process');
@@ -40,7 +40,10 @@ if (!target) {
   process.exit(1);
 }
 
-const url = `https://github.com/optiqor/optiqor-cli/releases/download/v${VERSION}/optiqor_${VERSION}_${target}.tar.gz`;
+const releaseBaseUrl = `https://github.com/optiqor/optiqor-cli/releases/download/v${VERSION}`;
+const archiveName = `optiqor_${VERSION}_${target}.tar.gz`;
+const url = `${releaseBaseUrl}/${archiveName}`;
+const checksumsUrl = `${releaseBaseUrl}/checksums.txt`;
 const vendorDir = path.join(__dirname, '..', 'vendor');
 fs.mkdirSync(vendorDir, { recursive: true });
 
@@ -63,20 +66,101 @@ const get = (u) =>
 
 const pipelineP = promisify(pipeline);
 
-(async () => {
+class FatalInstallError extends Error {
+  constructor(message) {
+    super(message);
+    this.name = 'FatalInstallError';
+  }
+}
+
+const unlinkIfExists = (filePath) => {
+  try {
+    fs.unlinkSync(filePath);
+  } catch (err) {
+    if (err.code !== 'ENOENT') {
+      throw err;
+    }
+  }
+};
+
+const responseText = async (res) => {
+  const chunks = [];
+  for await (const chunk of res) {
+    chunks.push(Buffer.from(chunk));
+  }
+  return Buffer.concat(chunks).toString('utf8');
+};
+
+const checksumForArchive = (checksumsText, name) => {
+  for (const rawLine of checksumsText.split(/\r?\n/)) {
+    const line = rawLine.trim();
+    if (!line) {
+      continue;
+    }
+    const match = line.match(/^([a-fA-F0-9]{64})\s+\*?(.+)$/);
+    if (match && path.basename(match[2].trim()) === name) {
+      return match[1].toLowerCase();
+    }
+  }
+  return null;
+};
+
+const sha256File = (filePath) =>
+  new Promise((resolve, reject) => {
+    const hash = crypto.createHash('sha256');
+    fs.createReadStream(filePath)
+      .on('data', (chunk) => hash.update(chunk))
+      .on('error', reject)
+      .on('end', () => resolve(hash.digest('hex')));
+  });
+
+const verifyArchiveChecksum = async (filePath, name, checksumsText) => {
+  const expected = checksumForArchive(checksumsText, name);
+  if (!expected) {
+    throw new FatalInstallError(`checksum entry not found for ${name}`);
+  }
+
+  const actual = await sha256File(filePath);
+  if (actual !== expected) {
+    unlinkIfExists(filePath);
+    throw new FatalInstallError(
+      `checksum mismatch for ${name}: expected ${expected}, got ${actual}`,
+    );
+  }
+};
+
+const install = async () => {
   try {
     console.log(`optiqor: downloading binary for ${key}...`);
     const res = await get(url);
     await pipelineP(res, fs.createWriteStream(tarballPath));
+    const checksums = await responseText(await get(checksumsUrl));
+    await verifyArchiveChecksum(tarballPath, archiveName, checksums);
     // tar -xzf using system tar (avoids adding tar npm dep).
     execFileSync('tar', ['-xzf', tarballPath, '-C', vendorDir], { stdio: 'inherit' });
-    fs.unlinkSync(tarballPath);
+    unlinkIfExists(tarballPath);
     console.log('optiqor: ready. Run `optiqor --version` to verify.');
   } catch (err) {
+    unlinkIfExists(tarballPath);
     console.error('optiqor: failed to install binary:', err.message);
+    if (err instanceof FatalInstallError) {
+      console.error('optiqor: refusing to use an unverified release archive.');
+      process.exit(1);
+    }
     console.error('optiqor: this is non-fatal — build from source if needed:');
     console.error('  go install github.com/optiqor/optiqor-cli/cmd/optiqor@latest');
     // Exit 0 so npm install does not abort entirely.
     process.exit(0);
   }
-})();
+};
+
+if (require.main === module) {
+  install();
+}
+
+module.exports = {
+  checksumForArchive,
+  sha256File,
+  verifyArchiveChecksum,
+  FatalInstallError,
+};
diff --git a/npm/postinstall.test.js b/npm/postinstall.test.js
new file mode 100644
index 0000000..b572246
--- /dev/null
+++ b/npm/postinstall.test.js
@@ -0,0 +1,85 @@
+const assert = require('assert/strict');
+const fs = require('fs');
+const os = require('os');
+const path = require('path');
+const test = require('node:test');
+
+const {
+  FatalInstallError,
+  checksumForArchive,
+  sha256File,
+  verifyArchiveChecksum,
+} = require('./postinstall');
+
+const withTempFile = (contents) => {
+  const dir = fs.mkdtempSync(path.join(os.tmpdir(), 'optiqor-postinstall-'));
+  const filePath = path.join(dir, 'archive.tar.gz');
+  fs.writeFileSync(filePath, contents);
+  return { dir, filePath };
+};
+
+test('checksumForArchive returns the SHA-256 for the exact archive name', () => {
+  const checksums = [
+    'aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa  optiqor_1.2.3_linux_amd64.tar.gz',
+    'bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb  optiqor_1.2.3_darwin_arm64.tar.gz',
+  ].join('\n');
+
+  assert.equal(
+    checksumForArchive(checksums, 'optiqor_1.2.3_darwin_arm64.tar.gz'),
+    'bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb',
+  );
+});
+
+test('checksumForArchive supports star-prefixed checksum filenames', () => {
+  const checksums =
+    'cccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccc *optiqor_1.2.3_linux_arm64.tar.gz';
+
+  assert.equal(
+    checksumForArchive(checksums, 'optiqor_1.2.3_linux_arm64.tar.gz'),
+    'cccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccc',
+  );
+});
+
+test('verifyArchiveChecksum accepts a matching archive hash', async () => {
+  const { dir, filePath } = withTempFile('trusted archive');
+  try {
+    const hash = await sha256File(filePath);
+    await verifyArchiveChecksum(filePath, 'archive.tar.gz', `${hash}  archive.tar.gz`);
+    assert.equal(fs.existsSync(filePath), true);
+  } finally {
+    fs.rmSync(dir, { recursive: true, force: true });
+  }
+});
+
+test('verifyArchiveChecksum deletes a mismatched archive and fails loudly', async () => {
+  const { dir, filePath } = withTempFile('tampered archive');
+  try {
+    await assert.rejects(
+      verifyArchiveChecksum(
+        filePath,
+        'archive.tar.gz',
+        'dddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddd  archive.tar.gz',
+      ),
+      FatalInstallError,
+    );
+    assert.equal(fs.existsSync(filePath), false);
+  } finally {
+    fs.rmSync(dir, { recursive: true, force: true });
+  }
+});
+
+test('verifyArchiveChecksum fails loudly when the archive is missing from checksums.txt', async () => {
+  const { dir, filePath } = withTempFile('trusted archive');
+  try {
+    await assert.rejects(
+      verifyArchiveChecksum(
+        filePath,
+        'archive.tar.gz',
+        'eeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeee  other.tar.gz',
+      ),
+      FatalInstallError,
+    );
+  } finally {
+    fs.rmSync(dir, { recursive: true, force: true });
+  }
+});
diff --git a/package.json b/package.json
index ecb38c2..a53ddb0 100644
--- a/package.json
+++ b/package.json
@@ -26,6 +26,7 @@
   },
   "scripts": {
     "postinstall": "node ./npm/postinstall.js",
+    "test:npm": "node --test npm/postinstall.test.js",
     "test": "node -e \"require('./npm/index.js')\""
   },
   "files": [