diff --git a/npm/install.js b/npm/install.js
index 0e900b0..652daec 100644
--- a/npm/install.js
+++ b/npm/install.js
@@ -1,19 +1,18 @@
 #!/usr/bin/env node
 // postinstall: downloads the correct platform binary from GitHub Releases.
 
+"use strict";
+
 const { execSync } = require("child_process");
 const fs = require("fs");
 const https = require("https");
 const os = require("os");
 const path = require("path");
-const { createGunzip } = require("zlib");
 
 const REPO = "supermodeltools/cli";
 const BIN_DIR = path.join(__dirname, "bin");
 const BIN_PATH = path.join(BIN_DIR, process.platform === "win32" ? "supermodel.exe" : "supermodel");
 
-const VERSION = require("./package.json").version;
-
 const PLATFORM_MAP = {
   darwin: "Darwin",
   linux: "Linux",
@@ -30,21 +29,6 @@ function fail(msg) {
   process.exit(1);
 }
 
-const platform = PLATFORM_MAP[process.platform];
-const arch = ARCH_MAP[os.arch()];
-
-if (!platform) fail(`Unsupported platform: ${process.platform}`);
-if (!arch) fail(`Unsupported architecture: ${os.arch()}`);
-
-const ext = process.platform === "win32" ? "zip" : "tar.gz";
-const archive = `supermodel_${platform}_${arch}.${ext}`;
-const tag = `v${VERSION}`;
-const url = `https://github.com/${REPO}/releases/download/${tag}/${archive}`;
-
-console.log(`[supermodel] Downloading ${archive} from GitHub Releases...`);
-
-fs.mkdirSync(BIN_DIR, { recursive: true });
-
 function download(url, dest, cb) {
   const file = fs.createWriteStream(dest);
   https.get(url, (res) => {
@@ -59,22 +43,55 @@ function download(url, dest, cb) {
   }).on("error", (err) => fail(err.message));
 }
 
-const tmpArchive = path.join(os.tmpdir(), archive);
-
-download(url, tmpArchive, () => {
-  if (ext === "tar.gz") {
-    execSync(`tar -xzf "${tmpArchive}" -C "${BIN_DIR}" supermodel`);
-  } else {
-    // Windows: Expand-Archive extracts all files, so extract to a temporary
-    // directory and copy only the binary.
-    const tmpDir = fs.mkdtempSync(path.join(os.tmpdir(), "supermodel-extract-"));
-    execSync(
-      `powershell -NoProfile -Command "Expand-Archive -Force -Path '${tmpArchive}' -DestinationPath '${tmpDir}'"`,
-    );
-    fs.copyFileSync(path.join(tmpDir, "supermodel.exe"), BIN_PATH);
-    fs.rmSync(tmpDir, { recursive: true, force: true });
+// extractZip extracts a .zip archive into tmpDir.
+// Tries native tar first (Windows 10+); falls back to PowerShell Expand-Archive
+// with a retry loop to handle transient Antivirus file locks.
+// Accepts an optional execFn for testing (defaults to execSync).
+function extractZip(archive, tmpDir, execFn) {
+  const exec = execFn || execSync;
+  try {
+    exec(`tar -xf "${archive}" -C "${tmpDir}"`);
+  } catch {
+    const psCommand =
+      `$RetryCount = 0; while ($RetryCount -lt 10) { try { Expand-Archive` +
+      ` -Force -Path '${archive}' -DestinationPath '${tmpDir}'; break }` +
+      ` catch { Start-Sleep -Seconds 1; $RetryCount++ } }`;
+    exec(`powershell -NoProfile -Command "${psCommand}"`);
   }
-  if (process.platform !== "win32") fs.chmodSync(BIN_PATH, 0o755);
-  fs.unlinkSync(tmpArchive);
-  console.log(`[supermodel] Installed to ${BIN_PATH}`);
-});
+}
+
+if (require.main === module) {
+  const platform = PLATFORM_MAP[process.platform];
+  const arch = ARCH_MAP[os.arch()];
+
+  if (!platform) fail(`Unsupported platform: ${process.platform}`);
+  if (!arch) fail(`Unsupported architecture: ${os.arch()}`);
+
+  const ext = process.platform === "win32" ? "zip" : "tar.gz";
+  const archive = `supermodel_${platform}_${arch}.${ext}`;
+  const tag = `v${require("./package.json").version}`;
+  const url = `https://github.com/${REPO}/releases/download/${tag}/${archive}`;
+  const tmpArchive = path.join(os.tmpdir(), archive);
+
+  console.log(`[supermodel] Downloading ${archive} from GitHub Releases...`);
+  fs.mkdirSync(BIN_DIR, { recursive: true });
+
+  download(url, tmpArchive, () => {
+    if (ext === "tar.gz") {
+      execSync(`tar -xzf "${tmpArchive}" -C "${BIN_DIR}" supermodel`);
+    } else {
+      const tmpDir = fs.mkdtempSync(path.join(os.tmpdir(), "supermodel-extract-"));
+      try {
+        extractZip(tmpArchive, tmpDir);
+        fs.copyFileSync(path.join(tmpDir, "supermodel.exe"), BIN_PATH);
+      } finally {
+        fs.rmSync(tmpDir, { recursive: true, force: true });
+      }
+    }
+    if (process.platform !== "win32") fs.chmodSync(BIN_PATH, 0o755);
+    fs.unlinkSync(tmpArchive);
+    console.log(`[supermodel] Installed to ${BIN_PATH}`);
+  });
+}
+
+module.exports = { extractZip };
diff --git a/npm/install.test.js b/npm/install.test.js
new file mode 100644
index 0000000..6d29a02
--- /dev/null
+++ b/npm/install.test.js
@@ -0,0 +1,133 @@
+// Tests for the Windows zip extraction logic in install.js.
+// Uses Node's built-in test runner (node:test, available since Node 18).
+
+"use strict";
+
+const { test } = require("node:test");
+const assert = require("node:assert/strict");
+const fs = require("fs");
+const os = require("os");
+const path = require("path");
+const { execSync } = require("child_process");
+const { extractZip } = require("./install");
+
+// createTestZip builds a real .zip containing a single file named "supermodel.exe"
+// using the system zip/tar command. Skips on platforms where neither is available.
+function createTestZip(t) {
+  const src = fs.mkdtempSync(path.join(os.tmpdir(), "supermodel-test-src-"));
+  const binary = path.join(src, "supermodel.exe");
+  fs.writeFileSync(binary, "fake binary");
+
+  const archive = path.join(os.tmpdir(), `supermodel-test-${process.pid}.zip`);
+  try {
+    // Use system zip or tar to build the archive.
+    try {
+      execSync(`zip -j "${archive}" "${binary}"`, { stdio: "pipe" });
+    } catch {
+      execSync(`tar -cf "${archive}" --format=zip -C "${src}" supermodel.exe`, { stdio: "pipe" });
+    }
+  } catch {
+    fs.rmSync(src, { recursive: true, force: true });
+    return null; // zip tooling not available — caller should skip
+  }
+  fs.rmSync(src, { recursive: true, force: true });
+  return archive;
+}
+
+test("extractZip extracts via tar when tar succeeds", () => {
+  const archive = createTestZip();
+  if (!archive) {
+    // Skip gracefully if zip tooling unavailable (e.g. minimal CI image).
+    console.log("  skipped: zip tooling not available");
+    return;
+  }
+  try {
+    const tmpDir = fs.mkdtempSync(path.join(os.tmpdir(), "supermodel-test-out-"));
+    try {
+      let called = null;
+      extractZip(archive, tmpDir, (cmd) => {
+        called = cmd;
+        // Only simulate tar; let the actual extraction happen via real execSync
+        // if this is the tar command.
+        if (cmd.startsWith("tar")) {
+          execSync(cmd, { stdio: "pipe" });
+        } else {
+          throw new Error("should not reach PowerShell");
+        }
+      });
+      assert.ok(called.startsWith("tar"), "should have called tar first");
+      const extracted = fs.readdirSync(tmpDir);
+      assert.ok(extracted.length > 0, "tmpDir should contain extracted files");
+    } finally {
+      fs.rmSync(tmpDir, { recursive: true, force: true });
+    }
+  } finally {
+    fs.unlinkSync(archive);
+  }
+});
+
+test("extractZip falls back to PowerShell when tar fails", () => {
+  const commands = [];
+  // Simulate tar failing; PowerShell "succeeds" (no-op).
+  extractZip("/fake/archive.zip", "/fake/tmpdir", (cmd) => {
+    commands.push(cmd);
+    if (cmd.startsWith("tar")) throw new Error("tar not available");
+    // PowerShell call — just record it, don't execute.
+  });
+
+  assert.equal(commands.length, 2, "should have attempted tar then PowerShell");
+  assert.ok(commands[0].startsWith("tar"), "first call should be tar");
+  assert.ok(commands[1].includes("powershell"), "second call should be PowerShell");
+  assert.ok(commands[1].includes("Expand-Archive"), "PowerShell command should use Expand-Archive");
+});
+
+test("extractZip PowerShell fallback includes retry loop", () => {
+  const commands = [];
+  extractZip("/fake/archive.zip", "/fake/tmpdir", (cmd) => {
+    commands.push(cmd);
+    if (cmd.startsWith("tar")) throw new Error("tar not available");
+  });
+
+  const psCmd = commands.find((c) => c.includes("powershell"));
+  assert.ok(psCmd, "PowerShell command should be present");
+  assert.ok(psCmd.includes("$RetryCount"), "should include retry counter");
+  assert.ok(psCmd.includes("Start-Sleep"), "should include sleep between retries");
+  assert.ok(psCmd.includes("-lt 10"), "should retry up to 10 times");
+});
+
+test("extractZip uses tar when both succeed — tar wins", () => {
+  const commands = [];
+  extractZip("/fake/archive.zip", "/fake/tmpdir", (cmd) => {
+    commands.push(cmd);
+    // Both would succeed; tar is tried first and doesn't throw.
+  });
+
+  assert.equal(commands.length, 1, "should only call tar when it succeeds");
+  assert.ok(commands[0].startsWith("tar"), "the single call should be tar");
+});
+
+test("extractZip passes archive and tmpDir paths into tar command", () => {
+  const archive = "/tmp/test.zip";
+  const tmpDir = "/tmp/extract-dir";
+  let tarCmd = null;
+  extractZip(archive, tmpDir, (cmd) => {
+    tarCmd = cmd;
+  });
+
+  assert.ok(tarCmd.includes(archive), "tar command should include archive path");
+  assert.ok(tarCmd.includes(tmpDir), "tar command should include tmpDir path");
+});
+
+test("extractZip passes archive and tmpDir paths into PowerShell fallback", () => {
+  const archive = "/tmp/test.zip";
+  const tmpDir = "/tmp/extract-dir";
+  const commands = [];
+  extractZip(archive, tmpDir, (cmd) => {
+    commands.push(cmd);
+    if (cmd.startsWith("tar")) throw new Error("tar failed");
+  });
+
+  const psCmd = commands[1];
+  assert.ok(psCmd.includes(archive), "PowerShell command should include archive path");
+  assert.ok(psCmd.includes(tmpDir), "PowerShell command should include tmpDir path");
+});
diff --git a/npm/package.json b/npm/package.json
index 6855d26..0734291 100644
--- a/npm/package.json
+++ b/npm/package.json
@@ -12,7 +12,8 @@
     "supermodel": "./bin.js"
   },
   "scripts": {
-    "postinstall": "node install.js"
+    "postinstall": "node install.js",
+    "test": "node --test install.test.js"
   },
   "files": [
     "bin.js",