diff --git a/CHANGELOG.md b/CHANGELOG.md
index bbb88cad..22426071 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -83,6 +83,11 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
   - Skips interactions.jsonl and molecules.jsonl in sync checks
   - These files are runtime state, not sync targets
 
+- **Windows npm postinstall file locking** (GH#670) - Windows install fix
+  - Fixed file handle not being released before extraction on Windows
+  - Moved write stream creation after redirect handling to avoid orphan streams
+  - Added delay after file close to ensure Windows releases file handle
+
 - **FatalErrorRespectJSON** (bd-28sq) - Consistent error output
   - All commands respect `--json` flag for error output
   - Errors return proper JSON structure when flag is set
diff --git a/npm-package/scripts/postinstall.js b/npm-package/scripts/postinstall.js
index 7e24059e..1403834e 100755
--- a/npm-package/scripts/postinstall.js
+++ b/npm-package/scripts/postinstall.js
@@ -50,14 +50,18 @@ function getPlatformInfo() {
   return { platformName, archName, binaryName };
 }
 
+// Small delay helper for Windows file handle release
+function delay(ms) {
+  return new Promise(resolve => setTimeout(resolve, ms));
+}
+
 // Download file from URL
 function downloadFile(url, dest) {
   return new Promise((resolve, reject) => {
     console.log(`Downloading from: ${url}`);
-    const file = fs.createWriteStream(dest);
 
     const request = https.get(url, (response) => {
-      // Handle redirects
+      // Handle redirects - must happen BEFORE creating write stream
       if (response.statusCode === 301 || response.statusCode === 302) {
         const redirectUrl = response.headers.location;
         console.log(`Following redirect to: ${redirectUrl}`);
@@ -70,27 +74,37 @@ function downloadFile(url, dest) {
         return;
       }
 
+      // Only create write stream after we know we have the final URL
+      const file = fs.createWriteStream(dest);
+
       response.pipe(file);
 
       file.on('finish', () => {
         // Wait for file.close() to complete before resolving
         // This is critical on Windows where the file may still be locked
-        file.close((err) => {
-          if (err) reject(err);
-          else resolve();
+        file.close(async (err) => {
+          if (err) {
+            reject(err);
+            return;
+          }
+          // On Windows, add a small delay to ensure file handle is fully released
+          if (os.platform() === 'win32') {
+            await delay(100);
+          }
+          resolve();
         });
       });
+
+      file.on('error', (err) => {
+        fs.unlink(dest, () => {});
+        reject(err);
+      });
     });
 
     request.on('error', (err) => {
       fs.unlink(dest, () => {});
       reject(err);
     });
-
-    file.on('error', (err) => {
-      fs.unlink(dest, () => {});
-      reject(err);
-    });
   });
 }