From b054b7c21a5a357309a98c8cc6c6cf5620243810 Mon Sep 17 00:00:00 2001
From: Charlie Tonneslan <cst0520@gmail.com>
Date: Wed, 18 Mar 2026 11:23:40 -0400
Subject: [PATCH] fix(filesystem): handle Windows EPERM on file rename when
 target is locked

On Windows, fs.rename() throws EPERM when the target file is locked
by another process (e.g., open in VS Code or Notepad). This breaks
both write_file and edit_file operations.

Added an atomicReplace() helper that falls back to copyFile + unlink
on Windows when rename fails with EPERM. This preserves the atomic
write semantics on Linux/macOS while working around the Windows
file locking limitation.

Fixes #3199
---
 src/filesystem/lib.ts | 22 ++++++++++++++++++++--
 1 file changed, 20 insertions(+), 2 deletions(-)

diff --git a/src/filesystem/lib.ts b/src/filesystem/lib.ts
index 17e4654cd5..d6cfd74b52 100644
--- a/src/filesystem/lib.ts
+++ b/src/filesystem/lib.ts
@@ -7,6 +7,24 @@ import { minimatch } from 'minimatch';
 import { normalizePath, expandHome } from './path-utils.js';
 import { isPathWithinAllowedDirectories } from './path-validation.js';
 
+/**
+ * Atomically replace a file using rename, with a Windows fallback.
+ * On Windows, fs.rename() throws EPERM when the target file is locked
+ * (e.g., open in an editor). Fall back to copyFile + unlink in that case.
+ */
+async function atomicReplace(tempPath: string, targetPath: string): Promise<void> {
+  try {
+    await fs.rename(tempPath, targetPath);
+  } catch (error) {
+    if ((error as NodeJS.ErrnoException).code === 'EPERM' && process.platform === 'win32') {
+      await fs.copyFile(tempPath, targetPath);
+      await fs.unlink(tempPath);
+    } else {
+      throw error;
+    }
+  }
+}
+
 // Global allowed directories - set by the main module
 let allowedDirectories: string[] = [];
 
@@ -171,7 +189,7 @@ export async function writeFileContent(filePath: string, content: string): Promi
       const tempPath = `${filePath}.${randomBytes(16).toString('hex')}.tmp`;
       try {
         await fs.writeFile(tempPath, content, 'utf-8');
-        await fs.rename(tempPath, filePath);
+        await atomicReplace(tempPath, filePath);
       } catch (renameError) {
         try {
           await fs.unlink(tempPath);
@@ -269,7 +287,7 @@ export async function applyFileEdits(
     const tempPath = `${filePath}.${randomBytes(16).toString('hex')}.tmp`;
     try {
       await fs.writeFile(tempPath, modifiedContent, 'utf-8');
-      await fs.rename(tempPath, filePath);
+      await atomicReplace(tempPath, filePath);
     } catch (error) {
       try {
         await fs.unlink(tempPath);