diff --git a/src/utils/permissions/__tests__/pathValidation.test.ts b/src/utils/permissions/__tests__/pathValidation.test.ts
new file mode 100644
index 0000000000..7c9716cae0
--- /dev/null
+++ b/src/utils/permissions/__tests__/pathValidation.test.ts
@@ -0,0 +1,203 @@
+import { describe, expect, mock, test } from 'bun:test'
+import { logMock } from '../../../../tests/mocks/log'
+import { debugMock } from '../../../../tests/mocks/debug'
+
+// Cut the bootstrap/state dependency chain (mock.module requirement).
+mock.module('src/utils/log.ts', logMock)
+mock.module('src/utils/debug.ts', debugMock)
+mock.module('bun:bundle', () => ({
+  feature: (_name: string) => false,
+}))
+
+// MACRO is a build-time define injected by `bun --define` (see
+// scripts/dev.ts → -d flags). Without it, `declare const MACRO` references
+// in source code resolve to `undefined` at runtime and crash any function
+// that touches `MACRO.VERSION` (e.g. `getBundledSkillsRoot` via
+// `checkReadableInternalPath`).
+// Setting it on globalThis lets the bare `MACRO` identifier resolve at
+// runtime in tests.
+;(globalThis as unknown as { MACRO: { VERSION: string } }).MACRO = {
+  VERSION: 'test',
+}
+
+const { validatePath } = await import('../pathValidation.js')
+const { getEmptyToolPermissionContext } = await import('../../../Tool.js')
+
+function makeContext(): ReturnType<typeof getEmptyToolPermissionContext> {
+  return getEmptyToolPermissionContext()
+}
+
+const isWindows = process.platform === 'win32'
+const describeIfWindows = isWindows ? describe : describe.skip
+
+// ─── MinGW path normalization (Windows) ──────────────────────────────────
+//
+// These tests pin the fix for a sandbox-escape class: on Windows, the Git
+// Bash shell interprets paths like `/c/Users/foo/bar.txt` as `C:\Users\foo\bar.txt`
+// (the C: drive). However, the Node `path` module treats such paths as
+// drive-relative absolute paths on the current drive, so:
+//   - path.isAbsolute('/c/Users/foo/bar.txt') === true   (on Windows)
+//   - path.resolve('D:\\project', '/c/Users/foo/bar.txt')
+//       === 'D:\\c\\Users\\foo\\bar.txt'                  (on Windows)
+//
+// That means without normalization, validatePath would compare
+// `D:\c\Users\foo\bar.txt` against the allowed-directories list, while
+// Git Bash actually writes to `C:\Users\foo\bar.txt` — a completely
+// different filesystem location. This is a TOCTOU/sandbox-escape bug.
+//
+// The fix runs `posixPathToWindowsPath` on Windows before resolution,
+// converting `/c/...` and `/cygdrive/c/...` to their `C:\...` form so the
+// validator's path space matches the shell's.
+/**
+ * Tests that `validatePath` normalizes MinGW-style absolute paths
+ * (`/c/Users/foo`, `/cygdrive/c/Users/foo`) to Windows paths
+ * (`C:\\Users\\foo`) on Windows. Without this, the validator runs in
+ * Windows host path space while the Git Bash shell runs in MinGW path
+ * space, leading to a sandbox-escape class — see the comment block
+ * at the top of this file for the full security rationale.
+ */
+describeIfWindows('validatePath MinGW path normalization', () => {
+  test('converts /c/Users/foo/file.txt to C:\\Users\\foo\\file.txt', () => {
+    const result = validatePath(
+      '/c/Users/foo/file.txt',
+      'D:\\project',
+      makeContext(),
+      'read',
+    )
+    // resolvedPath is the canonical form the validator (and ultimately the
+    // shell) operates on. It must be the Windows-style path, not the
+    // drive-relative form `D:\c\Users\foo\file.txt`.
+    expect(result.resolvedPath.replace(/\//g, '\\')).toBe(
+      'C:\\Users\\foo\\file.txt',
+    )
+  })
+
+  test('converts /cygdrive/c/Users/foo/file.txt to C:\\Users\\foo\\file.txt', () => {
+    const result = validatePath(
+      '/cygdrive/c/Users/foo/file.txt',
+      'D:\\project',
+      makeContext(),
+      'read',
+    )
+    expect(result.resolvedPath.replace(/\//g, '\\')).toBe(
+      'C:\\Users\\foo\\file.txt',
+    )
+  })
+
+  test('uppercases the drive letter', () => {
+    const result = validatePath(
+      '/d/work/file.txt',
+      'C:\\project',
+      makeContext(),
+      'read',
+    )
+    expect(result.resolvedPath.replace(/\//g, '\\')).toBe('D:\\work\\file.txt')
+  })
+
+  test('preserves Windows paths unchanged', () => {
+    // An already-Windows path should not be touched.
+    const result = validatePath(
+      'C:\\Users\\foo\\file.txt',
+      'D:\\project',
+      makeContext(),
+      'read',
+    )
+    expect(result.resolvedPath.replace(/\//g, '\\')).toBe(
+      'C:\\Users\\foo\\file.txt',
+    )
+  })
+
+  test('preserves relative paths (just flips slashes)', () => {
+    // Relative paths are not MinGW absolute paths; the conversion
+    // should be a no-op aside from slash direction. The path is then
+    // resolved against cwd by `validatePath`, which is expected behavior.
+    const result = validatePath(
+      'src/file.txt',
+      'D:\\project',
+      makeContext(),
+      'read',
+    )
+    expect(result.resolvedPath.replace(/\//g, '\\')).toBe(
+      'D:\\project\\src\\file.txt',
+    )
+  })
+
+  test('handles bare drive mount (no trailing path)', () => {
+    const result = validatePath('/c', 'D:\\project', makeContext(), 'read')
+    expect(result.resolvedPath.replace(/\//g, '\\')).toBe('C:\\')
+  })
+
+  test('handles drive root with trailing slash', () => {
+    const result = validatePath('/c/', 'D:\\project', makeContext(), 'read')
+    expect(result.resolvedPath.replace(/\//g, '\\')).toBe('C:\\')
+  })
+
+  test('handles deeply nested MinGW paths', () => {
+    const result = validatePath(
+      '/c/Users/me/Documents/project/src/index.ts',
+      'D:\\project',
+      makeContext(),
+      'read',
+    )
+    expect(result.resolvedPath.replace(/\//g, '\\')).toBe(
+      'C:\\Users\\me\\Documents\\project\\src\\index.ts',
+    )
+  })
+})
+
+// ─── Sandbox escape regression (Windows) ─────────────────────────────────
+//
+// This is the bug the MinGW-normalization fix exists to prevent: without
+// it, the validator compares `<currentDrive>:\c\Users\foo\file.txt` against
+// the allowed dirs, while bash writes to `C:\Users\foo\file.txt`. With the
+// fix, both sides of the comparison use the same `C:\Users\foo\file.txt`
+// location.
+//
+// We pin this by setting up a context where:
+//   - cwd is `D:\project` (and D:\project is allowed)
+//   - `C:\Users\foo` is NOT in any allowed directory
+// Then we check that `/c/Users/foo/sensitive.txt` is denied with a
+// resolvedPath pointing at C:\Users\foo — proving the validator now sees
+// the same path the shell will write to.
+/**
+ * Regression tests for the sandbox-escape class the MinGW-normalization
+ * fix prevents. Without the fix, a MinGW-style path like
+ * `/c/Users/foo/sensitive.txt` is resolved (by `path.resolve`) against
+ * the current drive (`D:\c\Users\foo\sensitive.txt`) and compared to
+ * the allowed-directories list — while Git Bash actually writes to
+ * `C:\Users\foo\sensitive.txt`. With the fix, both sides of the
+ * comparison use the same Windows path so a path the shell will write
+ * to but isn't in any allowed dir is denied.
+ */
+describeIfWindows('validatePath sandbox escape regression', () => {
+  test('MinGW path that escapes allowed dirs is denied at correct location', () => {
+    // Without the fix, this would resolve to `D:\c\Users\foo\sensitive.txt`
+    // and (if D:\ is broadly allowed) pass validation, while bash actually
+    // writes to `C:\Users\foo\sensitive.txt`. With the fix, the validator
+    // sees the correct path and denies it because C:\Users\foo is not in
+    // any allowed directory.
+    const result = validatePath(
+      '/c/Users/foo/sensitive.txt',
+      'D:\\project',
+      makeContext(),
+      'create',
+    )
+    expect(result.allowed).toBe(false)
+    // The resolvedPath should be at C:\Users\foo — not D:\c\Users\foo.
+    const normalized = result.resolvedPath.replace(/\//g, '\\')
+    expect(normalized.startsWith('C:\\Users\\foo')).toBe(true)
+    expect(normalized.startsWith('D:\\c\\')).toBe(false)
+  })
+
+  test('cygdrive path that escapes allowed dirs is denied at correct location', () => {
+    const result = validatePath(
+      '/cygdrive/c/Users/foo/sensitive.txt',
+      'D:\\project',
+      makeContext(),
+      'create',
+    )
+    expect(result.allowed).toBe(false)
+    const normalized = result.resolvedPath.replace(/\//g, '\\')
+    expect(normalized.startsWith('C:\\Users\\foo')).toBe(true)
+  })
+})
diff --git a/src/utils/permissions/pathValidation.ts b/src/utils/permissions/pathValidation.ts
index c8fdf7f8fb..76cf83b651 100644
--- a/src/utils/permissions/pathValidation.ts
+++ b/src/utils/permissions/pathValidation.ts
@@ -3,6 +3,7 @@ import { homedir } from 'os'
 import { dirname, isAbsolute, resolve } from 'path'
 import type { ToolPermissionContext } from '../../Tool.js'
 import { getPlatform } from '../../utils/platform.js'
+import { posixPathToWindowsPath } from '../windowsPaths.js'
 import {
   getFsImplementation,
   getPathsForPermissionCheck,
@@ -35,6 +36,12 @@ export type ResolvedPathCheckResult = PathCheckResult & {
   resolvedPath: string
 }
 
+/**
+ * Format a list of working directories for inclusion in permission-prompt
+ * messages. When the list is short (≤ {@link MAX_DIRS_TO_LIST}) all
+ * directories are listed; when longer, a truncated form is shown with
+ * the remaining count.
+ */
 export function formatDirectoryList(directories: string[]): string {
   const dirCount = directories.length
 
@@ -472,9 +479,27 @@ export function validatePath(
   }
 
   // Resolve path
-  const absolutePath = isAbsolute(cleanPath)
-    ? cleanPath
-    : resolve(cwd, cleanPath)
+  // SECURITY: On Windows, normalize MinGW-style absolute paths
+  // (`/c/Users/foo`, `/cygdrive/c/Users/foo`) to Windows paths
+  // (`C:\Users\foo`) BEFORE the isAbsolute/resolve step below.
+  //
+  // Without this, `path.isAbsolute('/c/Users/foo')` returns true on
+  // Windows (interpreted as a drive-relative absolute path) and
+  // `path.resolve(cwd, '/c/Users/foo')` produces `<currentDrive>:\c\Users\foo`
+  // — a completely different filesystem location from what Git Bash
+  // actually writes to (`C:\Users\foo`). The validator would compare
+  // the wrong path against the allowed-directories list, enabling a
+  // sandbox escape where `/c/Users/foo/sensitive.txt` passes validation
+  // when `C:\Users\foo\sensitive.txt` is not in any allowed dir.
+  //
+  // `posixPathToWindowsPath` is a no-op for already-Windows paths and
+  // for relative paths (it just flips slashes), so it's safe to apply
+  // unconditionally on Windows.
+  const pathForResolve =
+    getPlatform() === 'windows' ? posixPathToWindowsPath(cleanPath) : cleanPath
+  const absolutePath = isAbsolute(pathForResolve)
+    ? pathForResolve
+    : resolve(cwd, pathForResolve)
   const { resolvedPath, isCanonical } = safeResolvePath(
     getFsImplementation(),
     absolutePath,