diff --git a/package.json b/package.json
index 1d3396fd49..7d1528c169 100644
--- a/package.json
+++ b/package.json
@@ -13,7 +13,7 @@
     "check:fix": "biome check --write",
     "check-types": "turbo run check-types",
     "test": "turbo run test",
-    "skills:validate": "bun scripts/validate-skills.ts && bun test scripts/clawhub-ignore-policy.test.ts scripts/claude-mcp-config-policy.test.ts scripts/openai-tool-annotation-policy.test.ts scripts/public-skill-discovery-policy.test.ts",
+    "skills:validate": "bun scripts/validate-skills.ts && bun test scripts/clawhub-ignore-policy.test.ts scripts/claude-mcp-config-policy.test.ts scripts/openai-tool-annotation-policy.test.ts scripts/path-containment.test.ts scripts/public-skill-discovery-policy.test.ts",
     "kill": "lsof -ti:3002 | xargs kill -9 2>/dev/null || echo 'No processes found on port 3002'",
     "clean:cache": "rm -rf apps/*/.next apps/*/.swc apps/*/.turbo packages/*/.turbo tooling/*/.turbo .turbo node_modules/.cache",
     "restart": "bun kill && bun clean:cache && bun dev",
diff --git a/scripts/path-containment.test.ts b/scripts/path-containment.test.ts
new file mode 100644
index 0000000000..a4b26354a7
--- /dev/null
+++ b/scripts/path-containment.test.ts
@@ -0,0 +1,26 @@
+import { describe, expect, test } from 'bun:test'
+import { isPathInside } from './path-containment'
+
+describe('isPathInside', () => {
+  test.each([
+    ['windows separators', 'C:\\repo\\skills\\pascal-3d', 'C:\\repo\\skills\\pascal-3d\\a.md'],
+    ['posix separators', '/repo/skills/pascal-3d', '/repo/skills/pascal-3d/a.md'],
+    ['mixed separators', 'C:\\repo\\skills\\pascal-3d', 'C:/repo/skills/pascal-3d/a.md'],
+    ['nested directory', '/repo/skills/pascal-3d', '/repo/skills/pascal-3d/examples/a.md'],
+    ['drive root', 'C:\\', 'C:\\a.md'],
+    ['trailing separator on the parent', '/repo/skills/', '/repo/skills/a.md'],
+  ])('accepts a target inside the parent with %s', (_label, parent, target) => {
+    expect(isPathInside(parent, target)).toBe(true)
+  })
+
+  test.each([
+    ['a sibling sharing the name prefix', '/repo/skills', '/repo/skills-extra/a.md'],
+    ['a parent directory', '/repo/skills/pascal-3d', '/repo/skills/a.md'],
+    ['an unrelated directory', '/repo/skills', '/repo/other/a.md'],
+    ['the parent itself', '/repo/skills', '/repo/skills'],
+    ['a traversal out of the parent', '/repo/skills', '/repo/other/../skills-evil/a.md'],
+    ['a different drive', 'C:\\repo\\skills', 'D:\\repo\\skills\\a.md'],
+  ])('rejects %s', (_label, parent, target) => {
+    expect(isPathInside(parent, target)).toBe(false)
+  })
+})
diff --git a/scripts/path-containment.ts b/scripts/path-containment.ts
new file mode 100644
index 0000000000..f2afe9c8cb
--- /dev/null
+++ b/scripts/path-containment.ts
@@ -0,0 +1,16 @@
+/**
+ * Containment check for paths produced by `resolve()` / `join()`.
+ *
+ * A `${parentDir}/` string prefix is not portable: `resolve()` returns
+ * backslash-separated paths on Windows, so the prefix never matches there and
+ * every file that is genuinely inside the directory is reported as outside.
+ * Comparing on a normalized separator keeps the check identical on every
+ * platform.
+ */
+function normalizeSeparators(path: string): string {
+  return path.replace(/\\/g, '/').replace(/\/+$/, '')
+}
+
+export function isPathInside(parentDir: string, target: string): boolean {
+  return normalizeSeparators(target).startsWith(`${normalizeSeparators(parentDir)}/`)
+}
diff --git a/scripts/validate-skills.ts b/scripts/validate-skills.ts
index 88b8516dba..aafef47fd5 100644
--- a/scripts/validate-skills.ts
+++ b/scripts/validate-skills.ts
@@ -5,6 +5,7 @@ import { XMLParser, XMLValidator } from 'fast-xml-parser'
 import { validateClaudeMcpPolicy } from './claude-mcp-config-policy'
 import { validateClawHubIgnorePolicy } from './clawhub-ignore-policy'
 import { validateOpenAiToolAnnotationPacket } from './openai-tool-annotation-policy'
+import { isPathInside } from './path-containment'
 import {
   collectSkillDiscoveryEntries,
   validatePublicSkillDiscoverySurface,
@@ -384,7 +385,7 @@ for (const skillName of skillNames) {
         const target = match[1]!
         if (/^(?:https?:|mailto:|#)/.test(target)) continue
         const resolvedTarget = resolve(dirname(path), target.split('#')[0]!)
-        if (!resolvedTarget.startsWith(`${skillRoot}/`)) {
+        if (!isPathInside(skillRoot, resolvedTarget)) {
           fail(`${relative(root, path)} links outside its standalone skill bundle: ${target}`)
         }
       }
@@ -1079,7 +1080,7 @@ for (const field of ['composerIcon', 'logo']) {
     continue
   }
   const asset = resolve(root, value)
-  if (!asset.startsWith(`${root}/`) || !existsSync(asset) || !lstatSync(asset).isFile()) {
+  if (!isPathInside(root, asset) || !existsSync(asset) || !lstatSync(asset).isFile()) {
     fail(`OpenAI ${field} must reference an existing file inside the plugin`)
     continue
   }