diff --git a/packages/client/ui-primitives/src/FileTypeIcon.tsx b/packages/client/ui-primitives/src/FileTypeIcon.tsx
index 1fbcee6750..fc2e3c2667 100644
--- a/packages/client/ui-primitives/src/FileTypeIcon.tsx
+++ b/packages/client/ui-primitives/src/FileTypeIcon.tsx
@@ -44,7 +44,9 @@ export type FileTypeIconProps = IconProps & (
   }
 )
 
-const EXTENSION_TYPES: Readonly<Record<string, ClassifiedFileType>> = {
+// Maps, not objects: a basename or extension equal to an `Object.prototype` key
+// (`constructor`, `__proto__`) must miss instead of resolving the inherited member.
+const EXTENSION_TYPES: ReadonlyMap<string, ClassifiedFileType> = new Map(Object.entries({
   scss: 'code',
   sass: 'code',
   less: 'code',
@@ -101,13 +103,13 @@ const EXTENSION_TYPES: Readonly<Record<string, ClassifiedFileType>> = {
   ots: 'excel',
   fods: 'excel',
   numbers: 'excel',
-}
+}))
 
-const NAME_TYPES: Readonly<Record<string, ClassifiedFileType>> = {
+const NAME_TYPES: ReadonlyMap<string, ClassifiedFileType> = new Map(Object.entries({
   changelog: 'markdown',
   contributing: 'markdown',
   readme: 'markdown',
-}
+}))
 
 function basename(path: string): string {
   return path.slice(Math.max(path.lastIndexOf('/'), path.lastIndexOf('\\')) + 1)
@@ -140,8 +142,8 @@ export function classifyFileType(
   const name = basename(path).toLowerCase()
   const extension = fileExtension(name).toLowerCase()
   return classifyCodeFileType(name, extension, context)
-    ?? NAME_TYPES[name]
-    ?? EXTENSION_TYPES[extension]
+    ?? NAME_TYPES.get(name)
+    ?? EXTENSION_TYPES.get(extension)
     ?? 'other'
 }
 
diff --git a/packages/client/ui-primitives/src/code-file-types.ts b/packages/client/ui-primitives/src/code-file-types.ts
index 9b5ce75459..5bb1a5a628 100644
--- a/packages/client/ui-primitives/src/code-file-types.ts
+++ b/packages/client/ui-primitives/src/code-file-types.ts
@@ -61,7 +61,9 @@ export interface FileTypeProjectContext {
 
 const CODE_FILE_TYPE_SET = new Set<string>(CODE_FILE_TYPES)
 
-const FILE_NAME_TYPES: Readonly<Record<string, CodeFileType>> = {
+// Maps, not objects: a basename or extension equal to an `Object.prototype` key
+// (`constructor`, `__proto__`) must miss instead of resolving the inherited member.
+const FILE_NAME_TYPES: ReadonlyMap<string, CodeFileType> = new Map(Object.entries({
   '.bash_profile': 'shell',
   '.bashrc': 'shell',
   '.env': 'env',
@@ -90,7 +92,7 @@ const FILE_NAME_TYPES: Readonly<Record<string, CodeFileType>> = {
   'package.json': 'node',
   'podfile': 'ruby',
   'rakefile': 'ruby',
-}
+}))
 
 const FILE_NAME_PREFIX_TYPES: readonly (readonly [string, CodeFileType])[] = [
   ['dockerfile.', 'docker'],
@@ -109,7 +111,7 @@ const FILE_NAME_SUFFIX_TYPES: readonly (readonly [string, CodeFileType])[] = [
   ['.dockerfile', 'docker'],
 ]
 
-const EXTENSION_TYPES: Readonly<Record<string, CodeFileType>> = {
+const EXTENSION_TYPES: ReadonlyMap<string, CodeFileType> = new Map(Object.entries({
   'bash': 'shell',
   'c': 'c',
   'c++': 'cpp',
@@ -217,7 +219,7 @@ const EXTENSION_TYPES: Readonly<Record<string, CodeFileType>> = {
   'yml': 'yaml',
   'zig': 'zig',
   'zsh': 'shell',
-}
+}))
 
 const LINK_CODE_EXTENSIONS = new Set([
   'ts',
@@ -322,7 +324,7 @@ export function classifyCodeFileType(
   extension: string,
   context?: FileTypeProjectContext,
 ): CodeFileType | null {
-  const exact = FILE_NAME_TYPES[name]
+  const exact = FILE_NAME_TYPES.get(name)
   if (exact !== undefined) return exact
   for (const [prefix, type] of FILE_NAME_PREFIX_TYPES) {
     if (name.startsWith(prefix)) return type
@@ -334,5 +336,5 @@ export function classifyCodeFileType(
     const pubspec = Object.entries(context.files).find(([path]) => basename(path).toLowerCase() === 'pubspec.yaml')?.[1]
     if (pubspec?.includes('flutter:') === true) return 'flutter'
   }
-  return EXTENSION_TYPES[extension] ?? null
+  return EXTENSION_TYPES.get(extension) ?? null
 }
diff --git a/packages/client/ui-primitives/tests/file-type-icon.client.spec.tsx b/packages/client/ui-primitives/tests/file-type-icon.client.spec.tsx
index b39b22f0f2..e9c8367d59 100644
--- a/packages/client/ui-primitives/tests/file-type-icon.client.spec.tsx
+++ b/packages/client/ui-primitives/tests/file-type-icon.client.spec.tsx
@@ -4,6 +4,7 @@ import { afterEach, describe, expect, it } from 'vitest'
 import {
   classifyFileType, fileExtension, FileTypeIcon, type FileTypeKind,
 } from '@deepseek-ai/dsh-client-ui-primitives'
+import { classifyCodeFileType } from '../src/code-file-types.ts'
 import css from '../src/FileTypeIcon.module.css'
 
 afterEach(cleanup)
@@ -58,6 +59,30 @@ describe('classifyFileType', () => {
   })
 })
 
+describe('Object.prototype key names', () => {
+  const keys = ['constructor', '__proto__', 'toString', 'hasOwnProperty']
+
+  it.each(keys.flatMap(key => [
+    [key, 'other'],
+    [`src/${key.toUpperCase()}`, 'other'],
+    [`archive.${key}`, 'other'],
+    [`${key}.md`, 'markdown'],
+    [`${key}.ts`, 'typescript'],
+  ] as [string, Exclude<FileTypeKind, 'folder'>][]))('%s → %s', (path, type) => {
+    expect(classifyFileType(path)).toBe(type)
+  })
+
+  it.each(keys)('%s misses the code-file name and extension tables', (key) => {
+    expect(classifyCodeFileType(key, key)).toBeNull()
+    expect(classifyCodeFileType(key, 'ts')).toBe('typescript')
+  })
+
+  it.each(keys.flatMap(key => [key, `notes.${key}`]))('%s renders the generic file glyph', (path) => {
+    const { container } = render(<FileTypeIcon path={path} />)
+    expect(container.querySelector('svg')!.classList.contains(css.other!)).toBe(true)
+  })
+})
+
 describe('FileTypeIcon', () => {
   const types: FileTypeKind[] = [
     'code', 'excel', 'folder', 'html', 'image', 'markdown', 'other', 'pdf', 'ppt', 'video', 'word',