diff --git a/src/chunking/core.test.ts b/src/chunking/core.test.ts
index 5dcfdb4..9900fd3 100644
--- a/src/chunking/core.test.ts
+++ b/src/chunking/core.test.ts
@@ -20,9 +20,9 @@ describe('constants', () => {
 })
 
 describe('isSupportedLanguage', () => {
-  it('returns false for unknown languages (Unit 4 stub)', () => {
-    expect(isSupportedLanguage('typescript')).toBe(false)
-    expect(isSupportedLanguage('python')).toBe(false)
+  it('returns true for known languages and false for unknown ones', () => {
+    expect(isSupportedLanguage('typescript')).toBe(true)
+    expect(isSupportedLanguage('python')).toBe(true)
     expect(isSupportedLanguage('not-a-real-language')).toBe(false)
   })
 })
diff --git a/src/chunking/core.ts b/src/chunking/core.ts
index 78da8a5..3c72114 100644
--- a/src/chunking/core.ts
+++ b/src/chunking/core.ts
@@ -8,10 +8,7 @@
 // loads even when the package is not yet installed, falling back to the
 // line chunker in that case.
 
-// Stub for ALL_LANGUAGES until Unit 4 (language detection) lands.
-// Once `src/indexing/files.ts` exists, replace this with:
-//   import { ALL_LANGUAGES } from '../indexing/files.ts'
-const ALL_LANGUAGES: ReadonlySet<string> = new Set<string>()
+import { ALL_LANGUAGES } from '../languages.ts'
 
 export const RECURSION_DEPTH = 500
 export const MIN_CHUNK_SIZE = 50
diff --git a/src/indexing/cache.ts b/src/indexing/cache.ts
index bf20422..1b0bca2 100644
--- a/src/indexing/cache.ts
+++ b/src/indexing/cache.ts
@@ -20,10 +20,10 @@ import { chmodSync, existsSync, mkdirSync, readdirSync, realpathSync, rmSync } f
 import { readFile, stat } from 'node:fs/promises'
 import { homedir } from 'node:os'
 import { basename, dirname, join, normalize, relative } from 'node:path'
+import { getExtensions } from '../languages.ts'
 import { isGitUrl } from '../utils.ts'
 import { MAX_FILE_BYTES } from './create.ts'
 import { walkFiles } from './file-walker.ts'
-import { getExtensions } from './files.ts'
 import { CspIndex, DEFAULT_CONTENT, parseManifest } from './index.ts'
 
 /** Directory permissions for every cache directory (owner-only). NFR-003. */
diff --git a/src/indexing/create.ts b/src/indexing/create.ts
index a503954..446e75c 100644
--- a/src/indexing/create.ts
+++ b/src/indexing/create.ts
@@ -5,11 +5,11 @@ import type { Model } from './dense.ts'
 import { readFileSync, statSync } from 'node:fs'
 import { relative } from 'node:path'
 import { chunkSource } from '../chunking/chunk-source.ts'
+import { detectLanguage, getExtensions } from '../languages.ts'
 import { tokenize } from '../tokens.ts'
 import { ContentType } from '../types.ts'
 import { embedChunks, SelectableBasicBackend } from './dense.ts'
 import { walkFiles } from './file-walker.ts'
-import { detectLanguage, getExtensions } from './files.ts'
 import { Bm25Index, enrichForBm25 } from './sparse.ts'
 
 /** 1 MB max file size to read and index. */
diff --git a/src/indexing/files.test.ts b/src/languages.test.ts
similarity index 99%
rename from src/indexing/files.test.ts
rename to src/languages.test.ts
index 2090559..5a8c69c 100644
--- a/src/indexing/files.test.ts
+++ b/src/languages.test.ts
@@ -7,7 +7,7 @@ import {
   DOC_LANGUAGES,
   EXTENSION_TO_LANGUAGE,
   getExtensions,
-} from './files.ts'
+} from './languages.ts'
 
 describe('detectLanguage', () => {
   it('detects typescript from .ts', () => {
diff --git a/src/indexing/files.ts b/src/languages.ts
similarity index 100%
rename from src/indexing/files.ts
rename to src/languages.ts