diff --git a/.changeset/mts-cts-typescript-highlighting.md b/.changeset/mts-cts-typescript-highlighting.md
new file mode 100644
index 00000000..3c759bcb
--- /dev/null
+++ b/.changeset/mts-cts-typescript-highlighting.md
@@ -0,0 +1,5 @@
+---
+"hunkdiff": patch
+---
+
+Highlight `.mts` and `.cts` files as TypeScript instead of plain text.
diff --git a/src/core/diffFile.test.ts b/src/core/diffFile.test.ts
index 25740c36..2ed62988 100644
--- a/src/core/diffFile.test.ts
+++ b/src/core/diffFile.test.ts
@@ -36,6 +36,14 @@ describe("buildDiffFile", () => {
     expect(file.stats).toEqual({ additions: 2, deletions: 1 });
   });
 
+  test("derives TypeScript language for module and commonjs TypeScript files", () => {
+    const mtsFile = buildDiffFile(metadataFor("a\n", "b\n", "foo.mts"), "PATCH", 0, "src", null);
+    const ctsFile = buildDiffFile(metadataFor("a\n", "b\n", "foo.cts"), "PATCH", 1, "src", null);
+
+    expect(mtsFile.language).toBe("typescript");
+    expect(ctsFile.language).toBe("typescript");
+  });
+
   test("infers binary status from the patch when not given explicitly", () => {
     const binary = buildDiffFile(metadata, "Binary files a/x and b/x differ\n", 0, "src", null);
     expect(binary.isBinary).toBe(true);
diff --git a/src/core/diffFile.ts b/src/core/diffFile.ts
index a80103fb..8ff0ff71 100644
--- a/src/core/diffFile.ts
+++ b/src/core/diffFile.ts
@@ -1,6 +1,7 @@
-import { getFiletypeFromFileName, type FileDiffMetadata } from "@pierre/diffs";
+import { type FileDiffMetadata } from "@pierre/diffs";
 import { findAgentFileContext } from "./agent";
 import { patchLooksBinary } from "./binary";
+import { getFiletypeFromFileName } from "./fileLanguage";
 import { normalizeDiffMetadataPaths, normalizeDiffPath } from "./diffPaths";
 import type { FileSourceFetcher } from "./fileSource";
 import type { AgentContext, DiffFile, DiffLineMoveKinds } from "./types";
diff --git a/src/core/fileLanguage.test.ts b/src/core/fileLanguage.test.ts
new file mode 100644
index 00000000..490fe41a
--- /dev/null
+++ b/src/core/fileLanguage.test.ts
@@ -0,0 +1,17 @@
+import { describe, expect, test } from "bun:test";
+import { getFiletypeFromFileName } from "./fileLanguage";
+
+describe("custom file language registration", () => {
+  test("maps TypeScript module/commonjs extensions to typescript", () => {
+    expect(getFiletypeFromFileName("foo.mts")).toBe("typescript");
+    expect(getFiletypeFromFileName("foo.cts")).toBe("typescript");
+    expect(getFiletypeFromFileName("src/nested/foo.mts")).toBe("typescript");
+  });
+
+  test("preserves Pierre's built-in extension detection", () => {
+    expect(getFiletypeFromFileName("foo.ts")).toBe("typescript");
+    expect(getFiletypeFromFileName("foo.tsx")).toBe("tsx");
+    expect(getFiletypeFromFileName("foo.mjs")).toBe("javascript");
+    expect(getFiletypeFromFileName("foo.cjs")).toBe("javascript");
+  });
+});
diff --git a/src/core/fileLanguage.ts b/src/core/fileLanguage.ts
new file mode 100644
index 00000000..25f1dd3a
--- /dev/null
+++ b/src/core/fileLanguage.ts
@@ -0,0 +1,17 @@
+import {
+  getFiletypeFromFileName,
+  setCustomExtension,
+  type SupportedLanguages,
+} from "@pierre/diffs";
+
+// Pierre omits these TypeScript extensions, so register them before lookups or rendering.
+const HUNK_CUSTOM_EXTENSIONS: Record<string, SupportedLanguages> = {
+  mts: "typescript",
+  cts: "typescript",
+};
+
+for (const [extension, language] of Object.entries(HUNK_CUSTOM_EXTENSIONS)) {
+  setCustomExtension(extension, language);
+}
+
+export { getFiletypeFromFileName };