diff --git a/src/language-js/parse/oxc.js b/src/language-js/parse/oxc.js
index 79be36b7c..9472499df 100644
--- a/src/language-js/parse/oxc.js
+++ b/src/language-js/parse/oxc.js
@@ -4,6 +4,7 @@ import createError from "../../common/parser-create-error.js";
 import { tryCombinationsSync } from "../../utilities/try-combinations.js";
 import postprocess from "./postprocess/index.js";
 import createParser from "./utilities/create-parser.js";
+import { isDtsFile } from "./utilities/is-dts-file.js";
 import jsxRegexp from "./utilities/jsx-regexp.evaluate.js";
 import { shouldEnableJsx } from "./utilities/jsx-support.js";
 import {
@@ -103,10 +104,7 @@ function parseJs(text, options) {
 function getLanguageCombinations(text, options) {
   const filepath = options?.filepath;
 
-  if (
-    typeof filepath === "string" &&
-    filepath.toLowerCase().endsWith(".d.ts")
-  ) {
+  if (isDtsFile(filepath)) {
     return ["dts"];
   }
 
diff --git a/src/language-js/parse/utilities/is-dts-file.js b/src/language-js/parse/utilities/is-dts-file.js
new file mode 100644
index 000000000..25aa2da50
--- /dev/null
+++ b/src/language-js/parse/utilities/is-dts-file.js
@@ -0,0 +1,5 @@
+function isDtsFile(filepath) {
+  return typeof filepath === "string" && /\.d\.(?:ts|mts|cts)$/i.test(filepath);
+}
+
+export { isDtsFile };
diff --git a/src/language-js/parse/utilities/source-types.js b/src/language-js/parse/utilities/source-types.js
index 3d1cda81a..2d375ea79 100644
--- a/src/language-js/parse/utilities/source-types.js
+++ b/src/language-js/parse/utilities/source-types.js
@@ -13,8 +13,6 @@ export function getSourceType(filepath) {
     return;
   }
 
-  filepath = filepath.toLowerCase();
-
   if (/\.(?:mjs|mts)$/i.test(filepath)) {
     return SOURCE_TYPE_MODULE;
   }
diff --git a/src/language-js/parse/yuku.js b/src/language-js/parse/yuku.js
index e7f05eb12..dbeddceed 100644
--- a/src/language-js/parse/yuku.js
+++ b/src/language-js/parse/yuku.js
@@ -4,6 +4,7 @@ import createError from "../../common/parser-create-error.js";
 import { tryCombinationsSync } from "../../utilities/try-combinations.js";
 import postprocess from "./postprocess/index.js";
 import createParser from "./utilities/create-parser.js";
+import { isDtsFile } from "./utilities/is-dts-file.js";
 import jsxRegexp from "./utilities/jsx-regexp.evaluate.js";
 import { shouldEnableJsx } from "./utilities/jsx-support.js";
 import {
@@ -87,10 +88,7 @@ function parseJs(text, options) {
 function getLanguageCombinations(text, options) {
   const filepath = options?.filepath;
 
-  if (
-    typeof filepath === "string" &&
-    filepath.toLowerCase().endsWith(".d.ts")
-  ) {
+  if (isDtsFile(filepath)) {
     return ["dts"];
   }
 
diff --git a/tests/format/typescript/d-ts-files/__snapshots__/format.test.js.snap b/tests/format/typescript/d-ts-files/__snapshots__/format.test.js.snap
deleted file mode 100644
index a4b90ce13..000000000
--- a/tests/format/typescript/d-ts-files/__snapshots__/format.test.js.snap
+++ /dev/null
@@ -1,31 +0,0 @@
-// Jest Snapshot v1, https://jestjs.io/docs/snapshot-testing
-
-exports[`const-without-initializer.d.ts format 1`] = `
-====================================options=====================================
-parsers: ["typescript"]
-                                                      printWidth: 80 (default) |
-=====================================input======================================
-export const    version: string;
-
-=====================================output=====================================
-export const version: string;
-
-================================================================================
-`;
-
-exports[`const-without-initializer-in-namespace.d.ts format 1`] = `
-====================================options=====================================
-parsers: ["typescript"]
-                                                      printWidth: 80 (default) |
-=====================================input======================================
-export namespace builders {
-  const    breakParent: string;
-}
-
-=====================================output=====================================
-export namespace builders {
-  const breakParent: string;
-}
-
-================================================================================
-`;
diff --git a/tests/format/typescript/dts-files/__snapshots__/format.test.js.snap b/tests/format/typescript/dts-files/__snapshots__/format.test.js.snap
new file mode 100644
index 000000000..fa83c2036
--- /dev/null
+++ b/tests/format/typescript/dts-files/__snapshots__/format.test.js.snap
@@ -0,0 +1,91 @@
+// Jest Snapshot v1, https://jestjs.io/docs/snapshot-testing
+
+exports[`const-without-initializer.d.cts format 1`] = `
+====================================options=====================================
+parsers: ["typescript"]
+                                                      printWidth: 80 (default) |
+=====================================input======================================
+const    version: string;
+
+=====================================output=====================================
+const version: string;
+
+================================================================================
+`;
+
+exports[`const-without-initializer.d.mts format 1`] = `
+====================================options=====================================
+parsers: ["typescript"]
+                                                      printWidth: 80 (default) |
+=====================================input======================================
+export const    version: string;
+
+=====================================output=====================================
+export const version: string;
+
+================================================================================
+`;
+
+exports[`const-without-initializer.d.ts format 1`] = `
+====================================options=====================================
+parsers: ["typescript"]
+                                                      printWidth: 80 (default) |
+=====================================input======================================
+export const    version: string;
+
+=====================================output=====================================
+export const version: string;
+
+================================================================================
+`;
+
+exports[`const-without-initializer-in-namespace.d.cts format 1`] = `
+====================================options=====================================
+parsers: ["typescript"]
+                                                      printWidth: 80 (default) |
+=====================================input======================================
+namespace builders {
+  const    breakParent: string;
+}
+
+=====================================output=====================================
+namespace builders {
+  const breakParent: string;
+}
+
+================================================================================
+`;
+
+exports[`const-without-initializer-in-namespace.d.mts format 1`] = `
+====================================options=====================================
+parsers: ["typescript"]
+                                                      printWidth: 80 (default) |
+=====================================input======================================
+export namespace builders {
+  const    breakParent: string;
+}
+
+=====================================output=====================================
+export namespace builders {
+  const breakParent: string;
+}
+
+================================================================================
+`;
+
+exports[`const-without-initializer-in-namespace.d.ts format 1`] = `
+====================================options=====================================
+parsers: ["typescript"]
+                                                      printWidth: 80 (default) |
+=====================================input======================================
+export namespace builders {
+  const    breakParent: string;
+}
+
+=====================================output=====================================
+export namespace builders {
+  const breakParent: string;
+}
+
+================================================================================
+`;
diff --git a/tests/format/typescript/dts-files/const-without-initializer-in-namespace.d.cts b/tests/format/typescript/dts-files/const-without-initializer-in-namespace.d.cts
new file mode 100644
index 000000000..45057a87f
--- /dev/null
+++ b/tests/format/typescript/dts-files/const-without-initializer-in-namespace.d.cts
@@ -0,0 +1,3 @@
+namespace builders {
+  const    breakParent: string;
+}
diff --git a/tests/format/typescript/d-ts-files/const-without-initializer-in-namespace.d.ts b/tests/format/typescript/dts-files/const-without-initializer-in-namespace.d.mts
similarity index 100%
rename from tests/format/typescript/d-ts-files/const-without-initializer-in-namespace.d.ts
rename to tests/format/typescript/dts-files/const-without-initializer-in-namespace.d.mts
diff --git a/tests/format/typescript/dts-files/const-without-initializer-in-namespace.d.ts b/tests/format/typescript/dts-files/const-without-initializer-in-namespace.d.ts
new file mode 100644
index 000000000..6a3a1f34a
--- /dev/null
+++ b/tests/format/typescript/dts-files/const-without-initializer-in-namespace.d.ts
@@ -0,0 +1,3 @@
+export namespace builders {
+  const    breakParent: string;
+}
diff --git a/tests/format/typescript/dts-files/const-without-initializer.d.cts b/tests/format/typescript/dts-files/const-without-initializer.d.cts
new file mode 100644
index 000000000..2035369e2
--- /dev/null
+++ b/tests/format/typescript/dts-files/const-without-initializer.d.cts
@@ -0,0 +1 @@
+const    version: string;
diff --git a/tests/format/typescript/d-ts-files/const-without-initializer.d.ts b/tests/format/typescript/dts-files/const-without-initializer.d.mts
similarity index 100%
rename from tests/format/typescript/d-ts-files/const-without-initializer.d.ts
rename to tests/format/typescript/dts-files/const-without-initializer.d.mts
diff --git a/tests/format/typescript/dts-files/const-without-initializer.d.ts b/tests/format/typescript/dts-files/const-without-initializer.d.ts
new file mode 100644
index 000000000..56ebe816e
--- /dev/null
+++ b/tests/format/typescript/dts-files/const-without-initializer.d.ts
@@ -0,0 +1 @@
+export const    version: string;
diff --git a/tests/format/typescript/d-ts-files/format.test.js b/tests/format/typescript/dts-files/format.test.js
similarity index 100%
rename from tests/format/typescript/d-ts-files/format.test.js
rename to tests/format/typescript/dts-files/format.test.js