diff --git a/src/classes/Test.js b/src/classes/Test.js
index 0da162b..0ab3bcc 100644
--- a/src/classes/Test.js
+++ b/src/classes/Test.js
@@ -23,6 +23,14 @@ const INHERITED_PROPS = [
  * Represents a single test or a group of tests
  */
 export default class Test {
+	/**
+	 * Source file of each test definition, e.g. `{ label: "check.js", path: "file:///…/check.js" }`.
+	 * Populated by the environment, which cannot store it on the test object itself:
+	 * test definitions come from modules that may hold unrelated data.
+	 * @type {WeakMap<object, { label: string, path: string }>}
+	 */
+	static files = new WeakMap();
+
 	data = {};
 
 	constructor (test, parent) {
@@ -63,6 +71,12 @@ export default class Test {
 		}
 		Object.defineProperties(this, descriptors);
 
+		// Only set it if tagged: an own `file: undefined` would block inheriting the parent's below
+		let file = Test.files.get(test);
+		if (file) {
+			this.file = file;
+		}
+
 		if (typeof this.data === "function") {
 			this.getData = this.data;
 			this.data = {};
diff --git a/src/env/node.js b/src/env/node.js
index 9791f49..e985d62 100644
--- a/src/env/node.js
+++ b/src/env/node.js
@@ -208,8 +208,8 @@ async function rerun (options, urls) {
 
 				let test = module.default ?? Object.values(module);
 
-				if (Object.isExtensible(test)) {
-					test.file = old.test.file;
+				if (old.test.file && typeof test === "object") {
+					Test.files.set(test, old.test.file);
 				}
 
 				test = new Test(test, currentRoot.test);
@@ -342,16 +342,16 @@ export default {
 			[...loadedFiles].map(async url => {
 				let module = await import(url);
 				let test = module.default ?? module;
-				if (test && typeof test === "object" && Object.isExtensible(test) && !test.file) {
+				if (test && typeof test === "object") {
 					let fileUrl = new URL(url);
 					fileUrl.search = "";
-					test.file = {
+					Test.files.set(test, {
 						label: path.relative(
 							isDirectory ? location : path.dirname(location),
 							fileURLToPath(url),
 						),
 						path: fileUrl.href,
-					};
+					});
 				}
 			}),
 		);
diff --git a/tests/fixtures/plain-module.js b/tests/fixtures/plain-module.js
new file mode 100644
index 0000000..e470a21
--- /dev/null
+++ b/tests/fixtures/plain-module.js
@@ -0,0 +1,3 @@
+// A plain data module, not a test definition. Imported by ../module-tagging.js
+// to verify hTest doesn't tag it with source file metadata.
+export default { a: 1 };
diff --git a/tests/module-tagging.js b/tests/module-tagging.js
new file mode 100644
index 0000000..70be5bb
--- /dev/null
+++ b/tests/module-tagging.js
@@ -0,0 +1,27 @@
+import Test from "../src/classes/Test.js";
+import data from "./fixtures/plain-module.js";
+
+export default {
+	name: "Module tagging",
+	tests: [
+		{
+			name: "Non-test modules are left untouched",
+			description:
+				"The Node env tags every loaded module with its source file. Data modules must not be mutated: a `file` key would shadow a real one and leak into Object.keys().",
+			run: () => Object.keys(data),
+			expect: ["a"],
+		},
+		{
+			name: "Tests get their file from the registry",
+			description: "Source file metadata reaches the Test instance without touching the spec.",
+			run () {
+				let spec = { name: "Tagged" };
+				Test.files.set(spec, { label: "tagged.js", path: "file:///tagged.js" });
+				let test = new Test(spec);
+
+				return [test.file?.label, "file" in spec];
+			},
+			expect: ["tagged.js", false],
+		},
+	],
+};