diff --git a/lib/internal/test_runner/mock/mock.js b/lib/internal/test_runner/mock/mock.js
index 1aaefe222d926f..e492878479288c 100644
--- a/lib/internal/test_runner/mock/mock.js
+++ b/lib/internal/test_runner/mock/mock.js
@@ -513,7 +513,9 @@ class MockTracker {
       mockSpecifier, caller, null,
     );
     debug('module mock, url = "%s", format = "%s", caller = "%s"', url, format, caller);
-    validateOneOf(format, 'format', kSupportedFormats);
+    if (format) { // Format is not yet known for ambiguous files when detection is enabled.
+      validateOneOf(format, 'format', kSupportedFormats);
+    }
     const baseURL = URL.parse(url);
 
     if (!baseURL) {
diff --git a/lib/test/mock_loader.js b/lib/test/mock_loader.js
index 426604b45daed6..307ad980856094 100644
--- a/lib/test/mock_loader.js
+++ b/lib/test/mock_loader.js
@@ -143,26 +143,34 @@ async function load(url, context, nextLoad) {
   const baseURL = parsedURL ? parsedURL.href : url;
   const mock = mocks.get(baseURL);
 
+  const original = await nextLoad(url, context);
   debug('load hook, mock = %o', mock);
   if (mock?.active !== true) {
-    return nextLoad(url);
+    return original;
   }
 
   // Treat builtins as commonjs because customization hooks do not allow a
   // core module to be replaced.
-  const format = mock.format === 'builtin' ? 'commonjs' : mock.format;
+  // Also collapse 'commonjs-sync' and 'require-commonjs' to 'commonjs'.
+  const format = (
+    original.format === 'builtin' ||
+    original.format === 'commonjs-sync' ||
+    original.format === 'require-commonjs') ? 'commonjs' : original.format;
 
-  return {
+  const result = {
     __proto__: null,
     format,
     shortCircuit: true,
-    source: await createSourceFromMock(mock),
+    source: await createSourceFromMock(mock, format),
   };
+
+  debug('load hook finished, result = %o', result);
+  return result;
 }
 
-async function createSourceFromMock(mock) {
+async function createSourceFromMock(mock, format) {
   // Create mock implementation from provided exports.
-  const { exportNames, format, hasDefaultExport, url } = mock;
+  const { exportNames, hasDefaultExport, url } = mock;
   const useESM = format === 'module';
   const source = `${testImportSource(useESM)}
 if (!$__test.mock._mockExports.has('${url}')) {