diff --git a/packages/docusaurus-bundler/src/__tests__/compiler.test.ts b/packages/docusaurus-bundler/src/__tests__/compiler.test.ts
new file mode 100644
index 000000000000..72936ed26f6a
--- /dev/null
+++ b/packages/docusaurus-bundler/src/__tests__/compiler.test.ts
@@ -0,0 +1,187 @@
+/**
+ * Copyright (c) Facebook, Inc. and its affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+import {describe, expect, it} from 'vitest';
+import fs from 'node:fs/promises';
+import path from 'node:path';
+import {tmpdir} from 'node:os';
+import {compile, formatStatsErrorMessage} from '../compiler';
+import {getCurrentBundler} from '../currentBundler';
+
+describe('formatStatsErrorMessage', () => {
+  it('includes the source module and location', () => {
+    const result = formatStatsErrorMessage({
+      errors: [
+        {
+          moduleIdentifier: '/project/src/pages/thing.xyz',
+          file: 'main.js',
+          moduleName: './src/pages/thing.xyz',
+          loc: '1:0',
+          message: 'Module parse failed: Unexpected character',
+        },
+      ],
+      warnings: [],
+    });
+    expect(result).toContain('./src/pages/thing.xyz:1:0');
+    expect(result).toContain('Module parse failed: Unexpected character');
+    expect(result).not.toContain('/project/');
+    expect(result).not.toContain('main.js');
+  });
+
+  it('uses the file when no source module is provided', () => {
+    expect(
+      formatStatsErrorMessage({
+        errors: [{file: 'styles.css', message: 'Invalid CSS'}],
+        warnings: [],
+      }),
+    ).toContain('styles.css');
+  });
+
+  it('keeps errors without source metadata', () => {
+    expect(
+      formatStatsErrorMessage({
+        errors: [{message: 'A plain error'}],
+        warnings: [],
+      }),
+    ).toContain('A plain error');
+  });
+
+  it('keeps the source of each formatted error', () => {
+    const result = formatStatsErrorMessage({
+      errors: [
+        {moduleName: './first.js', message: 'First error'},
+        {moduleName: './second.js', message: 'Second error'},
+      ],
+      warnings: [],
+    });
+    expect(result).toContain('./first.js');
+    expect(result).toContain('First error');
+    expect(result).toContain('./second.js');
+    expect(result).toContain('Second error');
+    expect(result).toContain('--------------------------');
+  });
+
+  it('keeps the matching source when syntax errors filter other errors', () => {
+    const result = formatStatsErrorMessage({
+      errors: [
+        {moduleName: './other.js', message: 'Another error'},
+        {moduleName: './broken.js', message: 'Syntax error: Invalid token'},
+      ],
+      warnings: [],
+    });
+    expect(result).toContain('./broken.js');
+    expect(result).toContain('Syntax error: Invalid token');
+    expect(result).not.toContain('./other.js');
+    expect(result).not.toContain('Another error');
+  });
+
+  it('preserves module-resolution details when adding the source', () => {
+    const result = formatStatsErrorMessage({
+      errors: [
+        {
+          moduleName: './src/index.js',
+          loc: '1:0-24',
+          message: [
+            "Module not found: Error: Can't resolve 'buffer'",
+            'BREAKING CHANGE: webpack < 5 included polyfills by default.',
+            'Configure resolve.fallback to provide a polyfill.',
+          ].join('\n'),
+        },
+      ],
+      warnings: [],
+    });
+    expect(result).toContain('./src/index.js:1:0-24');
+    expect(result).toContain("Can't resolve 'buffer'");
+    expect(result).toContain('BREAKING CHANGE:');
+    expect(result).toContain('Configure resolve.fallback');
+  });
+
+  it('preserves Sass installation guidance when adding the source', () => {
+    const result = formatStatsErrorMessage({
+      errors: [
+        {
+          moduleName: './src/styles.scss',
+          message:
+            "./src/styles.scss\nCannot find module 'sass'\nRequire stack:",
+        },
+      ],
+      warnings: [],
+    });
+    expect(result).toContain('./src/styles.scss');
+    expect(result).toContain(
+      'To import Sass files, you first need to install sass.',
+    );
+    expect(result).toContain('npm install sass');
+  });
+
+  it('returns undefined when there are no errors', () => {
+    expect(formatStatsErrorMessage(undefined)).toBeUndefined();
+    expect(formatStatsErrorMessage({errors: [], warnings: []})).toBeUndefined();
+  });
+});
+
+describe.each(['webpack', 'rspack'] as const)(
+  '%s error diagnostics',
+  (name) => {
+    it('preserves real module-resolution diagnostics', async () => {
+      await using dir = await fs.mkdtempDisposable(
+        path.join(await fs.realpath(tmpdir()), 'docusaurus-compiler-'),
+      );
+      await fs.mkdir(path.join(dir.path, 'src'));
+      await fs.writeFile(
+        path.join(dir.path, 'src/index.js'),
+        "import 'buffer';",
+      );
+      const currentBundler = await getCurrentBundler({
+        siteConfig: {webpack: name === 'webpack' ? {} : undefined},
+      });
+      const compilation = compile({
+        currentBundler,
+        configs: [
+          {
+            mode: 'development',
+            context: dir.path,
+            entry: './src/index.js',
+            output: {path: path.join(dir.path, 'build')},
+          },
+        ],
+      });
+      await expect(compilation).rejects.toThrow('./src/index.js');
+      await expect(compilation).rejects.toThrow(
+        name === 'webpack' ? 'resolve.fallback' : "Can't resolve 'buffer'",
+      );
+    });
+
+    it('identifies the source of a real module parse failure', async () => {
+      await using dir = await fs.mkdtempDisposable(
+        path.join(await fs.realpath(tmpdir()), 'docusaurus-compiler-'),
+      );
+      await fs.mkdir(path.join(dir.path, 'src'));
+      await fs.writeFile(
+        path.join(dir.path, 'src/index.js'),
+        "import './thing.xyz';",
+      );
+      await fs.writeFile(path.join(dir.path, 'src/thing.xyz'), '🔥 invalid');
+      const currentBundler = await getCurrentBundler({
+        siteConfig: {webpack: name === 'webpack' ? {} : undefined},
+      });
+      await expect(
+        compile({
+          currentBundler,
+          configs: [
+            {
+              mode: 'development',
+              context: dir.path,
+              entry: './src/index.js',
+              output: {path: path.join(dir.path, 'build')},
+            },
+          ],
+        }),
+      ).rejects.toThrow('./src/thing.xyz');
+    });
+  },
+);
diff --git a/packages/docusaurus-bundler/src/legacy/formatWebpackMessages.js b/packages/docusaurus-bundler/src/legacy/formatWebpackMessages.js
index 8e66cb8aab24..ec87f8f9a58a 100644
--- a/packages/docusaurus-bundler/src/legacy/formatWebpackMessages.js
+++ b/packages/docusaurus-bundler/src/legacy/formatWebpackMessages.js
@@ -122,12 +122,26 @@ function formatMessage(message) {
   return message.trim();
 }
 
+/**
+ * Adds source metadata after cleanup, which relies on diagnostic line positions.
+ * @param {import("webpack").StatsError} error
+ */
+function formatError(error) {
+  const message = formatMessage(error);
+  const source = error.moduleName ?? error.file;
+  if (!source) {
+    return message;
+  }
+  const sourceLocation = error.loc ? `${source}:${error.loc}` : source;
+  return `${sourceLocation}\n${message}`;
+}
+
 /**
  * @param {import("webpack").Stats.ToJsonOutput} json.
  * @returns {{ errors: string[], warnings: string[] }}
  */
 module.exports = function formatWebpackMessages(json) {
-  const formattedErrors = json.errors.map(formatMessage);
+  const formattedErrors = json.errors.map(formatError);
   const formattedWarnings = json.warnings.map(formatMessage);
   const result = {errors: formattedErrors, warnings: formattedWarnings};
   if (result.errors.some(isLikelyASyntaxError)) {