diff --git a/packages/wxt/src/builtin-modules/__tests__/unimport.test.ts b/packages/wxt/src/builtin-modules/__tests__/unimport.test.ts
new file mode 100644
index 000000000..cd5784343
--- /dev/null
+++ b/packages/wxt/src/builtin-modules/__tests__/unimport.test.ts
@@ -0,0 +1,97 @@
+import { describe, expect, it } from 'vitest';
+import { unimportPlugin } from '../unimport';
+import type { WxtResolvedUnimportOptions } from '../../types';
+
+const options = (): WxtResolvedUnimportOptions => ({
+  disabled: false,
+  eslintrc: { enabled: false, filePath: '', globalsPropValue: true },
+  presets: [{ from: 'wxt/browser', imports: ['browser'] }],
+});
+
+const transform = (
+  plugin: ReturnType<typeof unimportPlugin>,
+  code: string,
+  id: string,
+) => {
+  const transform = plugin.transform;
+  const handler =
+    typeof transform === 'function' ? transform : transform!.handler;
+  return handler.call({} as any, code, id) as Promise<
+    { code: string; map: { mappings: string; sources: string[] } } | undefined
+  >;
+};
+
+/**
+ * Sourcemap `mappings` are grouped by generated line (`;`), then by segment
+ * (`,`). A line-level sourcemap only has one segment per line, so counting
+ * segments tells us whether column information was included.
+ */
+const segmentsPerLine = (mappings: string) =>
+  mappings.split(';').map((line) => (line === '' ? 0 : line.split(',').length));
+
+describe('Unimport Module', () => {
+  describe('unimportPlugin', () => {
+    const id = '/src/utils/example.ts';
+    const code = [
+      'export function getExtensionId() {',
+      '  const id = browser.runtime.id;',
+      '  return id.toUpperCase();',
+      '}',
+      '',
+    ].join('\n');
+
+    it('should inject auto-imports', async () => {
+      const res = await transform(unimportPlugin(options()), code, id);
+
+      expect(res?.code).toContain("import { browser } from 'wxt/browser';");
+      expect(res?.code).toContain('browser.runtime.id');
+    });
+
+    it('should generate a sourcemap with column-level mappings', async () => {
+      const res = await transform(unimportPlugin(options()), code, id);
+
+      // Without `hires`, every line collapses to a single segment, which makes
+      // coverage tools treat the entire module as one statement.
+      // See https://github.com/wxt-dev/wxt/issues/2604
+      expect(Math.max(...segmentsPerLine(res!.map.mappings))).toBeGreaterThan(
+        1,
+      );
+    });
+
+    it('should include the module ID as the sourcemap source', async () => {
+      const res = await transform(unimportPlugin(options()), code, id);
+
+      expect(res!.map.sources).toEqual([id]);
+    });
+
+    it('should not transform files that do not use auto-imports', async () => {
+      const res = await transform(
+        unimportPlugin(options()),
+        'export const one = 1;\n',
+        id,
+      );
+
+      expect(res).toBeUndefined();
+    });
+
+    it('should not transform excluded files', async () => {
+      const res = await transform(
+        unimportPlugin(options()),
+        code,
+        '/node_modules/example/index.js',
+      );
+
+      expect(res).toBeUndefined();
+    });
+
+    it('should not transform non-JS files', async () => {
+      const res = await transform(
+        unimportPlugin(options()),
+        code,
+        '/src/a.css',
+      );
+
+      expect(res).toBeUndefined();
+    });
+  });
+});
diff --git a/packages/wxt/src/builtin-modules/unimport.ts b/packages/wxt/src/builtin-modules/unimport.ts
index 8e9ec12d2..f4ac740be 100644
--- a/packages/wxt/src/builtin-modules/unimport.ts
+++ b/packages/wxt/src/builtin-modules/unimport.ts
@@ -8,7 +8,8 @@ import type {
   EslintConfigVersion,
 } from '../types';
 import { type Unimport, createUnimport, toExports } from 'unimport';
-import UnimportPlugin from 'unimport/unplugin';
+import { defaultExcludes, defaultIncludes } from 'unimport/unplugin';
+import { createFilter, type FilterPattern, type Plugin } from 'vite';
 
 export default defineWxtModule({
   name: 'wxt:built-in:unimport',
@@ -64,11 +65,61 @@ export default defineWxtModule({
 
     // Add vite plugin
     addViteConfig(wxt, () => ({
-      plugins: [UnimportPlugin.vite(wxt.config.imports)],
+      plugins: [unimportPlugin(wxt.config.imports)],
     }));
   },
 });
 
+/**
+ * Vite plugin that injects auto-imports into modules.
+ *
+ * Equivalent to `UnimportPlugin.vite(options)`, but it generates a high
+ * resolution sourcemap. Unimport's unplugin calls `MagicString#generateMap()`
+ * with no options, which produces a line-level map with no `source`. Since
+ * auto-imports are injected at the top of the file, tools that rely on columns
+ * (like V8 code coverage) then see the entire module as a single statement,
+ * silently dropping it from coverage reports.
+ *
+ * `hires: 'boundary'` keeps columns accurate and is cheaper than `hires: true`.
+ *
+ * If unimport passes these options itself, this plugin can be dropped in favor
+ * of `UnimportPlugin.vite(options)` again.
+ *
+ * @see https://github.com/wxt-dev/wxt/issues/2604
+ * @see https://github.com/unjs/unimport/issues/562
+ */
+export function unimportPlugin(options: WxtResolvedUnimportOptions): Plugin {
+  // `include`/`exclude` aren't part of `UnimportOptions`, but unimport's
+  // unplugin accepts them, so keep honoring them.
+  const { include = defaultIncludes, exclude = defaultExcludes } = options as {
+    include?: FilterPattern;
+    exclude?: FilterPattern;
+  };
+  const filter = createFilter(include, exclude);
+  const unimport = createUnimport(options);
+  let initialized: Promise<void> | undefined;
+  const init = () => (initialized ??= unimport.init());
+
+  return {
+    name: 'wxt:unimport',
+    // Run after plugins that compile files to JS, like `@vitejs/plugin-vue`.
+    enforce: 'post',
+    buildStart: init,
+    async transform(code, id) {
+      if (!filter(id)) return;
+
+      await init();
+      const injected = await unimport.injectImports(code, id);
+      if (!injected.s.hasChanged()) return;
+
+      return {
+        code: injected.code,
+        map: injected.s.generateMap({ hires: 'boundary', source: id }),
+      };
+    },
+  };
+}
+
 async function getImportsDeclarationEntry(
   unimport: Unimport,
 ): Promise<WxtDirFileEntry> {
diff --git a/packages/wxt/src/testing/wxt-vitest-plugin.ts b/packages/wxt/src/testing/wxt-vitest-plugin.ts
index 6e5eed922..be69ef4d9 100644
--- a/packages/wxt/src/testing/wxt-vitest-plugin.ts
+++ b/packages/wxt/src/testing/wxt-vitest-plugin.ts
@@ -13,7 +13,7 @@ import {
   resolveAppConfig,
 } from '../core/builders/vite/plugins';
 import { InlineConfig } from '../types';
-import UnimportPlugin from 'unimport/unplugin';
+import { unimportPlugin } from '../builtin-modules/unimport';
 import { registerWxt, wxt } from '../core/wxt';
 
 /**
@@ -44,7 +44,7 @@ export async function WxtVitest(
     resolveAppConfig(wxt.config),
     extensionApiMock(wxt.config),
   ];
-  plugins.push(UnimportPlugin.vite(wxt.config.imports));
+  plugins.push(unimportPlugin(wxt.config.imports));
 
   return plugins;
 }