diff --git a/src/index.spec.ts b/src/index.spec.ts
index fc9d548..cac7126 100644
--- a/src/index.spec.ts
+++ b/src/index.spec.ts
@@ -189,4 +189,244 @@ describe('gulpTateru', () => {
     expect(error).toBeInstanceOf(PluginError);
     expect((error as PluginError).message).toContain('Streaming not supported');
   });
+
+  it('should handle null files by passing them through', async () => {
+    const stream = gulpTateruCli();
+    const nullFile = new Vinyl({
+      path: 'test.json',
+      contents: null,
+    });
+
+    const result = await new Promise((resolve) => {
+      stream.once('data', resolve);
+      stream.write(nullFile);
+      stream.end();
+    });
+
+    expect(result).toBe(nullFile);
+  });
+
+  it('should handle errors from core function', async () => {
+    const stream = gulpTateruCli();
+
+    // Create a config that will cause core() to fail
+    // Using an invalid cwd path should cause an error in core
+    const invalidConfigFile = new Vinyl({
+      path: 'test-config.json',
+      cwd: '/absolutely/nonexistent/path/that/should/cause/error',
+      base: '/test',
+      contents: Buffer.from(
+        JSON.stringify({
+          // Valid JSON structure but invalid paths/config for tateru-cli core
+          environments: {
+            dev: { minify: false },
+          },
+          translations: {},
+          pages: {
+            test: {
+              template: 'nonexistent.html.twig',
+              output: 'test.html',
+            },
+          },
+        })
+      ),
+    });
+
+    const errorPromise = new Promise((resolve) => {
+      stream.once('error', resolve);
+    });
+
+    stream.write(invalidConfigFile);
+    stream.end();
+
+    const error = await errorPromise;
+
+    expect(error).toBeInstanceOf(PluginError);
+    expect((error as PluginError).plugin).toBe('gulp-tateru-cli');
+  });
+
+  it('should work with env option independently', async () => {
+    const { count } = await new Promise<{
+      count: number;
+    }>((resolve) => {
+      let n = 0;
+
+      gulp
+        .src('./tateru.config.json', { cwd: 'test/fixtures' })
+        .pipe(gulpTateruCli({ env: 'dev' }))
+        .on('data', () => {
+          n++;
+        })
+        .on('end', () => {
+          resolve({ count: n });
+        });
+    });
+
+    expect(count).toBe(6);
+  });
+
+  it('should apply both formatter and minify functions together', async () => {
+    const formatter: Formatter = async (contents, fileType) =>
+      `FORMATTED (${fileType}): ${contents}`;
+    const minify: Minify = async (contents) => contents.replace(/\s+/g, ' ');
+
+    const { generatedFile } = await new Promise<{
+      generatedFile: string;
+    }>((resolve) => {
+      let generatedFile: string;
+
+      gulp
+        .src('./tateru.config.json', { cwd: 'test/fixtures' })
+        .pipe(
+          gulpTateruCli({
+            formatter,
+            minify,
+            page: 'about',
+            env: 'prod',
+          })
+        )
+        .on('data', (file) => {
+          generatedFile = file.contents.toString();
+        })
+        .on('end', () => {
+          resolve({ generatedFile });
+        });
+    });
+
+    expect(generatedFile).toContain('FORMATTED (html):');
+    expect(generatedFile).not.toMatch(/\s{2,}/); // No multiple spaces
+  });
+
+  it('should handle different file types correctly', async () => {
+    const formatterCalls: { contents: string; fileType: string }[] = [];
+    const formatter: Formatter = async (contents, fileType) => {
+      formatterCalls.push({ contents, fileType: fileType ?? 'unknown' });
+      return Promise.resolve(contents);
+    };
+
+    await new Promise<void>((resolve) => {
+      gulp
+        .src('./tateru.config.json', { cwd: 'test/fixtures' })
+        .pipe(gulpTateruCli({ formatter }))
+        .on('data', () => {
+          // Just collect files
+        })
+        .on('end', () => {
+          resolve();
+        });
+    });
+
+    const fileTypes = formatterCalls.map((call) => call.fileType);
+    expect(fileTypes).toContain('html');
+    expect(fileTypes).toContain('txt');
+    expect(fileTypes).toContain('xml');
+    expect(fileTypes).toContain('webmanifest');
+  });
+
+  it('should set correct vinyl file properties', async () => {
+    const files: Vinyl[] = [];
+
+    await new Promise<void>((resolve) => {
+      gulp
+        .src('./tateru.config.json', { cwd: 'test/fixtures' })
+        .pipe(gulpTateruCli({ page: 'about' }))
+        .on('data', (file) => {
+          files.push(file);
+        })
+        .on('end', () => {
+          resolve();
+        });
+    });
+
+    expect(files).toHaveLength(1);
+    const file = files[0];
+
+    expect(file).toBeInstanceOf(Vinyl);
+    expect(file.contents).toBeInstanceOf(Buffer);
+    expect(file.path).toBeTruthy();
+    expect(file.base).toBeTruthy();
+    expect(file.cwd).toBeTruthy();
+  });
+
+  it('should handle nonexistent page gracefully', async () => {
+    const { count } = await new Promise<{
+      count: number;
+    }>((resolve) => {
+      let n = 0;
+
+      gulp
+        .src('./tateru.config.json', { cwd: 'test/fixtures' })
+        .pipe(gulpTateruCli({ page: 'nonexistent-page' }))
+        .on('data', () => {
+          n++;
+        })
+        .on('end', () => {
+          resolve({ count: n });
+        });
+    });
+
+    // Should generate 0 files for nonexistent page
+    expect(count).toBe(0);
+  });
+
+  it('should handle nonexistent language gracefully', async () => {
+    const { count } = await new Promise<{
+      count: number;
+    }>((resolve) => {
+      let n = 0;
+
+      gulp
+        .src('./tateru.config.json', { cwd: 'test/fixtures' })
+        .pipe(gulpTateruCli({ lang: 'nonexistent-lang' }))
+        .on('data', () => {
+          n++;
+        })
+        .on('end', () => {
+          resolve({ count: n });
+        });
+    });
+
+    // Should generate 0 files for nonexistent language
+    expect(count).toBe(0);
+  });
+
+  it('should handle async formatter and minify functions', async () => {
+    const formatter: Formatter = async (contents) => {
+      // Simulate async operation
+      await new Promise((resolve) => setTimeout(resolve, 1));
+      return `ASYNC_FORMATTED: ${contents}`;
+    };
+
+    const minify: Minify = async (contents) => {
+      // Simulate async operation
+      await new Promise((resolve) => setTimeout(resolve, 1));
+      return contents.replace(/\s+/g, '');
+    };
+
+    const { generatedFile } = await new Promise<{
+      generatedFile: string;
+    }>((resolve) => {
+      let generatedFile: string;
+
+      gulp
+        .src('./tateru.config.json', { cwd: 'test/fixtures' })
+        .pipe(
+          gulpTateruCli({
+            formatter,
+            minify,
+            page: 'about',
+            env: 'prod',
+          })
+        )
+        .on('data', (file) => {
+          generatedFile = file.contents.toString();
+        })
+        .on('end', () => {
+          resolve({ generatedFile });
+        });
+    });
+
+    expect(generatedFile).toContain('ASYNC_FORMATTED:');
+    expect(generatedFile).not.toMatch(/\s+/);
+  });
 });