diff --git a/src/converters/document.js b/src/converters/document.js
index ef0730d..f34ae46 100644
--- a/src/converters/document.js
+++ b/src/converters/document.js
@@ -176,6 +176,10 @@ async function convertMdToDocx(inputPath, outputPath) {
   await fs.writeFile(outputPath, buffer);
 }
 
+async function copyFileContents(inputPath, outputPath) {
+  await fs.copyFile(inputPath, outputPath);
+}
+
 async function extractPdfPageTexts(inputPath) {
   const data = new Uint8Array(await fs.readFile(inputPath));
   const doc = await pdfjsLib.getDocument({ data }).promise;
@@ -223,6 +227,8 @@ export function registerDocumentConverters() {
   register({ family: 'document', sourceFormat: 'pdf', targetFormat: 'md', convert: convertPdfToTxt });
   register({ family: 'document', sourceFormat: 'docx', targetFormat: 'md', convert: convertDocxToMd });
   register({ family: 'document', sourceFormat: 'md', targetFormat: 'docx', convert: convertMdToDocx });
+  register({ family: 'document', sourceFormat: 'md', targetFormat: 'txt', convert: copyFileContents });
+  register({ family: 'document', sourceFormat: 'txt', targetFormat: 'md', convert: copyFileContents });
   register({ family: 'document', sourceFormat: 'docx', targetFormat: 'pdf', convert: convertDocxToPdf });
   register({ family: 'document', sourceFormat: 'pdf', targetFormat: 'txt', convert: convertPdfToTxt });
   register({ family: 'document', sourceFormat: 'pdf', targetFormat: 'html', convert: convertPdfToHtml });
diff --git a/test/converters/document.test.js b/test/converters/document.test.js
index 62009a4..d86aec9 100644
--- a/test/converters/document.test.js
+++ b/test/converters/document.test.js
@@ -141,4 +141,30 @@ describe('document converters', () => {
     expect(result.value).toContain('first point');
     expect(result.value).toContain('<li>');
   }, 20000);
+
+  it('copies MD content unchanged to TXT', async () => {
+    const inputPath = path.join(tmpDir, 'fixture-for-txt.md');
+    const content = '# Heading\n\nSome **bold** text.\n';
+    await fs.writeFile(inputPath, content);
+    const outputPath = path.join(tmpDir, 'from-md.txt');
+    const entry = resolve('md', 'txt');
+
+    await entry.convert(inputPath, outputPath);
+
+    const output = await fs.readFile(outputPath, 'utf8');
+    expect(output).toBe(content);
+  });
+
+  it('copies TXT content unchanged to MD', async () => {
+    const inputPath = path.join(tmpDir, 'fixture-for-md-from-txt.txt');
+    const content = 'Plain text content.\n';
+    await fs.writeFile(inputPath, content);
+    const outputPath = path.join(tmpDir, 'from-txt.md');
+    const entry = resolve('txt', 'md');
+
+    await entry.convert(inputPath, outputPath);
+
+    const output = await fs.readFile(outputPath, 'utf8');
+    expect(output).toBe(content);
+  });
 });