feat(filesystem): support base64 write content

src/filesystem/__tests__/structured-content.test.ts

@@ -123,6 +123,27 @@ describe('structuredContent schema compliance', () => {
     });
   });
 
+  describe('write_file', () => {
+    it('should decode content_base64 before writing the file', async () => {
+      const filePath = path.join(testDir, 'base64-write.md');
+      const content = '# Test\n\n`inline code` with "quotes" and ${template} literals.\n';
+
+      const result = await client.callTool({
+        name: 'write_file',
+        arguments: {
+          path: filePath,
+          content_base64: Buffer.from(content, 'utf-8').toString('base64')
+        }
+      });
+
+      expect(result.structuredContent).toBeDefined();
+      expect(await fs.readFile(filePath, 'utf-8')).toBe(content);
+
+      const structuredContent = result.structuredContent as { content: unknown };
+      expect(structuredContent.content).toContain('Successfully wrote');
+    });
+  });
+
   describe('list_directory (control - already working)', () => {
     it('should return structuredContent.content as a string', async () => {
       const result = await client.callTool({

src/filesystem/index.ts

@@ -112,7 +112,8 @@ const ReadMultipleFilesArgsSchema = z.object({
 
 const WriteFileArgsSchema = z.object({
   path: z.string(),
-  content: z.string(),
+  content: z.string().optional(),
+  content_base64: z.string().optional(),
 });
 
 const EditOperation = z.object({
@@ -346,14 +347,23 @@ server.registerTool(
       "Handles text content with proper encoding. Only works within allowed directories.",
     inputSchema: {
       path: z.string(),
-      content: z.string()
+      content: z.string().optional(),
+      content_base64: z.string().optional().describe("Base64-encoded UTF-8 content. Use when content contains characters that are hard to serialize safely in JSON.")
     },
     outputSchema: { content: z.string() },
     annotations: { readOnlyHint: false, idempotentHint: true, destructiveHint: true }
   },
   async (args: z.infer<typeof WriteFileArgsSchema>) => {
     const validPath = await validatePath(args.path);
-    await writeFileContent(validPath, args.content);
+    const content = args.content_base64 !== undefined
+      ? Buffer.from(args.content_base64, "base64").toString("utf-8")
+      : args.content;
+
+    if (content === undefined) {
+      throw new Error("Must provide either content or content_base64");
+    }
+
+    await writeFileContent(validPath, content);
     const text = `Successfully wrote to ${args.path}`;
     return {
       content: [{ type: "text" as const, text }],