fix: return binary files as MCP embedded resource instead of BASE64 prefix fixes #9

The previous implementation prefixed base64 with "BASE64:" in a text
response. This updates the response to use the proper MCP embedded
resource format:

  { type: "resource", resource: { uri, mimeType, blob } }

Changes:
- types.ts: extend ToolOutput content union to allow resource items
- file-operations.ts:
  - getMimeType() maps common extensions to MIME types, falling back
    to application/octet-stream
  - MIME_TYPES table covers PDF, ZIP, images, Office formats, audio/video
  - Binary files are now returned as an EmbeddedResource with:
      uri:      obsidian://<vault>/<path>
      mimeType: detected from file extension
      blob:     base64-encoded raw bytes from the Buffer
  - Tool descriptions updated to document the resource response shape

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

src/tools/file-operations.ts

@@ -40,6 +40,30 @@ function isBinaryContent(buf: Buffer): boolean {
   return nonPrintable / sample.length > 0.1;
 }
 
+/** Map common file extensions to MIME types */
+const MIME_TYPES: Record<string, string> = {
+  pdf:  'application/pdf',
+  zip:  'application/zip',
+  gz:   'application/gzip',
+  tar:  'application/x-tar',
+  png:  'image/png',
+  jpg:  'image/jpeg',
+  jpeg: 'image/jpeg',
+  gif:  'image/gif',
+  webp: 'image/webp',
+  svg:  'image/svg+xml',
+  mp3:  'audio/mpeg',
+  mp4:  'video/mp4',
+  docx: 'application/vnd.openxmlformats-officedocument.wordprocessingml.document',
+  xlsx: 'application/vnd.openxmlformats-officedocument.spreadsheetml.sheet',
+  pptx: 'application/vnd.openxmlformats-officedocument.presentationml.presentation',
+};
+
+function getMimeType(filePath: string): string {
+  const ext = filePath.split('.').pop()?.toLowerCase() ?? '';
+  return MIME_TYPES[ext] ?? 'application/octet-stream';
+}
+
 /**
  * Register all file operation tools
  */
@@ -151,7 +175,7 @@ export async function registerFileOperationTools(server: ObsidianMCPServer): Pro
   // T031: Read note tool
   server.registerTool(
     'obsidian_read_note',
-    'Read the content of a note from the Obsidian vault. Specify either the note name (file) or full path (path). For large files (e.g. PDFs), use max_chars and offset to read in chunks and avoid exceeding context limits. Binary files (ZIP, images, compiled files, etc.) are automatically detected and returned as a base64-encoded string prefixed with "BASE64:" — the client must base64-decode the value to recover the original binary content.',
+    'Read the content of a note from the Obsidian vault. Specify either the note name (file) or full path (path). For large files (e.g. PDFs), use max_chars and offset to read in chunks and avoid exceeding context limits. Binary files (ZIP, images, PDFs, etc.) are automatically detected and returned as an MCP embedded resource with a uri, mimeType, and base64-encoded blob field instead of plain text.',
     {
       type: 'object',
       properties: {
@@ -174,7 +198,7 @@ export async function registerFileOperationTools(server: ObsidianMCPServer): Pro
       },
     },
     createToolHandler(
-      'Read the content of a note. Binary files are returned as a base64-encoded string prefixed with "BASE64:" — decode it to recover the original binary content.',
+      'Read the content of a note. Binary files are returned as an MCP embedded resource (type: "resource") with uri, mimeType, and a base64-encoded blob field.',
       {
         type: 'object',
         properties: {
@@ -209,13 +233,21 @@ export async function registerFileOperationTools(server: ObsidianMCPServer): Pro
         const result = await executeObsidianCommandBinary('read', cmdArgs);
         handleCLIResult(result, { operation: 'read_note', identifier: sanitized.file || sanitized.path });
 
-        // Detect binary content from the raw buffer and return as base64
+        // Detect binary content from the raw buffer and return as MCP resource
         if (result.stdoutBuffer && isBinaryContent(result.stdoutBuffer)) {
+          const identifier = sanitized.file || sanitized.path as string;
+          const vaultName = process.env.OBSIDIAN_VAULT ?? 'vault';
+          const uri = `obsidian://${encodeURIComponent(vaultName)}/${encodeURIComponent(identifier)}`;
+          const mimeType = getMimeType(identifier);
           return {
             content: [
               {
-                type: 'text',
-                text: `BASE64:${result.stdoutBuffer.toString('base64')}`,
+                type: 'resource' as const,
+                resource: {
+                  uri,
+                  mimeType,
+                  blob: result.stdoutBuffer.toString('base64'),
+                },
               },
             ],
           };

src/utils/types.ts

@@ -16,10 +16,10 @@ export interface ToolInput {
  * Successful tool output
  */
 export interface ToolOutput {
-  content: Array<{
-    type: 'text';
-    text: string;
-  }>;
+  content: Array<
+    | { type: 'text'; text: string }
+    | { type: 'resource'; resource: { uri: string; mimeType?: string; blob: string } }
+  >;
   isError?: false;
 }