diff --git a/server.mjs b/server.mjs
index c9472eb..9631ede 100644
--- a/server.mjs
+++ b/server.mjs
@@ -343,7 +343,10 @@ async function saveImage({ dataUrl, model, prefix }) {
   const filename = `${prefix}-${safeModel}-${Date.now()}.png`;
   const filepath = path.join(OUTPUT_DIR, filename);
   await fs.writeFile(filepath, bytes);
-  return { filepath, filename, bytes: bytes.length };
+  // Return Buffer under `bytes` (NOT Buffer.length) — the caller's
+  // `bytes.toString('base64')` in the inline image block needs a real Buffer,
+  // not a Number. `size` holds the byte length for metadata/logging.
+  return { filepath, filename, bytes, size: bytes.length };
 }
 
 // ---------- MCP server ----------
@@ -507,11 +510,11 @@ server.setRequestHandler(CallToolRequestSchema, async (req) => {
       const { dataUrl, provider, mime } = await callOpenRouter({
         prompt, model, aspect, transparent, imageUrls,
       });
-      const { filepath, filename, bytes } = await saveImage({
+      const { filepath, filename, bytes, size } = await saveImage({
         dataUrl, model, prefix: transparent ? 'transparent' : 'opaque',
       });
       process.stderr.write(
-        `[gen] OK provider=${provider} mime=${mime} ${Date.now() - t0}ms ${filepath} ${bytes}b\n`,
+        `[gen] OK provider=${provider} mime=${mime} ${Date.now() - t0}ms ${filepath} ${size}b\n`,
       );
       // Return multipart content: image inline (base64) + text metadata.
       // The image block lets MCP clients (Cursor, Claude Desktop, Figma) render
@@ -527,7 +530,7 @@ server.setRequestHandler(CallToolRequestSchema, async (req) => {
         refs_used: imageUrls.length,
         filepath,
         filename,
-        bytes,
+        bytes: size,
         latency_ms: Date.now() - t0,
       };
       return {
@@ -580,7 +583,7 @@ server.setRequestHandler(CallToolRequestSchema, async (req) => {
   } catch (e) {
     process.stderr.write(`[gen-ERR] ${e.stack || e}\n`);
     return {
-      content: [{ type: 'text', text: `Error: ${e.message}\n\nStack: ${(e.stack||'').toString().slice(0, 1000)}` }],
+      content: [{ type: 'text', text: `Error: ${e.message}` }],
       isError: true,
     };
   }