diff --git a/packages/@ant/computer-use-mcp/src/toolCalls.ts b/packages/@ant/computer-use-mcp/src/toolCalls.ts
index eb68b0bb3..d7b796a94 100644
--- a/packages/@ant/computer-use-mcp/src/toolCalls.ts
+++ b/packages/@ant/computer-use-mcp/src/toolCalls.ts
@@ -37,6 +37,19 @@
 import type { CallToolResult } from "@modelcontextprotocol/sdk/types.js";
 import { randomUUID } from "node:crypto";
 
+/** Detect actual image MIME type from base64 data using magic bytes. */
+function detectMimeFromBase64(b64: string): string {
+  // First byte is enough to distinguish PNG (0x89) from JPEG (0xFF)
+  const c = b64.charCodeAt(0);
+  if (c === 0x89) return "image/png";
+  if (c === 0xFF) return "image/jpeg";
+  // RIFF = WebP
+  if (c === 0x52) return "image/webp";
+  // GIF
+  if (c === 0x47) return "image/gif";
+  return "image/png";
+}
+
 import { getDefaultTierForApp, getDeniedCategoryForApp, isPolicyDenied } from "./deniedApps.js";
 import type {
   ComputerExecutor,
@@ -2162,7 +2175,7 @@ async function handleScreenshot(
         {
           type: "image",
           data: shot.base64,
-          mimeType: "image/jpeg",
+          mimeType: detectMimeFromBase64(shot.base64),
         },
       ],
       screenshot: shot,
@@ -2231,7 +2244,7 @@ async function handleScreenshot(
       {
         type: "image",
         data: shot.base64,
-        mimeType: "image/jpeg",
+        mimeType: detectMimeFromBase64(shot.base64),
       },
     ],
     // Piggybacked for serverDef.ts to stash on InternalServerContext.
@@ -2310,7 +2323,7 @@ async function handleZoom(
 
   // Return the image. NO `.screenshot` piggyback — this is the invariant.
   return {
-    content: [{ type: "image", data: zoomed.base64, mimeType: "image/jpeg" }],
+    content: [{ type: "image", data: zoomed.base64, mimeType: detectMimeFromBase64(zoomed.base64) }],
   };
 }
 
diff --git a/src/utils/computerUse/wrapper.tsx b/src/utils/computerUse/wrapper.tsx
index 05a1f81fd..5571f0437 100644
--- a/src/utils/computerUse/wrapper.tsx
+++ b/src/utils/computerUse/wrapper.tsx
@@ -30,6 +30,7 @@ import { getSessionId } from '../../bootstrap/state.js'
 import { ComputerUseApproval } from '../../components/permissions/ComputerUseApproval/ComputerUseApproval.js'
 import type { Tool, ToolUseContext } from '../../Tool.js'
 import { logForDebugging } from '../debug.js'
+import { detectImageFormatFromBase64 } from '../imageResizer.js'
 import {
   checkComputerUseLock,
   tryAcquireComputerUseLock,
@@ -330,7 +331,7 @@ export function getComputerUseMCPToolOverrides(
                 type: 'image' as const,
                 source: {
                   type: 'base64' as const,
-                  media_type: item.mimeType ?? 'image/jpeg',
+                  media_type: item.mimeType ?? detectImageFormatFromBase64(item.data),
                   data: item.data,
                 },
               }