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fix: correct detectMimeFromBase64 to decode raw bytes from base64

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The charCodeAt approach compared raw byte magic numbers against
base64-encoded characters which never matched. Decode first 12 raw
bytes and check standard magic byte signatures directly.
Fixes API 400 on Windows (JPEG) and macOS (PNG) screenshots.
This commit is contained in:
unraid
2026-04-14 19:54:11 +08:00
parent 637c9081f6
commit ee369549a8
1 changed files with 14 additions and 9 deletions
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23
packages/@ant/computer-use-mcp/src/toolCalls.ts
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@@ -37,16 +37,21 @@
import type { CallToolResult } from "@modelcontextprotocol/sdk/types.js";
import { randomUUID } from "node:crypto";
/** Detect actual image MIME type from base64 data using magic bytes. */
/** Detect actual image MIME type from base64 data by decoding the 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";
// Decode first 12 raw bytes (16 base64 chars is enough) and check standard magic bytes.
// PNG: 89 50 4E 47
// JPEG: FF D8 FF
// RIFF+WEBP: "RIFF" at 0..3 + "WEBP" at 8..11
// GIF: "GIF" at 0..2
const raw = Buffer.from(b64.slice(0, 16), "base64");
if (raw[0] === 0x89 && raw[1] === 0x50 && raw[2] === 0x4e && raw[3] === 0x47) return "image/png";
if (raw[0] === 0xff && raw[1] === 0xd8 && raw[2] === 0xff) return "image/jpeg";
if (
raw[0] === 0x52 && raw[1] === 0x49 && raw[2] === 0x46 && raw[3] === 0x46 && // RIFF
raw[8] === 0x57 && raw[9] === 0x45 && raw[10] === 0x42 && raw[11] === 0x50 // WEBP
) return "image/webp";
if (raw[0] === 0x47 && raw[1] === 0x49 && raw[2] === 0x46) return "image/gif";
return "image/png";
}