[BUG] Claude Desktop (Windows on ARM): extensions with server.type "uv" fail with "Checksum verification failed"; managed uv download never completes

Environment

• Claude Desktop 2.31226.1.0, Windows 11 on ARM64 (MSIX package Claude_2.31226.1.0_arm64)
• Extension: local .mcpb, manifest_version 0.4, server.type: "uv", entry point src/server.py, with pyproject.toml

Steps to reproduce

1. On a Windows on ARM PC, install any .mcpb with "server": {"type": "uv", "entry_point": "src/server.py"}.
2. Install the extension, or enable it.

Expected

Claude Desktop downloads its managed uv and starts the server.

Actual

• Installation stops with: "Couldn't install extension … Checksum verification failed".
• After the extension is enabled, the server fails with the same error (three retries per start), also in the shared pool for Cowork and Code sessions.
• The folder %LOCALAPPDATA%\Packages\Claude_pzs8sxrjxfjjc\LocalCache\Roaming\Claude\uv-runtime\uv-0.9.7-win32-arm64 is created but stays empty.

Log (mcp-server-<name>.log):

[error] Checksum verification failed
  at Mb (...\Claude_2.31226.1.0_arm64__pzs8sxrjxfjjc\app\resources\app.asar\.vite\build\index.chunk-v4ACsSJK.js:13:840830)
[error] Couldn't start for Cowork and Code sessions. Error: Checksum verification failed

Evidence that the network and the artifact are fine

On the same machine and network, https://github.com/astral-sh/uv/releases/download/0.9.7/uv-aarch64-pc-windows-msvc.zip downloads in full (19,483,618 bytes), and its SHA-256 4482ad2544544e1966b6d933d38e27368ce0307739d293322c10459f698a1629 matches Astral's published .sha256. The checksum check or the platform mapping in Claude Desktop is likely wrong for win32-arm64 (cf. #30717, where a linux-arm64 binary was requested on darwin-arm64).

Workaround

server.type: "node" with a launcher that runs a uv the extension fetches and verifies itself works on the same machine.