Miniflare Durable Objects polyfill conflicts with Sentry instrumentation

[AbhiPrasad]

Which Cloudflare product(s) does this pertain to?

Miniflare

What versions & operating system are you using?

MacOS (but should apply everywhere), Wrangler compat-date 2025-03-21

Please provide a link to a minimal reproduction

getsentry/sentry-mcp#134 - clone the repo at this commit

Describe the Bug

Sentry has instrumentation for @modelcontextprotocol/sdk package, released in 9.13.0 of the Sentry SDKs.

import { McpServer } from '@modelcontextprotocol/sdk/server/mcp.js';

// Create an MCP server
const server = new McpServer({
  name: 'Demo',
  version: '1.0.0',
});

// Use the instrumented server in your application
const instrumentedServer = Sentry.wrapMcpServerWithSentry(server);

Under the hood, this monkeypatches the MCP instance via Proxies: https://github.com/getsentry/sentry-javascript/blob/57893e2e61a2677fffd8bfc226f0ed60097500f3/packages/core/src/mcp-server.ts#L49

It seems like Miniflare is using a polyfill for private class fields/methods. The MCP transport that the cloudflare agents library configure uses private fields/methods. The polyfill minflare seems to work by doing WeakMap<Transport, PrivateField> mappings.

The Sentry instrumentation intercepts whatever transport is passed into the MCP server, Proxys it and forwards it to the MCP server. The problem is that the proxying is actually changing the identity of the transport, which will break the WeakMap.

Is it possible we can improve this polyfill so we don't break proxies? We rely on this for instrumentation, and theres not a better way to patch the library for now.

As an alternative, we could disable the patching just in miniflare, but we'd need a good way to detect this.

Please provide any relevant error logs

The error message we get is not very useful:

3:24:55 PM [vite] Internal server error: Cannot read from private field
@sentry/mcp-cloudflare:dev:       at async ProxyServer.fetch (file:///home/dcramer/src/sentry-mcp/node_modules/.pnpm/[email protected]/node_modules/miniflare/src/workers/core/proxy.worker.ts:173:11)
^C    ...Finishing writing to cache...   

ref: getsentry/sentry-mcp#136

[mhart]

FWIW the polyfill is from esbuild – that's how it transforms private fields (including the WeakMap stuff)

[petebacondarwin]

I have dug around a bit and it seems that the problem is actually with the build of the Cloudflare Agents library.
These WeakMaps are being injected into the distributable code that is then being imported in the sentry-mcp repo above.
You can find the code at packages/mcp-cloudflare/node_modules/agents/dist/mcp/index.js line 71.

This is the line that probably needs fixing: https://github.com/cloudflare/agents/blob/main/tsconfig.base.json#L14
From the typescript docs ES2021 or below maps private fields to WeakMaps in place of #.

[petebacondarwin]

I created an issue on the Agents repo: cloudflare/agents#264

[threepointone]

Fixed in [email protected]

[AbhiPrasad]

thank you all!