Ping messages not forwarded to end-client in v0.1.22+

[EliavLunar]

Starting from mcp-remote v0.1.22, ping messages initiated by MCP servers are not being properly forwarded all the way to the end-client, making it impossible for MCP servers to use ping mechanism to detect client liveness when clients connect via mcp-remote. This appears to be a regression as v0.1.21 and earlier work correctly.

Reproduction

Minimal reproduction available at: https://github.com/TheLunarCompany/mcp-remote-ping

Steps:

1. Clone the repo and run npm install
2. Start MCP server (terminal 1): npm run server (sends ping to connected clients every 5s, disconnect clients after 3 failed pings)
3. Run client (terminal 2) with working version: npm run client 0.1.21 ✅
4. Then run client with affected version: npm run client 0.1.22 ❌

Expected behavior

Clients should respond to ping messages from server indefinitely.

Actual behavior (v0.1.22+)

• Only first ping message is responded correctly, subsequent pings fail to reach end-client (they do reach mcp-remote-fallback-test probe client)
• mcp-remote logs error in client for these subsequent pings (TypeError: Cannot read properties of undefined (reading 'method'))
• Server disconnects client after 3 failed pings (~15 seconds)

Environment

• OS: macOS Sequoia 15.5
• Hardware: MacBook Pro 16-inch 2021, Apple M1 Pro
• Node version: v22.9.0
• @modelcontextprotocol/sdk: 1.17.4 (both client and server)
• Tested mcp-remote versions:
  • working: 0.1.18, 0.1.21
  • affected: 0.1.22, 0.1.23, 0.1.24, 0.1.27, 0.1.29

This breaking change prevents proper connection health monitoring in production MCP servers when clients are using mcp-remote (e.g. when connecting from Claude Desktop).

Lastly, thank you for this fantastic package and for all your hard work! If helpful, I am happy to discuss a PR that adds a failing test.

[EliavLunar]

Opened a fix PR: #158