Support for MCP v2

[nazrhom]

Hi, I am working on an MCP server. Since MCP v2 is coming out soon(ish) I started building with the v2-alpha client.
From my understanding the current version of mcpcat is not compatible with the v2 sdk, is that correct? If so, I was wondering if you have an estimate on when support for that might be available.

Thanks for your great work!

[naji247]

Let me check, we have tests running on beta releases usually.

[nazrhom]

I was investigating this with claude. Ill post it's reasoning about it being incompatible here in case it may help:

  I read both sides of the contract and they're incompatible at the front door.

  What mcpcat requires (compatibility.ts, isCompatibleServerType): it gates on the legacy high-level method:
  typeof server.tool !== "function"  →  rejected
  It checks for a method literally named tool (the old server.tool(...) API), plus server._registeredTools. It does not accept
  registerTool. mcpcat targets @modelcontextprotocol/sdk v1.11+.

  What you actually have (@modelcontextprotocol/server@2.0.0-alpha.2 — note: a different package than /sdk). The v2 McpServer
  class (index.d.mts:498) exposes:
  - registerTool(), registerResource(), registerPrompt() — but no tool() method at all (grep: 0 hits). The legacy alias was
  dropped in v2.
  - readonly server: Server, private _registeredTools ✅ (those two match)

Also later on it reported

Risk: v2 registers tool handlers lazily (_toolHandlersInitialized, setToolRequestHandlers) and uses one
  consolidated CallToolRequest handler — mcpcat's low-level tracing wraps that handler, and its tool-injection (the mcpcat
  "context" tool) assumes v1 internals

[naji247]

I suspect they're moving to a middleware model which will make our lives easier. We make sure we're compatible with every official release before they're GA-ed, so not long!

FWIW the main change between v1 and v2 is making stateless HTTP transport the default. I would actually suggest using a finalized MCP version and building it with stateless mode in mind, then transition as I suspect it wouldn't be much effort. My 2 cents :)

[nazrhom]

Yeah, fair enough. Switching between v1 and v2 seems pretty simple. Thanks for the quick reply and looking into this!