ACP + MCP on same stream

[nalbion]

I like the idea of ACP, and think that I can make it work on the same transport/stream as my MCP server, but if your protocolVersion in the initialize messages ever changes from an int to a string I'll be in trouble.

I think it would be better if ACP was an extension of MCP or had an extra protool: "acp" field in each message

[ConradIrwin]

We initially protyped a version of this, but it turns out that agents typically already have good support for connecting to MCP servers; and it's relatively invasive to have a "special" MCP server type that requires passing a connection around all over the codebase.

So, although it's marginally less efficient (in the edge case your ACP client is also an MCP server); it's much easier to work in a world where the MCP connections are MCP connections and ACP connections are ACP connections.