Versions: @cloudflare/think 0.20.2 and agents 0.28.1. Still reproduces on 0.21.0 and 0.29.0, and the inspected main at fc7f59f has the same code.

What happens

Agent.shouldSendProtocolMessages() is documented as: "When this returns false for a connection, that connection will not receive any protocol text frames — neither on connect nor via broadcasts." The base Agent honours it: no identity, state or MCP frames are sent. Think doesn't:

1. Think's onConnect wrapper (_setupProtocolHandlers) sends _buildIdleConnectMessages() (the whole transcript) or _notifyStreamResuming() to every new connection. That wrapper is installed in onStart, so it runs before the base Agent's wrapper sets the per-connection protocol flag. _preStream.park() can also send cf_agent_stream_pending there.
2. _broadcastChat() and _broadcast() call this.broadcast() with no protocol filter. The base Agent's _broadcastProtocol() excludes connections where isConnectionProtocolEnabled() is false.

Reproduction

export class FramesAgent extends Think<Env> {
  getModel() { return scripted(() => text("one two three four five")); }
  shouldSendProtocolMessages() { return false; }   // every connection opts out
  onMessage() {}                                    // the socket speaks the app's own protocol
  async turn() { return this.runTurn({ input: "hi" }); }
}

Open a WebSocket to /agents/frames-agent/x, then call turn() over RPC. The socket receives:

on connect:  cf_agent_chat_messages
during turn: cf_agent_use_chat_response ×12, cf_agent_chat_messages ×2

Expected: none of those frames. No cf_agent_identity, cf_agent_state or cf_agent_mcp_servers arrive, which is correct.

Workaround: override broadcast() to exclude the app's sockets, and replace the private _buildIdleConnectMessages and _notifyStreamResuming on the instance.

Proposed fix

• In Think's onConnect wrapper, skip the transcript, resume and park frames when !this.shouldSendProtocolMessages(connection, ctx). Asking the hook directly is needed because the flag isn't set yet at that point.
• In _broadcastChat() and _broadcast(), exclude connections where !this.isConnectionProtocolEnabled(c), as _broadcastProtocol() does. Alternatively, route both through _broadcastProtocol().

Patching those three places in the 0.21.0 build gives zero frames on the opted-out socket. In-browser clients using useAgentChat are unaffected, because they keep the default true.