Remote sessions can connect MCP servers from mcp_set_servers after the turn starts

What

MCP servers are external tools Claude Code connects to. A program driving Claude Code through the SDK can add them with an mcp_set_servers request. In remote sessions, those connects can now finish after Claude Code has replied, instead of holding up the reply.

• Whether to defer is decided by a new check that replaces the old mcpNonBlocking value. The check returns "off" unless CLAUDE_CODE_REMOTE is set. It returns "on" only when the server flag tengu_ccr_mcp_set_servers_defer_connect is true, or tengu_ccr_mcp_set_servers_defer_connect_relay for relay-marked sessions. Both flags fall back to false, and nothing has been read about either for this release. Otherwise the check returns "cold" or "off", and only "on" defers.
• A new deferredMcpServersJoin step runs before the next turn and waits for deferred servers that are still connecting. It waits only in relay-marked sessions, for at most 8000 ms after the deferred start, and not when the main thread was requested. The older sdkMcpServersJoin step still exists.
• The startup wait for MCP servers in headless (non-interactive) mode gains onlyServerNames and alsoWaitFor options. There is a 500 ms cap for servers waited on only by name. Before, it could only wait on local servers and on remote servers named with waitRemoteServerNames.
• Each apply of mcp_set_servers records whether it was deferred, the check's result and whether the session was relay-marked.

Why

In cloud or remote sessions, adding MCP servers late no longer has to block the reply, and the first turn waits only a bounded time for them. Because the behaviour depends on server flags with a false fallback, it may not be active for you.