MCP servers enabled via /mcp mid-session don't become available until restart

Problem

When you enable an MCP server mid-session using the /mcp command, the tool schemas are not injected into the running conversation. The server appears enabled in the /mcp UI, but Claude cannot actually use any of its tools until you restart Claude Code.

This makes /mcp effectively a config editor rather than a runtime toggle.

Steps to Reproduce

1. Start Claude Code with only Context7 MCP enabled (GitHub + Supabase disabled)
2. Verify Context7 tools work — they do
3. Run /mcp and enable GitHub + Supabase MCP servers
4. Try to use a GitHub or Supabase MCP tool — fails with "No such tool available"
5. Restart Claude Code — tools now work

Expected Behavior

Enabling an MCP server via /mcp should make its tools available immediately in the current session, without requiring a restart. The tool schemas should be hot-loaded into the active conversation context.

Actual Behavior

The /mcp command updates the configuration but does not inject the newly enabled server's tool schemas into the running session. Tools remain unavailable until a full restart.

Why This Matters

This is the main friction point for managing context usage with MCP servers:

• Users who want to keep context lean can't selectively enable MCPs on-demand
• The only workflow is: quit → change config → restart → lose conversation context
• This defeats the purpose of having a runtime /mcp toggle

Combined with #23787 (lazy-loading MCP schemas), true mid-session MCP management would let users run lean by default and pull in heavy tool sets only when needed — without losing their conversation.

Environment

• Claude Code CLI
• macOS (Darwin 25.2.0)
• Multiple MCP servers configured (Context7, GitHub, Supabase)