What
Claude Code saves what it learns from MCP servers (Model Context Protocol servers, which give Claude extra tools) in a discovery cache. The cache is a stored copy, so Claude Code does not have to ask the servers again. The cache's file name and lookup key now record whether the MCP Apps host is enabled, which gives one copy for each case. When a cache entry is written or deleted, both copies are now handled.
CLAUDE_CODE_MCP_APPS_HOST is what turns on the io.modelcontextprotocol/ui extension capability.
Why
When the MCP Apps host is turned on or off, Claude Code no longer reuses discovery results that were saved under the other setting.
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Whether `CLAUDE_CODE_MCP_APPS_HOST` is an environment variable or a setting, and how a user turns it on, is not shown.