What
MCP servers are add-ons that give Claude extra tools. Programs that drive Claude Code through its SDK (software development kit) or from a remote host send it control requests, which are instructions such as which MCP servers to use. The mcp_set_servers control request now accepts an optional denied_tools list:
- Each entry is a single MCP tool name written as
mcp__<server>__<tool>, and that tool is blocked for the session. - The list is passed on when Claude Code reconnects its MCP servers. It is not behind a flag.
- A list of more than 2,048 names, or any other bad value, is ignored with the warning "denied_tools must be an array of at most 2,048 single MCP tool names", and the session keeps the list it already had.
- The error-code description gains
side_question_busy.
Separately, network errors from the gateway that sandboxed sessions use to reach the internet are now split in two. A 502 answer to a connection attempt is reported as connect_unreachable, meaning the host was allowed but the gateway could not reach it. Other 4xx and 5xx answers stay connect_rejected, now meaning a policy denial. Before, every such answer was connect_rejected, described as "policy denial or upstream failure".
Why
A program hosting Claude Code can now keep chosen MCP tools away from Claude for one session without removing the whole server, and a bad list does not wipe out the existing one. When a network request fails in a sandbox, the error now shows whether the host was blocked by policy or simply could not be reached.