You'll noticeTier: how much it should matter to you
3Useful: my rating, 1 to 5
1Signal: worth watching, 1 to 5
HooksArea: what it touches
Bug FixesKind: in v2.1.281,
Bug FixesSection of the release
What
An mcp_tool hook is a hook (a user-configured action that runs automatically at certain moments) that calls a tool on an MCP server, meaning an external program that adds tools to Claude Code. If that server was still connecting, the hook used to be skipped at once as "not connected". Now the hook waits for the server to connect:
It waits up to the hook's own timeout or the MCP connect timeout.
Once the server is retrying a reconnect, the wait is capped at 5 seconds.
One overall deadline covers both the wait and the tool call itself.
While waiting, it reports that it is waiting up to a number of milliseconds for the named server to finish connecting. Some events do not wait, including Notification, SessionStart, SessionEnd and Setup.
Why
Hooks that fire early, before a slow server is ready, no longer silently do nothing. Hooks on events like SessionStart still run without waiting, so starting or ending a session is not held up by a slow server.
Read from
Names in the bundlemcp_tool
Since it was published
The entry above is what we published on the day. These lines were added later, as Anthropic's own pages caught up, and they sit beside the original rather than replacing it.
Added sinceA small documentation edit on Automate actions with hooks touched a line naming mcp_tool after this was published.* `"type": "mcp_tool"`: call a tool on a configured MCP server. See [MCP tool hooks](/docs/en/hooks#mcp-tool-hook-fields).hooks-guidesee the edit
Confirmed sinceAnthropic's documentation has since written up mcp_tool, on mid-conversation-system-messages.`tool_addition` and `tool_removal` are content blocks in the `content` array of a `role: "system"` message, and they can be mixed with `text` blocks in the same message. The message follows the placement rules for any mid-conversation syst…build-with-claude/mid-conversation-system-messagessee the edit
How sure we are
Two sources agreeTwo things we can check say the same as this entry.
Anthropic's documentation agreesAnthropic's documentation has since written up mcp_tool, on mid-conversation-system-messages.
Anthropic's release notes agreeFixed mcp_tool hooks on blocking events (PreToolUse and similar) being skipped while their MCP server was still connecting; they now wait…
