Source Intelligence
Follow Discord

DisclaimerUnofficial, and not affiliated with Anthropic. Nearly all of this is read straight out of what ships: npm bundles, captured prompts, published docs. Anthropic's own notes go in verbatim, marked as theirs. The rest is my reading, and every entry carries the strings behind it. If one looks wrong, vote it down and say why.

← All of v2.1.242 Home All releases ←olderv2.1.241 v2.1.243newer→
Claude Code v2.1.242

Plugin-registered tools are served over a local MCP server

Under the hood
Useful2 Signal3
Plugins

Tools a plugin registers reach the model through a local loopback MCP server.

What

Tools a plugin registers are exposed by starting an HTTP server bound to 127.0.0.1 on a random path, which is then registered as an MCP server scoped to that plugin. This is how plugin tools reach the model.

Details
  • Calling a tool that no handler answered returns an error result telling the plugin author to add a tool.call handler for the tool's full name, in the form mcp__<plugin>__<tool>.
  • The port is chosen at runtime and logged.
  • Behind the tengu_plugin_hooks_modules rollout flag.
Evidence

loopback MCP server listening on 127.0.0.1:

Strings lifted out of the shipped bundle, so the claim above can be checked against them.

See this entry in the whole of v2.1.242 →