Eugene Plexus — site-host

The piece of Eugene Plexus that runs a machine's tools for Workbench: a local MCP host whose policy is final on that machine.

• Eugene's own file server, one per folder registered on the machine: list_directory, read_text and, on a writable folder, write_text.

• Local MCP servers a machine administrator adds at the machine.

Its contract is openapi/site-host.yaml in specs; the design is docs/design/remote-nodes.md §3.4 and §6.2.

What it is, and what it is not

It decides. Every request reaches it as one MCP request of the 2026-07-28 revision, relayed by the machine's agent from the root's queue. The host checks the server, the person and the tool against its own policy before anything runs, and records the call in an audit log kept on the machine. Nothing upstream can make it run what it refuses.

• Default deny. A server is off until its owner turns it on (a folder's server is on while the folder is registered), and a person may use only the tools the owner named for them.

• Destructive tools need a standing pre-approval. A tool the server does not mark read-only is treated as able to change something, as MCP itself defaults.

• A system server (one that could alter the operating system) cannot be turned on without an administrator's consent recorded at the machine.

On a Job Site (site mode) the host keeps its own folders, its own list of who may use what, and its own audit log, and takes changes to them only from the owner the site pinned when it joined. On an ordinary LAN node (node mode) Eugene's owner writes policy from the console, and the root's grant travels with each call.

It is not a server anyone dials. It listens on loopback only and talks to nobody but its agent. It runs in an OS account of its own, never inside the agent's privileged supervisor, and the agent installs it at a pinned commit. Built that way so it can later ship as an unprivileged install of its own.

Related MCP server: Local Workspace MCP

Development

uv venv --python 3.12 .venv
uv pip install -e ".[dev]"
pytest
ruff check . && ruff format --check . && mypy src/
python scripts/codegen.py   # after bumping SPECS_REF

File tools run only under a Windows service or Linux system install (EUGENE_PLEXUS_APP_ACCOUNT_KIND is windows_service or systemd): the folder code holds handles and refuses links, and on Linux confines each operation with Landlock.

Licence

Apache-2.0. Contributions are signed off under the DCO (git commit -s); see CONTRIBUTING.md.