Cos Server

A small, harness-agnostic Docker control plane. It runs workloads — one-shot jobs and long-lived services — on the local Docker daemon, with arc-agnostic ownership, lifecycle, and reaping layered on top. All state lives in container labels (cos.managed=true), so there's no sidecar database.

Its primary front end is an MCP server: any MCP client (e.g. the arc agent runtime) connects over streamable-HTTP and gets container_* tools. The same core library is usable directly from Python and from the cos CLI.

Designed against agent-runtime/v2/_design/0024-container-orchestration-and-job-dispatch.md.

Why

Some capabilities can't (or shouldn't) run in a host process — heavy engines with hostile installs. The answer is "the environment is a dependency": ship a recipe (an image), and dispatch a job into a container. This service is the thing that runs those containers.

⚠️ Security / trust model — read before exposing this

cos drives the Docker daemon, which is root-equivalent on a standard install. Treat it accordingly.

Hardened (2026-07):

• Bind-mount sources are validated — /var/run/docker.sock, /, /proc, /sys, /dev, /etc, /var/run (and symlink/.. tricks) are rejected, so the mount-the-socket host-escape is closed.
• Every container gets pids_limit, no-new-privileges, and default cpu/mem caps (a spec's own limits override) — fork-bomb / OOM / setuid- escalation are shut off without breaking stock images.
• network=none default, loopback-only port publishing, host/container:* network modes rejected, correctly-quoted command/stdin (no shell injection).

Still open — know these:

• The MCP server is unauthenticated. Anything that can reach 127.0.0.1:8770 — any local process, and (absent an Origin/Host check) a malicious web page via DNS-rebinding — can drive Docker. Never bind it to anything but loopback, and never expose the port. It is a control plane for a single trusted local user, not a multi-tenant service.
• Not yet a full sandbox for untrusted code. cap_drop=ALL + read-only rootfs + a workspace mount allowlist remain a future opt-in hardened profile. The catastrophic vectors are closed; for genuinely untrusted binaries wait for that profile. Full analysis + mitigation log: _code_review/02-security-audit.md and _mitigation/ in the agent-runtime repo.

Concepts

• EnvSpec — how to obtain the image: image (pull), build (a context), or base + provision (a base image + setup steps, synthesized into a Dockerfile).
• WorkloadSpec — env + command + stdin + mounts + env vars + limits + network (default none) + lifecycle (ephemeral | persistent).
• Jobs run once, return {exit_code, stdout, stderr}, and auto-remove.
• Services are persistent, named, and reconnected by label (find-or-create).
• Networks — put cooperating containers on a user-defined network and they reach each other by name over Docker's embedded DNS. A persistent container named X is reachable at hostname cos-X. none (sandbox) and bridge (host-reachable, no inter-container DNS) remain the built-in modes; host and container:* are rejected.
• Images — build_image builds a named, cos.managed-labeled image ONCE (from a context dir, an inline Dockerfile, or base+provision); reference it from many containers via image=<tag> (build-once, run-many). image_list / image_remove manage them.
• GC — gc reclaims managed cruft: stopped containers, empty networks, and images not backing any container. Never touches running containers or unmanaged resources. All builds (including the base+provision cache) are labeled managed, so nothing accumulates unreclaimably.

Multi-container example

cos network create appnet
cos run python:3.11-slim --network appnet --cmd "python -m http.server 8000"  # (as a service via MCP container_ensure)
# a second container on appnet reaches the first at http://cos-<name>:8000
cos network ls

Over MCP: network_create / network_list / network_remove, plus network=<name> on container_run / container_ensure.

Build once, run many + clean up

cos image build myapp:latest --context ./myapp   # build + label the image once
cos run myapp:latest --cmd "..."                 # reference it by tag, N times
cos image ls
cos gc                                           # reclaim stopped/empty/unused

Over MCP: image_build / image_list / image_remove and gc.

Quick start

pip install -e ".[mcp]"          # core + MCP server
cos ping                         # check the daemon
cos run alpine:3.19 --cmd "echo hello"
cos serve --port 8770            # run the MCP server (streamable-HTTP)

Point an MCP client at it (arc example):

arc mcp add container --transport http --url http://127.0.0.1:8770/mcp

Status

v1: core library + Docker backend + MCP server + CLI. Native REST API + a programmatic Python client are deferred until an engine dispatcher needs a non-MCP path (see _deviations/).

License

MIT.