According to its GitHub repository, Coasts uses a Coastfile at the project root to reference an existing docker-compose setup then runs coast build to produce an image supporting multiple isolated runtimes (https://github.com/coast-guard/coasts). The system bind-mounts host project roots and worktrees into the container while using a separate /workspace mount; worktree switches are handled via umount, bind mount, and rshared propagation so inner Docker daemons receive updates. Agents execute commands against specific coast instances for testing without altering the original docker-compose.

Dynamic ports are defined in the Coastfile and exposed to the host while coast checkout employs socat to bind canonical ports such as 3000 or 5432 for hardcoded dependencies or webhook testing (https://news.ycombinator.com/item?id=47575417). The coast lookup command lets agents identify their assigned coast from within a worktree directory to run targeted exec operations such as tests. Coasts remains harness-agnostic for any host-side agent framework.

Configuration options include volume topologies, omission of unneeded services and volumes, host-side sharing of selected services, and per-service strategies for worktree assignment changes including none, hot, restart, or rebuild (https://github.com/coast-guard/coasts). Documentation videos and a demo video illustrate the runtime isolation approach originally developed after early 2024 experiments with Codex and Conductor.