• Actions Taken:

    • Created a jail/ directory to house a new, scripted agent environment based on the JAIL_PROTOTYPE.md design.
    • Implemented a flake.nix to provide a consistent development shell with colima, docker, and other necessary tools.
    • Created a run.sh script to launch a jailed container using a pre-built nixos/nix image, which installs tools like socat, dtach, and bun on startup.
    • Created an agent.ts script to programmatically send commands to the jailed container and receive output.
    • Wrote jail/README.md to document the new, simplified workflow.

  • Friction/Success Points:

    • The development process was a lengthy and iterative debugging session that uncovered multiple layers of issues.
    • Initial Approach (Failure): The first attempt to build a custom Docker image using nix build on macOS failed due to Linux-specific dependencies (virtiofsd) that could not be built on Darwin.
    • Second Approach (Failure): The next attempt involved running the nix build command inside a temporary nixos/nix container. This failed due to a nested virtualization issue where the build process required KVM, which was unavailable inside the container.
    • Third Approach (Success): The final, successful approach abandoned building a custom image altogether. Instead, we use a standard nixos/nix image and install the required tools at runtime. This proved to be far more robust and portable.
    • Networking Debugging: Solved a series of networking issues, from realizing Colima required a --network-address flag to expose an IP, to correcting the docker run port mapping.
    • Docker Context: The DOCKER_HOST environment variable was not set correctly, preventing the docker CLI from connecting to the Colima daemon. The final solution was to add a shellHook to flake.nix to export this variable automatically.
    • Shell Interaction: The agent script was initially unable to capture command output because the interactive shell in the container would echo the command back, prematurely triggering the end-of-command logic. This was resolved by making the container’s shell non-interactive.

  • Lessons Learned:

    • Building Linux Docker images with Nix on macOS is fraught with platform compatibility issues. Using a pre-built Linux image and installing packages at runtime is a much more reliable pattern.
    • For programmatic control of a shell, a non-interactive shell (bash -l) is vastly superior to an interactive one (bash -li), as it provides a clean I/O stream without terminal echo.
    • Automatically configuring the environment (like setting DOCKER_HOST in a shellHook) is critical for creating a smooth and reproducible developer experience.
    • The debugging process, while frustrating, was essential for arriving at a simple and robust final solution. Each failure revealed a deeper layer of the problem and led to a better design.