{
  pkgs,
  ...
}:
let
  script = ../services/llama-cpp/llama-cpp-xmrig-pause.py;
  python = pkgs.python3;

  # SmolLM-135M Q2_K: 85MB, modern GGUFv3, generates ~30 tok/s on one CPU
  # thread — slow enough that a 200-token request keeps the process busy for
  # several seconds, fast enough that tests don't crawl.
  tinyModel = pkgs.fetchurl {
    url = "https://huggingface.co/QuantFactory/SmolLM-135M-GGUF/resolve/main/SmolLM-135M.Q2_K.gguf";
    hash = "sha256-DX46drPNJILNba21xfY2tyE0/yPWgOhz43gJdeSYKh4=";
  };
in
pkgs.testers.runNixOSTest {
  name = "llama-cpp-xmrig-pause";

  nodes.machine =
    { pkgs, ... }:
    {
      environment.systemPackages = [
        pkgs.python3
        pkgs.procps
        pkgs.curl
        pkgs.llama-cpp
      ];

      # Mock xmrig as a simple sleep process that can be stopped/started.
      systemd.services.xmrig = {
        description = "Mock xmrig miner";
        serviceConfig = {
          ExecStart = "${pkgs.coreutils}/bin/sleep infinity";
          Type = "simple";
        };
        wantedBy = [ "multi-user.target" ];
      };
    };

  testScript = ''
    import time

    PORT = 18088
    MODEL = "${tinyModel}"
    PYTHON = "${python}/bin/python3"
    SCRIPT = "${script}"

    # Tuned for test speed while remaining realistic.
    # POLL_INTERVAL=1 keeps detection latency low.
    # GRACE_PERIOD=5 is long enough to verify "stays stopped" but short enough
    # that the full test completes in ~2 minutes.
    # CPU_THRESHOLD=10 is low because the VM has limited cores and the model
    # is small — but any active inference still saturates a core.
    POLL_INTERVAL = "1"
    GRACE_PERIOD = "5"
    CPU_THRESHOLD = "10"

    infer_counter = 0

    def send_completion(n_predict=200):
        """Fire a completion request in the background via a transient systemd unit."""
        global infer_counter
        infer_counter += 1
        name = f"infer-{infer_counter}"
        machine.succeed(
            f"systemd-run --unit={name} --property=Type=exec "
            f"curl -sf -X POST http://127.0.0.1:{PORT}/completion "
            f"-H 'Content-Type: application/json' "
            f"-d '{{\"prompt\": \"Once upon a time in a land far away there lived\", \"n_predict\": {n_predict}}}'"
        )
        return name

    def wait_inference_done(unit_name, timeout=60):
        """Wait for a background inference request to finish."""
        machine.wait_until_fails(
            f"systemctl is-active {unit_name}",
            timeout=timeout,
        )

    start_all()
    machine.wait_for_unit("multi-user.target")
    machine.wait_for_unit("xmrig.service")

    with subtest("Start llama-server"):
        machine.succeed(
            f"systemd-run --unit=llama-server "
            # Single inference thread to maximise per-core CPU%, which is
            # what the monitor measures.  Keeps token generation slow enough
            # (~30 tok/s) that a 200-token request sustains load for seconds.
            f"llama-server --model {MODEL} --port {PORT} --ctx-size 512 -t 1 -np 1"
        )
        machine.wait_until_succeeds(
            f"curl -sf http://127.0.0.1:{PORT}/health",
            timeout=30,
        )
        machine.succeed("pgrep -x llama-server")

    with subtest("Start pause monitor"):
        machine.succeed(
            f"systemd-run --unit=llama-xmrig-pause "
            f"--setenv=POLL_INTERVAL={POLL_INTERVAL} "
            f"--setenv=GRACE_PERIOD={GRACE_PERIOD} "
            f"--setenv=CPU_THRESHOLD={CPU_THRESHOLD} "
            f"{PYTHON} {SCRIPT}"
        )
        # The monitor needs two consecutive polls to compute a CPU delta.
        # Wait for baseline to stabilise.
        time.sleep(3)

    with subtest("xmrig stays running while llama-server is idle"):
        machine.succeed("systemctl is-active xmrig")

    with subtest("xmrig stopped during prompt processing"):
        unit = send_completion(n_predict=200)
        machine.wait_until_fails("systemctl is-active xmrig", timeout=20)

    with subtest("xmrig remains stopped during grace period after inference ends"):
        wait_inference_done(unit)
        # Inference just finished.  The monitor will need 1-2 polls to detect
        # idle, then the grace period starts.  Checking 2s after completion
        # is well within the 5s grace window.
        time.sleep(2)
        machine.fail("systemctl is-active xmrig")

    with subtest("xmrig resumes after grace period expires"):
        # Already idle since previous subtest.  Grace period (5s) plus
        # detection delay (~2 polls) means xmrig should restart within ~8s.
        machine.wait_until_succeeds("systemctl is-active xmrig", timeout=15)

    with subtest("Sequential prompts do not cause xmrig flapping"):
        # First prompt — stop xmrig
        unit1 = send_completion(n_predict=200)
        machine.wait_until_fails("systemctl is-active xmrig", timeout=20)
        wait_inference_done(unit1)

        # Brief idle gap — shorter than grace period
        time.sleep(2)

        # Second prompt arrives before grace period expires, resetting it
        unit2 = send_completion(n_predict=200)
        time.sleep(3)

        # xmrig must still be stopped
        machine.fail("systemctl is-active xmrig")

        wait_inference_done(unit2)
        machine.wait_until_succeeds("systemctl is-active xmrig", timeout=15)

    with subtest("xmrig stays stopped during sustained inference"):
        unit = send_completion(n_predict=500)
        machine.wait_until_fails("systemctl is-active xmrig", timeout=20)

        # Stay busy longer than the grace period to prove continuous
        # activity keeps xmrig stopped indefinitely.
        time.sleep(8)
        machine.fail("systemctl is-active xmrig")

        wait_inference_done(unit)
        machine.wait_until_succeeds("systemctl is-active xmrig", timeout=15)
  '';
}