Behaviour every backend implements so a UI (Smart Cell, Livebook, …) can discover its configurable settings and probe reachability without hard-coding per-backend knowledge.

Smart Cell sketch

for module <- AtpClient.backends() do
  %{
    key:    module.config_key(),
    label:  module.label(),
    fields: module.config_schema()
  }
end

# Once the user fills the form:
AtpClient.StarExec.verify(
  base_url: "https://...",
  username: "...",
  password: "..."
)
# => :ok | {:error, term}

verify/1 accepts the same keyword list every other backend call accepts, so the cell can probe without first writing values into Application env.

Two error channels

query/2 returns {:ok, %AtpClient.Result{}} or {:error, term()}, and the two mean different things:

• {:ok, result} is the system's verdict: it ran and asserted an SZS status, which result.status carries. The no-success values are results too — :timeout, :gave_up and :input_error all arrive here.
• {:error, reason} means no status was obtained. The reasons a reader produces are enumerated in AtpClient.Verdict.failure/0 — output with no status line and no known pattern, a status word outside the ontology, a crash signature. Everything else is a transport or session error: connection refused, an HTTP status, a session that would not open, missing configuration, a missing binary. Their shapes are backend-specific and documented on each backend; match {:error, _} last.

A common idiom:

case Backend.query(problem, opts) do
  {:ok, %AtpClient.Result{status: status}} -> {:verdict, status}
  {:error, {:prover_not_found, _}} -> :missing_binary
  {:error, {:unrecognized_output, _}} -> :bad_parse
  {:error, reason} -> {:transport, reason}
end

Cancellation contract

Each backend treats the death of the process calling query/2 as a request to abort. Implementations must (a) release external resources — OS children, network connections, remote jobs — and (b) clean up state held in helper processes. Death-as-cancellation is the only required cancellation mechanism; backends may additionally expose explicit handles, but they are not required to.

The intended use is Process.exit(task_pid, :kill) from a supervisor task running query/2: the host kills the Task, the backend tears the upstream work down, and no resource is left running.