Error Handling

Configure error policies, handle failures, and recover gracefully.

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Setup How errors flow Error directives from actions Handling errors in cmd/2 The five error policies Supervision and restarts Next steps

Setup

Mix.install([
  {:jido, "~> 2.0"}
])

Jido.start_link(name: MyApp.Jido)

Define a failing action and a simple agent to use throughout this guide.

defmodule MyApp.FailingAction do
  use Jido.Action,
    name: "failing_action",
    description: "An action that always fails",
    schema: []

  @impl true
  def run(_params, _context) do
    {:error, "something went wrong"}
  end
end
defmodule MyApp.CounterAgent do
  use Jido.Agent,
    name: "counter_agent",
    schema: [
      count: [type: :integer, default: 0]
    ]
end

How errors flow

Actions return {:error, reason} from their run/2 callback. The execution strategy catches the error and wraps it into a %Jido.Agent.Directive.Error{} struct containing a Jido.Error and a context atom like :instruction.

The agent struct is never corrupted by a failed action. cmd/2 returns the original state alongside the error directive in its directives list.

When running inside an AgentServer, the server processes each Error directive through the configured error_policy. The policy decides whether to log, stop, emit a signal, or run custom recovery logic.

Error directives from actions

When an action returns {:error, reason}, the strategy wraps it into a directive.

agent = MyApp.CounterAgent.new()
{agent, directives} = MyApp.CounterAgent.cmd(agent, MyApp.FailingAction)

IO.inspect(directives, label: "Directives")

The directives list contains a %Jido.Agent.Directive.Error{error: %Jido.Error{...}, context: :instruction}. The agent’s state remains unchanged.

Handling errors in cmd/2

Pattern match on the directives list to detect errors before they reach the runtime.

{agent, directives} = MyApp.CounterAgent.cmd(agent, MyApp.FailingAction)

errors =
  Enum.filter(directives, &match?(%Jido.Agent.Directive.Error{}, &1))

case errors do
  [] -> IO.puts("All actions succeeded")
  [%{error: err} | _] -> IO.puts("Failed: #{err.message}")
end

This gives you full control at the pure-function layer without needing a running AgentServer.

The five error policies

Error policies apply when an agent runs inside Jido.AgentServer. You set the policy with the :error_policy option on start_link/1.

:log_only (default)

Logs the error and continues processing. The agent stays alive with its state unchanged.

{:ok, pid} =
  Jido.AgentServer.start_link(
    jido: MyApp.Jido,
    agent: MyApp.CounterAgent,
    error_policy: :log_only
  )

The server calls Logger.error("Agent <id> [instruction]: something went wrong") and moves on to the next queued signal.

:stop_on_error

Logs the error and stops the agent process. Use this when any failure is unacceptable and the agent should not continue.

{:ok, pid} =
  Jido.AgentServer.start_link(
    jido: MyApp.Jido,
    agent: MyApp.CounterAgent,
    error_policy: :stop_on_error
  )

The process exits with reason {:agent_error, %Jido.Error{}}. A supervisor can restart it if configured to do so.

### {:max_errors, n}

Counts errors and stops the agent after n accumulated failures. Errors below the threshold are logged as warnings.

{:ok, pid} =
  Jido.AgentServer.start_link(
    jido: MyApp.Jido,
    agent: MyApp.CounterAgent,
    error_policy: {:max_errors, 5}
  )

Each error increments state.error_count. Below the limit, the server logs "Agent <id> error 3/5: ..." and continues. At the threshold, the process stops with reason {:max_errors_exceeded, 5}.

### {:emit_signal, dispatch_cfg}

Emits a jido.agent.error signal via Jido.Signal.Dispatch and continues processing. The signal includes the error message, context, and agent ID.

{:ok, pid} =
  Jido.AgentServer.start_link(
    jido: MyApp.Jido,
    agent: MyApp.CounterAgent,
    error_policy: {:emit_signal, {:pubsub, topic: "agent_errors"}}
  )

The dispatched signal looks like this:

%Jido.Signal{
  type: "jido.agent.error",
  source: "/agent/<id>",
  data: %{
    error: "something went wrong",
    context: :instruction,
    agent_id: "<id>"
  }
}

The dispatch runs in a supervised task so it does not block the agent.

Custom function

Pass a 2-arity function that receives the %Jido.Agent.Directive.Error{} and the server’s internal %Jido.AgentServer.State{}. Return {:ok, state} to continue or {:stop, reason, state} to shut down.

custom_handler = fn error_directive, state ->
  IO.puts("Custom handler: #{error_directive.error.message}")
  {:ok, state}
end

{:ok, pid} =
  Jido.AgentServer.start_link(
    jido: MyApp.Jido,
    agent: MyApp.CounterAgent,
    error_policy: custom_handler
  )

If your function raises or throws, the error is logged and the agent continues. This prevents a buggy error handler from crashing the agent process.

Supervision and restarts

Error policies handle expected failures - actions that return {:error, reason}. Unexpected crashes - exceptions, bad pattern matches, killed processes - bypass error policies entirely.

Jido.AgentServer processes run under a DynamicSupervisor with a :one_for_one strategy. If the GenServer process crashes, the supervisor restarts it according to its restart configuration.

Keep this distinction clear: use error policies for application-level failures you can anticipate, and rely on supervision for infrastructure-level crashes you cannot.

Next steps

Now that you know how to handle failures, explore related patterns.

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