Using Oban for MCP Tasks

Oban is a natural fit for MCP tasks. Job states map onto task statuses, PostgreSQL persistence works across nodes, and Oban Pro workflows enable chained task execution.

ConduitMCP has two task implementations, one per protocol era:

New code should target the modern path. The legacy section documents the frozen store for servers that still serve 2025-11-25 clients. A server serving both eras keeps both.

Runnable examples

• SQLite (single-node, dependency-light, both eras): examples/oban_tasks_server/ — full mix project; cd in and iex -S mix. Examples.ObanTasks.ExtStore is a modern ConduitMcp.Ext.Tasks.Store.
• Postgres (multi-node, production shape, legacy store): examples/oban_task_store.ex — drop-in reference.

Modern era: the tasks extension

Enable the extension and, for durability across nodes, point it at your own ConduitMcp.Ext.Tasks.Store (see Tasks Extension):

config :conduit_mcp, :ext_tasks, store: MyApp.ExtTaskStore

The tool

The tool returns {:task, attrs}. The library stores the task first, then calls :start with its id; enqueue the job there. The job can never see a missing task.

tool "analyze_dataset", "Run a long-running analysis" do
  param :dataset_id, :string, "Dataset ID", required: true

  handle fn _conn, %{"dataset_id" => dataset_id} ->
    {:task,
     ttl_ms: :timer.hours(1),
     poll_interval_ms: 2_000,
     status_message: "Queued",
     start: fn task_id ->
       {:ok, _job} =
         %{"task_id" => task_id, "dataset_id" => dataset_id}
         |> MyApp.AnalysisWorker.new()
         |> Oban.insert()
     end}
  end
end

The client must declare io.modelcontextprotocol/tasks in the request's client capabilities, otherwise it gets -32021. It then polls tasks/get, sending Mcp-Name: <taskId>, or listens for changes (see Status notifications).

The worker

The worker reports through ConduitMcp.Ext.Tasks, addressing the task by id:

defmodule MyApp.AnalysisWorker do
  use Oban.Worker, queue: :mcp_tasks, max_attempts: 3, unique: [period: 300, keys: [:task_id]]

  alias ConduitMcp.Ext.Tasks

  @impl Oban.Worker
  def perform(%Oban.Job{args: %{"task_id" => id, "dataset_id" => dataset_id}}) do
    case Tasks.status(id) do
      {:ok, %{status: "cancelled"}} ->
        {:cancel, "cancelled by client"}

      {:ok, %{status: "working"}} ->
        :ok = Tasks.progress(id, "Analyzing #{dataset_id}")
        report = MyApp.Analysis.run(dataset_id)
        _ = Tasks.complete(id, %{"content" => [%{"type" => "text", "text" => report}]})
        :ok

      {:ok, %{status: "input_required"}} ->
        {:snooze, 5}

      _terminal_or_expired ->
        {:cancel, "task gone"}
    end
  end
end

Terminal states are final: after cancelled, a late complete/2 returns {:error, :terminal} instead of overwriting it. Answers to require_input/3 arrive in Tasks.status/1's input_responses once every outstanding key is answered. Each key is single-use over the task's life.

Status notifications

Instead of polling, a modern client can name its tasks in the taskIds filter of subscriptions/listen and receive notifications/tasks (the full task state) on every change. The default notifier, ConduitMcp.Ext.Tasks.ListenNotifier, delivers them only to streams whose principal owns the task.

To also wake a snoozed job when the client answers, configure your own notifier and keep listen delivery by calling the default from it:

config :conduit_mcp, :ext_tasks, store: MyApp.ExtTaskStore, notifier: MyApp.TaskNotifier

defmodule MyApp.TaskNotifier do
  @behaviour ConduitMcp.Ext.Tasks.Notifier

  @impl true
  def task_changed(task) do
    ConduitMcp.Ext.Tasks.ListenNotifier.task_changed(task)
    if task.status == "working", do: MyApp.Jobs.wake(task.task_id)
  end
end

task_changed/1 runs in the process that made the change, and an exception in it is logged and swallowed. Delivery is node-local; see Subscriptions for fan-out across a cluster. notifier: nil turns notifications off.

The store

An Oban-backed ConduitMcp.Ext.Tasks.Store keeps the task record next to its job:

• Owner checks. get/2, update/3 and cancel/2 must answer {:error, :not_found} for another owner, exactly as for a missing id.
• Atomicity. Every read-modify-write must be atomic (a version column with compare-and-swap, SELECT … FOR UPDATE, a transaction). Otherwise a client's tasks/update can overwrite the worker's completed.
• No list. There is no tasks/list; task ids are bearer tokens.

Examples.ObanTasks.ExtStore in examples/oban_tasks_server/ implements this over SQLite with a version column.

Legacy era: ConduitMcp.Tasks (frozen)

ConduitMcp.Tasks makes the storage layer pluggable via the ConduitMcp.Tasks.Store behaviour — flip the framework from its default in-memory ETS store to an Oban-backed store with one config line:

config :conduit_mcp, :tasks_store, MyApp.ObanTaskStore

The legacy tasks/get, tasks/cancel, tasks/result, and tasks/list JSON-RPC routes dispatch through the configured store without any handler changes.

Status Mapping

Setup

Add to your deps:

{:oban, "~> 2.18"}

Migration

defmodule MyApp.Repo.Migrations.CreateMcpTasks do
  use Ecto.Migration

  def change do
    create table(:mcp_tasks, primary_key: false) do
      add :task_id, :string, primary_key: true
      add :oban_job_id, :integer
      add :status, :string, default: "working"
      add :method, :string
      add :result, :map
      add :metadata, :map, default: %{}
      timestamps()
    end

    create index(:mcp_tasks, [:status])
    create index(:mcp_tasks, [:oban_job_id])
  end
end

Schema

defmodule MyApp.McpTask do
  use Ecto.Schema
  import Ecto.Changeset

  @primary_key {:task_id, :string, autogenerate: false}
  schema "mcp_tasks" do
    field :oban_job_id, :integer
    field :status, :string, default: "working"
    field :method, :string
    field :result, :map
    field :metadata, :map, default: %{}
    timestamps()
  end

  def changeset(task, attrs) do
    task
    |> cast(attrs, [:task_id, :oban_job_id, :status, :method, :result, :metadata])
    |> validate_required([:task_id])
    |> validate_inclusion(:status, ~w(working input_required completed failed cancelled))
  end
end

Worker

defmodule MyApp.McpTaskWorker do
  use Oban.Worker,
    queue: :mcp_tasks,
    max_attempts: 3,
    unique: [period: 300, fields: [:args], keys: [:task_id]]

  # Allowlist of handler names → modules. NEVER resolve a client-supplied
  # string to a module via String.to_existing_atom/1 + apply/3.
  @handlers %{"analysis" => MyApp.AnalysisHandler}

  @impl Oban.Worker
  def perform(%Oban.Job{attempt: attempt, max_attempts: max, args: %{"task_id" => task_id} = args}) do
    update_status(task_id, "working")

    case execute(args) do
      {:ok, result} ->
        update_task(task_id, %{status: "completed", result: result})
        :ok

      {:error, reason} ->
        # Only mark "failed" on the final attempt; let earlier retries stay
        # "working" so the client doesn't see status flicker on each retry.
        if attempt >= max do
          update_task(task_id, %{status: "failed", result: %{"error" => inspect(reason)}})
        end

        {:error, reason}

      {:input_required, schema} ->
        update_task(task_id, %{
          status: "input_required",
          metadata: %{"schema" => schema}
        })
        {:snooze, 300}  # Snooze 5 min, retry after user provides input
    end
  end

  defp execute(%{"handler" => handler} = args) do
    case Map.fetch(@handlers, handler) do
      {:ok, module} -> module.execute(args)
      :error -> {:error, "unknown handler: #{handler}"}
    end
  end

  defp update_status(task_id, status), do: update_task(task_id, %{status: status})

  # Returns {:ok, task} | {:error, changeset} | {:error, :not_found} so callers
  # can react to a failed write instead of silently dropping it.
  defp update_task(task_id, updates) do
    case MyApp.Repo.get(MyApp.McpTask, task_id) do
      nil -> {:error, :not_found}
      task -> MyApp.Repo.update(MyApp.McpTask.changeset(task, updates))
    end
  end
end

Task Store

A ConduitMcp.Tasks.Store implementation. The framework dispatches every tasks/* route through this module once configured.

defmodule MyApp.ObanTaskStore do
  @behaviour ConduitMcp.Tasks.Store

  import Ecto.Query
  @repo MyApp.Repo

  # Task insert, job insert, and the oban_job_id back-link all commit in one
  # transaction. A crash mid-sequence rolls back — no orphaned task, no
  # unlinked job (which would leave cancel/1 unable to reach the job).
  def create_with_job(task_id, worker_args, opts \\ []) do
    attrs = %{task_id: task_id, status: "working", method: worker_args["method"]}
    job_args = Map.put(worker_args, "task_id", task_id)
    worker = Keyword.get(opts, :worker, MyApp.McpTaskWorker)

    Ecto.Multi.new()
    |> Ecto.Multi.insert(:task, MyApp.McpTask.changeset(%MyApp.McpTask{}, attrs))
    |> Oban.insert(:job, worker.new(job_args))
    |> Ecto.Multi.update(:link, fn %{task: task, job: job} ->
      MyApp.McpTask.changeset(task, %{oban_job_id: job.id})
    end)
    |> @repo.transaction()
    |> case do
      {:ok, %{link: task}} -> {:ok, to_map(task)}
      {:error, _step, reason, _changes} -> {:error, reason}
    end
  end

  def get(task_id) do
    case @repo.get(MyApp.McpTask, task_id) do
      nil -> {:error, :not_found}
      task -> {:ok, to_map(task)}
    end
  end

  def cancel(task_id) do
    case @repo.get(MyApp.McpTask, task_id) do
      nil ->
        {:error, :not_found}

      task ->
        # Don't crash if the job already finished — log a non-:ok return.
        with id when not is_nil(id) <- task.oban_job_id,
             {:error, reason} <- Oban.cancel_job(id) do
          require Logger
          Logger.warning("Oban.cancel_job(#{id}) failed: #{inspect(reason)}")
        end

        # Return the updated task per the Store `cancel/1` contract
        # ({:ok, task} | {:error, :not_found}), not a bare :ok.
        case @repo.update(MyApp.McpTask.changeset(task, %{status: "cancelled"})) do
          {:ok, updated} -> {:ok, to_map(updated)}
          {:error, changeset} -> {:error, changeset}
        end
    end
  end

  def list(opts \\ []) do
    query = from(t in MyApp.McpTask, order_by: [desc: t.inserted_at])
    query = if s = opts[:status], do: where(query, [t], t.status == ^s), else: query
    query = if l = opts[:limit], do: limit(query, ^l), else: query
    @repo.all(query) |> Enum.map(&to_map/1)
  end

  defp to_map(task) do
    %{
      "task_id" => task.task_id,
      "status" => task.status,
      "result" => task.result,
      "metadata" => task.metadata,
      "created_at" => task.inserted_at
    }
  end
end

Usage in MCP Server

Register the store once in config:

# config/config.exs
config :conduit_mcp, :tasks_store, MyApp.ObanTaskStore

Then write tools that use the standard ConduitMcp.Tasks API and the task/2 helper — the storage layer is transparent:

defmodule MyApp.MCPServer do
  use ConduitMcp.Server

  tool "analyze_dataset", "Run long-running analysis" do
    task_support :supported
    scope "analysis:run"
    param :dataset_id, :string, "Dataset ID", required: true

    handle fn _conn, %{"dataset_id" => dataset_id} ->
      task_id = ConduitMcp.Tasks.generate_id()

      {:ok, _} = MyApp.ObanTaskStore.create_with_job(task_id, %{
        # Must be a key registered in the worker's @handlers allowlist
        # (see above) — never the raw module name from the client.
        "handler" => "analysis",
        "dataset_id" => dataset_id
      })

      # Returns {:ok, %{"content" => [...], "_meta" => %{"task" => %{"id" => task_id}}}}.
      # Clients see the task id in the spec-compliant _meta.task.id shape.
      task(task_id, "Analysis started")
    end
  end
end

Why Oban?

• Multi-node: All nodes share the PostgreSQL-backed job queue
• Retries: Failed tasks retry automatically (configurable)
• Uniqueness: Prevent duplicate tasks with unique option
• Observability: Oban Web dashboard shows task status in real-time
• Cancellation: Oban.cancel_job/1 maps directly to MCP task cancellation
• Snooze: {:snooze, seconds} handles input_required → wait for user input → resume
• Oban Pro Workflows: Chain dependent tasks (e.g., fetch → transform → analyze)