diff --git a/lib/anubis/server/session/scheduler.ex b/lib/anubis/server/session/scheduler.ex
index 84225710..ca8a7894 100644
--- a/lib/anubis/server/session/scheduler.ex
+++ b/lib/anubis/server/session/scheduler.ex
@@ -183,25 +183,17 @@ defmodule Anubis.Server.Session.Scheduler do
 
   defp do_handle_request(module, %{"method" => "tools/call"} = request, frame, _method) do
     tool_name = get_in(request, ["params", "name"])
+    arguments = get_in(request, ["params", "arguments"])
 
-    :telemetry.span(
-      [:anubis_mcp | Telemetry.event_server_tool_call()],
-      %{tool: tool_name},
-      fn ->
-        result = module.handle_request(request, frame)
-        {result, %{tool: tool_name, is_error: tool_call_error?(result)}}
-      end
-    )
+    Telemetry.span_tool_call(tool_name, arguments, fn ->
+      module.handle_request(request, frame)
+    end)
   end
 
   defp do_handle_request(module, request, frame, _method) do
     module.handle_request(request, frame)
   end
 
-  defp tool_call_error?({:error, _reason, _frame}), do: true
-  defp tool_call_error?({:reply, %{"isError" => true}, _frame}), do: true
-  defp tool_call_error?(_result), do: false
-
   defp decode_task_result({:reply, response, %Frame{} = frame}, inflight, state) do
     Telemetry.execute(
       Telemetry.event_server_response(),
diff --git a/lib/anubis/server/session/tasks.ex b/lib/anubis/server/session/tasks.ex
index f507b7ad..dc89372e 100644
--- a/lib/anubis/server/session/tasks.ex
+++ b/lib/anubis/server/session/tasks.ex
@@ -11,6 +11,7 @@ defmodule Anubis.Server.Session.Tasks do
   alias Anubis.Server.Handlers.Tasks, as: TasksHandler
   alias Anubis.Server.Session.ServerRequests
   alias Anubis.Server.Task, as: McpTask
+  alias Anubis.Telemetry
 
   @default_task_ttl 60_000
   @max_task_ttl to_timeout(hour: 1)
@@ -401,10 +402,14 @@ defmodule Anubis.Server.Session.Tasks do
     request = %{request | "params" => Map.delete(params, "task")}
 
     server_module = state.server_module
+    tool_name = params["name"]
+    arguments = params["arguments"]
 
     worker =
       Task.Supervisor.async_nolink(state.task_supervisor, fn ->
-        Handlers.handle(request, server_module, frame)
+        Telemetry.span_tool_call(tool_name, arguments, fn ->
+          Handlers.handle(request, server_module, frame)
+        end)
       end)
 
     ttl_timer = Process.send_after(self(), {:task_expired, task.id}, ttl)
diff --git a/lib/anubis/telemetry.ex b/lib/anubis/telemetry.ex
index 4788f667..d55af558 100644
--- a/lib/anubis/telemetry.ex
+++ b/lib/anubis/telemetry.ex
@@ -1,6 +1,8 @@
 defmodule Anubis.Telemetry do
   @moduledoc false
 
+  @default_capture_tool_payload false
+
   @doc """
   Execute a telemetry event with the Anubis MCP namespace.
 
@@ -14,6 +16,61 @@ defmodule Anubis.Telemetry do
     :telemetry.execute([:anubis_mcp | event_name], measurements, metadata)
   end
 
+  @doc """
+  Wraps a tool call handler invocation in the `[:server, :tool_call]`
+  telemetry span.
+
+  Shared by the synchronous scheduler dispatch and the task-augmented
+  `tools/call` worker path so both surface identical span data regardless
+  of which route a given request took.
+
+  The span's `:start` metadata always carries `tool`; the `:stop` metadata
+  always carries `tool` and `is_error`. When
+  `:telemetry_capture_tool_payload` is enabled (defaults to `false`), the
+  `:start` metadata also carries `arguments` and the `:stop` metadata also
+  carries `result`. See `pages/testing.md` for the rationale behind the
+  opt-in default.
+
+  ## Examples
+
+      iex> Anubis.Telemetry.span_tool_call("get_weather", %{"city" => "NYC"}, fn -> :ok end)
+      :ok
+  """
+  @spec span_tool_call(String.t() | nil, map() | nil, (-> result)) :: result when result: var
+  def span_tool_call(tool_name, arguments, fun) do
+    capture_payload? =
+      Application.get_env(:anubis_mcp, :telemetry_capture_tool_payload, @default_capture_tool_payload)
+
+    start_metadata =
+      if capture_payload? do
+        %{tool: tool_name, arguments: arguments}
+      else
+        %{tool: tool_name}
+      end
+
+    :telemetry.span(
+      [:anubis_mcp | event_server_tool_call()],
+      start_metadata,
+      fn ->
+        result = fun.()
+        is_error = tool_call_error?(result)
+
+        stop_metadata =
+          if capture_payload? do
+            %{tool: tool_name, is_error: is_error, result: result}
+          else
+            %{tool: tool_name, is_error: is_error}
+          end
+
+        {result, stop_metadata}
+      end
+    )
+  end
+
+  defp tool_call_error?({:error, _reason, _frame}), do: true
+  defp tool_call_error?({:reply, %{"isError" => true}, _frame}), do: true
+  defp tool_call_error?(_result), do: false
+
   # Define event name constants to ensure consistency
 
   # Client events
diff --git a/pages/testing.md b/pages/testing.md
index 8ba72ea5..241f9e91 100644
--- a/pages/testing.md
+++ b/pages/testing.md
@@ -176,6 +176,56 @@ end
 
 The `start: true` option forces the HTTP transport to boot even though no Phoenix endpoint is serving during tests. One integration test per server is usually enough; the per-component behavior belongs in the unit tests above.
 
+## Observing tool calls
+
+Every `tools/call` request — including task-augmented calls dispatched via
+the `tasks/` worker path — is wrapped in a `:telemetry.span/3` under
+`[:anubis_mcp, :server, :tool_call]`. By default the span's metadata carries
+only the tool name and whether the call errored — enough to build dashboards
+and alerts, not enough to answer "what was this client actually asking, and
+what did we return."
+
+Set `:telemetry_capture_tool_payload` to opt into the full payload:
+
+```elixir
+# config/config.exs (or runtime.exs)
+config :anubis_mcp, :telemetry_capture_tool_payload, true
+```
+
+With the flag enabled, the `:start` event's metadata gains `arguments` (the
+raw `params.arguments` map from the request) and the `:stop` event's
+metadata gains `result` (the value your callback returned). `arguments` is
+only present on `:start`, and `result`/`is_error` only on `:stop` — attach to
+both events if you need both:
+
+```elixir
+require Logger
+
+:telemetry.attach_many(
+  "log-tool-payloads",
+  [
+    [:anubis_mcp, :server, :tool_call, :start],
+    [:anubis_mcp, :server, :tool_call, :stop]
+  ],
+  fn
+    [:anubis_mcp, :server, :tool_call, :start], _measurements, %{tool: tool, arguments: args}, _config ->
+      Logger.info("tool_call_start", tool: tool, arguments: args)
+
+    [:anubis_mcp, :server, :tool_call, :stop], _measurements, %{tool: tool, result: result}, _config ->
+      Logger.info("tool_call_stop", tool: tool, result: inspect(result))
+  end,
+  nil
+)
+```
+
+The flag defaults to `false`. Tool arguments and results are the
+highest-cardinality, highest-PII surface in the request lifecycle — this
+mirrors the `opt_in` requirement level OpenTelemetry's GenAI semantic
+conventions assign to the equivalent `gen_ai.tool.call.arguments` /
+`gen_ai.tool.call.result` span attributes. Only enable it where you also
+control retention and redaction of whatever sink receives the telemetry
+event (a Datadog trace, a log pipeline, a database).
+
 ## Next steps
 
 - [Building a Server](building-a-server.md) documents the callbacks tested here.
diff --git a/test/anubis/server/session_test.exs b/test/anubis/server/session_test.exs
index fbd2fa12..3f0e76bc 100644
--- a/test/anubis/server/session_test.exs
+++ b/test/anubis/server/session_test.exs
@@ -744,6 +744,89 @@ defmodule Anubis.Server.SessionTest do
     end
   end
 
+  describe "tool_call telemetry opt-in payload capture" do
+    setup do
+      original_config = Application.fetch_env(:anubis_mcp, :telemetry_capture_tool_payload)
+
+      on_exit(fn ->
+        case original_config do
+          {:ok, value} -> Application.put_env(:anubis_mcp, :telemetry_capture_tool_payload, value)
+          :error -> Application.delete_env(:anubis_mcp, :telemetry_capture_tool_payload)
+        end
+      end)
+
+      test_pid = self()
+      handler_id = "test-tool-call-payload-#{System.unique_integer([:positive])}"
+
+      :telemetry.attach_many(
+        handler_id,
+        [
+          [:anubis_mcp, :server, :tool_call, :start],
+          [:anubis_mcp, :server, :tool_call, :stop]
+        ],
+        fn
+          [:anubis_mcp, :server, :tool_call, :start], _m, meta, _c -> send(test_pid, {:tool_call_start, meta})
+          [:anubis_mcp, :server, :tool_call, :stop], _m, meta, _c -> send(test_pid, {:tool_call_stop, meta})
+        end,
+        nil
+      )
+
+      on_exit(fn -> :telemetry.detach(handler_id) end)
+
+      transport_name = Registry.transport_name(TasksStubServer, StubTransport)
+      start_supervised!({StubTransport, name: transport_name}, id: :payload_transport)
+      task_sup = Registry.task_supervisor_name(TasksStubServer)
+      start_supervised!({Task.Supervisor, name: task_sup}, id: :payload_task_sup)
+
+      session_id = "tool-call-payload-#{System.unique_integer([:positive])}"
+      session_name = Registry.session_name(TasksStubServer, session_id)
+
+      session =
+        start_supervised!(
+          {Session,
+           session_id: session_id,
+           server_module: TasksStubServer,
+           name: session_name,
+           transport: [layer: StubTransport, name: transport_name],
+           task_supervisor: task_sup},
+          id: :payload_session
+        )
+
+      init_msg = init_request("2025-03-26", %{"name" => "TestClient", "version" => "1.0.0"})
+      assert {:ok, _} = GenServer.call(session, {:mcp_request, init_msg, %{}})
+
+      init_notification = build_notification("notifications/initialized", %{})
+      assert :ok = GenServer.cast(session, {:mcp_notification, init_notification, %{}})
+
+      %{session: session}
+    end
+
+    test "arguments/result are absent from span metadata when the flag is unset (default)", %{session: session} do
+      Application.delete_env(:anubis_mcp, :telemetry_capture_tool_payload)
+
+      request = build_request("tools/call", %{"name" => "no_tasks", "arguments" => %{"x" => 1}}, 1)
+
+      assert {:ok, _} = GenServer.call(session, {:mcp_request, request, %{}})
+
+      assert_receive {:tool_call_start, start_meta}, 500
+      refute Map.has_key?(start_meta, :arguments)
+
+      assert_receive {:tool_call_stop, stop_meta}, 500
+      refute Map.has_key?(stop_meta, :result)
+    end
+
+    test "arguments/result are present in span metadata when the flag is enabled", %{session: session} do
+      Application.put_env(:anubis_mcp, :telemetry_capture_tool_payload, true)
+
+      request = build_request("tools/call", %{"name" => "no_tasks", "arguments" => %{"x" => 1}}, 2)
+
+      assert {:ok, _} = GenServer.call(session, {:mcp_request, request, %{}})
+
+      assert_receive {:tool_call_start, %{tool: "no_tasks", arguments: %{"x" => 1}}}, 500
+      assert_receive {:tool_call_stop, %{tool: "no_tasks", is_error: false, result: _result}}, 500
+    end
+  end
+
   describe "terminate/2 on supervisor-initiated stop" do
     defp start_supervised_session(server_module, session_id) do
       transport_name = Registry.transport_name(server_module, StubTransport)
diff --git a/test/anubis/server/tasks_test.exs b/test/anubis/server/tasks_test.exs
index b4f0d93d..ab3f9979 100644
--- a/test/anubis/server/tasks_test.exs
+++ b/test/anubis/server/tasks_test.exs
@@ -46,6 +46,36 @@ defmodule Anubis.Server.TasksTest do
 
       assert decoded["error"]["code"] == -32_601
     end
+
+    test "emits [:anubis_mcp, :server, :tool_call] telemetry for the worker's async execution", %{session: session} do
+      test_pid = self()
+      handler_id = "test-task-augmented-tool-call-#{System.unique_integer([:positive])}"
+
+      :telemetry.attach_many(
+        handler_id,
+        [
+          [:anubis_mcp, :server, :tool_call, :start],
+          [:anubis_mcp, :server, :tool_call, :stop]
+        ],
+        fn
+          [:anubis_mcp, :server, :tool_call, :start], _m, meta, _c -> send(test_pid, {:task_tool_call_start, meta})
+          [:anubis_mcp, :server, :tool_call, :stop], _m, meta, _c -> send(test_pid, {:task_tool_call_stop, meta})
+        end,
+        nil
+      )
+
+      on_exit(fn -> :telemetry.detach(handler_id) end)
+
+      decoded = create_task_call(session, "always_fails", %{"reason" => "kaboom"}, "req-1")
+      task_id = decoded["result"]["task"]["taskId"]
+
+      # The worker runs asynchronously — wait for span :stop rather than
+      # asserting immediately after the CreateTaskResult reply.
+      assert_receive {:task_tool_call_start, %{tool: "always_fails"}}, 500
+      assert_receive {:task_tool_call_stop, %{tool: "always_fails", is_error: true}}, 500
+
+      SyncHelpers.await_state(session, fn state -> not Map.has_key?(state.tasks, task_id) end)
+    end
   end
 
   describe "tasks/get" do