diff --git a/README.md b/README.md
index 1edff73..1e7349a 100644
--- a/README.md
+++ b/README.md
@@ -162,7 +162,8 @@ session exposes the typed JSON turn as `session.last_turn` or
 Usage totals sum reported nonnegative integers from recorded turns. A field
 is `nil` if it was never reported; the whole total is `nil` if no recorded
 turn included usage. Nonzero exits leave the session unchanged and do not
-contribute to usage totals.
+contribute to usage totals. Each recorded send updates a running total;
+reading it takes constant time and does not parse the retained stdout again.
 
 Session options are the exec options in the table under
 [Exec options](#exec-options) and apply to every turn. Resumed turns run
@@ -200,6 +201,12 @@ event. `CodexWrapper.exec/2` and `CodexWrapper.review/1` still return
 `{:ok, %Result{success: false}}` for a nonzero exit; runner errors return
 `{:error, reason}`.
 
+Status, error, and terminal evidence cover the entire event stream. A new
+`turn.started` resets only the final message and usage; it does not erase an
+earlier reported outcome. For example, a completed turn followed by a new
+start still has `status: :completed`, but its message and usage are `nil`
+until the new turn supplies them. See `CodexWrapper.Turn` for each field's scope.
+
 Usage has the keys `:input_tokens`, `:cached_input_tokens`,
 `:cache_write_input_tokens`, `:output_tokens` and `:reasoning_output_tokens`.
 Only nonnegative integers are retained; missing or invalid counters are
diff --git a/lib/codex_wrapper/session.ex b/lib/codex_wrapper/session.ex
index d2f9021..8382eb5 100644
--- a/lib/codex_wrapper/session.ex
+++ b/lib/codex_wrapper/session.ex
@@ -37,6 +37,7 @@ defmodule CodexWrapper.Session do
           session_id: String.t() | nil,
           history: [Result.t()],
           last_turn: Turn.t() | nil,
+          usage_total: Turn.usage() | nil,
           exec_opts: keyword()
         }
 
@@ -44,6 +45,7 @@ defmodule CodexWrapper.Session do
     :config,
     :session_id,
     :last_turn,
+    :usage_total,
     history: [],
     exec_opts: []
   ]
@@ -101,6 +103,8 @@ defmodule CodexWrapper.Session do
   and through `last_turn/1`. The returned result and history remain raw
   `%Result{}` values. Inspect the projection's `status` for the JSON turn
   outcome, which may be `:failed` even when codex exited 0.
+  Each recorded turn also updates the running token totals returned by
+  `total_usage/1`, using the same projection.
 
   Returns:
 
@@ -126,7 +130,8 @@ defmodule CodexWrapper.Session do
         session
         | session_id: session_id,
           history: session.history ++ [result],
-          last_turn: turn
+          last_turn: turn,
+          usage_total: sum_usage(turn.usage, session.usage_total)
       }
 
       {:ok, updated, result}
@@ -189,7 +194,12 @@ defmodule CodexWrapper.Session do
   def total_cost(%__MODULE__{}), do: nil
 
   @doc """
-  Sum the reported token usage across recorded turns.
+  Get the reported token usage accumulated by `send/3`.
+
+  Reads the running totals in constant time without parsing raw result
+  history. `send/3` updates the totals when it records a turn; the raw
+  `%Result{}` history remains unchanged. A new or resumed session has no
+  totals until a recorded turn reports usage.
 
   Returns `nil` when no turn reported a usage map. Otherwise returns the
   five usage fields from `CodexWrapper.Turn`, summing only reported
@@ -201,11 +211,7 @@ defmodule CodexWrapper.Session do
   and do not contribute to these totals.
   """
   @spec total_usage(t()) :: Turn.usage() | nil
-  def total_usage(%__MODULE__{history: history}) do
-    Enum.reduce(history, nil, fn result, total ->
-      result |> Turn.from_result() |> Map.fetch!(:usage) |> sum_usage(total)
-    end)
-  end
+  def total_usage(%__MODULE__{usage_total: total}), do: total
 
   @doc """
   Get the typed projection of the last recorded turn, if any.
diff --git a/lib/codex_wrapper/turn.ex b/lib/codex_wrapper/turn.ex
index 4eed61e..b79ce86 100644
--- a/lib/codex_wrapper/turn.ex
+++ b/lib/codex_wrapper/turn.ex
@@ -1,6 +1,6 @@
 defmodule CodexWrapper.Turn do
   @moduledoc """
-  A pure projection of a command's `--json` output.
+  A pure projection of a command's `--json` output, which can contain several turns.
 
   `from_result/1` starts no process and leaves the raw `CodexWrapper.Result`
   unchanged. Ordinary text messages do not need to contain JSON:
@@ -9,23 +9,36 @@ defmodule CodexWrapper.Turn do
       turn = CodexWrapper.Turn.from_result(result)
       turn.final_message
 
-  The last `turn.completed` or `turn.failed` determines the native outcome.
-  A top-level `error` fails the turn until a later terminal event supersedes
-  it. A nonzero process exit always makes `status` `:failed`, including after
-  `turn.completed`. Without a terminal event or error, status is `:no_terminal`
-  when the process exited zero or its exit code is unknown.
-
-  `final_message` is the last completed `agent_message` text after the last
-  `turn.started`. `thread_id` is the first nonblank string ID announced by
-  `thread.started`. Usage comes from the last terminal event of the last
-  turn. Only the five documented token fields are projected, each as a
-  nonnegative integer or `nil`; missing usage is `nil`, not zero.
-
-  `error` retains native error data, or is `nil` when no native error was
-  reported. A process failure alone does not fabricate an error object.
-  `events` keeps all parsed events, including unknown fields and invalid
-  usage values. `terminal_event` retains the last native terminal event,
-  independently of process exit and subsequent top-level errors.
+  ## Field scopes
+
+  `status`, `error`, and `terminal_event` summarize the **entire event stream**.
+  The last `turn.completed` or `turn.failed` determines the native outcome;
+  a top-level `error` fails it until a later terminal event supersedes that
+  error. A new `turn.started` does not clear these three fields. Therefore
+  `:completed` means the stream's last reported terminal outcome completed,
+  not that its last started turn necessarily finished. `:no_terminal` means
+  no terminal event or error was reported anywhere in the stream, provided
+  the process did not exit nonzero.
+
+  `exit_code` describes the whole process. A nonzero exit always makes
+  `status` `:failed`, including after `turn.completed`. `error` retains native
+  error data or `nil`; process failure alone does not fabricate an error
+  object. `terminal_event` remains the last native terminal event regardless
+  of process exit or subsequent top-level errors. `thread_id` is the first
+  nonblank string ID announced by `thread.started` anywhere in the stream.
+  `events` keeps every parsed event, including unknown fields and invalid
+  usage values.
+
+  `final_message` and `usage` describe only the **last started turn**. Both
+  reset to `nil` at each `turn.started`. The message is its last completed
+  `agent_message` text, and usage comes from its last terminal event. If no
+  `turn.started` appears, these fields use the whole stream. Only the five
+  documented token fields are projected, each as a nonnegative integer or
+  `nil`; missing usage is `nil`, not zero.
+
+  For example, `turn.started`, `turn.completed`, `turn.started` produces
+  `status: :completed` and retains the earlier `terminal_event`, while
+  `final_message` and `usage` are `nil`. The new turn has no reported outcome.
   """
 
   alias CodexWrapper.{JsonLineEvent, Result}
diff --git a/test/codex_wrapper/session_test.exs b/test/codex_wrapper/session_test.exs
index 39398ee..ed1ab18 100644
--- a/test/codex_wrapper/session_test.exs
+++ b/test/codex_wrapper/session_test.exs
@@ -3,8 +3,28 @@ defmodule CodexWrapper.SessionTest do
 
   alias CodexWrapper.{Config, JsonLineEvent, Result, Session}
 
+  # Synthetic NDJSON supplied as the prompt makes each test's turn output
+  # explicit while exercising the normal Session.send/3 recording path.
+  defmodule UsageRunner do
+    @behaviour CodexWrapper.Runner
+
+    @impl true
+    def run(_binary, _args, opts, _timeout), do: {:ok, {Keyword.fetch!(opts, :stdin), 0}}
+  end
+
   defp event(type, data), do: %JsonLineEvent{event_type: type, data: data, raw: ""}
 
+  defp usage_session do
+    Session.new(Config.new(binary: "synthetic-codex", runner: UsageRunner))
+  end
+
+  defp record_usage(session, usage) do
+    event = %{"type" => "turn.completed"}
+    event = if usage == :missing, do: event, else: Map.put(event, "usage", usage)
+    {:ok, session, _result} = Session.send(session, Jason.encode!(event))
+    session
+  end
+
   describe "new/2" do
     test "creates session with config" do
       config = Config.new()
@@ -13,6 +33,7 @@ defmodule CodexWrapper.SessionTest do
       assert session.session_id == nil
       assert session.history == []
       assert session.last_turn == nil
+      assert session.usage_total == nil
       assert session.exec_opts == []
     end
 
@@ -34,6 +55,7 @@ defmodule CodexWrapper.SessionTest do
       config = Config.new()
       session = Session.resume(config, "abc-123")
       assert Session.session_id(session) == "abc-123"
+      assert Session.total_usage(session) == nil
     end
 
     test "with exec opts" do
@@ -90,26 +112,25 @@ defmodule CodexWrapper.SessionTest do
 
   describe "total_usage/1" do
     test "returns nil when there is no reported usage" do
-      session = Session.new(Config.new())
+      session = usage_session()
       assert Session.total_usage(session) == nil
 
-      result = Result.from_cmd({~s({"type":"turn.completed"}), 0})
-      assert Session.total_usage(%{session | history: [result]}) == nil
+      session = record_usage(session, :missing)
+      assert Session.turn_count(session) == 1
+      assert Session.total_usage(session) == nil
     end
 
     test "sums known counters while preserving unknown values and zero" do
       usages = [
         %{"input_tokens" => 12, "cached_input_tokens" => 0, "output_tokens" => 3},
+        :missing,
         %{"input_tokens" => 5, "cache_write_input_tokens" => 2, "output_tokens" => -1},
-        %{"input_tokens" => "9", "cached_input_tokens" => nil, "output_tokens" => 1.5}
+        %{"input_tokens" => "9", "cached_input_tokens" => nil, "output_tokens" => 1.5},
+        %{"reasoning_output_tokens" => false}
       ]
 
-      results =
-        Enum.map(usages, fn usage ->
-          Result.from_cmd({Jason.encode!(%{"type" => "turn.completed", "usage" => usage}), 0})
-        end)
-
-      session = %{Session.new(Config.new()) | history: results}
+      session = Enum.reduce(usages, usage_session(), &record_usage(&2, &1))
+      assert Session.turn_count(session) == length(usages)
 
       assert Session.total_usage(session) == %{
                input_tokens: 17,
@@ -121,8 +142,7 @@ defmodule CodexWrapper.SessionTest do
     end
 
     test "a reported empty usage map retains unknown counters" do
-      result = Result.from_cmd({~s({"type":"turn.completed","usage":{}}), 0})
-      session = %{Session.new(Config.new()) | history: [result]}
+      session = record_usage(usage_session(), %{})
 
       assert Session.total_usage(session) == %{
                input_tokens: nil,
@@ -132,6 +152,37 @@ defmodule CodexWrapper.SessionTest do
                reasoning_output_tokens: nil
              }
     end
+
+    test "repeated reads use recorded totals independently of raw stdout history" do
+      session =
+        usage_session()
+        |> record_usage(%{"input_tokens" => 12, "output_tokens" => 0})
+        |> record_usage(%{"input_tokens" => 5})
+
+      expected = Session.total_usage(session)
+      assert expected.input_tokens == 17
+      assert expected.output_tokens == 0
+
+      # A consumer may redact retained raw output after recording a turn.
+      # Replaying these Results would lose the recorded usage entirely.
+      history =
+        Enum.map(session.history, fn %Result{} = result -> %{result | stdout: "redacted"} end)
+
+      session = %{session | history: history}
+
+      for _ <- 1..3, do: assert(Session.total_usage(session) == expected)
+      assert Session.history(session) == history
+
+      session = record_usage(session, %{"input_tokens" => 1, "cache_write_input_tokens" => 0})
+
+      assert Session.total_usage(session) == %{
+               expected
+               | input_tokens: 18,
+                 cache_write_input_tokens: 0
+             }
+
+      assert Enum.take(Session.history(session), 2) == history
+    end
   end
 
   describe "turns/1" do
diff --git a/test/codex_wrapper/session_turn_test.exs b/test/codex_wrapper/session_turn_test.exs
index b178b99..7241782 100644
--- a/test/codex_wrapper/session_turn_test.exs
+++ b/test/codex_wrapper/session_turn_test.exs
@@ -157,7 +157,9 @@ defmodule CodexWrapper.SessionTurnTest do
   end
 
   describe "typed session server access" do
-    test "reports usage and preserves the previous turn when a later call fails", %{tmp_dir: dir} do
+    test "preserves usage and the previous turn on exit and thread mismatch failures", %{
+      tmp_dir: dir
+    } do
       lines = [
         started(@uuid),
         ~s({"type":"turn.completed","usage":{"input_tokens":8,"output_tokens":0}})
@@ -177,6 +179,48 @@ defmodule CodexWrapper.SessionTurnTest do
       assert SessionServer.total_usage(pid) == turn.usage
       assert SessionServer.history(pid) == [result]
       assert SessionServer.turn_count(pid) == 1
+
+      mismatched_lines = [
+        started(@other),
+        ~s({"type":"turn.completed","usage":{"input_tokens":100,"output_tokens":9}})
+      ]
+
+      fake(dir, mismatched_lines, nil, 0)
+
+      assert {:error, {:thread_mismatch, @uuid, @other, %Result{}}} =
+               SessionServer.send_message(pid, "third")
+
+      assert SessionServer.last_turn(pid) == turn
+      assert SessionServer.total_usage(pid) == turn.usage
+      assert SessionServer.history(pid) == [result]
+      assert SessionServer.turn_count(pid) == 1
+    end
+
+    test "repeated usage reads do not replay retained raw output", %{tmp_dir: dir} do
+      lines = [
+        started(@uuid),
+        ~s({"type":"turn.completed","usage":{"input_tokens":8,"output_tokens":0}})
+      ]
+
+      config = fake(dir, lines, nil, 0)
+      {:ok, pid} = SessionServer.start_link(config: config)
+      assert {:ok, %Result{}} = SessionServer.send_message(pid, "first")
+      assert {:ok, %Result{}} = SessionServer.send_message(pid, "second")
+      expected = SessionServer.total_usage(pid)
+      assert expected.input_tokens == 16
+      assert expected.output_tokens == 0
+
+      # Preserve the recorded totals while redacting raw payloads. Re-parsing
+      # these Results instead of reading the aggregate would return nil.
+      :sys.replace_state(pid, fn session ->
+        history =
+          Enum.map(session.history, fn %Result{} = result -> %{result | stdout: "redacted"} end)
+
+        %{session | history: history}
+      end)
+
+      for _ <- 1..3, do: assert(SessionServer.total_usage(pid) == expected)
+      assert Enum.all?(SessionServer.history(pid), &(&1.stdout == "redacted"))
     end
   end
 
diff --git a/test/codex_wrapper/turn_test.exs b/test/codex_wrapper/turn_test.exs
index 0e80cdf..8f28bcd 100644
--- a/test/codex_wrapper/turn_test.exs
+++ b/test/codex_wrapper/turn_test.exs
@@ -109,6 +109,55 @@ defmodule CodexWrapper.TurnTest do
     assert Turn.from_result(result(events ++ [event("turn.started")])).final_message == nil
   end
 
+  test "a new turn clears its message and usage but retains the stream's reported outcome" do
+    usage = %{"input_tokens" => 12, "output_tokens" => 3}
+    error = %{"message" => "earlier failure"}
+
+    for {outcome, status, native_error} <- [
+          {event("turn.completed", %{"usage" => usage}), :completed, nil},
+          {event("turn.failed", %{"error" => error, "usage" => usage}), :failed, error}
+        ] do
+      events = [
+        event("thread.started", %{"thread_id" => "native-thread"}),
+        event("turn.started"),
+        message("earlier answer"),
+        outcome,
+        event("turn.started")
+      ]
+
+      turn = Turn.from_result(result(events))
+      assert turn.status == status
+      assert turn.error == native_error
+      assert turn.terminal_event.data == outcome
+      assert turn.final_message == nil
+      assert turn.usage == nil
+      assert turn.thread_id == "native-thread"
+      assert turn.events == JsonLineEvent.parse_lines(result(events).stdout)
+
+      failed_process = Turn.from_result(result(events, 7))
+      assert failed_process.status == :failed
+      assert failed_process.exit_code == 7
+      assert failed_process.terminal_event == turn.terminal_event
+      assert failed_process.error == native_error
+    end
+  end
+
+  test "a new turn retains a preceding top-level error until a terminal supersedes it" do
+    error = event("error", %{"message" => "earlier stream failure"})
+    events = [event("turn.completed"), error, event("turn.started")]
+    turn = Turn.from_result(result(events))
+
+    assert turn.status == :failed
+    assert turn.error == error
+    assert turn.terminal_event.event_type == "turn.completed"
+    assert turn.final_message == nil
+    assert turn.usage == nil
+
+    recovered = Turn.from_result(result(events ++ [event("turn.completed")]))
+    assert recovered.status == :completed
+    assert recovered.error == nil
+  end
+
   test "missing turn.started is allowed, and malformed final message does not reuse earlier text" do
     assert Turn.from_result(result([message("hello")])).final_message == "hello"