core/agent: suppress end-of-turn todo nag on retry when turn-start pair survived

src/plyngent/agent/chat.py

@@ -437,6 +437,12 @@ class ChatAgent:
             # have survived rollback (it looks like a committed tool batch).
             # Skip fresh injection — refresh_synthetic_todo_nags already updated
             # the content of the surviving pair above.
+            _turn_nag_skipped = (
+                self.todo_stack is not None
+                and not self.todo_stack.is_empty()
+                and _turn_nag_strategy == "synthetic_tool"
+                and _synthetic_todo_pair_after(self.messages, user_index)
+            )
             if (
                 self.todo_stack is not None
                 and not self.todo_stack.is_empty()
@@ -467,6 +473,7 @@ class ChatAgent:
                 max_context_tokens=self.max_context_tokens,
                 todo_stack=self.todo_stack,
                 todo_nag_strategy=_turn_nag_strategy,
+                turn_start_nag_skipped=_turn_nag_skipped,
                 directive_reminder_tokens=self.directive_reminder_tokens,
                 directive_reminder_text=self.directive_reminder_text,
                 reminder_last_band=self.reminder_last_band,

src/plyngent/agent/loop.py

@@ -360,6 +360,7 @@ async def run_chat_loop(  # noqa: C901, PLR0912 — multi-phase tool loop
     max_context_tokens: int = DEFAULT_CONTEXT_MAX_TOKENS,
     todo_stack: TodoStack | None = None,
     todo_nag_strategy: TodoNagStrategy = DEFAULT_TODO_NAG_STRATEGY,
+    turn_start_nag_skipped: bool = False,
     directive_reminder_tokens: int = DEFAULT_DIRECTIVE_REMINDER_TOKENS,
     directive_reminder_text: str | None = None,
     reminder_last_band: int = 0,
@@ -453,12 +454,19 @@ async def run_chat_loop(  # noqa: C901, PLR0912 — multi-phase tool loop
             if not has_tools:
                 if todo_stack is not None and todo_stack.needs_review() and not todo_review_injected:
                     todo_review_injected = True
-                    injected, nag_events = inject_todo_nag_for_stack_with_events(
-                        messages,
-                        todo_stack,
-                        kind="end_of_turn",
-                        strategy=todo_nag_strategy,
-                    )
+                    # On retry, the turn-start nag was skipped (pair survived
+                    # rollback).  The model already has the stack reminder
+                    # in-history — skip the end-of-turn duplicate.
+                    if turn_start_nag_skipped:
+                        injected = False
+                        nag_events: list[AgentEvent] = []
+                    else:
+                        injected, nag_events = inject_todo_nag_for_stack_with_events(
+                            messages,
+                            todo_stack,
+                            kind="end_of_turn",
+                            strategy=todo_nag_strategy,
+                        )
                     for nag_event in nag_events:
                         yield nag_event
                     if injected: