core/agent: skip duplicate synthetic todo nag injection on retry

src/plyngent/agent/chat.py

@@ -26,6 +26,7 @@ from .loop import DEFAULT_MAX_ROUNDS, run_chat_loop
 from .todo_nag import (
     DEFAULT_TODO_NAG_STRATEGY,
     inject_todo_nag_for_stack_with_events,
+    is_synthetic_todo_nag_call_id,
     parse_todo_nag_strategy,
     refresh_synthetic_todo_nags,
 )
@@ -114,6 +115,37 @@ def committed_prefix_end(messages: Sequence[AnyChatMessage], user_index: int) ->
     return end
 
 
+def _synthetic_todo_pair_after(
+    messages: Sequence[AnyChatMessage],
+    index: int,
+) -> bool:
+    """True when a synthetic todo nag pair sits at ``messages[index+1:index+3]``.
+
+    The pattern is ``AssistantChatMessage`` (all tool calls synthetic) followed
+    by a matching ``ToolChatMessage`` with a synthetic call id.
+    On retry after a failed turn, a previously injected turn-start nag may still
+    be in messages (survived rollback). This detects it so we can skip a fresh
+    injection.
+    """
+    if index + 2 >= len(messages):
+        return False
+    first = messages[index + 1]
+    if not isinstance(first, AssistantChatMessage):
+        return False
+    tool_calls = first.tool_calls
+    if tool_calls is UNSET or not tool_calls:
+        return False
+    if not all(
+        isinstance(call, AssistantFunctionToolCall) and is_synthetic_todo_nag_call_id(call.id) for call in tool_calls
+    ):
+        return False
+    synth_ids = {call.id for call in tool_calls if isinstance(call, AssistantFunctionToolCall)}
+    second = messages[index + 2]
+    if not isinstance(second, ToolChatMessage):
+        return False
+    return second.tool_call_id in synth_ids
+
+
 class ChatAgent:
     """Thin wrapper: chat client + optional tools + optional memory bind."""
 
@@ -398,14 +430,25 @@ class ChatAgent:
         turn_rounds = 0
         last_request = TokenUsage()
         try:
+            _turn_nag_strategy: TodoNagStrategy = self.todo_nag_strategy
             # Turn-start nag before first completion; yield events so CLI flushes
             # (synthetic_tool → ToolCall/Result chrome, not glued to text).
-            if self.todo_stack is not None and not self.todo_stack.is_empty():
+            # On retry, a synthetic_tool pair from a previous failed attempt may
+            # 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.
+            if (
+                self.todo_stack is not None
+                and not self.todo_stack.is_empty()
+                and (
+                    _turn_nag_strategy != "synthetic_tool" or not _synthetic_todo_pair_after(self.messages, user_index)
+                )
+            ):
                 _injected, nag_events = inject_todo_nag_for_stack_with_events(
                     self.messages,
                     self.todo_stack,
                     kind="turn_start",
-                    strategy=self.todo_nag_strategy,
+                    strategy=_turn_nag_strategy,
                 )
                 for nag_event in nag_events:
                     yield nag_event
@@ -423,7 +466,7 @@ class ChatAgent:
                 parallel_tools=self.parallel_tools,
                 max_context_tokens=self.max_context_tokens,
                 todo_stack=self.todo_stack,
-                todo_nag_strategy=self.todo_nag_strategy,
+                todo_nag_strategy=_turn_nag_strategy,
                 directive_reminder_tokens=self.directive_reminder_tokens,
                 directive_reminder_text=self.directive_reminder_text,
                 reminder_last_band=self.reminder_last_band,