refactor(agent): share the hidden interrupt placeholder row

message_sanitization.close_interrupted_tool_sequence and the
turn_api_call repetition-loop exit each spelled the same hidden
assistant row literal (content "", display_kind hidden, api_content
_INTERRUPTED_PLACEHOLDER), and the conversation loop's redirect built it
field by field. Add hidden_interrupt_placeholder_row() to the leaf
placeholders module and use it at all three sites so the shape cannot
drift between them.

Gate rP finding 6 (reuse).

agent/agent_runtime_helpers_placeholders.py

@@ -18,3 +18,9 @@ _INTERRUPTED_PLACEHOLDER = "[interrupt: no assistant output for this turn]"
 # The spelling shipped before #132949. Models reproduce it verbatim, so rows already carrying it
 # stay poisoned after the wording change; the replay filters reference this name to retire them.
 _LEGACY_INTERRUPTED_PLACEHOLDER = "[response interrupted]"
+
+
+def hidden_interrupt_placeholder_row() -> dict:
+    """A fresh assistant row that is invisible in transcripts but carries the placeholder as its
+    non-empty ``api_content``, so the pre-call sanitizer does not re-heal it every call (#88955)."""
+    return {"role": "assistant", "content": "", "display_kind": "hidden", "api_content": _INTERRUPTED_PLACEHOLDER}

agent/conversation_loop.py

@@ -343,15 +343,10 @@ def _apply_active_turn_redirect(agent: Any, messages: List[Dict[str, Any]], text
     # preserves alternation only — scaffold bytes must never land in it, since api_content
     # is substituted back into content on replay (#81841).
     if not (messages and messages[-1].get("role") == "assistant"):
-        placeholder: Dict[str, Any] = {"role": "assistant", "content": visible or ""}
-        if not visible:
-            placeholder["display_kind"] = "hidden"
-            # Hidden row, but a non-empty neutral api_content so the pre-call sanitizer
-            # does not re-heal it every call (#88955). Never _INTERRUPT_SCAFFOLD_MARKER:
-            # as assistant text the model echoes it (#81841).
-            from agent.agent_runtime_helpers_placeholders import _INTERRUPTED_PLACEHOLDER
-            placeholder["api_content"] = _INTERRUPTED_PLACEHOLDER
-        append_message(messages, placeholder)
+        # Hidden row with a neutral api_content (#88955). Never _INTERRUPT_SCAFFOLD_MARKER:
+        # as assistant text the model echoes it (#81841).
+        from agent.agent_runtime_helpers_placeholders import hidden_interrupt_placeholder_row
+        append_message(messages, {"role": "assistant", "content": visible} if visible else hidden_interrupt_placeholder_row())
     # Transcript shows the user's own words; the provider replays the scaffolded form.
     append_message(messages, {"role": "user", "content": text, "api_content": correction})
 

agent/message_sanitization.py

@@ -302,16 +302,11 @@ def close_interrupted_tool_sequence(messages: list, final_response: Any = None)
         return False
     text = final_response if isinstance(final_response, str) else ""
     from agent.message_metadata import append_message
-    from agent.agent_runtime_helpers_placeholders import _INTERRUPTED_PLACEHOLDER
+    from agent.agent_runtime_helpers_placeholders import _INTERRUPTED_PLACEHOLDER, hidden_interrupt_placeholder_row
 
     stripped = text.strip()
     if not stripped or stripped == _INTERRUPTED_PLACEHOLDER:
-        append_message(messages, {
-            "role": "assistant",
-            "content": "",
-            "display_kind": "hidden",
-            "api_content": _INTERRUPTED_PLACEHOLDER,
-        })
+        append_message(messages, hidden_interrupt_placeholder_row())
     else:
         append_message(messages, {"role": "assistant", "content": stripped})
     return True

agent/turn_api_call.py

@@ -15,7 +15,7 @@ import time
 from typing import Any, Dict, Optional
 
 from agent.error_classifier import FailoverReason
-from agent.agent_runtime_helpers_placeholders import _INTERRUPTED_PLACEHOLDER
+from agent.agent_runtime_helpers_placeholders import hidden_interrupt_placeholder_row
 from agent.message_metadata import append_message
 from agent.repetition_guard import REPETITION_LOOP_INTERRUPTED, is_runaway_repetition
 from agent.turn_failure_copy import site_copy, stamp_failure
@@ -198,10 +198,7 @@ def handle_api_interrupt(
         # The interrupted row is replayed next turn; looped bytes there re-seed the loop
         # (#112764). Same hidden shape as the redirect placeholder: nothing visible in the
         # transcript, a neutral api_content so the pre-call sanitizer does not re-heal it.
-        append_message(messages, {
-            "role": "assistant", "content": "", "display_kind": "hidden",
-            "api_content": _INTERRUPTED_PLACEHOLDER,
-        })
+        append_message(messages, hidden_interrupt_placeholder_row())
         final_response = REPETITION_LOOP_INTERRUPTED
     elif _partial:
         append_message(messages, {