diff --git a/api/streaming.py b/api/streaming.py
index 6d1c920b..27b1a9ff 100644
--- a/api/streaming.py
+++ b/api/streaming.py
@@ -5264,6 +5264,18 @@ def _merge_display_messages_after_agent_result(previous_display, previous_contex
             # before the agent runs. When the agent returns that same user turn
             # in result_messages, keep the durable checkpoint and append only
             # the assistant/tool delta.
+            # The eager checkpoint was written before _assign_stable_message_ids
+            # stamped the result rows, so it has no `id` while the context copy
+            # does — which would silently defeat id-based fork/truncate alignment
+            # for eager-mode users. Carry the minted id onto the kept checkpoint
+            # so display and context share it (#5564).
+            if (
+                isinstance(msg, dict)
+                and msg.get('id') is not None
+                and isinstance(merged[-1], dict)
+                and merged[-1].get('id') is None
+            ):
+                merged[-1]['id'] = msg['id']
             continue
         if (
             key is not None