diff --git a/engineering/engineering-email-intelligence-engineer.md b/engineering/engineering-email-intelligence-engineer.md
index 46b27c7f..b9390ef2 100644
--- a/engineering/engineering-email-intelligence-engineer.md
+++ b/engineering/engineering-email-intelligence-engineer.md
@@ -128,7 +128,16 @@ def reconstruct_thread(messages):
     - Quoted replies duplicate content (20-msg thread = ~4-5x token bloat)
     - Thread forks when people reply to different messages in the chain
     """
-    # Build reply graph from In-Reply-To and References headers
+    # Reject ambiguous identities before building or mutating the graph.
+    # Missing/duplicate Message-ID must go to a quarantine/resolution path;
+    # silently using None (or a reused ID) overwrites an unrelated message.
+    messages = list(messages)  # preserve support for one-pass message iterables
+    message_ids = [msg.get("message_id") for msg in messages]
+    if any(not isinstance(mid, str) or not mid.strip() for mid in message_ids):
+        raise ValueError("Every message needs a nonempty Message-ID")
+    if len(set(message_ids)) != len(message_ids):
+        raise ValueError("Duplicate Message-ID: resolve identity before reconstruction")
+
     graph = {}
     for msg in messages:
         parent_id = msg["in_reply_to"]