diff --git a/routes/history/history_routes.py b/routes/history/history_routes.py
index b822d2d95..363fadebe 100644
--- a/routes/history/history_routes.py
+++ b/routes/history/history_routes.py
@@ -397,10 +397,17 @@ def setup_history_routes(session_manager, upload_handler=None) -> APIRouter:
                 raise HTTPException(400, "Request body must be valid JSON")
             if not isinstance(body, dict):
                 raise HTTPException(400, "Request body must be a JSON object")
-            before_msg_id = body.get("before_msg_id") or body.get("message_id") or ""
-            if not isinstance(before_msg_id, str):
+            # Resolve before_msg_id / message_id alias with explicit presence checks.
+            # None (JSON null) and empty string are both treated as "not provided here"
+            # and fall through to the alias, preserving the original OR-chain semantics.
+            # Any other non-string value (False, 0, True, 123, list, …) is rejected
+            # immediately so falsy non-strings cannot bypass type validation.
+            raw_msg_id = body.get("before_msg_id") if "before_msg_id" in body else None
+            if raw_msg_id is None or raw_msg_id == "":
+                raw_msg_id = body.get("message_id")
+            if raw_msg_id is not None and not isinstance(raw_msg_id, str):
                 raise HTTPException(400, "Message ID must be a string")
-            before_msg_id = before_msg_id.strip()
+            before_msg_id = (raw_msg_id or "").strip()
             if "keep_count" not in body and not before_msg_id:
                 raise HTTPException(400, "keep_count or before_msg_id required")
             raw_count = body.get("keep_count", 0)
@@ -415,22 +422,22 @@ def setup_history_routes(session_manager, upload_handler=None) -> APIRouter:
             if keep_count < 0:
                 raise HTTPException(400, "keep_count must be a non-negative integer")
             deleted_sft_pairs: list[dict[str, str]] = []
-            if keep_count >= 0:
-                db = SessionLocal()
-                try:
-                    all_db_messages = db.query(DbChatMessage).filter(
-                        DbChatMessage.session_id == session_id
-                    ).order_by(DbChatMessage.timestamp).all()
-                    if before_msg_id:
-                        resolved_keep_count = _keep_count_before_message(all_db_messages, before_msg_id)
-                        if resolved_keep_count is None:
-                            raise HTTPException(404, "Message not found")
-                        keep_count = resolved_keep_count
-                    for pos, row in enumerate(all_db_messages):
-                        row._will_delete_for_sft = pos >= keep_count
-                    deleted_sft_pairs = _deleted_sft_pairs_from_db_rows(all_db_messages)
-                finally:
-                    db.close()
+            # keep_count is always >= 0 here; the block is unconditional.
+            db = SessionLocal()
+            try:
+                all_db_messages = db.query(DbChatMessage).filter(
+                    DbChatMessage.session_id == session_id
+                ).order_by(DbChatMessage.timestamp).all()
+                if before_msg_id:
+                    resolved_keep_count = _keep_count_before_message(all_db_messages, before_msg_id)
+                    if resolved_keep_count is None:
+                        raise HTTPException(404, "Message not found")
+                    keep_count = resolved_keep_count
+                for pos, row in enumerate(all_db_messages):
+                    row._will_delete_for_sft = pos >= keep_count
+                deleted_sft_pairs = _deleted_sft_pairs_from_db_rows(all_db_messages)
+            finally:
+                db.close()
             result = session_manager.truncate_messages(session_id, keep_count)
             _remove_deleted_sft_trace_rows(
                 owner=effective_user(request),
diff --git a/tests/test_history_truncate_validation.py b/tests/test_history_truncate_validation.py
index be8ea8173..9772037db 100644
--- a/tests/test_history_truncate_validation.py
+++ b/tests/test_history_truncate_validation.py
@@ -47,6 +47,7 @@ def saved_messages(database):
     {"keep_count": 1.9}, {"keep_count": -1}, {"keep_count": "bad"},
     {"keep_count": []}, {"keep_count": {}},
     {"before_msg_id": True}, {"before_msg_id": ["message"]},
+    {"before_msg_id": False}, {"before_msg_id": 0}, {"message_id": 0},
     [], None, "two", 2, False,
 ])
 def test_invalid_target_preserves_history(history_client, payload):