diff --git a/backend/app/engine/merge_query.py b/backend/app/engine/merge_query.py
index 4d473886..52a54d59 100644
--- a/backend/app/engine/merge_query.py
+++ b/backend/app/engine/merge_query.py
@@ -343,21 +343,23 @@ def _authorize(action: int, *_args: Any) -> int:
 
 
 def _read(conn: sqlite3.Connection, stmt: str, limits: QueryLimits) -> tuple[list[str], list[list[Any]], bool]:
-    """逐行读、边读边判上限：和查询层（data.engine.run_query）同一个口径，到了行数或字节上限就停，记 truncated。"""
+    """逐行读、边读边判上限：和查询层（data.engine.run_query）同一个口径——到了行数或字节上限之后
+    确实还有下一行才记 truncated，恰好取满不算（截断的合并结果下游会拒收，误报就是拒了完整的结果）。"""
     cursor = conn.execute(stmt)
     columns = [str(d[0]) for d in cursor.description or []]
     if not columns:
         raise MergeError("合并 SQL 没有返回任何列")
     rows: list[list[Any]] = []
     size = 0
-    truncated = False
+    truncated = full = False
     for row in cursor:
+        if full:
+            truncated = True
+            break
         values = [_jsonable(v) for v in row]
         rows.append(values)
         size += len(json.dumps(values, ensure_ascii=False, default=str))
-        if len(rows) >= limits.max_rows or size >= limits.max_bytes:
-            truncated = True
-            break
+        full = len(rows) >= limits.max_rows or size >= limits.max_bytes
     return columns, rows, truncated
 
 
diff --git a/backend/tests/test_merge_engine.py b/backend/tests/test_merge_engine.py
index 55bda2be..9acf20c9 100644
--- a/backend/tests/test_merge_engine.py
+++ b/backend/tests/test_merge_engine.py
@@ -213,6 +213,13 @@ def test_result_over_the_row_limit_is_truncated_and_warned():
     assert "result_truncated" in [w["code"] for w in out.warnings]
 
 
+def test_exactly_full_result_is_not_truncated():
+    """恰好取满上限、后面没有了：和查询层同一个口径，不算截断——截断的合并结果下游会拒收，误报就是把完整的结果拒了。"""
+    out = execute([sales(), visits()], "SELECT * FROM s CROSS JOIN v", limits=QueryLimits(max_rows=16))
+    assert out.truncated is False and len(out.rows) == 16
+    assert "result_truncated" not in [w["code"] for w in out.warnings]
+
+
 def test_byte_limit_matches_the_query_layer():
     out = execute([sales(), visits()], "SELECT * FROM s CROSS JOIN v", limits=QueryLimits(max_bytes=100))
     assert out.truncated is True and 0 < len(out.rows) < 16