diff --git a/app/harness/steps_json_traces.py b/app/harness/steps_json_traces.py
new file mode 100644
index 0000000..959a217
--- /dev/null
+++ b/app/harness/steps_json_traces.py
@@ -0,0 +1,51 @@
+"""把 predictions.steps_json 转成 RunLog.get_traces 的行形。
+
+infer_adhoc 的 traces 表为空，轨迹存于 steps_json（{thought, tool_call, tool_output}）。
+诊断管线经 get_traces 消费轨迹，故需此确定性转换适配。
+"""
+
+from __future__ import annotations
+
+import json
+from typing import Any
+
+
+def steps_json_to_trace_rows(
+    video_id: str, question_id: str, steps_json: str
+) -> list[dict[str, Any]]:
+    """将单题 steps_json 解析为 trace 行列表（step 从 0 递增）。
+
+    参数:
+        video_id: 视频 ID。
+        question_id: 题 ID。
+        steps_json: predictions.steps_json 原文（JSON 数组字符串）。
+
+    返回:
+        行字典列表，字段对齐 traces 表 schema；空/空数组返回 []。
+
+    关键实现细节:
+        - 工具名读 tool_call.tool（infer_adhoc 真实字段），对极少数历史
+          数据的 name 做 back-compat 回退。
+        - steps_json 非 JSON 数组时直接报错，不做兜底掩盖。
+    """
+    if not steps_json or not steps_json.strip():
+        return []
+    steps = json.loads(steps_json)
+    if not isinstance(steps, list):
+        raise ValueError(f"steps_json 非数组: {question_id}")
+    rows: list[dict[str, Any]] = []
+    for i, s in enumerate(steps):
+        call = s.get("tool_call") or {}
+        rows.append(
+            {
+                "video_id": video_id,
+                "question_id": question_id,
+                "step": i,
+                # infer_adhoc 用 "tool"；back-compat 兼容极少数 "name"
+                "tool_name": call.get("tool", call.get("name")),
+                "tool_args": call.get("args", {}),
+                "tool_output": s.get("tool_output"),
+                "thought": s.get("thought"),
+            }
+        )
+    return rows
diff --git a/tests/unit/test_steps_json_traces.py b/tests/unit/test_steps_json_traces.py
new file mode 100644
index 0000000..af750f0
--- /dev/null
+++ b/tests/unit/test_steps_json_traces.py
@@ -0,0 +1,30 @@
+import json
+
+from app.harness.steps_json_traces import steps_json_to_trace_rows
+
+
+def test_parses_tool_call_into_name_and_args():
+    # 真实 infer_adhoc steps_json 形态：tool_call={"tool":..., "args":...}（非 "name"）
+    steps = [
+        {
+            "thought": "看根节点",
+            "tool_call": {"tool": "view_node", "args": {"node_id": "v_L1_000"}},
+            "tool_output": "o0",
+        },
+        {
+            "thought": "搜索",
+            "tool_call": {"tool": "search_similar", "args": {"query": "gadget"}},
+            "tool_output": "hit",
+        },
+    ]
+    rows = steps_json_to_trace_rows("vid1", "q1", json.dumps(steps))
+    assert [r["step"] for r in rows] == [0, 1]
+    assert rows[0]["tool_name"] == "view_node"
+    assert rows[0]["tool_args"] == {"node_id": "v_L1_000"}
+    assert rows[0]["video_id"] == "vid1" and rows[0]["question_id"] == "q1"
+    assert rows[1]["tool_output"] == "hit"
+
+
+def test_empty_or_blank_steps_json_returns_empty():
+    assert steps_json_to_trace_rows("v", "q", "") == []
+    assert steps_json_to_trace_rows("v", "q", "[]") == []