fix(agent): serialize retrieval trace responses

ai_knowledge_base_v2/apps/backend/tests/test_knowledge_agent.py

@@ -1,6 +1,8 @@
 from __future__ import annotations
 
 import asyncio
+import json
+from datetime import datetime
 
 from sqlalchemy import create_engine
 from sqlalchemy.orm import Session
@@ -11,6 +13,7 @@ from app.models.knowledge import (
     Knowledge,
     KnowledgeChunk,
     KnowledgeManifest,
+    KnowledgeRetrievalLog,
     KnowledgeSection,
     KnowledgeSourceSnapshot,
     KnowledgeVersion,
@@ -140,6 +143,22 @@ def test_course_question_searches_chunk_and_reads_parent_section():
         assert any(item["tool"] == "read_knowledge_sections" for item in result.tool_trace)
 
 
+def test_tool_trace_serializes_published_datetime():
+    with _database() as db:
+        knowledge = _add_published_knowledge(db, knowledge_id=1, name="亲子课程")
+        version = db.get(KnowledgeVersion, knowledge.current_version_id)
+        assert version is not None
+        version.published_at = datetime(2026, 7, 13, 10, 30, 0)
+        db.commit()
+
+        result = asyncio.run(KnowledgeAgentService.build_result(db, question="课程里如何进行亲子沟通？"))
+        log = db.get(KnowledgeRetrievalLog, result.retrieval_log_id)
+        trace = json.loads(log.tool_trace)
+
+        catalog_call = next(item for item in trace if item["tool"] == "get_knowledge_catalog")
+        assert catalog_call["items"][0]["publishedAt"] == "2026-07-13T10:30:00"
+
+
 def test_contextual_follow_up_is_rewritten_before_agent_decision():
     with _database() as db:
         history = [