From 052ba7be0797e1e7fa17e0049c741ae5ba6d7c04 Mon Sep 17 00:00:00 2001
From: Eduard van Valkenburg <eavanvalkenburg@users.noreply.github.com>
Date: Fri, 13 Mar 2026 21:03:48 +0100
Subject: [PATCH] Python: normalize empty MCP tool output to null (#4683)

* Python: normalize empty MCP tool output to null

* Python: hardcode null for empty MCP output
---
 python/packages/core/agent_framework/_mcp.py | 5 ++---
 python/packages/core/tests/core/test_mcp.py  | 7 +++++--
 2 files changed, 7 insertions(+), 5 deletions(-)

diff --git a/python/packages/core/agent_framework/_mcp.py b/python/packages/core/agent_framework/_mcp.py
index 28c5f6db6a..81227c7e73 100644
--- a/python/packages/core/agent_framework/_mcp.py
+++ b/python/packages/core/agent_framework/_mcp.py
@@ -4,6 +4,7 @@ from __future__ import annotations
 
 import asyncio
 import base64
+import json
 import logging
 import re
 import sys
@@ -87,8 +88,6 @@ def _parse_prompt_result_from_mcp(
     Returns:
         A string representation of the prompt result.
     """
-    import json
-
     parts: list[str] = []
     for message in mcp_type.messages:
         content = message.content
@@ -194,7 +193,7 @@ def _parse_tool_result_from_mcp(
                 result.append(Content.from_text(str(item)))
 
     if not result:
-        result.append(Content.from_text(""))
+        result.append(Content.from_text("null"))
     return result
 
 
diff --git a/python/packages/core/tests/core/test_mcp.py b/python/packages/core/tests/core/test_mcp.py
index df3187673a..70aff972fe 100644
--- a/python/packages/core/tests/core/test_mcp.py
+++ b/python/packages/core/tests/core/test_mcp.py
@@ -195,13 +195,16 @@ def test_parse_tool_result_from_mcp_meta_not_in_string():
 
 
 def test_parse_tool_result_from_mcp_empty_content():
-    """Test that empty content produces list with empty text Content."""
+    """Test that empty MCP content normalizes to JSON null text content."""
     mcp_result = types.CallToolResult(content=[])
     result = _parse_tool_result_from_mcp(mcp_result)
     assert isinstance(result, list)
     assert len(result) == 1
     assert result[0].type == "text"
-    assert result[0].text == ""
+    assert result[0].text == "null"
+
+    function_result = Content.from_function_result(call_id="call_null", result=result)
+    assert function_result.result == "null"
 
 
 def test_parse_tool_result_from_mcp_audio_content():