From 03f8398cf3e2f9be9090bbb88ccbe83a8f109008 Mon Sep 17 00:00:00 2001
From: Zelys <zelys@dfkhelper.com>
Date: Sat, 4 Apr 2026 17:39:10 -0500
Subject: [PATCH] Add warning log when tool input JSON parsing fails

When a model returns malformed tool input that cannot be parsed as
JSON, the SDK silently falls back to an empty dict. This adds a
warning log with the tool name and the first 200 characters of the
raw input to aid debugging.

Fixes #2051
---
 src/strands/event_loop/streaming.py        |  5 +++++
 tests/strands/event_loop/test_streaming.py | 19 +++++++++++++++++++
 2 files changed, 24 insertions(+)

diff --git a/src/strands/event_loop/streaming.py b/src/strands/event_loop/streaming.py
index 0a1161135..f304eda7d 100644
--- a/src/strands/event_loop/streaming.py
+++ b/src/strands/event_loop/streaming.py
@@ -279,6 +279,11 @@ def handle_content_block_stop(state: dict[str, Any]) -> dict[str, Any]:
         try:
             current_tool_use["input"] = json.loads(current_tool_use["input"])
         except ValueError:
+            logger.warning(
+                "tool_name=<%s> raw_input=<%s> | failed to parse tool input as JSON; falling back to empty dict",
+                current_tool_use.get("name"),
+                current_tool_use["input"][:200],
+            )
             current_tool_use["input"] = {}
 
         tool_use_id = current_tool_use["toolUseId"]
diff --git a/tests/strands/event_loop/test_streaming.py b/tests/strands/event_loop/test_streaming.py
index 93f8d95f8..ff82ce76c 100644
--- a/tests/strands/event_loop/test_streaming.py
+++ b/tests/strands/event_loop/test_streaming.py
@@ -527,6 +527,25 @@ def test_handle_content_block_stop(state, exp_updated_state):
     assert tru_updated_state == exp_updated_state
 
 
+def test_handle_content_block_stop_invalid_json_logs_warning(caplog):
+    state = {
+        "content": [],
+        "current_tool_use": {"toolUseId": "123", "name": "my_tool", "input": "not valid json"},
+        "text": "",
+        "reasoningText": "",
+        "citationsContent": [],
+        "redactedContent": b"",
+    }
+
+    import logging
+
+    with caplog.at_level(logging.WARNING, logger="strands.event_loop.streaming"):
+        tru_updated_state = strands.event_loop.streaming.handle_content_block_stop(state)
+
+    assert tru_updated_state["content"] == [{"toolUse": {"toolUseId": "123", "name": "my_tool", "input": {}}}]
+    assert any("failed to parse tool input as JSON" in record.message for record in caplog.records)
+
+
 def test_handle_message_stop():
     event: MessageStopEvent = {"stopReason": "end_turn"}