fix: report malformed tool input JSON

strands-py/src/strands/event_loop/streaming.py

@@ -10,6 +10,7 @@
 
 from ..models.model import Model
 from ..tools import InvalidToolUseNameException
+from ..tools._validator import TOOL_INPUT_PARSE_ERROR_KEY
 from ..tools.tools import validate_tool_use_name
 from ..types._events import (
     CitationStreamEvent,
@@ -282,12 +283,22 @@ def handle_content_block_stop(state: dict[str, Any]) -> dict[str, Any]:
 
     if current_tool_use:
         if "input" not in current_tool_use:
-            current_tool_use["input"] = ""
-
-        try:
-            current_tool_use["input"] = json.loads(current_tool_use["input"])
-        except ValueError:
             current_tool_use["input"] = {}
+        else:
+            try:
+                current_tool_use["input"] = json.loads(current_tool_use["input"])
+            except ValueError as e:
+                logger.warning(
+                    "tool_name=<%s>, tool_use_id=<%s> | failed to parse tool input JSON",
+                    current_tool_use.get("name"),
+                    current_tool_use.get("toolUseId"),
+                )
+                current_tool_use["input"] = {
+                    TOOL_INPUT_PARSE_ERROR_KEY: (
+                        f"Invalid JSON in tool input for '{current_tool_use.get('name', 'unknown')}': {e}. "
+                        "Retry with a valid JSON object."
+                    )
+                }
 
         tool_use_id = current_tool_use["toolUseId"]
         tool_use_name = current_tool_use["name"]

strands-py/src/strands/tools/_validator.py

@@ -4,6 +4,8 @@
 from ..types.content import Message
 from ..types.tools import ToolResult, ToolUse
 
+TOOL_INPUT_PARSE_ERROR_KEY = "__strands_tool_input_parse_error__"
+
 
 def validate_and_prepare_tools(
     message: Message,
@@ -28,6 +30,22 @@ def validate_and_prepare_tools(
     # Avoid modifying original `tool_uses` variable during iteration
     tool_uses_copy = tool_uses.copy()
     for tool in tool_uses_copy:
+        parse_error = (
+            tool.get("input", {}).get(TOOL_INPUT_PARSE_ERROR_KEY) if isinstance(tool.get("input"), dict) else None
+        )
+        if parse_error:
+            tool_uses.remove(tool)
+            invalid_tool_use_ids.append(tool["toolUseId"])
+            tool_uses.append(tool)
+            tool_results.append(
+                {
+                    "toolUseId": tool["toolUseId"],
+                    "status": "error",
+                    "content": [{"text": f"Error: {parse_error}"}],
+                }
+            )
+            continue
+
         try:
             validate_tool_use(tool)
         except InvalidToolUseNameException as e:

strands-py/tests/strands/event_loop/test_streaming.py

@@ -5,6 +5,7 @@
 
 import strands
 import strands.event_loop
+from strands.tools._validator import TOOL_INPUT_PARSE_ERROR_KEY
 from strands.types._events import ModelStopReason, TypedEvent
 from strands.types.content import Message, Messages
 from strands.types.streaming import (
@@ -310,6 +311,25 @@ def test_handle_content_block_delta(event: ContentBlockDeltaEvent, event_type, s
     assert tru_callback_event == exp_callback_event
 
 
+def test_handle_content_block_stop_marks_malformed_tool_input(caplog):
+    state = {
+        "content": [],
+        "current_tool_use": {"toolUseId": "123", "name": "search", "input": '{"query": "unterminated'},
+        "text": "",
+        "reasoningText": "",
+        "citationsContent": [],
+        "redactedContent": b"",
+    }
+
+    with caplog.at_level("WARNING", logger="strands.event_loop.streaming"):
+        updated_state = strands.event_loop.streaming.handle_content_block_stop(state)
+
+    tool_input = updated_state["content"][0]["toolUse"]["input"]
+    assert TOOL_INPUT_PARSE_ERROR_KEY in tool_input
+    assert "Invalid JSON in tool input for 'search'" in tool_input[TOOL_INPUT_PARSE_ERROR_KEY]
+    assert "tool_name=<search>, tool_use_id=<123> | failed to parse tool input JSON" in caplog.text
+
+
 @pytest.mark.parametrize(
     ("state", "exp_updated_state"),
     [

strands-py/tests/strands/tools/test_validator.py

@@ -1,4 +1,5 @@
 from strands.tools import _validator
+from strands.tools._validator import TOOL_INPUT_PARSE_ERROR_KEY
 from strands.types.content import Message
 
 
@@ -49,3 +50,33 @@ def test_validate_and_prepare_tools():
     assert tru_tool_uses == exp_tool_uses
     assert tru_tool_results == exp_tool_results
     assert tru_invalid_tool_use_ids == exp_invalid_tool_use_ids
+
+
+def test_validate_and_prepare_tools_turns_malformed_input_into_tool_result():
+    message: Message = {
+        "role": "assistant",
+        "content": [
+            {
+                "toolUse": {
+                    "toolUseId": "t1",
+                    "name": "search",
+                    "input": {TOOL_INPUT_PARSE_ERROR_KEY: "Invalid JSON in tool input for 'search'"},
+                }
+            }
+        ],
+    }
+
+    tool_uses = []
+    tool_results = []
+    invalid_tool_use_ids = []
+
+    _validator.validate_and_prepare_tools(message, tool_uses, tool_results, invalid_tool_use_ids)
+
+    assert invalid_tool_use_ids == ["t1"]
+    assert tool_results == [
+        {
+            "toolUseId": "t1",
+            "status": "error",
+            "content": [{"text": "Error: Invalid JSON in tool input for 'search'"}],
+        }
+    ]