fix(streaming): handle tool use metadata in contentBlockDelta for non-standard models

Some Bedrock models (e.g., Kimi K2.5) send toolUseId and name in
contentBlockDelta rather than in contentBlockStart. The parser only
extracted input from the delta, leaving current_tool_use without those
fields and crashing in handle_content_block_stop.

- Capture toolUseId and name from delta when not already set by start
- Use .get("input", "") to avoid KeyError when input key is absent
- Add warning + skip for incomplete tool use blocks in stop handler

Fixes #1646

Author: Zelys-DFKH

src/strands/event_loop/streaming.py

@@ -210,10 +210,19 @@ def handle_content_block_delta(
     typed_event: ModelStreamEvent = ModelStreamEvent({})
 
     if "toolUse" in delta_content:
+        tool_use_delta = delta_content["toolUse"]
         if "input" not in state["current_tool_use"]:
             state["current_tool_use"]["input"] = ""
 
-        state["current_tool_use"]["input"] += delta_content["toolUse"]["input"]
+        state["current_tool_use"]["input"] += tool_use_delta.get("input", "")
+
+        # Some models provide toolUseId and name in contentBlockDelta instead of contentBlockStart.
+        # Capture them here if not already set from a prior contentBlockStart event.
+        if "toolUseId" not in state["current_tool_use"] and "toolUseId" in tool_use_delta:
+            state["current_tool_use"]["toolUseId"] = tool_use_delta["toolUseId"]
+        if "name" not in state["current_tool_use"] and "name" in tool_use_delta:
+            state["current_tool_use"]["name"] = tool_use_delta["name"]
+
         typed_event = ToolUseStreamEvent(delta_content, state["current_tool_use"])
 
     elif "text" in delta_content:
@@ -281,8 +290,16 @@ def handle_content_block_stop(state: dict[str, Any]) -> dict[str, Any]:
         except ValueError:
             current_tool_use["input"] = {}
 
-        tool_use_id = current_tool_use["toolUseId"]
-        tool_use_name = current_tool_use["name"]
+        tool_use_id = current_tool_use.get("toolUseId", "")
+        tool_use_name = current_tool_use.get("name", "")
+
+        if not tool_use_id or not tool_use_name:
+            logger.warning(
+                "Incomplete tool use block (missing toolUseId or name); skipping content block. "
+                "The model may be using a non-standard streaming format."
+            )
+            state["current_tool_use"] = {}
+            return state
 
         tool_use = ToolUse(
             toolUseId=tool_use_id,

tests/strands/event_loop/test_streaming.py

@@ -1422,3 +1422,110 @@ async def test_process_stream_keeps_tool_use_stop_reason_unchanged(agenerator, a
     last_event = cast(ModelStopReason, (await alist(stream))[-1])
 
     assert last_event["stop"][0] == "tool_use"
+
+
+def test_handle_content_block_delta_captures_tool_use_id_and_name_from_delta():
+    """Delta events that include toolUseId and name should populate current_tool_use."""
+    event = {"delta": {"toolUse": {"input": '{"x": 1}', "toolUseId": "abc123", "name": "output_slide"}}}
+    state = {"current_tool_use": {}}
+
+    updated_state, _ = strands.event_loop.streaming.handle_content_block_delta(event, state)
+
+    assert updated_state["current_tool_use"]["toolUseId"] == "abc123"
+    assert updated_state["current_tool_use"]["name"] == "output_slide"
+    assert updated_state["current_tool_use"]["input"] == '{"x": 1}'
+
+
+def test_handle_content_block_delta_does_not_override_existing_tool_use_id_and_name():
+    """toolUseId and name from contentBlockStart should not be overridden by a later delta."""
+    event = {"delta": {"toolUse": {"input": '{"x": 1}', "toolUseId": "from_delta", "name": "from_delta"}}}
+    state = {"current_tool_use": {"toolUseId": "from_start", "name": "from_start", "input": ""}}
+
+    updated_state, _ = strands.event_loop.streaming.handle_content_block_delta(event, state)
+
+    assert updated_state["current_tool_use"]["toolUseId"] == "from_start"
+    assert updated_state["current_tool_use"]["name"] == "from_start"
+
+
+def test_handle_content_block_delta_tool_use_without_input_key():
+    """A toolUse delta missing the input key should not raise KeyError."""
+    event = {"delta": {"toolUse": {}}}
+    state = {"current_tool_use": {"toolUseId": "t1", "name": "tool"}}
+
+    updated_state, _ = strands.event_loop.streaming.handle_content_block_delta(event, state)
+
+    assert updated_state["current_tool_use"]["input"] == ""
+
+
+def test_handle_content_block_stop_skips_incomplete_tool_use_missing_id(caplog):
+    """A tool use block missing toolUseId is skipped with a warning."""
+    import logging
+
+    state = {
+        "content": [],
+        "current_tool_use": {"name": "output_slide", "input": '{"x": 1}'},
+        "text": "",
+        "reasoningText": "",
+        "citationsContent": [],
+    }
+
+    with caplog.at_level(logging.WARNING, logger="strands.event_loop.streaming"):
+        updated_state = strands.event_loop.streaming.handle_content_block_stop(state)
+
+    assert updated_state["content"] == []
+    assert updated_state["current_tool_use"] == {}
+    assert "Incomplete tool use block" in caplog.text
+
+
+def test_handle_content_block_stop_skips_incomplete_tool_use_missing_name(caplog):
+    """A tool use block missing name is skipped with a warning."""
+    import logging
+
+    state = {
+        "content": [],
+        "current_tool_use": {"toolUseId": "abc123", "input": '{"x": 1}'},
+        "text": "",
+        "reasoningText": "",
+        "citationsContent": [],
+    }
+
+    with caplog.at_level(logging.WARNING, logger="strands.event_loop.streaming"):
+        updated_state = strands.event_loop.streaming.handle_content_block_stop(state)
+
+    assert updated_state["content"] == []
+    assert updated_state["current_tool_use"] == {}
+    assert "Incomplete tool use block" in caplog.text
+
+
+@pytest.mark.asyncio
+async def test_process_stream_tool_use_info_in_delta(agenerator, alist):
+    """Models that provide toolUseId and name in contentBlockDelta (not contentBlockStart) work correctly."""
+    response = [
+        {"messageStart": {"role": "assistant"}},
+        {"contentBlockStart": {"start": {}}},
+        {
+            "contentBlockDelta": {
+                "delta": {"toolUse": {"input": '{"title": "Test"}', "toolUseId": "xyz789", "name": "output_slide"}}
+            }
+        },
+        {"contentBlockStop": {}},
+        {"messageStop": {"stopReason": "tool_use"}},
+        {
+            "metadata": {
+                "usage": {"inputTokens": 5, "outputTokens": 10, "totalTokens": 15},
+                "metrics": {"latencyMs": 50},
+            }
+        },
+    ]
+
+    stream = strands.event_loop.streaming.process_stream(agenerator(response))
+    events = await alist(stream)
+    last_event = cast(ModelStopReason, events[-1])
+
+    stop_reason, message, _, _ = last_event["stop"]
+    assert stop_reason == "tool_use"
+    assert len(message["content"]) == 1
+    tool_use = message["content"][0]["toolUse"]
+    assert tool_use["toolUseId"] == "xyz789"
+    assert tool_use["name"] == "output_slide"
+    assert tool_use["input"] == {"title": "Test"}