Fix IndexError in streaming when ToolCallPartDelta is present (#1208)

* Fix IndexError in streaming when ToolCallPartDelta is present

When pydantic-ai streams incomplete tool calls (ToolCallPartDelta),
the event.index refers to the position in the internal _parts list,
but get_parts() filters out these incomplete parts. This causes an
IndexError when trying to access get_parts()[index].

The fix uses the internal _parts list directly for snapshots, since
that's what the event indices refer to.

Fixes #1207

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

* Add defensive check for None snapshot when accessing tool_call_id

---------

Co-authored-by: Claude <noreply@anthropic.com>

Author: zzstoatzz

src/marvin/engine/streaming.py

@@ -189,8 +189,14 @@ def _process_pydantic_event(
     tools_map: dict[str, Callable[..., Any]],
     end_turn_tools_map: dict[str, EndTurn],
 ) -> Event | None:
-    def _get_snapshot(index: int) -> ModelResponsePart:
-        return parts_manager.get_parts()[index]
+    def _get_snapshot(index: int) -> ModelResponsePart | None:
+        # Use the internal _parts list directly since event.index refers to that.
+        # get_parts() filters out ToolCallPartDelta objects which causes index mismatch.
+        # Note: _parts can contain both ModelResponsePart and ToolCallPartDelta
+        if index < len(parts_manager._parts):
+            return parts_manager._parts[index]
+        # This should never happen with valid events from pydantic-ai
+        return None
 
     # Handle Part Start Events
     if isinstance(event, PartStartEvent):
@@ -227,7 +233,7 @@ def _get_snapshot(index: int) -> ModelResponsePart:
                     tool_call_id=event.part.tool_call_id,
                 ),
                 snapshot=snapshot,
-                tool_call_id=snapshot.tool_call_id,
+                tool_call_id=snapshot.tool_call_id if snapshot else None,
                 tool=tools_map.get(event.part.tool_name),
             )
     # Handle Part Delta Events

tests/basic/engine/test_streaming.py

@@ -0,0 +1,54 @@
+"""Tests for the streaming module."""
+
+from pydantic_ai._parts_manager import ModelResponsePartsManager
+from pydantic_ai.messages import PartDeltaEvent, ToolCallPartDelta
+
+from marvin import Agent
+from marvin.engine.streaming import _process_pydantic_event
+
+
+def test_get_snapshot_with_incomplete_tool_call():
+    """Test that _get_snapshot handles ToolCallPartDelta correctly.
+
+    This tests the fix for issue #1207 where accessing get_parts()[index]
+    would raise IndexError when the part at that index is a ToolCallPartDelta
+    that gets filtered out by get_parts().
+    """
+    # Setup
+    parts_manager = ModelResponsePartsManager()
+    actor = Agent(name="test")
+    tools_map = {}
+    end_turn_tools_map = {}
+
+    # Create an incomplete tool call (ToolCallPartDelta with no tool name)
+    # This simulates what happens when streaming starts a tool call
+    parts_manager.handle_tool_call_delta(
+        vendor_part_id=0,
+        tool_name=None,  # No tool name yet - creates ToolCallPartDelta
+        args="",
+        tool_call_id=None,
+    )
+
+    # Verify the setup - we should have a ToolCallPartDelta in _parts
+    # but get_parts() should return empty
+    assert len(parts_manager._parts) == 1
+    assert len(parts_manager.get_parts()) == 0
+
+    # Create a PartDeltaEvent that references index 0
+    event = PartDeltaEvent(index=0, delta=ToolCallPartDelta(args_delta="{"))
+
+    # Process the event - this should NOT raise IndexError
+    # Before the fix, this would fail with "list index out of range"
+    result = _process_pydantic_event(
+        event=event,
+        actor=actor,
+        parts_manager=parts_manager,
+        tools_map=tools_map,
+        end_turn_tools_map=end_turn_tools_map,
+    )
+
+    # The result should be a ToolCallDeltaEvent with the snapshot
+    assert result is not None
+    assert hasattr(result, "snapshot")
+    # The snapshot should be the ToolCallPartDelta from _parts[0]
+    assert result.snapshot == parts_manager._parts[0]