Fix IndexError in streaming when ToolCallPartDelta is present

[zzstoatzz]

Summary

Fixes #1207 - IndexError when processing pydantic events in streaming

Problem

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].

Solution

1. Use the internal _parts list directly for snapshots, since that's what the event indices refer to
2. Added defensive check for None snapshots when accessing tool_call_id attribute
3. Updated type hint to reflect that _get_snapshot can return None (though this should never happen with valid events)

Test Plan

• Added unit test that reproduces and verifies the fix
• Existing tests pass
• Verified the exact error is reproduced without the fix

🤖 Generated with Claude Code

[Copilot]

Pull Request Overview

Fixes an IndexError that occurs when processing streaming tool calls in pydantic-ai events. The issue arises when ToolCallPartDelta objects are present but filtered out by get_parts(), causing a mismatch between event indices and accessible parts.

• Modified _get_snapshot() to use the internal _parts list directly instead of the filtered get_parts() method
• Added bounds checking to prevent accessing invalid indices
• Added comprehensive unit test to verify the fix and prevent regression

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
src/marvin/engine/streaming.py	Updated _get_snapshot() to access _parts directly with bounds checking
tests/basic/engine/test_streaming.py	Added test case reproducing the IndexError scenario with incomplete tool calls

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