fix(truncator): keep an all-system context in the system half

[he-yufeng]

What

ContextTruncator._split_system_rest splits a message list into (system_messages, rest). It initialized first_non_system = 0 and only updated it when it found a non-system message. When every message is a system message the loop never breaks, so first_non_system stays 0 and the split is reversed — the system messages end up in the rest half and the system half is empty.

The truncation strategies then treat those system messages as droppable turns. For example truncate_by_halving on four system messages returns only two:

msgs = [Message(role="system", content=f"S{i}") for i in range(4)]
ContextTruncator().truncate_by_halving(msgs)
# before: 2 messages (two system messages dropped)
# after:  4 messages (all preserved)

System messages must never be dropped by truncation. The fix defaults first_non_system to len(messages), so an all-system list stays entirely in the system half. Mixed inputs (the loop still breaks at the first non-system message) and empty inputs are unchanged.

Test plan

Added two tests in tests/agent/test_truncator.py:

• test_split_system_rest_with_only_system_messages — all-system input puts everything in the system half.
• test_halving_keeps_all_system_messages — truncate_by_halving keeps all four system messages.

Both fail on master and pass with this change.

python -m pytest tests/agent/test_truncator.py
# 37 passed

ruff check and ruff format --check pass on the changed files.

Summary by Sourcery

Ensure context truncation never drops system messages when splitting or halving message histories.

Bug Fixes:

• Correct splitting of message lists so all-system inputs are fully retained in the system portion instead of being misclassified as non-system messages.

Tests:

• Add tests verifying that all-system message lists remain entirely in the system half and that halving truncation preserves all system messages.

[gemini-code-assist]

Code Review

This pull request fixes a bug in the context truncator where an all-system message list was incorrectly split, potentially leading to system messages being dropped during truncation. The fix initializes the split index to the length of the messages, ensuring all-system lists are correctly handled. Corresponding unit tests have been added to verify this behavior. No review comments were provided, so there is no feedback to address.

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[sourcery-ai]

Hey - I've left some high level feedback:

• The new tests exercise behavior via the private helper _split_system_rest; consider adding/adjusting a test that validates the same invariant purely through the public truncation APIs so future refactors of the helper don’t require test changes.

Prompt for AI Agents

Please address the comments from this code review:

## Overall Comments
- The new tests exercise behavior via the private helper `_split_system_rest`; consider adding/adjusting a test that validates the same invariant purely through the public truncation APIs so future refactors of the helper don’t require test changes.

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