[seanmcrae]

Summary

Reject negative token counts in calculate_cost_by_tokens instead of silently returning a negative USD cost. Token counts represent usage quantities and cannot be negative; accepting them can invert a spend estimate and corrupt accounting.

Changes

• Raise ValueError with a clear message when num_tokens < 0.
• Add a regression test that asserts the invalid input is rejected.

Validation

• Regression test fails on base e7f7c1928084af55bb4955102327982401ed8649 because the function returns a negative cost instead of raising.
• PYTHONPATH=. .venv/bin/pytest -q tests/test_costs.py -k 'not claude': 94 passed, 2 deselected. The excluded pre-existing Claude cases invoke Anthropic's remote token-count API and require credentials.
• python -m compileall and git diff --check pass.
• flake8 tokencost/ tests/test_costs.py reports 8 existing findings, all reproduced against unchanged base files; the patch adds no lint finding. A full lint pass is not claimed.