Pydantic AI Updates: August 11, 2026

1. Pydantic AI v2.27.1 Unlocks Adaptive Thinking With Tool Output on Anthropic

Pydantic AI. Pydantic AI v2.27.1 enables adaptive thinking to work alongside Tool Output and forced tool choice on Anthropic models, a combination that previously could not be used together. The patch also fixes XaiStreamedResponse finish_reason mapping for xAI streaming responses, restores tool spans when argument validation fails, points offline web UI hosting at the self-contained chat UI build, and gates RetryPromptPart OpenTelemetry content on the include_content setting. Teams running agents on Anthropic with structured tool output and extended thinking are the main beneficiaries of the release. Source