Claude Opus 4.8 and Sonnet 5 exhibit increased tool call schema errors
July 4, 2026
Newer Anthropic models like Opus 4.8 and Sonnet 5 are failing to adhere to strict tool schemas by injecting invented keys into nested arrays. This regression in tool-calling reliability causes systematic rejections in environments like Pi, despite the actual content of the edits being correct.
HOW THIS AFFECTS YOU
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builderYou may need to implement more robust schema validation or instruction tuning to handle hallucinated fields in SOTA model tool calls.