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Newer Claude models — including the flagship Opus 4.8, not just small models — sometimes call Pi's edit tool with extra, invented fields in the nested edits[] array, so the arguments fail to match the schema and Pi rejects the tool call and asks for a retry.
Armin Ronacher found that newer Claude models, even the flagship Opus 4.8, sometimes invent extra fields when calling Pi's edit tool, causing schema validation to fail and the call to be rejected. ✦ AI generated
Armin Ronacher · Simon Willison's Weblog · 2026-07-04 · original ↗
The short version is that newer Claude models sometimes call Pi's edit tool with extra, invented fields in the nested edits[] array. And not Haiku or some small model: Opus 4.8. The edit itself is usually correct but the arguments do not match the schema as the model invents made-up keys and Pi thus rejects the tool call and asks to try again.
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It is specifically the newest, state-of-the-art Claude models — Opus 4.8 and Sonnet 5 — that show this malformed-tool-call problem, while none of the older models do, meaning the SOTA models are worse at this particular tool schema than their older siblings.Armin Ronacher · Simon Willison's Weblog · conf 70%Third-party coding harnesses like Pi may need to implement multiple edit tools so they can use whichever one performs best for the specific underlying model the user has selected.Simon Willison · Simon Willison's Weblog · conf 65%