Armin Ronacher found that Anthropic’s latest Opus 4.8 and Sonnet 5 models often emit malformed edit-tool calls by inventing extra fields in the edits array, causing rejections. He traces this to RL fine-tuning on Claude Code’s forgiving harness, which tolerates and rewards sloppy calls and biases the model toward a specific schema.

• Opus 4.8 and Sonnet 5 invent extra fields in tool calls (e.g. "requireUnique," "oldText2") up to 20% of the time in agentic multi-turn sessions, while older models and single-turn prompts don't show this.
• Ronacher attributes this to RL fine-tuning on Claude Code's own forgiving harness, which silently retries, coerces types, and strips unknown keys without penalizing the model, so it learns sloppy schemas get rewarded.
• The result is a newer, stronger model that's paradoxically worse at conforming to external/third-party tool schemas than its predecessor (Opus 4.5 adapted fine, Opus 4.8 doesn't).
• Anyone building on Anthropic's undocumented closed-source harness risks their own tool formats silently becoming "off-distribution," with failures only showing up after the fact.