Claude Fable 5 for Agents: Tool-Call Refusals, Cost vs GLM 5.2

1Key Takeaways

• Claude Fable 5 refused mid-tool-call on 11 of 44 coding-agent turns in our eval, on tasks as mundane as fixing a config default.
• The refusal arrives as stop_reason: "refusal" partway through generating the tool arguments, the truncated arguments still parse as valid JSON, and an agent loop that executes tool calls without checking the stop reason will happily write a half-finished file to disk.
• That behavior, not the price tag, is the first thing to engineer around when you put Fable 5 in an….