Claude Opus 5 Costs 3x More Than Opus 4.8 Unless You Disable Thinking

Claude Opus 5 and Opus 4.8 both cost $5 per million input tokens, but Opus 5's default adaptive thinking inflates output tokens by up to 3.1x on identical prompts. Disabling thinking via the API brings costs to parity with Opus 4.8, though Fable 5 rejects that parameter. The hidden thinking tokens account for 42-95% of output, making Opus 5's effective cost highly dependent on configuration.