# Claude Opus 4.8 vs Claude Fable 5 benchmark: prompt steering, cost, and latency results

I re-ran my April prompt steering benchmark on Claude Opus 4.8 \[1m\] and Claude Fable 5 \[1m\] looking at how effort level and prompt steering impacts token usage and token costs, and performance i.e. instruction following [https://ai.georgeliu.com/p/claude-opus-48-vs-claude-fable-5](https://ai.georgeliu.com/p/claude-opus-48-vs-claude-fable-5)

The benchmark harness runs 200x headless `claude -p` sessions and the analysis comes from session-metrics, a Claude Code skill I built with Claude Code itself - it parses session JSONL files into per-turn cost, cache, latency, and token breakdowns, and produced the comparison data and charts behind the article.

The results that surprised me:

* The model-specific inversion survived but moved. ultrathink is essentially free on Opus 4.8 at high effort (-2.6 percent cost) and the single most expensive wrapper on Fable 5 at the same effort (+31.0 percent, output up 53 percent).
* Fable 5 looks 2-3x more expensive than 4.8, but it sits on a 2x rate card. Divide by two and at high effort its token consumption is on par with 4.8. At medium it is genuinely \~43 percent heavier.
* Fable 5's medium effort cost MORE than high ($1.33 vs $1.14 per session). There is no economy rung hiding at medium.
* "Be concise" reliably breaks refactoring tasks. Across two model generations, compression wrappers made models drop required content from the explanation around otherwise-correct code (4.7 failed the same constraint in 4 of 10 cells in April, all concise/no-tools; 4.8 repeated it in June). If your prompt requires the answer to contain specific things, do not also tell the model to trim words.
* Instruction-following ranks across both runs: Opus 4.6 88/90, Fable 5 83/90, Opus 4.8 80/90, Opus 4.7 75/90. The oldest model is still the champion. Fable 5 hits "exactly 120 words" 10/10 but writes past character caps; 4.8 does the opposite.
* Both June models are much faster: 4.8 averages \~3 seconds per turn vs 6.6-19.5 for April's 4.6.



