Claude Opus 4.8 (max effort)
Anthropic · proprietary · writing benchmark
The short version
Claude Opus 4.8 (max effort) sits at #14 of 130 on ToneBench, in the upper tier, with a writing Elo of 2361 and an overall score of 88.5 out of 100. We measured it by having it write all 9 of our real YouTube scripts, five times each, then scoring every draft blind against our own finished versions. Here is how it shook out.
Within Anthropic it ranks 12th of 21. Claude Opus 5 (max effort) is the family's top writer here, about 288 Elo ahead, so this config trades some quality for whatever it saves you in cost or speed.
Among proprietary models, it comes in 12th of 83. That ranking is against the stronger half of the board: closed models still set the pace on voice fidelity here.
One thing that stands out: it is unusually consistent. Across its 45 drafts the overall score only moves ± 1.8 points run to run, versus a board median of ± 3.8. What you get on the first try is close to what you get every try.
What it does best is Length Discipline: it ranks 4th on that dimension at 93.0, well above the board average. Its softer spot is Visual Cues (86.8, 34th), which is the thing to watch if that metric matters most for your use.
It was uneven across the 9 scripts. Its best run was the founder announcement / personal origin story (90.6) and its weakest was the personal technical walkthrough / agentic workflow case study (87.1), a spread of about 4 points. Worth knowing if your writing skews toward one of those formats.
On cost, it runs about $0.435/task. Claude Opus 5 (low) scores higher for less money, at $0.115/task, so on pure value this config is not the frontier.
If you're tuning reasoning effort, we also tested Claude Opus 4.8 at other settings. None of them beat this config; the closest is Claude Opus 4.8 (default) (Elo 2123), so the only question is how much quality you want to trade for cost or speed.
Skill profile
The shape is the story: the further a corner reaches, the stronger that dimension. Biggest edge over the board: Length Discipline (+25.9); smallest edge: Substance & Value (+9.0).
Per-metric scores
Nine writing dimensions, each scored 0–100 and blended by the editorial weight shown. Rank is against all 130 current-ranked models. The thick bar is this model; the thin lines above and below are the current field's best model (darker) and average (lighter) on the same scale. The small ± number is its run-to-run variation.
Per-article scores
The same 9 real scripts every current-ranked model writes, scored individually. Different formats stress different skills.
Consistency: run-to-run variability
Every cell on this page is the mean of 5 independent runs per script; the ± numbers are the run-to-run standard deviation. Overall, Claude Opus 4.8 (max effort) varies by ± 1.8 points between runs versus a board median of ± 3.8, so it is noticeably steadier than the typical model here.
Its steadiest is Tone & Voice (± 1.3 vs ± 2.2 board median), which you can rely on run after run.
| Metric | This model | Board median | Verdict |
|---|---|---|---|
| Tone & Voice ? | 88.4 ± 1.3 | ± 2.2 | steadier than most |
| Writing Craft ? | 88.3 ± 1.5 | ± 2.0 | typical |
| Substance & Value ? | 87.7 ± 1.7 | ± 2.6 | typical |
| Flow & Emotion ? | 86.9 ± 1.7 | ± 2.6 | typical |
| YouTube Structure ? | 86.8 ± 2.0 | ± 3.6 | steadier than most |
| Hook ? | 89.9 ± 1.6 | ± 3.0 | steadier than most |
| Length Discipline ? | 93.0 ± 4.1 | ± 8.6 | steadier than most |
| Anti-Slop ? | 92.1 ± 1.6 | ± 2.6 | steadier than most |
| Visual Cues ? | 86.8 ± 2.8 | ± 4.7 | steadier than most |
Measured, not modeled
Numbers we log on every run and rarely talk about. None of these affect the writing scores; cost and latency are informational.
What each judge scored it
The published overall of 88.5 is the consensus of three family-disjoint judges scoring the same 45 stored drafts blind with the identical rubric. The highest and lowest judge differ by 5.6 points on its overall. The three judges essentially agree on this model. How the panel works: methodology.
How we ran Claude Opus 4.8 (max effort)
- Accessed viaClaude Code (Anthropic subscription)
- Exact model / route id
claude-opus-4-8atreasoning_effort: max - Generations5 independent runs × 9 scripts = 45 drafts, stored immutably after generation
- Run dates2026-07-29 → 2026-08-11
- Judges3-judge panel: Claude Opus 5, high effort (Anthropic) + DeepSeek V4 Flash (OpenRouter) + GPT-5.6 Sol, medium reasoning (Codex CLI): every draft scored independently by all three with the identical rubric, blind to model identity, evidence quotes required before each metric score; published score = per-metric consensus mean, verified with a Bradley-Terry refit (methodology)
- Samplingprovider-default sampling, no fine-tuning, no system-prompt changes; every current-ranked model gets the identical brief, style contract and research packet
- What stays privatethe exact rubric and any non-public reference material; source-publication timing is disclosed for exceptions (why)
Frequently asked questions
How good is Claude Opus 4.8 (max effort) at writing?
On ToneBench it ranks #14 of 130 with a writing Elo of 2361 and an overall score of 88.5/100. That score comes from writing our 9 real YouTube scripts five times each and scoring every draft blind against our own finished versions across nine writing dimensions.
Is Claude Opus 4.8 (max effort) the best Anthropic model for writing?
Not quite. Within Anthropic it ranks 12th of 21; Claude Opus 5 (max effort) is the family's best writer here.
Is Claude Opus 4.8 (max effort) good value for the money?
It costs about $0.435/task. Claude Opus 5 (low) scores higher for less, so it is not the value pick.
What are Claude Opus 4.8 (max effort)'s strengths and weaknesses?
Its strongest dimension is Length Discipline (4th on the board, 93.0). Its weakest is Visual Cues (34th on the board, 86.8). The full nine-metric breakdown is on this page.
Does Claude Opus 4.8 (max effort) write some formats better than others?
Yes. Its best of our 9 scripts was the founder announcement / personal origin story (90.6) and its weakest was the personal technical walkthrough / agentic workflow case study (87.1).
How consistent is Claude Opus 4.8 (max effort) between runs?
We run every script 5 times. Claude Opus 4.8 (max effort)'s overall score varies by about ±1.8 points between runs, versus a board median of ±3.8. That makes it one of the steadier models we test. The full per-metric variability table is on this page.
How was Claude Opus 4.8 (max effort) evaluated?
Via Claude Code (Anthropic subscription) using the exact model/route id claude-opus-4-8 at reasoning_effort max, run on 2026-07-29. 5 runs per script, provider-default sampling, no fine-tuning; every draft scored blind with a fixed rubric by a three-family judge panel. Full details in the 'How we ran it' section and the methodology.
Is Claude Opus 4.8 (max effort) open source?
No, it is a proprietary (closed-weights) model. Among proprietary models it ranks 12th of 83 for writing.