Grok 4.5 vs Claude Opus 4.8: Flagship Coding Test

Every incumbent eventually meets a challenger whose whole pitch is the price tag. Claude Opus 4.8 is Anthropic’s flagship and the model to beat in our arena — at publish time its high-effort variant holds the most community votes of anything we run. Grok 4.5 is xAI’s answer: a 500k-token context window at $2 input / $6 output per million tokens, a rate card written to undercut Opus’s $5/$25 before you finish reading it. The newcomer’s question is blunt — is the incumbent really four times better per output token? — and it happens to be a question you can watch get answered live.

The rate card, side by side

The output rate is where the gap compounds: $25 against $6 is roughly four to one. Generate the same 40k tokens of code and Opus bills a dollar where Grok bills 24 cents. But list price is only the opening bid — both models run thinking-effort dials, and reasoning tokens are tokens you pay for without ever pasting them into a project. That is why every output card in the arena prints cost per task, not cost per million: what did this exact build cost to produce.

What the incumbent is defending

Opus 4.8’s lead is not a marketing claim — it is a running tally of community votes on blind, one-shot outputs, and as this posts, Opus at high effort sits on top of it. The other thing the incumbent holds is range: six effort configurations mean the same model spans cheap, quick scaffolds at low and heavyweight reasoning runs at max and ultracode. That dial is a pricing weapon of its own — Opus at low effort competes on cost far harder than the $25 sticker suggests, while Opus at max stacks reasoning tokens on the full flagship rate.

What the challenger is betting on

• Price as strategy — at $6 output, retries, experiments and high-volume pipelines cost a quarter of the incumbent’s rate. Grok 4.5 can afford to lose some head-to-heads and still win the invoice.
• 500k context — two and a half times Opus’s window. Repo-sized prompts and long specs go in whole, no pruning pass first.
• Tempo — fast generation is half of xAI’s launch pitch, and the arena records generation time next to every output, so the claim is checkable.

Undercutting the leader is easy. Outbuilding it, one shot at a time, is the hard part.

Same prompt, one shot, judged blind

Our method is deliberately boring: every model gets the identical published prompt, one attempt, no retries, and the raw output runs live in your browser — bugs included. The pairing is already up: Grok 4.5 vs Opus 4.8 puts the challenger at high effort next to the incumbent at max on the landing-page build, and compare mode hides the names until after you vote. The leaderboard keeps the running verdict — community votes, cost per task and generation times for both, against the full field.

My take

The economics only run one way: if Grok 4.5 takes even a respectable minority of blind votes against Opus, its rate card makes it the default for anything high-volume, because a model at a quarter of the price does not need to win every round. The incumbent’s counter is that a broken build is expensive at any rate, and Opus earned its lead one working app at a time. Neither argument settles anything from a spec sheet — so open the arena, run the pair on your kind of task, and vote. If you are choosing a day-to-day coding model rather than refereeing flagships, the full best AI for coding breakdown covers the whole 20-family field.