Claude Opus 4.8: Anthropic's Flagship Reasoning Model
Claude Opus 4.8 is Anthropic's flagship reasoning model.
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Claude Opus 4.8 is Anthropic's flagship reasoning model.
For most of the Claude lineup's history, "Opus" has meant the most capable tier Anthropic offered.
As of roughly June 2026, that's no longer quite true - Claude Fable 5 now sits above it as a new, even more capable Mythos-class tier.
But Opus 4.8 hasn't been demoted to an afterthought.
It occupies a specific, still-important place in the lineup: the model built for hard reasoning, at a real cost saving versus Fable.
Since the Claude 3 family launched in March 2024, "Opus" has referred to the largest, most capable size tier in each Claude generation, with Sonnet as the balanced mid-tier and Haiku as the fast, lightweight tier.
Opus 4.8 continues that tradition as the current flagship reasoning model in the lineup.
Its defining technical feature is a 1M token context window by default - meaning it can hold roughly a million tokens' worth of text, code, or conversation in view at once without any special configuration.
That puts it on par with Fable 5's context size, even though Fable sits above it in the newer Mythos-class category.
Priced at roughly $5 input and $25 output per MTok, Opus 4.8 costs about half of Fable 5's rate, while remaining several times more expensive than Sonnet 5's introductory pricing.
Think of Opus 4.8 as the model for "this needs a careful, deliberate answer" work - not the absolute ceiling of what Claude can do, but a clear step up in reasoning depth from the everyday default.
Opus 4.8's positioning in the lineup is best understood by what sits on either side of it.
Below it, Claude Sonnet 5 is tuned to be the fast, cost-efficient default for everyday work, and as of its June 2026 launch it's strong enough to edge past Opus 4.8 on some knowledge-work benchmarks despite costing a fraction as much.
That's a meaningful nuance: Opus 4.8 is not automatically "smarter" than Sonnet 5 on every task, even though it costs more and is the older, more established reasoning tier.
Above it, Claude Fable 5 shares Opus 4.8's 1M token default context but adds always-on adaptive thinking and a larger 128K maximum output, at roughly double Opus's per-MTok cost.
The practical implication is that Opus 4.8 occupies a genuine middle ground for reasoning-heavy work: it's built to handle harder, more ambiguous problems than Sonnet 5 is optimized for, without the extra premium Fable 5 charges for its adaptive-thinking ceiling.
Where a task is hard enough that Sonnet 5's speed-tuned defaults start to show gaps, but not so demanding that it needs Fable's maximum context-plus-adaptive-reasoning combination, Opus 4.8 is the tier built for that gap.
Context window size and reasoning capability are related but distinct properties worth separating clearly.
Opus 4.8's 1M token window means it can consider a large amount of material at once - a long document, a sprawling set of notes, an extended conversation - but that's a different property from how carefully it reasons about what it's holding in view.
The two properties happen to be bundled together in Opus 4.8 (large context and strong reasoning), which is part of why it suits complex, document-heavy analysis particularly well.
Choosing between Opus 4.8 and its neighbors comes down to matching the task's actual difficulty to the tier, not defaulting upward "to be safe."
A task that's ambiguous, has multiple plausible interpretations, or requires weighing several factors against each other before answering is a good candidate for Opus 4.8 rather than Sonnet 5.
A task that additionally needs the largest possible context window, an extended single continuous output, or the extra reasoning depth of always-on adaptive thinking may be worth escalating further to Fable 5 - but many tasks that feel "hard" don't actually need that extra step up.
| Approach | Strength | Weakness | Best Fit |
|---|---|---|---|
| Claude Opus 4.8 | Strong reasoning at 1M context, roughly half Fable's cost | Costs more than Sonnet 5; behind Fable on output ceiling and adaptive thinking | Complex analysis and multi-step reasoning that doesn't need Fable's premium |
| Claude Sonnet 5 | Cheapest of the three heavier tiers; strong on coding/agentic benchmarks | Smaller context than Opus/Fable; less built for the hardest reasoning chains | Everyday work, including much of what used to require Opus |
| Claude Fable 5 | Largest context, output ceiling, and always-on adaptive thinking | Highest cost in the lineup | The hardest, largest tasks where Opus's reasoning still leaves gaps |
An important trend worth tracking: as newer mid-tier models like Sonnet 5 close the gap with Opus 4.8 on specific benchmarks, the "always escalate to Opus for anything hard" habit becomes less reliable over time.
Teams that periodically re-test whether Sonnet 5 alone now handles a task that used to require Opus can meaningfully cut costs without losing quality.
No - as of Fable 5's June 2026 launch, Opus 4.8 is the second-highest tier, sitting below the new Mythos-class Fable 5, though it remains the flagship reasoning model below that top tier.
A 1M token context window by default - the same size as Fable 5's default context, without needing any special configuration.
Roughly $5 input / $25 output per MTok - about half of Fable 5's rate, and several times Sonnet 5's introductory pricing.
When the task also needs Fable's larger 128K output ceiling, its always-on adaptive thinking, or simply the maximum capability Anthropic offers regardless of cost - Opus 4.8 alone is usually enough for hard reasoning that doesn't require those specific extras.
Not on every task - Sonnet 5 scores strongly on coding and agentic benchmarks and edges past Opus 4.8 on some knowledge-work tasks, despite being the cheaper, lighter tier.
It signals that Opus 4.8 is tuned specifically for careful, deliberate reasoning on complex or ambiguous problems, distinct from Sonnet 5's tuning toward fast, balanced everyday performance and Haiku 4.5's tuning toward speed and volume.
That always-on adaptive thinking behavior is specifically what distinguishes Fable 5 as a Mythos-class tier - Opus 4.8 is positioned as the flagship reasoning model below that, without carrying the same premium.
Since the Claude 3 family launched in March 2024, Opus has been the largest, most capable size tier in each Claude generation, with Sonnet and Haiku below it - Fable, added in June 2026, is the first tier to sit above Opus.
Not within the current lineup - it matches Fable 5's default context size, which sets it apart from Sonnet 5 and Haiku 4.5, both of which run with smaller context windows.
Defaulting to it out of habit for anything that seems hard, without first checking whether Sonnet 5 - now a strong, much cheaper default - already handles the task well enough.
Stack versions: Written against the Claude model lineup current as of ~June 2026 - Claude Fable 5, Claude Opus 4.8, Claude Sonnet 5 (the default), and Claude Haiku 4.5. Model names, pricing, and product features move quickly - verify current specifics at platform.claude.com/docs before relying on them.
Reviewed by Chris St. John·Last updated Jul 18, 2026