Claude Fable 5.2 vs Claude Opus 5.5: Six Index Points for 2.6× the Cost per Task

A single sentence in A​nthropic's own documentation settles most of this matchup, and it is not a marketing sentence: start with Claude Opus 5.5 for most workloads, and move to Claude Fable 5.2 when your evals on an Opus model at higher effort still fall short. That is the vendor telling you its $8 / $40 model is the exception and its $4 / $20 model is the rule. The numbers behind the exception are real — Claude Fable 5.2 is the strongest model either company ships by the independent measure, and on several agentic benchmarks it is not close. The numbers behind the rule are bigger: on Artificial Analysis's suite the cheaper model finishes an index task for $10.04 and the dearer one for $26.54, so the six-point gap between them costs 2.6× per completed task, and the default-effort mismatch between the two models means a naive A/B test will overstate the cheap one's advantage or the expensive one's cost depending on which way you forget to set it.

Both models are current, both ship a one-million-token context window and a 128K output ceiling, and both read text and images. Claude Opus 5.5 launched September 22, 2026 as the first model in the Claude 5.5 family and is still labelled latest on its own page; Claude Fable 5.2 arrived November 9, 2026 and is the current Fable. They are not generations of one line — Fable is the tier above Opus, priced accordingly — so the question is not which is newer but which tier your workload actually needs, and the honest answer for most workloads is the cheaper one.

The decision in three lines

• If your work is well-specified coding, routine agent runs, knowledge work, computer use, or anything on zero data retention, Claude Opus 5.5 is the correct default and the 2.6× is not buying you anything you will notice.

• If you have already pushed Claude Opus 5.5 to xhigh or max and your evals still fail, Claude Fable 5.2 is the only step up in the line, and the vendor claims it reaches Opus-parity on some coding work at roughly half the cost per task — which is the one case where the expensive model is also the economical one.

• If your workload is short, latency-sensitive chat, neither the tier nor the price cut matters much, because Fable 5.2 thinks before nearly every reply at its default effort and Opus 5.5 does not.

Two defaults that are not the same

The first trap in this comparison is invisible in a rate card. Claude Fable 5.2 defaults to high effort; Claude Opus 5.5 defaults to medium. A request that sets no effort therefore runs one level higher on the Fable model — the call succeeds, and the only evidence is a different cost, a different latency and a different answer. A​nthropic's migration documentation calls this out explicitly for anyone moving from Opus, and it is the single easiest way to produce a benchmark comparison that means nothing.

• Default effort — high on Claude Fable 5.2 vs medium on Claude Opus 5.5.

• Rate per million tokens — $8 in and $40 out vs $4 in and $20 out.

• Cache writes and reads — $10 / $16 / $0.20 vs $5 / $8 / $0.20 per million for five-minute writes, one-hour writes and cache reads.

• Batch pricing — $4 / $20 vs $2 / $10 per million.

• Fast mode — $16 / $80 vs $8 / $40 per million, both advertised at up to 2.5× speed.

• Comparative latency — “moderate” on both, per the vendor's own table; the only measured figure either has is Opus 5.5 at 115 output tokens per second.

• Data retention — 30-day required, no ZDR on Claude Fable 5.2 vs zero data retention available on Claude Opus 5.5.

• Knowledge cutoff — June 2026 on both; retirement floor — November 9, 2027 vs September 22, 2027.

• Batch output ceiling — 128K on Claude Fable 5.2 vs 128K, rising to 300K with the output-300k-2026-03-24 beta header, on Claude Opus 5.5.

Two of those lines decide real deployments before any benchmark is consulted. The retention line is a hard fork: Claude Fable 5.2 is a Covered Model that requires 30-day data retention and is not available under zero data retention unless A​nthropic expressly authorizes it, while Claude Opus 5.5 runs under ZDR. A team contractually barred from retaining prompts cannot use the stronger model at all, whatever its scores. The batch-output line is softer but concrete — the 300K extended-output beta is listed for Opus 5.5 and not for Fable 5.2, so a pipeline that emits enormous single documents from a batch job has one fewer option on the Fable side.

Where the gap is wide, and where it is a rounding error

A​nthropic's Claude Fable 5.2 launch post prints both models in the same columns, which makes it the only place the pair can be compared on identical hardware and settings. Everything below is vendor-reported and unreproduced; the effort setting is included wherever the post names one, because on these models it moves the number as much as the model does.

• Terminal-Bench-Science 0.1 — 74.6% for Claude Fable 5.2 vs 58.7% for Claude Opus 5.5: a 16-point gap, the widest on a standard benchmark.

• AutomationBench — 51.4% vs 42.5%; RoboDojo Sim — 64.9% vs 39.4%; RLI — 41.3% at high vs 21.3%.

• FrontierSWE v2 — 69.9% vs 62.3%; WANDR (relaxed) — 82.1% vs 72.3%.

• BenchCAD 2.1 in text — 83.5 vs 65.3; Chartography without tools — 73.4% vs 64.4%.

• Humanity's Last Exam with tools — 74.2% vs 67.7%.

• OSWorld 2.1 — 89.3% vs 87.2%: 2.1 points, which is roughly the noise band on a benchmark of that size.

• AA-Briefcase v1.1 — 66.1% rubric and 1,785 Elo for Claude Fable 5.2 vs 61.2% rubric and 1,800 Elo for Claude Opus 5.5. The rubric favors the Fable model and the Elo favors the Opus model. That reversal is worth staring at: on a judged comparison, the cheaper model wins the head-to-head while losing the checklist.

Read that set and the tier boundary becomes legible. The Fable advantage is concentrated in research-grade and long-horizon scientific and agentic work — Terminal-Bench-Science, RLI, RoboDojo, Chartography — and shrinks toward zero in computer-use-style environments where both models are already competent. If your workload lives in the OSWorld band, you are paying 2.6× for two points. If it lives in the Terminal-Bench-Science band, the cheaper model is genuinely short and no amount of effort on the Opus side closes a 16-point gap.

The cost-per-task arithmetic, worked

Per-token rates say Claude Fable 5.2 costs twice what Claude Opus 5.5 costs. That is only true if both models use the same number of tokens, and they do not. Artificial Analysis measures the pair on its own nine-evaluation suite and publishes the finished-task number:

• Intelligence Index — 65 for Claude Fable 5.2, ranked first of 74 models, vs 59 for Claude Opus 5.5, ranked fifth.

• Blended price at a 7:2:1 cache-hit / input / output mix — $5.74 vs $2.94 per million tokens, a 1.95× ratio.

• Cost per completed index task — $26.54 vs $10.04, a 2.64× ratio.

• Output tokens on that suite — 218 million for Claude Opus 5.5; Artificial Analysis publishes no speed or latency figure for Claude Fable 5.2 at all.

The gap between 1.95× and 2.64× is the whole lesson. The Fable model spends more tokens finishing the same task — which is what “think harder” looks like on a meter — so the per-token ratio understates what a completed unit of work costs by about a third. Every budget model built on list prices will be wrong in the same direction, and it will be wrong by a margin that grows with how much reasoning the task demands. Note also the missing measurement: the independent suite has an index score and a cost for Claude Fable 5.2 but no output-speed or latency figure, while Claude Opus 5.5 measures 115 tokens per second. For latency-sensitive products that asymmetry is itself a reason to wait.

A​nthropic's own framing of the trade is narrower and, read carefully, consistent with this. It says that at high effort on its FrontierCode evaluation the Fable model performs roughly on par with Claude Opus 5.5's best score at about half the cost per task, and that at low effort on Humanity's Last Exam it matches Opus 5.5's peak for under a tenth of the cost. Both claims are about cost per unit of capability, not cost per token, and both depend on picking an effort level deliberately. They also sit alongside a 16-point gap on Terminal-Bench-Science, which no effort setting on the Opus side closes — so the two statements are not in conflict, they are describing different tasks.

Crossing between them mid-conversation

One asymmetry is easy to miss until it bites. Claude Fable 5.2's thinking blocks are readable only by Claude Fable 5.2 and Claude Mythos 5.2. Move a conversation to Claude Opus 5.5 and the API drops those blocks before the model sees them: the request succeeds, the blocks are not billed, and the reasoning is simply gone for the turns that run on the other side. The permitted fallback targets for Claude Fable 5.2 are Claude Opus 4.8 and Claude Opus 5 — notably not Claude Opus 5.5.

The reverse direction is smoother: Claude Fable 5.2 reads thinking blocks from Claude Opus 5.5, Claude Sonnet 5.5, Claude Haiku 5.5, Claude Fable 5.1 and older Opus, Sonnet and Haiku models, so an escalation path that starts cheap and steps up keeps its reasoning intact. The practical shape follows from that: escalate into the Fable tier, do not fall back into it, and if your router moves conversations between the two, expect the descending direction to lose context rather than error.

One key, whichever way the routing rule points

The tier boundary here is exactly the kind of decision a gateway should hold rather than a code branch. Claude Opus 5.5 is live on OrcaRouter behind a single key at the provider's list price with zero markup, so the $4 / $20 card is what you pay and a vendor price change is live the same day rather than at the next invoice; a routing rule can keep Claude Opus 5.5 as the default and escalate to Claude Fable 5.2 only when a request is graded hard enough to clear the bar, which is the vendor's own guidance expressed as configuration instead of prose. A fallback chain covers the remaining failure mode — Claude Opus 5.5 is not a permitted fallback target for Fable 5.2, so a chain that ends on Claude Opus 4.8 or Claude Opus 5 fails closed in the way A​nthropic's service expects.

For accuracy: checked on November 10, 2026, OrcaRouter's public catalogue lists anthropic/claude-opus-5.5 but not Claude Fable 5.2, so today the escalation step is a model page you can read rather than a call you can make here. Opus 5.5 is live at its list price.

What would change the answer

Three things, and only one of them is about benchmarks. A price move is the first: if Fable 5.2's card were to fall near Opus 5.5's, the 16-point Terminal-Bench-Science gap would stop costing 2.6× and the tier boundary would dissolve for anyone without a retention constraint. Independent speed data is the second — A​nthropic advertises 30% faster output than Fable 5.1, and once the Fable model has a measured tokens-per-second figure, the latency argument for staying on Opus either holds up or evaporates. The third is your own eval suite, and it is the only one that matters for your workload; both models expose all five effort levels, and A​nthropic's guidance for each is the same: set effort explicitly, start at the default, and sweep rather than carrying a setting over from the other model.

Until then the split is simple enough to state without a table. Default to Claude Opus 5.5 for coding, routine agents, computer use, latency-sensitive products and anything under zero data retention. Pay for Claude Fable 5.2 where a failed run is expensive enough that a 16-point benchmark gap, or a research task that the Opus tier simply cannot finish at max effort, is worth $26.54 instead of $10.04 — and measure the finished-task cost on your own eval set before you commit, because the one number that reliably predicts which side of this line you are on is not the rate card.