Claude Opus 5.5 vs Claude Fable 5.1: Complete Comparison and Report on Pricing, Benchmarks, Effort Settings, and Whether the Cheaper Model Actually Matches the Larger One
Updated: 23 minutes ago
Anthropic released Claude Opus 5.5 on September 22, 2026, three weeks after Claude Fable 5.1, and priced it 60% below Fable's API rate while claiming it beats Fable on most of the benchmarks the company chose to publish. Anthropic's own launch page then adds a qualification most vendors do not volunteer: "at these levels of capability we've found that benchmark margins have become a less reliable guide to real-world differences. In our own use, the gap between Opus 5.5 and Claude Fable 5.1 is narrower than these scores suggest."
This report treats that tension, a cheaper model with better published scores and a vendor-issued warning not to trust the margin, as the subject, and works through the pricing, the benchmarks, and the specific conditions attached to each claim.
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RELEASE TIMELINE AND MODEL IDENTITY.
Model identifiers, lineage, and where each sits in Anthropic's current lineup.
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Attribute | Claude Opus 5.5 | Claude Fable 5.1 |
Release date | September 22, 2026 | September 1, 2026 |
Model ID | claude-opus-5-5 | claude-fable-5-1 |
Predecessor | Claude Opus 5 | Claude Fable 5 |
Status | Active (latest) | Active |
Retirement commitment | Not sooner than September 22, 2027 | Not before September 1, 2027 |
Restricted sibling | None disclosed | Claude Mythos 5.1, same weights, vetted access only |
Anthropic's own framing | "Frontier model for long-running coding agents, research, and professional knowledge work" | Anthropic's prior general-purpose flagship |
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Anthropic states plainly that Opus 5.5 is smaller and cheaper to serve than Fable 5.1, without disclosing a parameter count for either model. Sonnet 5.5 and Haiku 5.5 are described as coming "in the coming weeks," which means the rest of Anthropic's lineup is expected to receive the same treatment shortly, and any comparison built today may need revisiting once those ship.
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PRICING AND THE COST STRUCTURE CHANGE.
Full rate cards, including a cache discount that does not move in the direction the headline suggests.
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Rate per 1M tokens | Claude Opus 5.5 | Claude Fable 5.1 |
Input | $4 | $10 |
Output | $20 | $50 |
Cache read | $0.20 | $0.25 |
Cache read as % of input | 5% | 2.5% |
Cache write (5 min) | $5 | $12.50 |
Cache write (1 hour) | $8 | $20 |
Batch API | 50% discount on input and output | Not confirmed to support the same discount in this comparison |
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Opus 5.5's input and output rates sit 60% below Fable 5.1's on every line, and its absolute cache-read price is also lower in dollar terms. But the discount ratio runs the other way: Fable 5.1's $0.25 cache read is 2.5% of its $10 input rate, while Opus 5.5's $0.20 cache read is 5% of its $4 input rate. A workload built around heavy cache reuse gets a proportionally deeper discount on Fable 5.1 than on Opus 5.5, even though the absolute number is smaller on Opus 5.5.
Anthropic states that Opus 5.5's total operating cost runs roughly 40% below Opus 5's, a figure built from two separate effects: the 20% cut in per-token rates from Opus 5's $5/$25, and a claimed reduction in how many tokens the model needs to complete a typical task. The 40% figure describes the comparison against Opus 5, not against Fable 5.1, and no equivalent single "total cost" percentage against Fable 5.1 was published in the material reviewed for this report.
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CONTEXT WINDOW, OUTPUT LIMITS, AND BREAKING API CHANGES.
Specifications are nearly identical; the API contract underneath them is not.
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Specification | Claude Opus 5.5 | Claude Fable 5.1 |
Context window | 1,000,000 tokens | 1,000,000 tokens |
Maximum output | 128,000 tokens | 128,000 tokens |
Batch API max output (beta) | 300,000 tokens | Not offered |
Knowledge cutoff | June 2026 | June 2026 |
Thinking | Adaptive, always on, cannot be disabled | Adaptive, always on |
Default effort | Medium | High on API and Claude Code, Medium on Claude.ai and Cowork |
Comparative latency | Moderate | Slower, the slowest in the Claude lineup |
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Opus 5.5 introduces breaking changes for anyone migrating an existing integration: thinking can no longer be turned off at all, a forced tool_choice setting has been removed, the default effort level dropped from high to medium, and the computer-use tool type changed. None of these require a new account or a new API key, but each can silently change output on a request that previously specified the old parameters.
Fable 5.1 remains the more latency-costly of the two by a wide margin, at roughly 66 tokens per second against a cross-model median several times faster, a figure this outlet documented in earlier coverage and unchanged since. Opus 5.5's latency is described only as "moderate" in Anthropic's own documentation, with no specific tokens-per-second figure published for direct comparison.
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BENCHMARK METHODOLOGY AND THE EFFORT-LEVEL CAVEAT.
What Anthropic's own launch material discloses about how these numbers were produced.
Anthropic states that its Opus 5.5 results use adaptive thinking at max effort unless otherwise noted, and that its Terminal-Bench 4.0 comparison specifically pits Opus 5.5 at xhigh effort against GPT-6 Astra at high effort, OpenAI's own reported figure, describing both as "each model's highest score." That is a same-methodology comparison in the sense that both figures represent a ceiling, but it is not a same-effort-level comparison, and a team evaluating at a fixed effort setting on both models should not expect to reproduce the published gap.
A second disclosed caveat concerns safety routing during evaluation. Opus 5.5 was tested with production safeguards enabled, and where those safeguards intervened, cybersecurity tasks were completed by Claude Opus 4.8 and biology and frontier LLM development tasks were completed by Claude Opus 5, not by Opus 5.5 itself. Anthropic states this likely reduces Opus 5.5's own performance on those specific benchmark categories, which means the published scores in those categories understate the model's own capability rather than overstate it, the same direction of bias this outlet has documented on Fable 5.1's own safeguard-affected benchmarks.
Anthropic also reports a standard error of ±2.6 points for Opus 5.5 on Terminal-Bench 4.0, against ±1.6 to 2 points for other Claude models, and separately notes that a public leaderboard using 5 trials per task and the Claude Code harness reports Opus 5 at 51.8%, while Anthropic's own setup reproduces the same model at 52.3%, within noise. That reproducibility check is a data point in Anthropic's favor on measurement consistency, at least for Opus 5's own score.
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THE INTELLIGENCE INDEX DISCREPANCY ACROSS PUBLICATION DATES.
A benchmark this outlet has cited before now shows materially different numbers for the same two models.
Independent evaluator Artificial Analysis places Opus 5.5 at 58 on its Intelligence Index at max effort, describing it as the highest score the index has recorded, with Fable 5.1 and GPT-6 Astra tied at 53 each. This outlet's own earlier coverage of Fable 5.1, published shortly after that model's September 1 launch, cited Artificial Analysis placing Fable 5.1 first of 196 models at a score of 66, with GPT-6 Astra at 61.2.
Those two sets of figures, both attributed to Artificial Analysis, are not reconcilable at face value: Fable 5.1 moved from a reported 66 to 53, and Astra from 61.2 to 53, over three weeks with no announced change to either model. The most likely explanation is that Artificial Analysis revised its Intelligence Index methodology or rescaled it between the two measurement dates, a normal occurrence for an actively maintained benchmark suite, but no changelog documenting that revision was located for this report. Readers comparing Intelligence Index figures across articles published weeks apart should treat the absolute score as tied to its publication date rather than as a stable, cross-time measurement, and should compare same-date scores against each other rather than mixing scores from different snapshots of the index.
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DIRECTLY COMPARABLE AGENTIC CODING SCORES.
Anthropic's own published table, same benchmark suite, same testing round.
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Benchmark | Claude Opus 5.5 | Claude Fable 5.1 | Claude Opus 5 |
Terminal-Bench 4.0 (Anthropic's reported figure) | 66.4% | 55.8% | 52.3% |
Terminal-Bench 4.0 (Artificial Analysis independent figure) | 59.6% | Not published in this comparison | Roughly 48.6% (11 points below Opus 5.5, per Artificial Analysis) |
FrontierCode v1.1 Main | 54.4% | 50.3% | 48.0% |
AutomationBench | 40.0% | Not published in this comparison | Not published in this comparison |
Terminal-Bench-Science 0.1 | 58.7% | 52.6% | Not published in this comparison |
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Two different Terminal-Bench 4.0 figures for Opus 5.5, 66.4% from Anthropic's own reporting and 59.6% from Artificial Analysis's independent measurement, appear across sources published within hours of each other on launch day. Both cannot be describing the identical test configuration; the more likely explanation, consistent with the effort-level caveat above, is that the two figures reflect different effort settings or harness configurations rather than a factual contradiction. Neither source reviewed for this report reconciled the two numbers explicitly, so this comparison flags both rather than selecting one as authoritative.
On the rows where Fable 5.1 has a directly published figure, Opus 5.5 leads on every one, including a 27.9-point relative gain Fable 5.1 itself had reported over its own predecessor Fable 5 on Terminal-Bench-Science, a gain Opus 5.5's 58.7% now exceeds outright.
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COST PER COMPLETED TASK: A NAMED REAL-WORLD CASE.
Anthropic's own worked example, rather than a benchmark score.
Anthropic's launch material describes a specific case: both models were used to rewrite a portion of HAProxy's codebase, with both rewrites passing nearly all of HAProxy's own regression tests. Opus 5.5 finished in 9.5 hours against 12 hours for Fable 5.1, and cost 51% less to run. Anthropic separately states that at its default effort level, Opus 5.5 beats GPT-6 Astra on FrontierCode at roughly 20% of Astra's cost per task, matches Astra on Terminal-Bench 4.0 for about 40% of the cost, and beats GPT-5.6 Sol by 11 points on CursorBench for about a third of the cost.
These are vendor-selected examples rather than a systematic sample, and no independent party's re-run of the HAProxy case was located for this report. The specific, named, third-party codebase involved makes it more checkable in principle than an aggregate benchmark score, but it has not yet been independently checked.
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WHAT FABLE 5.1 STILL OFFERS.
The two things Opus 5.5's launch does not replace.
Fable 5.1 remains the only one of the two models with a documented restricted-access sibling: Claude Mythos 5.1, running on the same weights without cyber and bio classifiers, reaches vetted organizations through Project Glasswing and the Cyber and Life Sciences Verification Programs, a capability this outlet has covered in detail separately. No equivalent sibling has been disclosed for Opus 5.5 as of this report.
Fable 5.1's proportionally deeper cache discount, 2.5% of input against Opus 5.5's 5%, means a workload dominated by cache hits rather than fresh tokens may still favor Fable 5.1 on a pure cache-read cost basis, even at Fable's higher absolute rate, depending on the exact ratio of cached to uncached tokens in a given pipeline.
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WRITING STYLE AS A STATED DESIGN GOAL.
A qualitative claim Anthropic ties directly to customer complaints about Fable 5.1.
Anthropic states that the Opus 5.5 release specifically addresses customer feedback on cost, efficiency, and communication quality, with particular attention to financial services, law, and software development, and separate coverage frames this as an explicit promise to reduce what one outlet's headline calls "Claudish" writing, a reference to a recognizable, verbose house style attributed to recent Claude models. No independent, quantified evaluation of writing quality was located for this report; the improvement is currently a vendor claim rather than a measured result.
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SAFETY EVALUATION CONTEXT: PACING THE FRONTIER.
The release's stated relationship to Anthropic's own safety posture.
This is Anthropic's first model release since CEO Dario Amodei's public statement calling for "pacing the frontier," a stated intention to slow capability progress deliberately so that safety practices remain ahead of what a model can do rather than catching up afterward. Anthropic states that external evaluators, including METR and Frontier Design, tested Opus 5.5 before release, in addition to the internal safeguard routing described earlier in this report.
Whether this specific release reflects a measurably different pace than Anthropic's prior launches is not something this report can independently verify; it is presented here as Anthropic's own stated context for the release, alongside the concrete safeguard-routing details that are checkable against the benchmark tables above.
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AVAILABILITY AND SUBSCRIBER CHANGES.
Where each model runs, and what changed for existing subscribers on launch day.
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Channel | Claude Opus 5.5 | Claude Fable 5.1 |
Claude API | Yes | Yes |
Amazon Bedrock | Yes | Yes |
Google Cloud | Yes | Yes |
Microsoft Foundry | Yes | Yes |
Claude Platform on AWS | Yes | Yes |
Claude apps and Claude Code | Available same day as release | Available same day as release |
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Alongside the Opus 5.5 launch, Anthropic raised five-hour usage limits by 20% on Pro, Max, Team, and seat-based Enterprise plans, and gave subscription users a rate limit reset. No equivalent subscriber-facing change was announced alongside Fable 5.1's own September 1 launch in the material reviewed for this report.
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WORKLOAD ALLOCATION CRITERIA.
What the vendor's own hedge means for an actual choice between the two.
Anthropic's own model-selection guidance recommends Opus 5.5 for most work, reserving Fable 5.1 for harder tasks that need more reasoning, and keeping Opus 5 available mainly for teams already integrated against it. Given that Anthropic itself states the real-world gap is narrower than the benchmark margins suggest, the practical read is that Opus 5.5 is very likely to be at least as good as Fable 5.1 on typical work at a fraction of the cost, while Fable 5.1's actual edge, if any, concentrates on the harder, more open-ended tasks where Anthropic's own hedge implies benchmarks are least trustworthy as a guide.
A workable test for a specific pipeline is the one Anthropic's own launch case implies: run the same real task on both models at the effort level intended for production, not at max effort on one and default on the other, and compare wall-clock time and total cost rather than the published benchmark percentage. Given the unresolved Terminal-Bench 4.0 discrepancy between Anthropic's 66.4% and Artificial Analysis's 59.6% for the same model, and the Intelligence Index's apparent rescaling since this outlet's earlier coverage of Fable 5.1, no benchmark table in this report, including Anthropic's own, should be treated as a stable number to plan a multi-month deployment against without a same-day, same-effort re-verification first.
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