Claude Fable 5.1 vs Opus 5.5: Benchmarks, Cost & Verdict

Fable 5.1 vs Opus 5.5 looks like a lopsided fight on paper. Anthropic launched Opus 5.5 on September 22, 2026, and its launch table puts the cheaper model ahead of Fable 5.1 on every row. Developer forums quickly asked the obvious question: if the $4 model wins everything, why keep paying for the $10 one?

The benchmark sweep overstates the gap. Independent testing puts the two models level at their default settings, and Fable 5.1 still holds a narrow, real set of jobs where it is the better call.

This guide separates Anthropic's numbers from independent ones, compares cost per task instead of cost per token, and ends with a plain rule for choosing.

Fable 5.1 vs Opus 5.5 at a glance: same core specs, a 2.5x price gap

Fable 5.1 launched on September 1, three weeks before Opus 5.5, and the two models share a context window, an output limit, and a knowledge cutoff. What separates them is price and the kind of work Anthropic built each one for.

Table 1: Claude Fable 5.1 and Claude Opus 5.5 specifications and API list pricing, as of September 2026. Sources: Claude Platform documentation and Anthropic launch posts.

With the headline context, output, and knowledge-cutoff specs identical, the decision comes down to whether Fable 5.1's results justify paying 2.5 times as much per input and output token.

Opus 5.5 wins all 9 launch benchmarks, but the gap is thinner than it looks

Opus 5.5 scores higher than Fable 5.1 on every benchmark in Anthropic's launch table. Only Terminal-Bench 4.0 pairs a large lead with a published error margin it clears comfortably.

Table 2: Claude Opus 5.5 vs Claude Fable 5.1 on Anthropic's launch benchmarks, vendor-reported. Source: Anthropic, "Introducing Claude Opus 5.5."

Keep four things in mind when reading this table:

• Terminal-Bench 4.0 has the clearest disclosed margin: the published standard error is ±2.6 points for Opus 5.5 and ±1.6 to 2 points for the other Claude models, so its 10.6-point lead sits well outside the noise. Only one other row, Terminal-Bench-Science, has a published error margin, and its 6.1-point gap sits against ±3.5 to 5 points per model. The remaining rows carry no published uncertainty, so read the small gaps on Chartography, OSWorld 2.0, and Humanity's Last Exam (2.1 points or less) with care.
• The settings aren't fully matched: Opus 5.5 ran at max effort on everything except Terminal-Bench 4.0, which used xhigh, and Anthropic doesn't state the effort level used for Fable 5.1. Opus 5.5 also ran with its production safeguards on, so some cybersecurity tasks were completed by Opus 4.8 and some biology tasks by Opus 5, which Anthropic says likely lowered its scores.
• Anthropic says the real gap is smaller: at this capability level, it says benchmark margins have become a weaker guide to real-world differences, and the gap between the two models is narrower than the scores suggest.
• Fable 5.1's own launch table isn't comparable: that earlier post used different benchmark versions, including GDPval-AA v2 and CursorBench 3.2.0, so don't mix its Fable figures with the rows above.

At default settings, Fable 5.1 and Opus 5.5 score the same

On independent testing, the two models tie when each runs at its default effort. The Artificial Analysis comparison scores both models at every effort level on the same harness, which makes it the fairest head-to-head available. Its cost per task is a weighted average across the tasks in its Intelligence Index, so read it as a like-for-like benchmark cost, not a forecast of your own bill.

Table 3: Artificial Analysis Intelligence Index v4.3.2 and weighted average cost per index task at each effort level, independently verified, as of September 2026. Default API efforts are medium for Opus 5.5 and high for Fable 5.1. Source: Artificial Analysis.

Three results matter most.

• The defaults tie: Opus 5.5 at medium and Fable 5.1 at high both score 51. Opus gets there for $1.34 per task against $3.91, about a third of the cost.
• Opus 5.5 at high beats Fable 5.1 at max: a score of 54 for $1.82 per task tops Fable's best of 53 at $7.63, roughly a quarter of the price.
• Fable 5.1 wins at the bottom of the ladder: at low effort it scores 47 to Opus 5.5's 42, the only rung where Fable leads, though at more than four times the cost per task.

The ladder also shows where Fable tops out. Its xhigh and max settings both land at 53, while Opus 5.5 keeps climbing to 58. Opus 5.5 is also faster: Artificial Analysis measured Opus 5.5 at 78 output tokens per second at medium effort, against 56 for Fable 5.1 at high.

That settles one of the most-searched questions about this pair. Fable 5.1 at medium effort (49) does not beat Opus 5.5 at high effort (54), and it costs more per task.

Opus 5.5 costs about a third as much per task at default settings

Per token, Opus 5.5 is 60% cheaper than Fable 5.1 on input and output. Per task at default settings, the gap is wider, because Opus also finishes the same work in fewer tokens.

The cache line is the exception worth knowing. Cache reads cost $0.20 per million tokens on Opus 5.5 and $0.25 on Fable 5.1, only 20% apart. The reason is pricing: Fable 5.1's cache hits cost 2.5% of its input price, against the standard 10%. Agents that re-read a large context on every turn spend most of their budget on cache reads, so their per-token savings from switching are smaller than the headline suggests.

To make that concrete, here is one cache-heavy agent session priced at both models' list rates with identical token counts.

Table 4: Worked cost for one session with 200,000 uncached input tokens, 1.8M cache-read tokens, and 150,000 output tokens. Cache-write fees excluded. Calculated from Anthropic list pricing as of September 2026.

At identical token counts, Opus 5.5 comes in about 58% cheaper. In practice it also uses fewer output tokens at its default setting, which is why the independent per-task gap is closer to two thirds. For the full rate card and caching math, see our Fable 5.1 pricing breakdown.

One number circulating in launch coverage needs correcting. The launch-post claim that Opus 5.5 costs about 40% less to run compared with Opus 5, not with Fable 5.1. Some launch coverage blurred the two, and the real Fable comparison is the steeper one shown above.

Same-task tests favor Opus 5.5 on speed and reliability

When both models attempted the same real job, Opus 5.5 matched or beat Fable 5.1 on quality, and finished the migration faster and for less. These are the most direct comparisons available, since both models faced the same task and the same grader, though Anthropic designed and ran both tests.

Two direct comparisons come from Anthropic's launch post, both vendor-reported:

• Code migration: both models translated HAProxy, widely used traffic-balancing software, from C into Rust. Both rewrites passed nearly all of HAProxy's own regression tests, but Opus 5.5 finished in 9.5 hours against 12 for Fable 5.1, and cost 51% less.
• Research reports: each model wrote a quarterly earnings report from a copy of the web where the source release was hard to find, with a grader checking every figure and quote. Across effort settings, 16 of 18 Opus 5.5 reports passed. Fable 5.1 did not pass the bar in any attempt.

Independent coding results point the same way. On Snorkel AI's task set of expert-built terminal tasks, Opus 5.5 passed 68% of its task attempts against 49% for Fable 5.1. Snorkel found the two models fail differently: Fable 5.1 tended to stop early or fail to recover from errors mid-run, while Opus 5.5's most common failure was a malformed first response that its test harness could not parse.

Also read our Claude Fable 5.1 vs Opus 5 breakdown to see how the two compare on the same tasks before you commit to either.

Fable 5.1 is worth testing for three kinds of work

Fable 5.1 is the better pick for a short list of jobs. Anthropic's documentation names the first, independent testing supports the second, and Anthropic's safeguard policies explain the third. The evidence for each is thinner than the benchmark table, so treat these as reasons to test Fable rather than guarantees.

1. Fable 5.1 is the fallback when Opus 5.5 falls short at higher effort

Anthropic's model selection guidance says to start with Opus 5.5 for most workloads and use Fable 5.1 for demanding reasoning and long-horizon agentic work, or when your evaluations on Opus 5.5 at higher effort still fall short. That last condition is the practical test: treat Fable 5.1 as the model you try after Opus has had a fair attempt at high or xhigh effort.

The launch examples for Fable 5.1 fit that profile. Investment firm Millennium, quoted by Anthropic, reported that Fable 5.1 found the cause of a rare crash its engineers had failed to explain for years, after every other model it tried had missed it.

2. Fable 5.1 scores slightly higher at low effort

Fable 5.1 holds a small edge when the model gets one quick try. It leads Opus 5.5 at low effort on the Artificial Analysis index, 47 to 42. Separately, Snorkel measured it nominally ahead on first-attempt pass rate, 61.5% to 60.7%, though Snorkel doesn't state the effort setting it used. Both leads are narrow, and the low-effort one costs more than four times as much per task, so this matters only for short tasks where a wrong first answer is expensive.

3. Fable 5.1 allows vulnerability discovery that Opus 5.5 reroutes

Anthropic now permits vulnerability discovery on Fable 5.1 for defensive work, though not exploit development. Opus 5.5 is stricter here. Its launch post says users can find and fix bugs in their own code, but most other cybersecurity tasks are rerouted to Opus 4.8. That gap may narrow, since the company plans to expand its Cyber Verification Program to include Opus 5.5. Vetted cybersecurity and life sciences teams have a third option: Claude Mythos 5.1, which Anthropic describes as the same model as Fable 5.1 with fewer restrictions, available only through its trusted access programs.

If you're weighing Fable against OpenAI's flagship instead of Anthropic's cheaper tier, our GPT-6 Astra comparison covers that matchup.

Opus 5.5 fits most work, and Fable 5.1 fits a few specialist jobs

For most work, Opus 5.5 is the better default on both quality and cost. Fable 5.1 is the specialist.

Table 5: Recommended model by type of work, based on vendor-reported and independent results cited in this guide.

Start on Opus 5.5 and escalate to Fable 5.1 for the hard cases

The Fable 5.1 vs Opus 5.5 verdict is simple: start on Opus 5.5, and escalate to Fable 5.1 only when a hard, open-ended problem still defeats Opus at higher effort. Opus 5.5 wins all nine benchmarks Anthropic published and came out ahead in both of Anthropic's same-task tests. On independent testing, it ties Fable 5.1 at default settings for about a third of the cost per task. Fable 5.1's clearest advantages are as a fallback for problems Opus can't crack at higher effort, and for security research that Opus 5.5's safeguards route to an older model.

Both models are available on Emergent, so you can build your apps and internal tools on either one by describing what you want. Most builds belong on Opus 5.5, with Fable 5.1 reserved for projects that are research-heavy from day one, or builds that have stalled on Opus. If your app needs its own AI features, the Universal LLM Key gives it one credential for Claude, GPT, and Gemini with unified billing.

Start Building on Emergent.