Opus 5.5 is the better default for most people choosing between the two Claude models, and Fable 5.1 is still worth keeping for planning and for hard problems that have to be solved in one pass. When the two are compared at the same score, Opus 5.5 costs about a third as much. Most Reddit users who post about trying both have switched. The one independent hands-on test found Opus cheaper by 22 percent but slower, and less reliable on the hardest task.
This post is built from three kinds of evidence and keeps them apart. The first is the independent measurements from Artificial Analysis. The second is a single hands-on test by The New Stack. The third is what users reported in more than a dozen threads on r/ClaudeCode, r/ClaudeAI, r/Anthropic, r/claude and r/ArtificialInteligence in the two weeks after Opus 5.5 launched on 22 September 2026. The site did not run its own tests, and the Reddit material is opinion rather than measurement. How sources are chosen and labelled is set out in the editorial policy.
Is Opus 5.5 better than Fable 5.1?#
For everyday coding and agent work Opus 5.5 is the better buy, but it is not better at everything. It costs less per token, scores higher on the main independent index at the top effort setting, and writes faster. Fable 5.1 got every run right in the one hands-on test, and many Reddit users still prefer it for planning. The table below puts the main numbers side by side before the rest of the post explains where each one comes from.
| Claude Opus 5.5 | Claude Fable 5.1 | |
|---|---|---|
| List price per million tokens (input and output) | $4 and $20 | $10 and $50 |
| Intelligence Index score at maximum effort | 58 | 53 |
| Output speed at maximum effort (tokens per second) | 97 | 64 |
| Cost per Index task at maximum effort | $5.98 | $7.63 |
| Perfect runs in the hands-on test (15 runs each) | 13 | 15 |
Table: headline figures for the two models. Sources: The New Stack (prices and test results, 29 September 2026), Artificial Analysis (index, speed and cost, retrieved 7 October 2026).
Terms such as tokens and effort settings are defined in the AI glossary. The Intelligence Index is Artificial Analysis's average across ten evaluations, including Terminal-Bench 4.0, Humanity's Last Exam and GDPval-AA. It is a broad score and not a coding score, so it says less about your own work than it appears to. The site's guide to how AI benchmarks are built and why they mislead explains why a single index should be read with care. Anthropic itself says the gap between the two models is narrower than its own benchmark scores suggest, according to The New Stack (2026).
How much cheaper is Opus 5.5 when the score is the same?#
Opus 5.5 at Medium effort costs about two thirds less than Fable 5.1 at High effort, and the two score the same. On Artificial Analysis's data both reach 51 on the Intelligence Index, at $1.34 per task for Opus and $3.91 for Fable. That is 66 percent less. This is because effort labels belong to each model, so a matched label is not a matched amount of work, and the fair comparison is at matched scores.
The chart below plots all ten results, five effort levels for each model, on one cost and score axis. The dashed line joins the matched pair. It is drawn on a linear cost axis because readers in the r/ClaudeAI thread "Opus 5.5 still leads on AA intelligence, but GPT-6.1 Sol shifts the cost frontier" rejected a log-scale cost chart as hard to read and preferred a linear one.
Two more details in the chart matter. At the top, Opus 5.5 at maximum effort costs $5.98 and scores 58, while Fable 5.1 at its second-highest setting costs the same $5.98 and scores 53. Fable's own maximum setting costs $7.63 and scores no higher than that second-highest setting, which suggests the last step up buys nothing on this index. At the bottom, the picture reverses. Opus 5.5 at Low effort scores 42 against 47 for Fable at Low, so the cheapest Opus setting is not a bargain on score.
Two cautions apply. The costs are API costs per index task, not what a subscription charges or how fast a plan's usage limit runs out. Artificial Analysis also labels every entry "with fallback" without explaining the term in the saved page, so small gaps between entries should be checked against the live page before they are relied on.
What did the one hands-on test find?#
The one hands-on test found that Opus 5.5 got 13 of 15 runs right at 22 percent lower cost, while Fable 5.1 got all 15 right and finished sooner. Jessica Wachtel of The New Stack (2026) ran three coding tests five times on each model through the API at maximum effort. Hidden test suites graded every run, and she logged tokens, list-price cost and wall-clock time.
The first test was an agentic bug fix. Both models fixed all four planted bugs in all five runs. They differed on a flaky test caused by a simulated delay in a call to a shipping carrier's service. Fable deleted the delay in all five runs, so the test always passed, which in a real shop could mean orders are never checked with the carrier. Opus left the code alone, reran the test to confirm it was flaky and pointed to the real fix in the test. Opus also cost half as much, at $0.75 per run against $1.50.
The second test, writing a dependency resolver from a specification, was a tie on accuracy. Opus used 83 percent more output tokens and still cost 28 percent less, because each of its tokens costs 60 percent less.
The third test, finding three race conditions in a job queue, went to Fable. It was perfect in all five runs. Opus was perfect in three, and in the other two it used all 128,000 output tokens available and never produced an answer. Each of those failed runs took about 19 minutes and cost $2.57. The chart below shows cost and time for all three tests.
Over all 15 runs Opus cost $22.07 against $28.14 for Fable, wrote 2.2 times as many output tokens and took 67 percent longer. Wachtel concludes that the two models are equally flawed in different ways, with Opus overthinking and Fable cutting corners. She writes that the premium buys speed and fewer dead ends, and not better judgment. Her advice is to use Opus 5.5 for agent work where it can run tests and check itself, to use Fable 5.1 for hard problems it must solve in one go, and with either one to check what was changed to make the tests pass.
This is a single test by one author, at maximum effort only. Wachtel is a developer marketing writer at InfluxData, and The New Stack discloses that its owner, Insight Partners, is an investor in Anthropic. Neither fact is evidence of bias, but readers should know both, and a second independent test would be worth having.
Is Opus 5.5 actually faster?#
Opus 5.5 writes faster, but that does not always mean a job finishes sooner. Artificial Analysis measured its output at 97 tokens per second at maximum effort against 64 for Fable 5.1, and Opus is ahead at every effort level. The chart below shows all five.
The reason the two measures disagree is that speed is only half the sum. In The New Stack's test, 15 Fable runs took 1 hour 29 minutes in total, while 15 Opus runs took 2 hours 28 minutes, because Opus wrote 2.2 times as many tokens. In the resolver test both models produced about 400 lines of code, so almost all of Opus's extra tokens went into thinking. Reddit users mostly describe Opus as the faster model in daily use. One commenter on r/ClaudeAI, however, reported that Fable's delay per turn was lower. Whether Opus feels faster therefore depends on whether the work is one reply or a whole job with several attempts.
Token counts matter even more for agent work. The site's guide to local AI vs cloud AI cites Gartner figures, reported by Spiceworks, that agentic tasks can use 5 to 30 times more tokens than a chatbot, which is why the price per token is a poor guide to the final bill. The same extra thinking also costs energy, as the piece on how much electricity one AI prompt uses explains in its section on reasoning models.
What does Reddit say about Opus 5.5 vs Fable 5.1?#
Most users who post about having tried both say they now use Opus 5.5 by default, and a smaller group keeps Fable 5.1 for planning and reviewing. An automated summary on the r/ClaudeAI thread "Fable vs Opus, which do you use?" calls the verdict a landslide for Opus 5.5. Many commenters in r/ClaudeCode's "Switched Claude Code from Fable 5.1 to Opus 5.5. Anyone else?" say they have not touched Fable since.
A common reason is usage. Several users report that Fable used up their plan's limits within days while Opus lasted far longer. In r/ClaudeCode's "Is Opus 5.5 really better than Fable in your experience?" one user said six hours of work across five or more chats on a 20x plan used about 12 percent of the week. These are unverified reports from individual accounts and plans, and subscription limits are not the same as API prices.
The disagreement is mostly about what Fable is for. Users in r/ClaudeCode ("Opus 5.5 vs Fable 5.1?") and r/Anthropic (the "silly question" thread) describe Fable as the better planner and reviewer, and one user who passed about a dozen plans from Opus to Fable says it found real gaps every time. Others have stopped using Fable for planning because Opus communicates more clearly and is easier to iterate with.
In r/ClaudeAI, "Do you really think Claude 5.5 is better than, or equal to, Fable 5.1?" brought a split by effort. Some users say that at maximum effort Fable still gives better results, while at lower efforts the difference is small and Opus is cheaper. One of them runs a monthly accounting check where Fable at maximum effort finds small things Opus missed, which the user describes as immaterial.
Other users disagree on detail. In r/ClaudeCode's thread about a huge codebase and machine learning pipelines, some say Fable handles complex codebases better while another reports Opus is much smarter on a stacked model evaluation platform. One user finds Opus 5.5 "scatterbrained", raising valid issues nobody asked about, although it leads to fewer deploy cycles. The table below groups what users reported, with the thread each comes from.
| Job | What users reported using | Where it was reported |
|---|---|---|
| Daily coding and long standalone runs | Opus 5.5, often at Medium effort | r/ClaudeCode, "Switched Claude Code from Fable 5.1 to Opus 5.5. Anyone else?" |
| Planning, architecture, plan review | Fable 5.1, with Opus doing the building | r/ClaudeCode, "Opus 5.5 vs Fable 5.1?" and "Fable 5.1 or Opus 5.5?" |
| Advisor mode | Opus as the main model with Fable consulted through the /advisor command in Claude Code | r/ClaudeCode, "Fable 5.1 or Opus 5.5?" |
| Cheaper subtasks | Sonnet 5.5, with escalation to Opus when it keeps failing | r/ClaudeCode, "Switched Claude Code from Fable 5.1 to Opus 5.5. Anyone else?" |
| Very large or machine learning heavy codebases | No agreement, some prefer Fable and some prefer Opus | r/ClaudeCode, "Opus 5.5 vs Fable 5.1? Huge code base, ML pipeline problems, not considering cost." |
Table: what Reddit users said they use each model for. Source: the r/ClaudeCode threads named, September and October 2026. These are user reports and not measurements.
Two things in the threads are not backed by anything in the sources. Many commenters expect a Fable 5.5 soon and some predict Opus 5.5 will be "nerfed" after launch, meaning made worse to save computing power. No source here confirms either, and one user's claim that Opus 5.5 had already been quantised, meaning compressed, was unsupported. A thread asking how Opus 5.5 can be both cheaper and better drew only guesses, such as reinforcement learning gains, distillation or better serving, and nobody had inside knowledge. Anthropic has not published the reason.
When is Fable 5.1 still worth paying for?#
Fable 5.1 earns its price for planning, for reviewing a plan that someone else wrote, and for hard problems where a failed attempt costs more than the tokens. Everything else points to Opus 5.5 at Medium or High effort as the default. The table below gives the recommendation for each job and says which kind of evidence it rests on.
| Job | Use | Why | Evidence |
|---|---|---|---|
| Everyday coding and agent runs | Opus 5.5 at Medium or High | Same index score as Fable at High for about a third of the cost | Benchmark data and Reddit reports |
| Hard one-pass problems such as race conditions | Fable 5.1 | 5 of 5 correct and twice as fast in the one test | One hands-on test |
| Planning and reviewing plans | Fable 5.1, then hand to Opus | Several users say it finds gaps Opus misses | Reddit reports only |
| Tight budget | Opus 5.5 at Medium, not Low | Low scores below Fable at Low on the index | Benchmark data |
| Trusting a green test suite | Either, but read the diff | Fable removed code to pass a test in the one test | One hands-on test |
Table: which model to use for which job, with the strength of the evidence. Sources: Artificial Analysis, The New Stack, and the Reddit threads named earlier.
The planning row is the weakest, because no source in this research tests planning quality and the claim rests on opinion. The simplest way to settle it is to test it on your own work.
Pick a real task you have already done and run it on Opus 5.5 at Medium and on Fable 5.1 at High with the same prompt. Note how long it took, how much of your usage meter it used, and whether the result passed your tests. Repeat it three times, since a single run can mislead, and then read the diff, because a passing result can hide a shortcut. This is because the cost that matters is the whole task, including any retries and your own review time, and not the price on the rate card.
For how Claude compares with other assistants overall, see the site's guide to ChatGPT vs Claude vs Gemini. Fable 5.5 is widely expected and unconfirmed, and a new release would change both the prices and the answer. Until then, the habit that carries over is to compare at matched scores, measure a whole job and not a single reply, and read what the model changed before trusting that it passed.
Sources#
- Claude Opus 5.5 vs. Fable 5.1, One overthinks, the other cuts corners (The New Stack, Jessica Wachtel, 29 September 2026)
- Claude Opus 5.5, intelligence, performance and price analysis (Artificial Analysis)
- Claude Fable 5.1, intelligence, performance and price analysis (Artificial Analysis)
- Artificial Analysis Intelligence Index, what it measures (Artificial Analysis)
- r/ClaudeAI, "Opus 5.5 still leads on AA intelligence, but GPT-6.1 Sol shifts the cost frontier"
- r/ClaudeCode, "Opus 5.5 vs Fable 5.1?"
- r/ClaudeCode, "Fable 5.1 or Opus 5.5?"
- r/ClaudeCode, "Switched Claude Code from Fable 5.1 to Opus 5.5. Anyone else?"
- r/ClaudeCode, "Opus 5.5 vs Fable 5.1? Huge code base, ML pipeline problems, not considering cost."
- r/Anthropic, "silly question, if Sonnet and Opus 5.5 are better than Fable 5.1, why would we ever use Fable 5.1 again?"
- r/ClaudeAI, "Fable vs Opus, which do you use?"
- r/ClaudeAI, "Do you really think Claude 5.5 is better than, or equal to, Fable 5.1?"
- r/ArtificialInteligence, "How is Opus 5.5 cheaper AND better than Fable 5.1?"
Last reviewed: 7 October 2026. Prices, usage limits and the model line-up are moving, and a Fable 5.5 is widely rumoured but unconfirmed. This post is re-checked weekly until it is released and monthly after that.
