Claude Fable 5.1 or Opus 5.5: when the pricier model is worth it

People search "Claude Fable or Opus" when they already run Opus 5.5 and wonder whether to move hard tasks to the pricier Fable 5.1. Below: an exact price comparison, the cost of a single task and an agent session, and a formula that shows when Fable could pay off. The honest conclusion: on the data we have, Opus 5.5 is the better deal, and Fable is worth using only after testing it on your own tasks.

Prices and index

Prices are US dollars per 1M tokens, Zukko price list as of October 6, 2026.

The strength index is the Artificial Analysis Intelligence Index, an aggregate benchmark score shown in the Zukko catalog. Fable 5.1 scores lower than Opus 5.5.

Note the vendor price row: at list prices Fable costs 2.5 times more than Opus, while on Zukko it costs 8.7 times more. The discount on Opus 5.5 is deeper, so the gap on Zukko is wider than in the official price list.

The current list and prices are at GET https://api.zukko.pro/v1/models or on the "Models" page in your dashboard.

Claude Fable or Opus: what one task costs

A single request

Task: 30,000 input tokens (code or a document), 3,000 output tokens, no cache.

• Opus 5.5: 30,000 × $0.4 / 1,000,000 + 3,000 × $2 / 1,000,000 = $0.012 + $0.006 = $0.018
• Fable 5.1: 30,000 × $3.48 / 1,000,000 + 3,000 × $17.4 / 1,000,000 = $0.1044 + $0.0522 = $0.1566

The difference is $0.1386 per task. A thousand such tasks a month: $18 on Opus 5.5 and $156.60 on Fable 5.1.

Another way to see it: for the price of one Fable request you can make 8.7 identical requests to Opus (0.1566 / 0.018 = 8.7) — for instance, run Opus several times and pick the best answer, or add an extra verification pass.

An agent session with cache

Take a 30-step Claude Code session: context starts at 20,000 tokens and grows by 5,000 per step, each step has 300 tokens of fresh input and 1,500 tokens of output. At step k the previous step's context is read from cache and the new 5,000 tokens are written to it. Over the session: cache read 29 × 20,000 + 5,000 × (0 + 1 + … + 28) = 580,000 + 5,000 × 406 = 2,610,000 tokens; cache write 20,000 + 29 × 5,000 = 165,000; fresh input 30 × 300 = 9,000; output 30 × 1,500 = 45,000.

Formula: fresh input × input price + cache read × read price + cache write × write price + output × output price.

The Fable 5.1 session costs 8.37 times more (2.201715 / 0.262905). Fable's cache is expensive too: the cache writes alone cost more than the whole Opus session.

The calculation holds for the Anthropic format (/v1/messages, Claude Code, Anthropic SDK): only there does the gateway see how much input came from cache. In the OpenAI format all Claude input is billed at the input price.

When Fable could pay off

A model that costs more per token pays off in only one case: it solves the task noticeably more often, and a failed answer is expensive for you. As a formula:

• Pₒ and P_f are the failure rates of Opus 5.5 and Fable 5.1 on your tasks (measure them, don't guess);
• L is what one failure costs you: engineer time to fix it, a re-run, the consequences of a mistake;
• ΔC is the per-task price difference. For the single request above ΔC = $0.1386; for the agent session it is $2.201715 − $0.262905 = $1.93881.

A worked substitution. If a failed agent session costs you $20 (say, the time to investigate and redo it), Fable has to cut the failure rate by at least 1.93881 / 20 ≈ 0.097, i.e. 9.7 percentage points. If a failure costs $5, it has to cut it by 38.8 points (1.93881 / 5 ≈ 0.388) — a huge gap that is hard to expect without data.

The formula does not say Fable is better or worse. It tells you what quality difference you need to see on your own tasks for the extra cost to make sense. Fable 5.1 has the lower strength index (53 vs 58), so there is no prior reason to expect it to be more accurate.

How to test on your tasks

1. Pick 20–30 tasks where you know the right result: closed bugs, verified documents, tasks with tests.
2. Run them in the same agent or script on claude-opus-5-5 and claude-fable-5-1.
3. For each model compute the share of solved tasks and total cost. Actual charges are in the dashboard under "Usage".
4. Plug the measured Pₒ and P_f into the formula above with your own estimate of L.

To keep the experiment cheap, create a separate key in the dashboard↗: allow only claude-fable-5-1 and set a daily dollar limit. On production keys you can do the opposite and leave Fable off the allowed list: a request to a model outside the list returns 403 model_not_allowed_for_key, so an accidental expensive session can't happen. How limits work is described in the docs↗, and connecting Fable 5.1 with an example of such a key is covered in the article Claude Fable 5.1 via API↗.

It is convenient to compute each request's cost straight from the response:

from anthropic import Anthropic

client = Anthropic(base_url="https://api.zukko.pro", api_key="fetch_...")

PRICES = {  # $ per 1M: input, output, cache read, cache write
    "claude-opus-5-5": (0.4, 2, 0.038, 0.425),
    "claude-fable-5-1": (3.48, 17.4, 0.3, 3.663),
}

def run(model: str, prompt: str) -> tuple[str, float]:
    resp = client.messages.create(
        model=model,
        max_tokens=4096,
        messages=[{"role": "user", "content": prompt}],

Conclusion

On price and on the strength index, Opus 5.5 wins: Fable 5.1 charges 8.7 times as much per input and output token, and Opus 5.5 scores higher, 58 vs 53. A sensible setup is Opus 5.5 by default, with Fable 5.1 only for task types where your own run showed a difference large enough by the formula above. Without such a test, the extra cost has nothing behind it.

FAQ

How much more does Fable 5.1 cost than Opus 5.5?

On input and output, exactly 8.7 times ($3.48 vs $0.4 and $17.4 vs $2 per 1M). On cache read about 7.89 times, on cache write about 8.62 times.

Is Fable stronger than Opus?

By the strength index, no: 53 for Fable 5.1 vs 58 for Opus 5.5. The index is an average across benchmarks, so results on specific tasks can differ; that is what a test run checks.

Can I block Fable on a production key?

Yes. Give the key an allowed-models list without claude-fable-5-1, and requests to it will get 403 model_not_allowed_for_key.

Does caching work with Fable?

Yes, at its own prices: read $0.3, write $3.663 per 1M. The cache discount applies in the Anthropic format.