Claude Fable 5.1 or Sonnet 5.5: when a big price gap is justified

The "Claude Fable 5 vs Claude Sonnet" choice usually comes down to money. The current versions as of October 18, 2026 are Fable 5.1 and Sonnet 5.5, and those are what we compare: Fable 5.1 costs noticeably more, and you want to know what exactly you are paying for. We will not promise that one model "writes code better": quality on your task can only be learned by testing. Price, cache, context and intelligence index, though, can be compared honestly. Below are the numbers as of October 18, 2026, a cost calculation for the same task, and a rule for justifying the expensive model. The current list and prices are at GET https://api.zukko.pro/v1/models or on the Models page in the dashboard.

Model comparison

Zukko prices, $ per 1M tokens. Intelligence index is the Artificial Analysis Intelligence Index.

The index gap is 5 points; the gap in input and output price is much wider ($7 / $35 versus $0.4 / $2). What Claude Fable 5.1 costs per unit of work depends on the task, so we calculate concrete examples.

Example 1: a single request without cache

Assumptions: 20,000 input tokens (a document plus instructions), 3,000 output tokens.

Formula: input × input price + output × output price.

• Fable 5.1: 20,000 × 7 / 1,000,000 + 3,000 × 35 / 1,000,000 = 0.14 + 0.105 = $0.245.
• Sonnet 5.5: 20,000 × 0.4 / 1,000,000 + 3,000 × 2 / 1,000,000 = 0.008 + 0.006 = $0.014.

The difference is $0.231 per request. For one-off analysis that is pocket change; for a stream of thousands of requests it is a noticeable sum.

Example 2: an agent session with cache

Now an agent that works with a large context for 40 steps in a row. Assumptions per step: 50,000 tokens are read from the cache, 2,000 new tokens are written to the cache, 500 tokens are fresh input, 600 tokens are output. The format is Anthropic (/v1/messages, Claude Code, the Anthropic SDK), where the gateway sees cache reads and writes.

Step formula: fresh input × input price + cache read × read price + cache write × write price + output × output price.

Fable 5.1:

• read: 50,000 × 0.7 / 1,000,000 = 0.035;
• write: 2,000 × 8.75 / 1,000,000 = 0.0175;
• fresh input: 500 × 7 / 1,000,000 = 0.0035;
• output: 600 × 35 / 1,000,000 = 0.021;
• total $0.077 per step, $3.08 for 40 steps.

Sonnet 5.5:

• read: 50,000 × 0.05 / 1,000,000 = 0.0025;
• write: 2,000 × 0.5 / 1,000,000 = 0.001;
• fresh input: 500 × 0.4 / 1,000,000 = 0.0002;
• output: 600 × 2 / 1,000,000 = 0.0012;
• total $0.0049 per step, $0.196 for 40 steps.

Note that for Fable the cache write ($8.75) is considerably more than its own input ($7). In long sessions with an ever-growing context the share of writes in the spend is significant.

Example 3: the same agent in the OpenAI format

If you call Claude through /v1/chat/completions, the gateway gets no cache breakdown and all input is billed at the input price. The same 52,500 input tokens per step (50,000 + 2,000 + 500) plus 600 output tokens:

• Fable 5.1: 52,500 × 7 / 1,000,000 + 600 × 35 / 1,000,000 = 0.3675 + 0.021 = $0.3885 per step, $15.54 for 40 steps.
• Sonnet 5.5: 52,500 × 0.4 / 1,000,000 + 600 × 2 / 1,000,000 = 0.021 + 0.0012 = $0.0222 per step, $0.888 for 40 steps.

Takeaway: for Claude in agent tasks, the request format affects cost more than the choice between these two models. If an agent runs on Claude, use the Anthropic format. More on the difference between formats in Claude via /v1/messages or /v1/chat/completions: what happens to the cache↗.

A rule: how many attempts equal one Fable attempt

Simple arithmetic helps judge when the expensive model is justified. If one attempt on Sonnet 5.5 costs $0.196 and on Fable 5.1 $3.08, Fable pays off only if Sonnet fails to solve the task within 3.08 / 0.196 ≈ 15.7 attempts. For most tasks, repeating the request or refining the prompt is cheaper. But your own time counts too: if every failed attempt needs a manual review, the cheap model may cost more in hours. So this rule is settled only by measurement.

How to choose on your own examples

1. Collect 20 typical tasks from your work and a success criterion (tests pass, the answer matches a reference, a reviewer accepts the change).
2. Run both models on all tasks.
3. Compute the cost per solved task: total cost / number of successes.

A script for such a test with the Anthropic SDK:

import os
import anthropic

client = anthropic.Anthropic(
    base_url="https://api.zukko.pro",  # the Anthropic SDK appends /v1 itself
    api_key=os.environ["ZUKKO_API_KEY"],
)
PRICES = {  # $ per 1M tokens as of Oct 18, 2026: input, output, cache read, cache write
    "claude-fable-5-1": (7, 35, 0.7, 8.75),
    "claude-sonnet-5-5": (0.4, 2, 0.05, 0.5),
}

def solve(model, prompt):
    r = client.messages.create(

If the success difference on your tasks is not noticeable, paying for Fable makes no sense. If it is, count the cost of a success, not the cost of a request.

And what about Opus 5.5

For completeness: claude-opus-5-5 costs $0.8 / $4 per 1M tokens (cache read 0.08, write 1) with an intelligence index of 58. By the table that is cheaper than Fable 5.1 and higher on the index. So if you are considering Fable for the "strongest model", add Opus 5.5 to your test too. A detailed comparison of those two is in Claude Fable 5.1 or Opus 5.5↗, and connecting Fable and controlling spend is in Claude Fable 5.1 via API↗.

How to cap spend during a test

An expensive model in an agent session can spend money quickly. Create a separate key in the Zukko dashboard↗ with a daily limit, for example a few dollars, and an allowed_models list of the two models in your test. Limits are soft: a request started before the ceiling runs to the end, so leave a margin. Details are in the limits and cache documentation↗. Payment is available via SBP (the payment page adds 8.5% on top), cryptocurrency and MasterCard cards.

FAQ

How much does Claude Fable 5.1 cost?

As of October 18, 2026 on Zukko: $7 per 1M input tokens and $35 per 1M output tokens, cache read $0.7, cache write $8.75. The official price is $10 and $50. The final cost of a task depends on input volume, answer length and cache; calculate with the formula above.

Is Fable 5.1 stronger than Sonnet 5.5?

By the intelligence index, yes: 53 versus 48. But the index is a general metric, and your task may not differ between these models at all. Test on your own examples and count the cost of a success.

Can I keep both models in one project?

Yes, one key works with all models. A typical scheme is Sonnet 5.5 by default, Fable or Opus only for tasks the cheaper model failed. This scheme is called a model cascade.

Where do I see current prices?

In the GET https://api.zukko.pro/v1/models response and on the Models page in the dashboard. Prices change, and the table in this article reflects October 18, 2026.