Claude Opus 5.5 vs Fable 5.1: Which Lane, What It Costs

Opus 5.5 and Fable 5.1 differ by ~4.2x on Keyo. What each row costs, when the step up pays for itself, and the cheaper lanes on the same key.

The short answer

Start on claude-opus-5-5. It is the workhorse Claude lane at ~$1.76 / $8.82 per 1M in/out. Move work to claude-fable-5-1 (~$7.35 / $36.77) only where the harder model measurably changes the output — planner-tier reasoning, long multi-step agent runs, or flagship drafting you will publish. Everything else is a 4.2x bill for quality you may not be able to measure.

Side by side

Model ID	Input / output per 1M	Per 100K in / out	Typical role
`claude-opus-5-5`	~$1.76 / $8.82	$0.176 / $0.882	Hard agent and coding escalations
`claude-fable-5-1`	~$7.35 / $36.77	$0.735 / $3.677	Planner-tier flagship work

Both are Claude-class chat rows on the same OpenAI-compatible endpoint: POST /v1/chat/completions against https://www.keyoapi.xyz/v1, model string in the body. Switching between them is one model= change — no new SDK, no second invoice.

What the 4.2x actually buys

Run a month of realistic traffic — 10M input and 2M output tokens:

claude-opus-5-5    10M in × $1.76  = $17.60
                    2M out × $8.82 = $17.64   ≈ $35.24 / month

claude-fable-5-1   10M in × $7.35  = $73.50
                    2M out × $36.77 = $73.54  ≈ $147.04 / month

That is about $111.80 more per month, or 4.2x, for the same volume. The question is not which model is better in the abstract — it is whether your evaluation shows Fable 5.1 passing the cases Opus 5.5 fails. If you have not run that evaluation, you are paying the premium blind.

A practical split: keep drafting, classification, and routine agent turns on claude-opus-5-5, and route only the slices that fail your eval to claude-fable-5-1. If the failure rate is small, the blended cost stays close to the Opus row.

Cheaper lanes on the same key

Neither Claude row is the cheapest way to buy tokens on Keyo. If the task does not actually need top-tier Claude reasoning, the same base URL and key serve:

claude-haiku-5-5 — ~$0.07 / $0.37, the fastest, cheapest Claude 5.5 lane
claude-sonnet-5 — ~$0.44 / $2.21, everyday Claude coding and production work
claude-opus-5 — ~$1.10 / $5.52, the earlier Opus row, still listed
gpt-6-luna — ~$0.03 / $0.15, high-volume drafting and classifiers

Prototype on a fixed $0 catalog ID (the :free suffix) first — those calls draw down your $10 welcome credit at fair-use limits. See /free-models.

Check the bill on your own traffic

Every response carries a usage object — read prompt_tokens and completion_tokens and multiply by the table above. Both rows take the same call:

curl https://www.keyoapi.xyz/v1/chat/completions \
  -H "Authorization: Bearer YOUR_KEY" \
  -H "Content-Type: application/json" \
  -d '{"model":"claude-opus-5-5","messages":[{"role":"user","content":"hi"}]}'

Swap model to claude-fable-5-1 to compare the two on the same prompt. Rates here are indicative sell prices — confirm the live rows on /pricing/claude-opus-5-5 and /pricing/claude-fable-5-1 before you commit volume. The full Claude comparison sits on /claude-api-pricing, and the per-token walkthrough on Claude API Pricing per Token.