Claude Fable 5.1 vs Opus 5: Run Cost, Not Benchmarks

Searching “claude fable vs opus” in September 2026 surfaces benchmark roundups and Reddit threads — mostly comparing Fable 5, the previous release, with Opus 4.8 or July’s Opus 5. The model everyone is choosing between now, Claude Fable 5.1 (released September 1, 2026), barely appears, and no ranking page answers the question that sets your invoice: which model is cheaper per run of your workload, not per benchmark point. This article compares Fable 5.1 and Opus 5 the way a billing meeting would, with every price captured on September 4, 2026.

Two questions this keyword keeps asking, answered up front. Is Claude Fable better than Opus? There is no “better” verdict — Anthropic’s own scores split by task, so this page gives a dimension breakdown with scenario attribution instead of a ranking. Is Claude Fable more expensive than Opus? Yes on the headline rates: as of September 4, 2026, Fable 5.1 bills $45 per 1M output tokens against Opus 5’s $22.50 — about 2× — and $9 against $4.50 on input. That ratio is arithmetic on two price rows, derived and dated here, not a figure either vendor publishes. One exception runs the other way: cache reads cost $0.225 per 1M on Fable 5.1, half of Opus 5’s $0.45. Whether that ever offsets a 2× output price is what this article walks through — and how to answer it for your own traffic, not a benchmark table. We run Modellix, an API gateway that lists both models behind one key, so we have a commercial interest when Modellix appears; rates were read from the Modellix LLM price page and Anthropic’s own pages on September 4, 2026, and we flag wherever a number could not be verified.

The short answer: close on paper, different bills in practice

Anthropic’s benchmark table puts Fable 5.1 and Opus 5 within a few points on most rows (next section). At a 20-point gap this would be an easy call; at this one it is economic. Both models share a 1M-token window and flat-rate billing; what differs is the rate card and where your tokens land on it.

Dimension	Claude Fable 5.1	Claude Opus 5
Anthropic list price (input / output, per 1M)	$10.00 / $50.00	$5.00 / $25.00
Modellix rate, Sept 4, 2026 (input / output)	$9.00 / $45.00	$4.50 / $22.50
Cache read per 1M (Modellix, Sept 4, 2026)	$0.225	$0.45
Context window / pricing mode	1M tokens / flat-rate	1M tokens / flat-rate
Terminal-Bench 4.0 (Anthropic)	55.8%	52.3%
CursorBench 3.2.0 (Anthropic)	73.4%	70.0%
Switching cost	One string, same key	One string, same key
How to verify for your workload	Run both, compare logs	Run both, compare logs

Read the last two rows before the scores. If testing a model meant a new account and re-plumbed SDKs, benchmarks would be the pragmatic default; when the two models differ by one string on the same key, your own comparison takes an afternoon. That is the second half of this article.

First, which models are we actually comparing

Version drift does most of the work in this SERP: June’s pieces pit Fable 5 against Opus 4.8, July’s against the then-new Opus 5, and both are stale since Claude Fable 5.1 shipped September 1, 2026. This article compares the two current flagship IDs as billed today: anthropic/claude-fable-5.1 and anthropic/claude-opus-5.

Anthropic positions them like this. The Opus 5 announcement calls Opus 5 “a thoughtful and proactive model that comes close to the frontier intelligence of Claude Fable 5 at half the price” — the deliberate default for everyday difficult work. The Fable 5.1 announcement puts Fable 5.1 at the top of the rate card: Anthropic’s most capable model for long-running agentic coding, knowledge work, and research. Those are positions, not a verdict — “half the price” is exactly why the run-cost question exists.

Three facts keep the comparison honest (Modellix catalog, September 4, 2026, consistent with Anthropic’s pages):

Same context window, same billing shape. Both accept up to 1M tokens, both flat-rate — no tiered surcharge for long contexts. Neither has a bigger window or different tiering.
Same modality shape. Both list text and image input, text output.
Fable 5.1 kept Fable 5’s $10/$50 rate but cut cache reads. Per Anthropic’s Fable model page, cache reads now cost “$0.25 per million tokens, 75% less than Fable 5”. Anthropic separately estimates that cut lowers Fable 5.1’s typical-workload cost by about 25% versus Fable 5, and highly agentic workloads by up to approximately 45%. Those percentages are Anthropic’s, about Fable 5 → 5.1, not gateway discounts.

The full Fable 5.1 rate card — four billing dimensions, cache columns, the Fable 5 migration math — lives in our Claude Fable 5.1 pricing guide. This article answers the other question: which flagship should a given workload run on.

What the official benchmarks actually show

The scores below are verbatim from Anthropic’s benchmark table on its Fable 5.1 announcement (accessed September 4, 2026), which lists Fable 5.1, Fable 5, Opus 5, and GPT-5.6 Sol side by side. We quote only Anthropic’s numbers — third-party reruns (Tessl, BenchLM, DataCamp, Toloka, YouTube) are excluded by policy.

Benchmark (Anthropic, Fable 5.1 page)	Fable 5.1	Opus 5
Agentic scientific research — Terminal-Bench-Science 0.1	52.6%	29.0%
Agentic coding — Terminal-Bench 4.0	55.8%	52.3%
Multidisciplinary reasoning — Humanity’s Last Exam	60.9%	56.6%
Agentic coding — CursorBench 3.2.0	73.4%	70.0%

The Humanity’s Last Exam row shows Anthropic’s published run without tool use; the second configuration Anthropic publishes for this benchmark is not reproduced here.

Three observations before anyone turns scores into spend:

The big gap is on scientific research, not everyday work. Terminal-Bench-Science 0.1 (52.6% vs 29.0%) is the one clear separation. On the coding and reasoning rows most API traffic touches, the spread is 3.4 to 4.3 points.
Anthropic flags noise itself. Its Terminal-Bench-Science footnote reports a standard error of ±3.5–4.5 points per model and says its setup reproduces public leaderboard figures “within noise.” A single-digit gap sits inside that band; treat it as approximate.
Scores describe capability, not your bill. A model 3 points higher can be the costlier choice per successful run at 2× the token price — or cheaper, if it finishes in one attempt where the other needs two.

The price gap: ~2× output, half-price cache reads

The two current rows on the Modellix LLM price page, captured September 4, 2026:

Model (Modellix ID)	Input / 1M	Output / 1M	Cached read / 1M	Discount
Claude Fable 5.1 (`anthropic/claude-fable-5.1`)	$9.00	$45.00	$0.225	10% OFF
Claude Opus 5 (`anthropic/claude-opus-5`)	$4.50	$22.50	$0.45	10% OFF

Modellix shows the official list price and its rate side by side; figures above are Modellix’s rates. Anthropic list prices ($10/$50 and $5/$25) were confirmed on Anthropic’s pages the same day. Every price is a September 4, 2026 snapshot — the discount column changed for all four Anthropic rows within the past 24 hours, so re-check before you budget.

Screenshot of the Anthropic rows on the Modellix LLM pricing table on September 4, 2026: Claude Fable 5.1 at $9.00/$45.00 and Claude Opus 5 at $4.50/$22.50 per 1M tokens, both rows marked 10% OFF

Source of the two rows: Modellix’s “Models and pricing” list, captured September 4, 2026.

Three structural facts fall out of these rows:

Fable 5.1 is about 2× Opus 5 on input and output ($45 vs $22.50 output, $9 vs $4.50 input). “2×” is arithmetic on the two rows, dated September 4, 2026 — Anthropic publishes no Fable-vs-Opus ratio, and this is not a claim beyond today’s numbers.
Cache reads are the one line where Fable 5.1 is cheaper — half price ($0.225 vs $0.45). This holds at both layers: Anthropic prices Fable 5.1 reads at $0.25 (0.025× its input) while other Claude models follow the standard 0.1×-of-input rule that puts Opus 5 reads at $0.50, per Anthropic’s pricing documentation.
Both rows carry the same 10% OFF label today, so the gateway discount shaves both sides of the ratio equally.

For the full Fable rate card see the Claude Fable 5.1 pricing guide; the wider Anthropic lineup is in our Claude API pricing guide and the 28-model catalog in the LLM API pricing comparison. This article keeps two rows because the flagship decision does not need the rest of the table.

When a 2× output price is the cheaper run

Two worked examples with illustrative token assumptions we invented and Modellix’s September 4, 2026 rates; change the assumptions and the math changes.

Scenario A — a cache-dominant agent loop. A long-running agent re-reads a large stable context every turn. Say a 400K-token context written once, then re-read from cache 59 times, plus 2K fresh input and 1.5K output per call, 60 calls. The cache reads alone: Fable 5.1 = 59 × 400K × $0.225/1M ≈ $5.31, Opus 5 = 59 × 400K × $0.45/1M ≈ $10.62 — Fable saves ~$5.31 there, while its 2× rates add ~$1.80 (first-call input), $0.54 (fresh input), and $2.02 (output) over Opus’s equivalents. Net: Fable 5.1 ≈ $14.04 vs Opus 5 ≈ $14.99 — narrowly cheaper. (Caveat: cache columns apply only when tokens bill as cached reads; Modellix’s docs don’t document how caching is triggered.)

Scenario B — fresh context, output-heavy one-shot. 30K input, 6K output, one call, no reuse: Fable 5.1 = 30K × $9/1M + 6K × $45/1M = $0.54; Opus 5 = 30K × $4.50/1M + 6K × $22.50/1M = $0.27. Exactly half. No cache to rescue the premium.

Scenario C — retries, the hidden variable. Every failed run re-bills the whole call, and retries cost 2× as much on Fable 5.1. If Fable’s higher scores mean fewer attempts per success, the per-success cost narrows or flips; if Opus 5 rarely fails, you are paying 2× for nothing. That is why the next two sections exist.

The takeaway: Fable 5.1 suits cache-read-dominant, low-output, failure-expensive workloads; Opus 5 suits fresh-context, high-output, high-volume traffic. The dividing line is a ratio only your own usage supplies.

Switching between them is a one-string edit

Every benchmark site on this SERP assumes the hard part of comparing models is access. It is not, when both live behind one API key: on the Modellix LLM gateway, anthropic/claude-fable-5.1 and anthropic/claude-opus-5 differ by exactly one string in the model field — same key, same base URL, same SDK, same invoice. All four Anthropic IDs carry the same 10% OFF label, so the discount treatment does not change either. “Try it yourself” only works when trying costs one line edit — the precondition benchmark sites cannot give you.

Two ID-management details keep the test clean:

Pin fixed IDs for the experiment. Modellix’s docs echo Anthropic’s guidance: for evaluations or regression tests, use a fixed model ID. Compare the two concrete IDs above, not a moving target.
Use ~ aliases to track a family. Modellix also publishes stable aliases — ~anthropic/claude-fable-latest and ~anthropic/claude-opus-latest — that follow new releases without a code change. The trade-off, per Modellix’s alias note: the alias changes target as a family evolves and is “billed at the current target rate.” Alias for production drift-tracking; fixed IDs for a controlled A/B.

Concept diagram of running Claude Fable 5.1 and Claude Opus 5 side by side on one Modellix API key, switched by a one-string edit, with per-request billing streams

The comparison setup this article recommends: two Anthropic flagship IDs on one Modellix key, a one-string switch between them, and per-request billing streams on the same account. Illustrative Modellix artwork, not a product screenshot.

Once you have picked a side, pointing an Anthropic-protocol client at it is the remaining step — Claude Code and the Anthropic SDK reach the gateway through ANTHROPIC_BASE_URL without /v1, covered end to end in our Anthropic base URL guide.

Run each on your workload, then compare real bills

This is the step no benchmark can do for you. Modellix exposes per-request usage logs at GET /v1/logs — for every request, the model, token counts, and the exact billed cost. The fields that settle this comparison (per the Modellix LLM API docs, September 4, 2026):

{
  "model": { "provider": "anthropic", "model_name": "claude-fable-5.1" },
  "prompt_tokens": 4210,
  "completion_tokens": 1530,
  "cached_tokens": 398000,
  "cost": 0.1123
}

Illustrative shape — field names from the Modellix LLM API docs, values invented for display.

Run a representative batch of your real traffic on each model and pull both logs. Three things appear that no review can show:

Your actual cache-read share. Whether your requests bill mostly cached reads or fresh input and output is a cached_tokens column away. Cache-dominant → Fable 5.1’s $0.225 read does real work; otherwise you pay the 2× output rate with no offset.
Real per-request cost, not estimates. Sum the cost field per batch and the debate ends with a number. The optional X-Mdlx-User-Id header (8–128 characters) tags requests per end user so one team can A/B without mixing bills.
Retry economics, observed. Log task outcomes alongside the request log and Scenario C stops being hypothetical.

Both protocols work for the test, so use whatever client you already run: Anthropic Messages (https://llm.modellix.ai, no /v1) or OpenAI-compatible Chat Completions (https://llm.modellix.ai/v1). One honest boundary: Modellix publishes no SLA, uptime, or upstream-fallback guarantees and is a distribution layer, not the model maker — nothing here claims either model is faster or more stable through the gateway; what the gateway adds is measurability.

Read the Request Logs Reference

See the full GET /v1/logs response shape, headers, and per-request billing fields in the Modellix LLM documentation.

View Docs

Fable vs Sonnet, GPT-5.6, and the subscription meter

Quick boundaries for the long-tail versions of this query:

Fable vs Sonnet is a different axis. Sonnet 5 ($2/$10 list) is the production workhorse a full tier below both flagships. Between Fable 5.1 and Sonnet 5 the question is not “which is better” but “does this workload need the frontier tier” — the three-model rate card is in our Claude API pricing guide.
Fable vs GPT-5.6 crosses vendors. Anthropic’s table lists GPT-5.6 Sol as a reference column, but cross-vendor selection is a different decision than picking between two Claude models on one key. The 28-model catalog in our LLM API pricing comparison handles that.
Subscriptions and the API are separate meters. Fable 5.1 is available to Claude Pro and Max subscribers per Anthropic’s model page, and subscription caps do not share tokens with API billing — this article does not tell you to route around a subscription to save money. Choosing a client at all is covered by our Claude Code alternatives guide; here we only choose a model ID.

How to choose: Opus 5, Fable 5.1, or neither

Three decision paths cover most traffic:

Opus 5 for high-volume, varied work: fresh-context requests, long outputs, extraction and classification at scale, routine agent loops on moderate context. At half the token price it is the default until your logs show a reason to leave.
Fable 5.1 when the task is hard, long, or failure-expensive: deep multi-step coding, research, anything where a wrong answer means a full paid retry. Its score deltas on Terminal-Bench 4.0 and CursorBench 3.2.0 are small but consistently its way, and cache-heavy sessions blunt the price gap.
Neither when the workload never needed the frontier tier. Sonnet 5 and Haiku 4.5 exist so average traffic does not pay flagship prices; the cheapest correct model is the right one.

Two caveats before you commit spend. Our token scenarios are illustrative, and Anthropic’s scores are self-reported — only a logged A/B on your traffic settles it. Prices also drift: the Modellix discount column changed for all four Anthropic rows between September 3 and 4, 2026, so treat every figure here as a dated snapshot and re-check the Modellix LLM price page before budgeting.

Compare Fable 5.1 vs Opus 5 on Real Usage

Log in to switch between anthropic/claude-fable-5.1 and anthropic/claude-opus-5 on one key and pull per-request cost logs for your own A/B.

Frequently Asked Questions

Is Claude Fable better than Opus?

Only by dimension and scenario. On Anthropic’s numbers, Fable 5.1 leads Opus 5 on Terminal-Bench-Science 0.1 (52.6% vs 29.0%) and edges it on Terminal-Bench 4.0, Humanity’s Last Exam, and CursorBench 3.2.0 (table above) — while costing about 2× per token. Whether it “wins” depends if your workload needs that edge badly enough to pay for it.

Is Claude Fable more expensive than Opus?

Yes on headline rates as of September 4, 2026: $45 vs $22.50 output and $9 vs $4.50 input per 1M — about 2× — while cache reads run the other way ($0.225 vs $0.45). The 2× figure is derived from that date’s two price rows, not a vendor-published number.

Is Claude Fable really that good?

The only scores this article repeats are Anthropic’s own: 52.6% Terminal-Bench-Science 0.1, 55.8% Terminal-Bench 4.0, 73.4% CursorBench 3.2.0, Opus 5 within a few points on coding. Whether that is “good” for your budget is the run-cost question above.

What is Claude Fable?

Anthropic’s frontier model tier, positioned above Opus for the hardest long-running agentic coding and knowledge work. Fable 5.1 (September 2026) is the current release — API model anthropic/claude-fable-5.1 (Anthropic writes claude-fable-5-1) — sharing Opus 5’s 1M window and flat-rate billing at $10/$50 list, $9/$45 on Modellix as of September 4, 2026.

Is Fable faster or more stable than Opus on the API?

We do not publish an answer, and you should distrust anyone who does without receipts. Modellix discloses no SLA, uptime, or upstream-fallback guarantees. What is documented is billing: per-request cost and token logs at GET /v1/logs, so you measure runs instead of taking a vendor’s word.

Can I run both models under one API key and compare costs?

Yes — that is what a gateway is for. Both IDs share one key and base URL (Messages at https://llm.modellix.ai without /v1, Chat Completions at https://llm.modellix.ai/v1), so switching is a one-string edit; run a batch on each, pull the logs, compare summed cost.

Fable 5 or Fable 5.1?

Fable 5.1, starting fresh: same $10/$50 list as Fable 5 with cache reads cut 75% (Anthropic estimates ~25% lower typical cost, up to ~45% for highly agentic work — its figures). Pin Fable 5 only if a regression suite must not move; the migration arithmetic is in our Claude Fable 5.1 pricing guide.

Model availability, pricing, and discount labels change without notice; Anthropic’s pages are the authority on the models themselves. All prices above were captured September 4, 2026 from the Modellix LLM price page, with Anthropic list prices and benchmark figures cross-checked against Anthropic’s Fable 5.1 announcement, the Fable model page, Anthropic’s Opus 5 announcement, and Anthropic’s pricing documentation. Benchmark scores are Anthropic’s self-reported evaluations, not independent results. Modellix is an API aggregator with a commercial interest in this comparison and is not affiliated with Anthropic; verify current rates and scores against live sources before committing spend. Access language models from Anthropic, OpenAI, Google, DeepSeek, Qwen, and more through a single API key at modellix.ai.