Model comparison
Claude Fable 5.1 vs GPT-5.5 Pro
Anthropic against OpenAI, compared on context, price, and verified benchmark results.
Catalog record checked September 3, 2026Individual provider fields may changeEvidence confidence: High — see receipts below
Anthropic
Claude Fable 5.1
Frontier
OpenAI
GPT-5.5 Pro
Frontier
| Specification | Claude Fable 5.1 | GPT-5.5 Pro |
|---|---|---|
| Provider | Anthropic | OpenAI |
| Tier | Frontier | Frontier |
| Context window | 1M | Winner: 1.05M |
| Max output | 128K | 128K |
| Input / 1M tokens | Winner: $10 | $30 |
| Output / 1M tokens | Winner: $50 | $180 |
| Weights | Closed | Closed |
| Parameters | Not disclosedUnverified | Not disclosedUnverified |
| Reasoning levels | adaptive | none, low, medium, high, xhigh, max |
| Modalities | text, image | text, image |
| API model id | claude-fable-5-1 | gpt-5.5-pro |
| Released | September 1, 2026 | April 23, 2026 |
| Artificial Analysis Intelligence Index (2026-08-14) | 66 | Not verifiedUnverified |
| Artificial Analysis Intelligence Index [xhigh] (2026-08-14) | Not verifiedUnverified | 60 |
| Humanity's Last Exam (2026-09-01) | 65 | Not verifiedUnverified |
| Terminal-Bench 2.1 (2026-09-05) | 91.4 | Not verifiedUnverified |
| DeepSWE 1.1 (2026-09-03) | 67.4 | Not verifiedUnverified |
Prices are USD per million tokens at standard rates, excluding batch and caching discounts. Bold indicates the better figure where one is objectively better. Values we could not confirm from the provider are shown as “Not verified” rather than estimated.
Benchmark receipts
- 2026-09-05: Artificial Analysis Terminal-Bench v2.1 (independent, Terminus 2, max effort — current AA leader)Agentic terminal work: multi-step tasks executed in a sandboxed shell environment. Comparability: comparable with caveat — Scores across Terminal-Bench major versions (v1 vs v2) are NOT comparable; harness configuration (container, time limits) also shifts results.
- 2026-09-03: OpenAI GPT-6 Astra launch chart (independent cross-vendor; Fable 5.1 not on the official Datacurve leaderboard)Agentic software engineering: pass@1 on real repo tasks with reported cost per completed task. Comparability: directly comparable
- 2026-09-01: Anthropic announcement comparison table (vendor, with tools; 60.9 no tools)Frontier-knowledge ceiling: extremely hard multi-domain questions written to be near-impossible without deep expertise. Comparability: comparable with caveat — Subset/tool configurations (text-only vs with-tools) differ between vendors and materially change scores; must match configuration to compare.
- 2026-08-14: Artificial AnalysisComposite index blending reasoning, knowledge, and coding evals into one 0–100 score. Comparability: directly comparable — AA occasionally rebaselines the index scale between snapshots — a score captured on one date is only comparable to same-snapshot scores (check measuredAt).
Pricing tiers: Claude Fable 5.1: Priced with Fable 5 at $10/$50 per MTok; cache reads cut 75% to $0.25/MTok. Adaptive thinking always on (default effort high); knowledge cutoff Jun 2026. Sibling Mythos 5.1 launched the same day. · GPT-5.5 Pro: Standard $30/$180 per MTok; no cached-input discount. Batch $15/$90.
Claude Fable 5.1
Claude Fable 5.1 is Anthropic's September 2026 flagship refresh, debuting at the top of LiveBench and the Artificial Analysis Intelligence Index.
Best for
- Frontier agents
- Hardest reasoning work
- Fable 5 workloads at the same price
Watch out
Early-life model: leaderboard position is from launch-week boards; recheck before committing spend.
GPT-5.5 Pro
GPT-5.5 Pro is OpenAI's highest-effort tier for maximal accuracy on the hardest tasks.
Best for
- Maximum-accuracy reasoning
- Research
- Hard enterprise workloads
Watch out
At $30/$180 per MTok it is the most expensive OpenAI tier; reserve for work that needs it.
When the cheaper one wins
Claude Fable 5.1 is cheaper on output at $50 per million tokens against $180 for GPT-5.5 Pro — about 3.6×. Use the cheaper tier for classification, extraction, summarisation, and any task where the expensive model’s extra score does not change the accepted output. The expensive one only pays if your hardest task actually fails on the cheap tier. These are standard-tier API rates, excluding batch and cache discounts.
Run the model pickerEvidence confidence: High
How strong and complete the evidence behind this comparison is — not a prediction of which model is better.
- Pricing verified on both sides — Input and output rates are verified for both models.
- 4/5 core specs verified on both sides — Not published for at least one side: parameter count.
- 1 shared named benchmark with differing scores — Measured on: Artificial Analysis Intelligence Index.
- Verified within the last 90 days — Newest catalog check was 8 days ago.
- Both models carry source citations — Each side has at least two catalog sources on record.
Source receipts
Each catalog figure was checked against the provider or an independent second source on the date shown.
- Claude Fable 5.1: Anthropic pricing — Fable 5.1/Mythos 5.1 $10/$50 (accessed 2026-09-03)
- Claude Fable 5.1: Anthropic models overview — Fable 5.1 (1M context, 128K output) (accessed 2026-09-03)
- Claude Fable 5.1: Artificial Analysis — Fable 5.1 Intelligence Index 66 #1 (accessed 2026-09-01)
- GPT-5.5 Pro: OpenAI — Introducing GPT-5.5 (accessed 2026-08-29)
- GPT-5.5 Pro: OpenAI API pricing (gpt-5.5-pro $30/$180) (accessed 2026-08-29)
Related comparisons
- Claude Fable 5.1 vs Claude Haiku 4.5
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- Claude Fable 5.1 vs Claude Opus 5
- Claude Fable 5.1 vs Claude Sonnet 5
- Claude Fable 5.1 vs DeepSeek V4 Pro
- Claude Fable 5.1 vs Gemini 3.1 Pro
Diving deeper on one model? Claude Fable 5.1 · GPT-5.5 Pro
Common questions
Claude Fable 5.1 vs GPT-5.5 Pro
Answered from the verified figures on this page rather than general guidance.
Is Claude Fable 5.1 or GPT-5.5 Pro cheaper for input?
Claude Fable 5.1 is cheaper at $10 per million input tokens, against $30 for GPT-5.5 Pro — roughly 3.0× the price. Output tokens usually dominate a real bill, so weigh the output rate more heavily than the input rate. These are base rates; Claude Fable 5.1 has tiered pricing: Priced with Fable 5 at $10/$50 per MTok; cache reads cut 75% to $0.25/MTok. Adaptive thinking always on (default effort high); knowledge cutoff Jun 2026. Sibling Mythos 5.1 launched the same day. GPT-5.5 Pro has tiered pricing: Standard $30/$180 per MTok; no cached-input discount. Batch $15/$90.
Is Claude Fable 5.1 or GPT-5.5 Pro cheaper for output?
Claude Fable 5.1 is cheaper at $50 per million output tokens, against $180 for GPT-5.5 Pro — roughly 3.6× the price. Output tokens usually dominate a real bill, so weigh the output rate more heavily than the input rate. These are base rates; Claude Fable 5.1 has tiered pricing: Priced with Fable 5 at $10/$50 per MTok; cache reads cut 75% to $0.25/MTok. Adaptive thinking always on (default effort high); knowledge cutoff Jun 2026. Sibling Mythos 5.1 launched the same day. GPT-5.5 Pro has tiered pricing: Standard $30/$180 per MTok; no cached-input discount. Batch $15/$90.
Which has the larger context window, Claude Fable 5.1 or GPT-5.5 Pro?
GPT-5.5 Pro accepts 1.05M tokens against 1M for Claude Fable 5.1. This only matters if you routinely send very long documents or large codebases.
Do Claude Fable 5.1 and GPT-5.5 Pro support the same reasoning levels?
Claude Fable 5.1 exposes adaptive, while GPT-5.5 Pro exposes none, low, medium, high, xhigh, max.
Should I use Claude Fable 5.1 or GPT-5.5 Pro?
Both sit in the frontier tier, so the choice usually comes down to price and context rather than capability. Claude Fable 5.1 suits frontier agents; GPT-5.5 Pro suits maximum-accuracy reasoning.
Next step
Choosing between them
The verified figures that separate this pair, computed from the catalog rather than restated boilerplate.
- Input price: Claude Fable 5.1 costs $10 per 1M tokens versus $30 for GPT-5.5 Pro — a 3x difference at the headline tier.
- Context: GPT-5.5 Pro takes 1.05M against 1M for Claude Fable 5.1 — only decisive if your prompts approach the smaller window.
- Measured capability: Claude Fable 5.1 leads Artificial Analysis Intelligence Index 66 to 60 (measured 2026-08-14).
Choosing a harness rather than a model — Cursor, Copilot, Claude Code, Windsurf, or Lovable? Compare agentic harnesses · Latest releases.