Model comparison

Claude Fable 5.1 vs GPT-5.5

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

Frontier

AI model capability comparison
Specification	Claude Fable 5.1	GPT-5.5
Provider	Anthropic	OpenAI
Tier	Frontier	Frontier
Context window	1M	Winner: 1.05M
Max output	128K	128K
Input / 1M tokens	$10	Winner: $5
Output / 1M tokens	$50	Winner: $30
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
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	58
Humanity's Last Exam (2026-09-01)	Winner: 65	52.2
Terminal-Bench 2.1 (2026-09-05)	Winner: 91.4	83.4
DeepSWE 1.1 (2026-09-03)	67.4	Not verifiedUnverified
SWE-bench Verified (2026-09-01)	Not verifiedUnverified	82.6
GPQA Diamond (2026-07-27)	Not verifiedUnverified	93.5

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: vals.ai SWE-bench Verified leaderboard (independent, mini-swe-agent bash-only); official SWE-bench not publishedReal GitHub issue resolution: does the model's patch pass the hidden tests. Comparability: comparable with caveat — Post-audit vendor claims and pre-audit scores sit on different task trust levels; scaffolding (agent harness, compute budget) also dominates results. Never aggregate across scaffolds.
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).
2026-07-27: Kimi K3 model card (vendor-reported, xhigh); announcement charts show 93.6 — minor conflict preservedAgentic 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.
2026-04-23: OpenAI — Introducing GPT-5.5 (vendor, with tools; 41.4 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.

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: Standard $5/$30 per MTok; >272K input billed 2x in / 1.5x out. Cached input $0.50/MTok; batch $2.50/$15.

FrontierRecord checked September 3, 2026

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.

FrontierRecord checked September 3, 2026

GPT-5.5

GPT-5.5 is OpenAI's previous flagship — strong on Terminal-Bench 2.0 (82.7%) and 1M-token context.

Best for

Reasoning
Agentic coding
Long-context work

Watch out

Superseded by GPT-5.6 Sol on quality; still a capable, widely integrated model.

When the cheaper one wins

GPT-5.5 is cheaper on output at $30 per million tokens against $50 for Claude Fable 5.1 — about 1.7×. 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.

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Evidence 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.
3 shared named benchmarks with differing scores — Measured on: Artificial Analysis Intelligence Index, Humanity's Last Exam, Terminal-Bench 2.1.
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: OpenAI — Introducing GPT-5.5 (accessed 2026-08-29)
GPT-5.5: OpenAI API pricing (gpt-5.5 $5/$30) (accessed 2026-08-29)

Related comparisons

Diving deeper on one model? Claude Fable 5.1 · GPT-5.5

Common questions

Claude Fable 5.1 vs GPT-5.5

Answered from the verified figures on this page rather than general guidance.

Is Claude Fable 5.1 or GPT-5.5 cheaper for input?

GPT-5.5 is cheaper at $5 per million input tokens, against $10 for Claude Fable 5.1 — roughly 2.0× 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 has tiered pricing: Standard $5/$30 per MTok; >272K input billed 2x in / 1.5x out. Cached input $0.50/MTok; batch $2.50/$15.

Is Claude Fable 5.1 or GPT-5.5 cheaper for output?

GPT-5.5 is cheaper at $30 per million output tokens, against $50 for Claude Fable 5.1 — roughly 1.7× 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 has tiered pricing: Standard $5/$30 per MTok; >272K input billed 2x in / 1.5x out. Cached input $0.50/MTok; batch $2.50/$15.

Which has the larger context window, Claude Fable 5.1 or GPT-5.5?

GPT-5.5 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 support the same reasoning levels?

Claude Fable 5.1 exposes adaptive, while GPT-5.5 exposes none, low, medium, high, xhigh, max.

Should I use Claude Fable 5.1 or GPT-5.5?

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 suits reasoning.

Next step

Choosing between them

The verified figures that separate this pair, computed from the catalog rather than restated boilerplate.

Input price: GPT-5.5 costs $5 per 1M tokens versus $10 for Claude Fable 5.1 — a 2x difference at the headline tier.
Context: GPT-5.5 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 58 (measured 2026-08-14).

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