Claude Fable 5.1 vs Mercury 2
Updated October 21, 2026. 39 benchmarks have a result for both. Coding is ranked on the Vibe Coding Index; every other board is shown as published.
vs boards won of 39 shared
Decision reading
Pick Claude Fable 5.1 for coding: it scores 89.1 on the Vibe Coding Index against 0.0 for Mercury 2.
Pick Mercury 2 when output tokens cost 66.7x less.
Model AClaude Fable 5.1Anthropic89.1/1002 independent benchmarksNo cost per solved taskModel BMercury 2Inception0.0/1001 independent benchmarkNo cost per solved task
- Shared results
- 39
- Claude Fable 5.1 only
- 85
- Mercury 2 only
- 3
- Like-for-like categories
- 7 of 9
Shared results can support a head-to-head reading. Results only one model has describe coverage, not superiority.
Which one for your work
Each card is decided by one number both models carry. Where only one has it, the card says so and makes no call.
Coding
Vibe Coding Index, four public coding benchmarks
Claude Fable 5.1
Claude Fable 5.1 scores 89.1 against 0.0.
2 and 1 benchmarks behind the scores
Intelligence
AA Intelligence Index
Claude Fable 5.1
Claude Fable 5.1 leads, 53.4 against 10.8.
Same board, both measured
Agentic work
AA Agentic Index
Claude Fable 5.1
Claude Fable 5.1 leads, 57.9 against 1.6.
Same board, both measured
Reasoning
GPQA Diamond
No call
Only Claude Fable 5.1 has a GPQA Diamond result, 93.4%.
Directional only
Knowledge
MMLU Pro
No call
Only Claude Fable 5.1 has a MMLU Pro result, 92.4%.
Directional only
Web design
Design Arena: websites
Claude Fable 5.1
Claude Fable 5.1 leads, 1313 against 1036.
Same board, both measured
Long documents
Listed context window
Claude Fable 5.1
Claude Fable 5.1 takes 1M tokens against 128K.
Listed by OpenRouter
Everyday cost
1K input + 500 output tokens
Mercury 2
Claude Fable 5.1 $0.035, Mercury 2 $0.0006 per turn.
List prices
At a glance
- Coding index
- Claude Fable 5.1 89.1 from 2 benchmarks, Mercury 2 0.0 from 1 benchmark.
- Widest category
- Agentic work, 56.3 points apart on average over 1 shared benchmark.
- Widest single gap
- LMArena WebDev: Claude Fable 5.1 1743 against Mercury 2 1171, 77.5 points apart on a 0 to 100 scale.
- Output price
- Claude Fable 5.1 $50.00 against Mercury 2 $0.75 per million output tokens, 66.7x apart.
- Context window
- Claude Fable 5.1 takes 1M tokens, Mercury 2 128K.
Benchmark by benchmark
First the score: boards each model leads, per category, where both have a result. Then every board, a category to a fold; bars are each result on a 0 to 100 scale, so compare the two in a row, not one row with another.
| Category | Claude Fable 5.1 | Bars | Mercury 2 | Ties |
|---|---|---|---|---|
| Coding | 9 | 0 | ||
| Intelligence | 1 | 0 | ||
| Agentic work | 1 | 0 | ||
| Reasoning | 5 | 0 | ||
| Knowledge | 3 | 0 | ||
| Web design | 8 | 0 | ||
| General | 12 | 0 | ||
| All shared boards | 39 | 0 |
CodingLike-for-likeFour of these boards feed the Vibe Coding Index90
| Benchmark | Claude Fable 5.1 | Mercury 2 | Leader |
|---|---|---|---|
| LiveBench coding | 97.7% | Not measured | Only Claude Fable 5.1 measured |
| LMArena WebDev | 1743 | 1171 | Claude Fable 5.1 |
| AA Coding Index | 81.6 | 31.1 | Claude Fable 5.1 |
| LiveCodeBench (Vals AI) | 90.5% | Not measured | Only Claude Fable 5.1 measured |
| Terminal-Bench 4 (Vals AI) | 58.1% | Not measured | Only Claude Fable 5.1 measured |
| Vibe Code Bench (Vals AI) | 90.3% | Not measured | Only Claude Fable 5.1 measured |
| Code migration (Vals AI) | 54.6% | Not measured | Only Claude Fable 5.1 measured |
| ProgramBench (Vals AI) | 7.0% | Not measured | Only Claude Fable 5.1 measured |
| IOI (Vals AI) | 90.8% | Not measured | Only Claude Fable 5.1 measured |
| Terminal-Bench 2.1 (Vals AI) | 85.0% | Not measured | Only Claude Fable 5.1 measured |
| SWE-bench Multimodal (Vals AI) | 61.3% | Not measured | Only Claude Fable 5.1 measured |
| Vibe Code Bench 1-100 (Vals AI) | 28.0% | Not measured | Only Claude Fable 5.1 measured |
| SkillsBench (Vals AI) | 61.5% | Not measured | Only Claude Fable 5.1 measured |
| CyberBench (Vals AI) | 70.4% | Not measured | Only Claude Fable 5.1 measured |
| SciCode | 63.1% | 37.7% | Claude Fable 5.1 |
| Terminal-Bench Hard (Artificial Analysis run) | Not measured | 26.5% | Only Mercury 2 measured |
| FrontierCode | 50.9% | Not measured | Only Claude Fable 5.1 measured |
| FrontierSWE | 56.3% | Not measured | Only Claude Fable 5.1 measured |
| GSO-Bench | 88.2% | Not measured | Only Claude Fable 5.1 measured |
| WeirdML | 92.9% | 43.2% | Claude Fable 5.1 |
| MirrorCode | 73.3% | Not measured | Only Claude Fable 5.1 measured |
| ALE-Bench | 2143.2 | 785.6 | Claude Fable 5.1 |
| LMArena Text: coding | 1532 | 1390 | Claude Fable 5.1 |
| LMArena Text: software and IT | 1530 | 1385 | Claude Fable 5.1 |
| LMArena WebDev: React | 1792 | 1160 | Claude Fable 5.1 |
| LMArena WebDev: HTML | 1684 | 1199 | Claude Fable 5.1 |
| LMArena WebDev: from an image | 1722 | Not measured | Only Claude Fable 5.1 measured |
| Neither model has a result on the other 11 boards here | |||
IntelligenceLike-for-likePublic boards, shown as published, kept out of the index10
| Benchmark | Claude Fable 5.1 | Mercury 2 | Leader |
|---|---|---|---|
| AA Intelligence Index | 53.4 | 10.8 | Claude Fable 5.1 |
| Epoch Capabilities Index | 166.5 | Not measured | Only Claude Fable 5.1 measured |
Agentic workLike-for-likePublic boards, shown as published, kept out of the index10
| Benchmark | Claude Fable 5.1 | Mercury 2 | Leader |
|---|---|---|---|
| AA Agentic Index | 57.9 | 1.6 | Claude Fable 5.1 |
| Design Arena: agents building web apps | 1318 | Not measured | Only Claude Fable 5.1 measured |
| Design Arena: agents building full-stack apps | 1326 | Not measured | Only Claude Fable 5.1 measured |
| Design Arena: agents building mobile apps | 1267 | Not measured | Only Claude Fable 5.1 measured |
| Design Arena: agents building Godot games | 1327 | Not measured | Only Claude Fable 5.1 measured |
| Design Arena: PowerPoint slides | 1328 | Not measured | Only Claude Fable 5.1 measured |
| Design Arena: spreadsheets | 1374 | Not measured | Only Claude Fable 5.1 measured |
| LiveBench research | 82.0% | Not measured | Only Claude Fable 5.1 measured |
| LiveBench browser | 100.0% | Not measured | Only Claude Fable 5.1 measured |
| LiveBench long agentic | 80.8% | Not measured | Only Claude Fable 5.1 measured |
| SREBench (Vals AI) | 22.9% | Not measured | Only Claude Fable 5.1 measured |
| Finance Agent (Vals AI) | 58.9% | Not measured | Only Claude Fable 5.1 measured |
| Tax Agent Bench (Vals AI) | 49.2% | Not measured | Only Claude Fable 5.1 measured |
| Legal Research Bench (Vals AI) | 55.3% | Not measured | Only Claude Fable 5.1 measured |
| Excel Modeling Benchmark (Vals AI) | 76.7% | Not measured | Only Claude Fable 5.1 measured |
| Terminal-Bench-Science (Vals AI) | 40.0% | Not measured | Only Claude Fable 5.1 measured |
| Time Horizon Index (Vals AI) | 63.3% | Not measured | Only Claude Fable 5.1 measured |
| CUA-bench (Vals AI) | 13.2% | Not measured | Only Claude Fable 5.1 measured |
| Robo-bench (Vals AI) | 5.8% | Not measured | Only Claude Fable 5.1 measured |
| τ²-Bench Telecom | Not measured | 70.8% | Only Mercury 2 measured |
| APEX-Agents | 68.6% | Not measured | Only Claude Fable 5.1 measured |
| Remote Labor Index | 17.9% | Not measured | Only Claude Fable 5.1 measured |
| SpireClimb | 2.0% | Not measured | Only Claude Fable 5.1 measured |
| Vending-Bench 2 | $5,422 | Not measured | Only Claude Fable 5.1 measured |
| LMArena Search | 1246 | Not measured | Only Claude Fable 5.1 measured |
| Neither model has a result on the other 14 boards here | |||
ReasoningLike-for-likePublic boards, shown as published, kept out of the index50
| Benchmark | Claude Fable 5.1 | Mercury 2 | Leader |
|---|---|---|---|
| GPQA Diamond | 93.4% | Not measured | Only Claude Fable 5.1 measured |
| LiveBench reasoning | 91.7% | Not measured | Only Claude Fable 5.1 measured |
| MysteryMechanism (Vals AI) | 47.8% | Not measured | Only Claude Fable 5.1 measured |
| MMMU Pro (Vals AI) | 90.6% | Not measured | Only Claude Fable 5.1 measured |
| GPQA Diamond (Artificial Analysis run) | 93.7% | 77.0% | Claude Fable 5.1 |
| Humanity's Last Exam (Artificial Analysis run) | 59.1% | 17.1% | Claude Fable 5.1 |
| ARC-AGI | 97.5% | Not measured | Only Claude Fable 5.1 measured |
| ARC-AGI-2 | 90.0% | Not measured | Only Claude Fable 5.1 measured |
| Chess puzzles | 48.0% | Not measured | Only Claude Fable 5.1 measured |
| Mystery game puzzles | 58.0% | Not measured | Only Claude Fable 5.1 measured |
| EBR-bench | 57.1% | Not measured | Only Claude Fable 5.1 measured |
| LMArena Text: hard prompts | 1528 | 1362 | Claude Fable 5.1 |
| LMArena Text: expert | 1544 | 1358 | Claude Fable 5.1 |
| LMArena Text: hard prompts in English | 1522 | 1374 | Claude Fable 5.1 |
| LMArena Vision: diagrams | 1343 | Not measured | Only Claude Fable 5.1 measured |
| LMArena Vision: homework | 1345 | Not measured | Only Claude Fable 5.1 measured |
| Neither model has a result on the other 5 boards here | |||
MathDirectional onlyPublic boards, shown as published, kept out of the index6 boards, none shared
| Benchmark | Claude Fable 5.1 | Mercury 2 | Leader |
|---|---|---|---|
| OTIS Mock AIME | 100.0% | Not measured | Only Claude Fable 5.1 measured |
| FrontierMath, tiers 1 to 3 | 90.2% | Not measured | Only Claude Fable 5.1 measured |
| FrontierMath, tier 4 | 87.8% | Not measured | Only Claude Fable 5.1 measured |
| ProofBench | 100.0% | Not measured | Only Claude Fable 5.1 measured |
| LMArena Text: math | 1531 | Not measured | Only Claude Fable 5.1 measured |
| LMArena Text: mathematical work | 1541 | Not measured | Only Claude Fable 5.1 measured |
| Neither model has a result on the other 7 boards here | |||
KnowledgeLike-for-likePublic boards, shown as published, kept out of the index30
| Benchmark | Claude Fable 5.1 | Mercury 2 | Leader |
|---|---|---|---|
| MMLU Pro | 92.4% | Not measured | Only Claude Fable 5.1 measured |
| LegalBench (Vals AI) | 88.5% | Not measured | Only Claude Fable 5.1 measured |
| TaxEval (Vals AI) | 76.0% | Not measured | Only Claude Fable 5.1 measured |
| MortgageTax (Vals AI) | 70.8% | Not measured | Only Claude Fable 5.1 measured |
| MedCode (Vals AI) | 53.5% | Not measured | Only Claude Fable 5.1 measured |
| SimpleQA Verified | 70.8% | Not measured | Only Claude Fable 5.1 measured |
| LMArena Text: science | 1532 | 1343 | Claude Fable 5.1 |
| LMArena Text: medicine and healthcare | 1531 | 1363 | Claude Fable 5.1 |
| LMArena Text: legal and government | 1514 | Not measured | Only Claude Fable 5.1 measured |
| LMArena Text: business and finance | 1493 | 1349 | Claude Fable 5.1 |
| Neither model has a result on the other 7 boards here | |||
Web designLike-for-likePublic boards, shown as published, kept out of the index80
| Benchmark | Claude Fable 5.1 | Mercury 2 | Leader |
|---|---|---|---|
| Design Arena: websites | 1313 | 1036 | Claude Fable 5.1 |
| Design Arena: UI components | 1338 | 998 | Claude Fable 5.1 |
| Design Arena: game dev | 1382 | 985 | Claude Fable 5.1 |
| Design Arena: data viz | 1344 | 1009 | Claude Fable 5.1 |
| Design Arena: 3D | 1393 | 982 | Claude Fable 5.1 |
| Design Arena: all code categories | 1336 | 1022 | Claude Fable 5.1 |
| Design Arena: SVG | 1301 | 998 | Claude Fable 5.1 |
| Design Arena: ASCII art | 1324 | 990 | Claude Fable 5.1 |
| LiveBench design | 83.4% | Not measured | Only Claude Fable 5.1 measured |
GeneralLike-for-likePublic boards, shown as published, kept out of the index120
| Benchmark | Claude Fable 5.1 | Mercury 2 | Leader |
|---|---|---|---|
| LiveBench data analysis | 80.3% | Not measured | Only Claude Fable 5.1 measured |
| Public Benefits Bench (Vals AI) | 74.9% | Not measured | Only Claude Fable 5.1 measured |
| SAGE (Vals AI) | 48.5% | Not measured | Only Claude Fable 5.1 measured |
| MedScribe (Vals AI) | 91.3% | Not measured | Only Claude Fable 5.1 measured |
| IFBench | Not measured | 69.8% | Only Mercury 2 measured |
| Long context reasoning (AA-LCR) | 85.3% | 43.7% | Claude Fable 5.1 |
| DTBench | 97.6% | Not measured | Only Claude Fable 5.1 measured |
| LMCA | 65.5% | Not measured | Only Claude Fable 5.1 measured |
| Furniture assembly | 70.0% | Not measured | Only Claude Fable 5.1 measured |
| LMArena Text | 1512 | 1354 | Claude Fable 5.1 |
| LMArena Text: instruction following | 1518 | 1328 | Claude Fable 5.1 |
| LMArena Text: creative writing | 1509 | 1286 | Claude Fable 5.1 |
| LMArena Vision | 1319 | Not measured | Only Claude Fable 5.1 measured |
| LMArena Text: English | 1506 | 1378 | Claude Fable 5.1 |
| LMArena Text: non-English | 1508 | 1330 | Claude Fable 5.1 |
| LMArena Text: Chinese | 1584 | 1414 | Claude Fable 5.1 |
| LMArena Text: German | 1499 | Not measured | Only Claude Fable 5.1 measured |
| LMArena Text: Japanese | 1562 | Not measured | Only Claude Fable 5.1 measured |
| LMArena Text: Korean | 1517 | Not measured | Only Claude Fable 5.1 measured |
| LMArena Text: French | 1523 | Not measured | Only Claude Fable 5.1 measured |
| LMArena Text: Spanish | 1520 | Not measured | Only Claude Fable 5.1 measured |
| LMArena Text: Russian | 1520 | 1304 | Claude Fable 5.1 |
| LMArena Text: Polish | 1525 | Not measured | Only Claude Fable 5.1 measured |
| LMArena Text: multi-turn | 1493 | 1357 | Claude Fable 5.1 |
| LMArena Text: longer queries | 1524 | 1329 | Claude Fable 5.1 |
| LMArena Text: writing and language | 1510 | 1300 | Claude Fable 5.1 |
| LMArena Text: entertainment and media | 1497 | 1314 | Claude Fable 5.1 |
| LMArena Vision: English | 1313 | Not measured | Only Claude Fable 5.1 measured |
| LMArena Vision: Chinese | 1391 | Not measured | Only Claude Fable 5.1 measured |
| LMArena Vision: reading text | 1327 | Not measured | Only Claude Fable 5.1 measured |
| LMArena Vision: humor | 1318 | Not measured | Only Claude Fable 5.1 measured |
| LMArena Vision: creative writing | 1330 | Not measured | Only Claude Fable 5.1 measured |
| Neither model has a result on the other 14 boards here | |||
Image generationNot comparablePublic boards, shown as published, kept out of the index0 boards, none shared
| Benchmark | Claude Fable 5.1 | Mercury 2 | Leader |
|---|---|---|---|
| Neither model has a result on 5 boards here | |||
What each workload costs
Three fixed token mixes at each model's listed price, so per-token rates become a bill.
Chat turn
1K input + 500 output tokens
Claude Fable 5.1$0.035Fits in one request
Mercury 2$0.0006Fits in one request
Mercury 2 costs less
Repository review
50K input + 3K output tokens
Claude Fable 5.1$0.650Fits in one request
Mercury 2$0.015Fits in one request
Mercury 2 costs less
Agent loop
200K input + 10K output tokens
Claude Fable 5.1$2.50Fits in one request
Mercury 2$0.058Exceeds the context window
Mercury 2 costs less
Listed OpenRouter rates, without cache or batch discounts. A workload that does not fit a model's context window would need splitting.
Specification differences
The leading value in a row is marked; a row with a missing value names no leader. Our two rows are marked in red.
| Specification | Anthropic Claude Fable 5.1 | Inception Mercury 2 |
|---|---|---|
| Maker | Anthropic | Inception |
| Weights | No public weights listed | No public weights listed |
| Released | September 1, 2026 | March 4, 2026 |
| Context window | 1M | 128K |
| Input $/Mtok | $10.00 | $0.25 |
| Output $/Mtok | $50.00 | $0.75 |
| Coding index | 89.1 | 0.0 |
| Independent measurements behind it | 2 | 1 |
| Cost per solved task | Not computable | Not computable |
| Source | OpenRouter listing | OpenRouter listing |
Questions
Is Claude Fable 5.1 better than Mercury 2 for coding?
Pick Claude Fable 5.1 for coding: it scores 89.1 on the Vibe Coding Index against 0.0 for Mercury 2. Pick Mercury 2 when output tokens cost 66.7x less.
Which is cheaper, Claude Fable 5.1 or Mercury 2?
Claude Fable 5.1 costs $10.00 per million input tokens and $50.00 per million output tokens; Mercury 2 costs $0.25 and $0.75. Cost per solved task is known for neither of them.
Which has the bigger context window, Claude Fable 5.1 or Mercury 2?
Claude Fable 5.1, with 1M tokens against 128K.
How many benchmarks is the Claude Fable 5.1 vs Mercury 2 comparison based on?
39 benchmarks have a result for both models as of October 21, 2026. Claude Fable 5.1's coding index rests on 2 independent benchmarks and Mercury 2's on 1. Where only one model has a result, the page says so instead of naming a winner.