STEALTHMARK · HEAD TO HEAD

All comparisons ↗

Claude Fable 5.1 vs Step 5 Preview

Claude Fable 5.1 leads Step 5 Preview by 24.0 points on StealthMark, 63.0 to 39.0. It leads on all 9 benchmarks. Claude Fable 5.1 scores higher on Newton's Mirror, with its largest category advantage in reflections.

See the difference ↘
All results ↗
All results ↗

Overall StealthMark

63.0/100Overall rank 7/53
39.0/100Overall rank 36/53

General Intelligence

60.6/100
37.1/100

Visual Intelligence

67.1/100
41.9/100

Reasoning efficiency

On every task, result quality and reasoning tokens are each compared with all measured models and weighed equally. Each dot is one model.

Single-step reasoning Result quality weighed against reasoning tokens on a single problem

Multi-step reasoning Result quality weighed against reasoning tokens across connected steps, as in agentic workflows

BENCHMARK BY BENCHMARK

Where each model leads

  • Claude Fable 5.1
  • Step 5 Preview
  • Other models
  • Median

DATASHEET

Specs and pricing

Both take about 1M tokens of context.

  • Claude Fable 5.1
  • Step 5 Preview
  • Other models
  • Median
PricePer 1M output tokens, input below
Claude Fable 5.1: $50$10 inputStep 5 Preview: No list price
Context windowTokens per request, max output below
Claude Fable 5.1: 1MTied128K max outputStep 5 Preview: 1MTied
Output speedTokens per second, our measurement
Claude Fable 5.1: 122tokens/sStep 5 Preview: Not measured
SizeTotal and active parameters
Claude Fable 5.1: Not disclosedStep 5 Preview: 600B27B active
Developer
AnthropicStepFun
Released
1 Sep 2026Not published
Origin
United StatesNot published
Input → output
Text, image → textText, image, video → text
Architecture
Not publishedSparse mixture-of-experts; 92 layers (88 MoE), 352 routed experts with 8 selected per token plus a shared expert; grouped-query attention with 512-token sliding windows and full attention every fourth layer; 3 multi-token-prediction layers; vision tower for image and video input

Full datasheets, every price and sourceClaude Fable 5.1 ↗Step 5 Preview ↗

Anticipating threats and planning ahead. Collect coins while avoiding ghosts. Ghosts move before the player each turn, and collisions cost a life. 3 levels in sequence, up to 66 turns in total. Earlier moves and outcomes carry forward, testing learning across levels.

Claude Fable 5.1 34.2/100Rank 38/492 of 3 levels

2 of 3 played
Foresight: complete playing field
Level 38 ptsTurn 17 / 20

Step 5 Preview 59.9/100Rank 42/491 of 3 levels

1 of 3 played
Foresight: complete playing field
Level 122 ptsTurn 28 / 28

Points per level

LevelClaude Fable 5.1Step 5 Preview
Level 1Second Thoughts
21 ptsOut of lives
22 ptsSurvived
Level 2Pressure Pot
Pending
Pending
Level 3Hide or Flight
8 ptsOut of lives
Pending

Cooperative planning and learning from experience. Coordinate two robots to push numbered cores onto matching sockets. Pressure plates hold doors open; moving either robot uses one turn. 5 levels in sequence, up to 152 turns in total. Earlier moves and outcomes carry forward, testing learning across levels.

Claude Fable 5.1 82.6/100Rank 6/50

Cleared 5 of 5
Latch: complete playing field
Level 520.6 ptsTurn 27 / 40

Step 5 Preview 40.7/100Rank 28/50

Cleared 4 of 5
Latch: complete playing field
Level 515 ptsTurn 35 / 40

Points per level

LevelClaude Fable 5.1Step 5 Preview
Level 1Hold the door
5.2 ptsCleared
5.2 ptsCleared
Level 2Crossfeed
9.8 ptsCleared
9 ptsCleared
Level 3Relay vault
19.2 ptsCleared
18.6 ptsCleared
Level 4Passing bay
18.2 ptsClearedFinished early · 3 beacons left
15.6 ptsClearedFinished early · 3 beacons left
Level 5Second thoughts
20.6 ptsCleared
15 ptsFinished early

Newton's Mirror

Same task instructions

Physics, perspective and recursive reflections, tested together. Explore the test ↗

The judgment

Claude Fable 5.1

The broad paint sequence, bilateral near-field sphere arrangement, overlaps and diminishing panel repetitions are recognizable; crimson is slightly too low. Small paint scale and basic shading provide material cues, although the steel response remains soft. Inaccurate light projections, missing outer chains and incomplete nested room imagery remain the principal limitations.

Step 5 Preview

The main sphere, airborne black and small colored features occupy broadly appropriate positions. Most of the defining mirror system is absent, and paint interaction remains only partially supported. The small paint features do not visibly establish a separate volume-scale violation.

Score breakdown

CategoryClaude Fable 5.1Step 5 Preview
State at 600 ms24.0/2520.0/25
Camera and placement14.0/1513.5/15
Reflections48.0/12020.0/120
Materials and scale7.5/107.0/10
Floor-damage reward0.0/100.0/10

Benchmark task

A spinning metal sphere and paint in a mirrored room test physical reasoning, perspective and consistency across repeated reflections.

Selfie

Different task instructions

Visual construction and creative interpretation.

The judgment

Claude Fable 5.1

The coral palette, friendly face, matte shading and cream arch create a coherent emblem. However, the mantle is elongated rather than compact, and the appendages spread mostly sideways and downward instead of forming an evenly spaced, flower-like starburst. Two thin hanging appendages weaken the thick-arm motif. Clean contours and balanced framing support the simple design.

Step 5 Preview

A charming diagonal climber and warm palette communicate the central idea. Simple rounded construction, crowded limb counting, uncertain footholds and an unattached upper shelf limit its physical coherence. Growth bands and the yarn trail add specificity, but the requested material distinctions and unravelled bootie remain underdeveloped.

Score breakdown

CategoryClaude Fable 5.1Step 5 Preview
Task & likeness6.5/106.0/10
Anatomy & construction7.0/104.5/10
Space & placement7.5/105.0/10
Light & reflections6.5/105.5/10
Purposeful detail6.0/105.5/10
Artistry & character7.0/105.0/10
Materials & effects6.5/106.0/10
Visual integrity8.0/105.0/10

Benchmark task

Claude Fable 5.1

A coral squid with spreading arms tests connected anatomy, balanced composition and soft surface shading.

Step 5 Preview

Character scenes test expression, anatomy, materials and coherent visual construction.

Mona Lisa

Same task instructions

Precise visual recall and portrait reconstruction.

The judgment

Claude Fable 5.1

The dark clothing, crossed hands and hazy landscape establish the Mona Lisa composition. Replication is limited by the frontal, elongated face, wide eyes and simplified smile. Rounded, parallel fingers and unclear wrist transitions weaken the hands. Warm facial shading gives some volume, but broad garment shapes and scattered crack marks substitute for convincing folds and painterly detail.

Step 5 Preview

A recognizable, coherent simplification with purposeful facial contours and differentiated fingers. These provide more construction and detail than the broad silhouette alone suggests. The oversized face, frontal seated pose, unresolved wrists and sparse landscape still substantially limit likeness, depth and finish.

Score breakdown

CategoryClaude Fable 5.1Step 5 Preview
Task & likeness5.5/104.0/10
Anatomy & construction6.0/104.0/10
Space & placement6.0/104.0/10
Light & reflections6.0/103.5/10
Purposeful detail5.0/104.0/10
Artistry & character5.5/104.0/10
Materials & effects6.0/103.0/10
Visual integrity6.5/103.5/10

Benchmark task

An SVG interpretation of the Mona Lisa tests facial likeness, anatomy and the composition of a familiar portrait.

Beach

Same task instructions

Creativity, perspective and visual beauty.

The judgment

Claude Fable 5.1

The colorful sunset, layered shoreline, animals, kite and beach toys fulfill the lively beach brief. Clean silhouettes and a balanced composition give the illustration character. Construction is simplified: the umbrella resembles a segmented fan, dolphins have weak water interaction, and the sun’s reflection forms a rigid widening wedge. Flat surfaces and sparse contact shadows limit depth.

Step 5 Preview

A recognizable sunset beach with clear warm-cool contrast and playful accessories. Basic animal construction, shallow foreground placement and schematic reflections limit spatial and physical credibility. Sparse descriptive detail and abruptly ended sky bands constrain the finish.

Score breakdown

CategoryClaude Fable 5.1Step 5 Preview
Task & likeness8.5/104.0/10
Anatomy & construction7.0/103.0/10
Space & placement7.0/103.5/10
Light & reflections7.0/103.5/10
Purposeful detail7.5/103.0/10
Artistry & character7.5/103.0/10
Materials & effects6.5/103.5/10
Visual integrity7.5/104.0/10

Benchmark task

A busy shoreline at sunset tests scene composition, animal forms and the relationship between light and water reflections.

Ground Truth

Same task instructions

Physical reasoning, light and visual reconstruction.

The judgment

Claude Fable 5.1

This benchmark has not been assessed yet.

Step 5 Preview

This benchmark has not been assessed yet.

Benchmark task

A plank, computer mouse and blueberry in a steel room test physical stability, spatial relationships, materials and lighting.

Raccoon

Same task instructions

Scene composition and interacting subjects.

The judgment

Claude Fable 5.1

The raccoon rides on the dog's back, and the dog's extended foreleg reaches the jet ski controls. Expressive faces, a clear silhouette and sweeping spray make the action readable. Limb and handlebar overlaps weaken construction; broad shading provides limited volume, and the glowing sun has little corresponding reflection on the water. Clean vector shapes support an appealing but simplified illustration.

Step 5 Preview

Recognizable animals, a bright palette and identifying accessories support a basic cartoon. Primitive anatomy, partial seated support, unresolved steering and weak hull-water contact limit the central action. Lighting and spray provide atmosphere without establishing convincing physical depth.

Score breakdown

CategoryClaude Fable 5.1Step 5 Preview
Task & likeness8.5/104.0/10
Anatomy & construction6.5/104.5/10
Space & placement7.0/103.0/10
Light & reflections5.5/103.5/10
Purposeful detail7.0/104.0/10
Artistry & character7.0/103.5/10
Materials & effects6.5/103.0/10
Visual integrity7.5/103.0/10

Benchmark task

A jet-ski scene pairs a canine driver with a raccoon passenger. It tests the placement and contact of interacting subjects, along with vehicle construction.

Pelican

Same task instructions

Subject construction and spatial relationships.

The judgment

Claude Fable 5.1

The pelican reads clearly, with an expressive face, prominent bill and appealing composition. Both legs connect to the body, but the far foot's pedal contact and the wing's steering grip remain unclear. Wheel alignment and background layers are coherent. Clean contours and feather lines give polish, while schematic machinery, limited surface texture and weak directional lighting constrain the finish.

Step 5 Preview

The pelican and bicycle are recognizable, with a pleasant palette and basic diffuse shading. The riding relationship fails physically: the bird is unsupported, the front wheel is unmounted, and pedalling is unresolved. Simple geometric construction and sparse material treatment further limit the drawing.

Score breakdown

CategoryClaude Fable 5.1Step 5 Preview
Task & likeness8.5/103.0/10
Anatomy & construction7.0/103.0/10
Space & placement7.5/103.0/10
Light & reflections6.0/103.0/10
Purposeful detail7.0/104.0/10
Artistry & character7.0/103.0/10
Materials & effects6.0/103.0/10
Visual integrity8.0/102.0/10

Benchmark task

A cycling pelican tests anatomy, riding posture and contact between the bird and the bicycle.