STEALTHMARK · HEAD TO HEAD

All comparisons ↗

Callstack Apex vs Step 5 Preview

Callstack Apex leads Step 5 Preview by 10.5 points on StealthMark, 49.5 to 39.0. It leads on 6 of the 8 benchmarks with results for both and Step 5 Preview on 2, including Latch. Step 5 Preview scores higher on Newton's Mirror, with its largest category advantage in reflections.

See the difference ↘
Model guide ↗
All results ↗

Overall StealthMark

49.5/100Overall rank 18/53
39.0/100Overall rank 36/53

General Intelligence

40.4/100
37.1/100

Visual Intelligence

59.6/100
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

  • Callstack Apex
  • Step 5 Preview
  • Other models
  • Median

DATASHEET

Specs and pricing

Callstack Apex scores higher on StealthMark. Step 5 Preview takes 4× more context.

  • Callstack Apex
  • Step 5 Preview
  • Other models
  • Median
PricePer 1M output tokens, input below
Callstack Apex: $3$0.50 inputStep 5 Preview: No list price
Context windowTokens per request, max output below
Callstack Apex: 256KStep 5 Preview: 1M4× larger
Output speedTokens per second, our measurement
Callstack Apex: 255tokens/sStep 5 Preview: Not measured
SizeTotal and active parameters
Callstack Apex: Not disclosedStep 5 Preview: 600B27B active
Developer
CallstackStepFun
Released
1 Oct 2026Not published
Origin
PolandNot published
Input → output
Text → textText, image, video → text
Architecture
Qwen foundation with Callstack domain post-training for React and React NativeSparse 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 sourceCallstack Apex ↗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.

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

LevelCallstack ApexStep 5 Preview
Level 1Second Thoughts
No run
22 ptsSurvived
Level 2Pressure Pot
No run
Pending
Level 3Hide or Flight
No run
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.

Callstack Apex 100.0/100Rank 42/501 of 5 levels

1 of 5 played
Latch: complete playing field
Level 15.2 ptsTurn 6 / 12

Step 5 Preview 40.7/100Rank 28/50

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

Points per level

LevelCallstack ApexStep 5 Preview
Level 1Hold the door
5.2 ptsCleared
5.2 ptsCleared
Level 2Crossfeed
Pending
9 ptsCleared
Level 3Relay vault
Pending
18.6 ptsCleared
Level 4Passing bay
Not played
15.6 ptsClearedFinished early · 3 beacons left
Level 5Second thoughts
Not played
15 ptsFinished early

Newton's Mirror

Same task instructions

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

The judgment

Callstack Apex

The principal sphere placement and paint contact sequence are supported. Isolated sphere and panel images provide limited optical structure, but missing lateral chains and side-floor continuations prevent a coherent repeating mirror room. The main and reflected spheres lack ordered curved nested imagery, while sparse planar emitter images and incomplete spherical emitter mapping leave most of the reflection system unsupported.

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

CategoryCallstack ApexStep 5 Preview
State at 600 ms20.0/2520.0/25
Camera and placement13.0/1513.5/15
Reflections10.0/12020.0/120
Materials and scale7.0/107.0/10
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

Callstack Apex

The cheerful yellow bird, cream eyes, orange features and miniature teal console closely match the brief. Clean block edges, restrained colors and soft shadows create an appealing composition. The body reads more like a gridded disc than a fully cubic sphere, and the outlined left wing has limited volume. The right wing reaches the cyan block, though its grip is ambiguous.

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

CategoryCallstack ApexStep 5 Preview
Task & likeness8.5/106.0/10
Anatomy & construction7.0/104.5/10
Space & placement7.5/105.0/10
Light & reflections7.5/105.5/10
Purposeful detail7.5/105.5/10
Artistry & character7.0/105.0/10
Materials & effects7.0/106.0/10
Visual integrity7.5/105.0/10

Benchmark task

Callstack Apex

A rounded bird built from small cubes explores a miniature machine. The scene tests block construction, expressive proportions, contact and soft lighting.

Step 5 Preview

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

Mona Lisa

Different task instructions

Precise visual recall and portrait reconstruction.

The judgment

Callstack Apex

The framed portrait recalls Mona Lisa through dark hair, folded arms and a hazy landscape. Replication is limited by wide, staring eyes, an elongated neck and a downturned mouth. The overlapping hands lack convincing finger and wrist construction. Soft tonal shading suggests volume, but pervasive blur obscures clothing, landscape and anatomical 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

CategoryCallstack ApexStep 5 Preview
Task & likeness3.5/104.0/10
Anatomy & construction4.0/104.0/10
Space & placement4.5/104.0/10
Light & reflections6.0/103.5/10
Purposeful detail4.5/104.0/10
Artistry & character4.5/104.0/10
Materials & effects5.5/103.0/10
Visual integrity5.5/103.5/10

Benchmark task

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

Beach

Different task instructions

Creativity, perspective and visual beauty.

The judgment

Callstack Apex

A cohesive sunset beach illustration with lively wildlife, playful accents, and an appealing warm-to-purple palette. Palm framing and the aligned sun reflection organize the scene, while connected silhouettes keep the activities readable. Schematic joints, ambiguous dolphin junctions, loosely matched reflections, and repetitive surface marks limit construction, lighting coherence, and material 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

CategoryCallstack ApexStep 5 Preview
Task & likeness8.5/104.0/10
Anatomy & construction4.5/103.0/10
Space & placement7.5/103.5/10
Light & reflections5.5/103.5/10
Purposeful detail7.5/103.0/10
Artistry & character6.5/103.0/10
Materials & effects7.0/103.5/10
Visual integrity4.0/104.0/10

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

Callstack Apex

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

Multiple instruction versions

Scene composition and interacting subjects.

The judgment

Callstack Apex

The raccoon clearly rides the dog's back, and the dog's paws reach the jet ski controls. Expressive faces, a sweeping tail and a strong sunset composition give the illustration character. Limbs remain connected, but the dog's foreleg joints and vehicle seating are loosely constructed. Angular splashes convey speed more successfully than convincing water displacement; lighting and reflections remain decorative.

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

CategoryCallstack ApexStep 5 Preview
Task & likeness9.0/104.0/10
Anatomy & construction7.0/104.5/10
Space & placement7.5/103.0/10
Light & reflections7.0/103.5/10
Purposeful detail8.5/104.0/10
Artistry & character7.5/103.5/10
Materials & effects7.0/103.0/10
Visual integrity8.0/103.0/10

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

Different task instructions

Subject construction and spatial relationships.

The judgment

Callstack Apex

An appealing cartoon with a distinctive pelican, expressive eye, warm bill and carefully drawn bicycle wheels. The wing reaches the handlebar, but the feet do not clearly engage the pedals, weakening the riding mechanics. Layered hills create depth; limited ground shadows weaken contact. Clean curves and purposeful feather details give the illustration polish.

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

CategoryCallstack ApexStep 5 Preview
Task & likeness8.5/103.0/10
Anatomy & construction6.5/103.0/10
Space & placement7.5/103.0/10
Light & reflections7.5/103.0/10
Purposeful detail8.0/104.0/10
Artistry & character8.0/103.0/10
Materials & effects7.5/103.0/10
Visual integrity8.0/102.0/10

Benchmark task

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