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 ↘Overall StealthMark
General Intelligence
Visual Intelligence
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
Games
SVG
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
- 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 ↗
Foresight
Play Foresight yourselfAnticipating 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

Points per level
Latch
Play Latch yourselfCooperative 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

Step 5 Preview 40.7/100Rank 28/50

Points per level
Newton's Mirror
Same task instructionsPhysics, perspective and recursive reflections, tested together. Explore the test ↗
Callstack Apex 32.7/100 Rank 23/53

Step 5 Preview 37.9/100 Rank 19/53

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
| Category | Callstack Apex | Step 5 Preview |
|---|---|---|
| State at 600 ms | 20.0/25 | 20.0/25 |
| Camera and placement | 13.0/15 | 13.5/15 |
| Reflections | 10.0/120 | 20.0/120 |
| Materials and scale | 7.0/10 | 7.0/10 |
| Floor-damage reward | 0.0/10 | 0.0/10 |
Benchmark task
A spinning metal sphere and paint in a mirrored room test physical reasoning, perspective and consistency across repeated reflections.
Selfie
Different task instructionsVisual construction and creative interpretation.
Callstack Apex 74.1/100 Rank 13/53

Step 5 Preview 52.0/100 Rank 40/53

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
| Category | Callstack Apex | Step 5 Preview |
|---|---|---|
| Task & likeness | 8.5/10 | 6.0/10 |
| Anatomy & construction | 7.0/10 | 4.5/10 |
| Space & placement | 7.5/10 | 5.0/10 |
| Light & reflections | 7.5/10 | 5.5/10 |
| Purposeful detail | 7.5/10 | 5.5/10 |
| Artistry & character | 7.0/10 | 5.0/10 |
| Materials & effects | 7.0/10 | 6.0/10 |
| Visual integrity | 7.5/10 | 5.0/10 |
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 instructionsPrecise visual recall and portrait reconstruction.
Callstack Apex 45.9/100 Rank 19/53

Step 5 Preview 38.4/100 Rank 27/53

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
| Category | Callstack Apex | Step 5 Preview |
|---|---|---|
| Task & likeness | 3.5/10 | 4.0/10 |
| Anatomy & construction | 4.0/10 | 4.0/10 |
| Space & placement | 4.5/10 | 4.0/10 |
| Light & reflections | 6.0/10 | 3.5/10 |
| Purposeful detail | 4.5/10 | 4.0/10 |
| Artistry & character | 4.5/10 | 4.0/10 |
| Materials & effects | 5.5/10 | 3.0/10 |
| Visual integrity | 5.5/10 | 3.5/10 |
Benchmark task
An SVG interpretation of the Mona Lisa tests facial likeness, anatomy and the composition of a familiar portrait.
Beach
Different task instructionsCreativity, perspective and visual beauty.
Callstack Apex 61.4/100 Rank 26/53

Step 5 Preview 33.5/100 Rank 48/53

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
| Category | Callstack Apex | Step 5 Preview |
|---|---|---|
| Task & likeness | 8.5/10 | 4.0/10 |
| Anatomy & construction | 4.5/10 | 3.0/10 |
| Space & placement | 7.5/10 | 3.5/10 |
| Light & reflections | 5.5/10 | 3.5/10 |
| Purposeful detail | 7.5/10 | 3.0/10 |
| Artistry & character | 6.5/10 | 3.0/10 |
| Materials & effects | 7.0/10 | 3.5/10 |
| Visual integrity | 4.0/10 | 4.0/10 |
Benchmark task
A busy shoreline at sunset tests scene composition, animal forms and the relationship between light and water reflections.
Ground Truth
Same task instructionsPhysical reasoning, light and visual reconstruction.
Callstack Apex 44.1/100 Rank 18/53

Step 5 Preview 33.3/100 Rank 35/53

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 versionsScene composition and interacting subjects.
Callstack Apex 76.8/100 Rank 4/53

Step 5 Preview 37.8/100 Rank 44/53

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
| Category | Callstack Apex | Step 5 Preview |
|---|---|---|
| Task & likeness | 9.0/10 | 4.0/10 |
| Anatomy & construction | 7.0/10 | 4.5/10 |
| Space & placement | 7.5/10 | 3.0/10 |
| Light & reflections | 7.0/10 | 3.5/10 |
| Purposeful detail | 8.5/10 | 4.0/10 |
| Artistry & character | 7.5/10 | 3.5/10 |
| Materials & effects | 7.0/10 | 3.0/10 |
| Visual integrity | 8.0/10 | 3.0/10 |
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 instructionsSubject construction and spatial relationships.
Callstack Apex 76.3/100 Rank 8/53
Step 5 Preview 30.0/100 Rank 51/53

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
| Category | Callstack Apex | Step 5 Preview |
|---|---|---|
| Task & likeness | 8.5/10 | 3.0/10 |
| Anatomy & construction | 6.5/10 | 3.0/10 |
| Space & placement | 7.5/10 | 3.0/10 |
| Light & reflections | 7.5/10 | 3.0/10 |
| Purposeful detail | 8.0/10 | 4.0/10 |
| Artistry & character | 8.0/10 | 3.0/10 |
| Materials & effects | 7.5/10 | 3.0/10 |
| Visual integrity | 8.0/10 | 2.0/10 |
Benchmark task
A cycling pelican tests anatomy, riding posture and contact between the bird and the bicycle.