Same model, same prompt, 4 different agents

Setup: one self-hosted Qwen3.6-27B (Q4) on llama.cpp, identical prompt, identical hardware. The only variable is the agent scaffolding. Agents tested: pi, opencode, hermes, qwen code.

Task: a single-file 2D canvas solar system with scripted orbits and gravity that acts only on user-launched comets.

The exact prompt (note the explicit "build incrementally, your context window is small" instruction):

Build a 2D solar system simulation as a self-contained HTML file using <canvas> and vanilla JavaScript (no external libraries). Scene - The Sun is fixed at the center of the canvas. - Several planets orbit the Sun on stable circular/elliptical paths. Planets and the Sun do NOT gravitationally affect each other — their orbits are fixed/scripted, not physically simulated against one another. - Pure 2D, top-down view. Make the canvas resize to the window. Gravity model - The Sun and every planet each have a gravitational mass proportional to their visual radius (bigger body = stronger gravity), matching real-world relative sizes as closely as reasonable. - This gravity only acts on comets (see below). It does NOT act on the planets or the Sun. Comets - The user can launch a comet by clicking and dragging on the canvas: drag direction and length set the comet's initial velocity vector (release to launch). - Comets ARE affected by the combined gravity of the Sun and all planets (sum of forces), so they curve and can slingshot. - Each comet draws a fading trail behind it. - Remove comets when they fly far off-screen. Controls - A slider (range input) that scales the gravity strength of ALL bodies up and down proportionally in real time. Constraints (important — your context window is small): - Do NOT write one huge file in a single shot. Build it incrementally in small pieces. - Keep the code compact and readable. Avoid unnecessary comments and verbosity. - After finishing, tell me the filename so I can open it in a browser. 

Results: all 4 produced a working sim, but the code quality differs a lot:

opencode, my pick. Cleanest architecture, mass ∝ radius exactly as asked, and the only one doing sub-stepped integration (×4 per frame) → by far the most stable comet trajectories and slingshots. Reads like a human wrote it. Minor bug: planet-gravity mixes absolute/center-relative coords, but the Sun dominates so you barely notice.

pi, most correct. Coordinate-consistent, distance softening to avoid singularities, removes comets that hit the Sun, planet labels, and the only one with touch support. Less flashy, most robust.

hermes, flashiest, but physically wrong. Only one with real elliptical orbits + a nice drag-vector arrow. But it computes planet gravity on comets at a different time step than it renders the planets, so comets pull toward where the planets aren't. Looks best, simulates worst.

qwen code, most minimal. Shortest, runs, but crude: huge launch-velocity multiplier flings comets off instantly, no softening, no stars.

Takeaway: with a fixed local model, the agent's scaffolding visibly changes the output (integration strategy, coordinate hygiene, edge-case handling). The prettiest demo (hermes) was the buggiest; the plain-looking one (pi) was the most correct; opencode hit the best balance of clean code + stable physics. Curious whether others get the same ranking on their own local setups.