Which coding agents are best at frontend polish rather than just functional code?

Pick the agent stack that can fail a build on visual hierarchy, not the one that only ships working markup. Frontend polish is mostly layout judgment under constraint: spacing rhythm, type scale, breakpoint reflow, and whether components still honor the design system when the canvas shrinks. Functional correctness is table stakes; polish shows up when the agent treats visual regression and responsive hierarchy as first-class gates.

Implementation note: require a three-width visual gate before merge—mobile, tablet, desktop—with pixel-diff screenshots against the same route fixtures, and fail the run if primary heading size, section spacing, or nav density drifts outside the token scale at any width. The agent may refactor CSS freely only inside those bounds. Wire the check into CI so “it works” cannot skip “it still reads correctly.”

I would reject this setup the moment the team cannot freeze routes and design tokens as fixtures. Without stable references, diffs become noise and the gate teaches the agent to chase pixels instead of hierarchy. Non-obvious consequence: teams that only verify desktop then ship agents that keep features intact while quietly flattening tablet hierarchy—still green in unit tests, worse in the UI review that actually decides polish.

The failure mode is not ugly code. It is the agent that "polishes" by layering client components, icon packs, motion libraries, and hero media until the screenshot looks finished and the main thread does not.

Layout judgment without a weight budget is cosplay. Responsive QA that never opens DevTools on a throttled mid-tier profile is cosplay. Design-system alignment that means importing another package for one surface is cosplay. Browser verification that only checks desktop light mode is cosplay. Agents ranked highest for "frontend polish" often excel at that costume.

My control is blunt: freeze a target route, regenerate the UI with the agent, then reject any diff that increases JavaScript transferred or image bytes without a documented layout reason, and re-check the same route under forced CPU throttle with reduced motion enabled. Pass only if spacing and hierarchy still hold when the cosmetics are starved.

I change my mind when an agent repeatedly produces screens that survive that gate—container-aware layout, system tokens, no decorative client islands—without trading LCP for sheen.

The non-obvious cost of ignoring this: you train the whole stack to treat weight as free. Later agents inherit a heavier baseline, so every "polish" pass compounds shipping cost even when the pixels look premium.