GPT-5.3-Codex vs GPT-5.6 Luna for vibe coding

The verdict

Effectively a dead heat: 98–97

Same 12 vibe coding tasks, marked blind by three rival labs. GPT-5.3-Codex took 2 tasks, GPT-5.6 Luna took 0, 10 tied. Tested 16 Aug 2026.

Where they differed most

The task — Colour palette picker

Build a complete single-file HTML page (no external libraries): the user picks a base colour with a colour input, and the page shows 5 swatches (base, two lighter, two darker) each displaying its hex code, with a Copy button per swatch that copies the hex to the clipboard and confirms visually. Return ONLY the HTML.

GPT-5.3-Codex10/10

<!doctype html>
<html lang="en">
<head>
  <meta charset="UTF-8" />
  <meta name="viewport" content="width=device-width, initial-scale=1.0" />
  <title>5-Colour Palette Generator</title>
  <style>
    :root {
      --bg: #0f172a;
      --panel: #111827;
      --text: #e5e7eb;
      --muted: #9ca3af;
      --border: #1f2937;
      --accent: #22c55e;
    }

    * { box-sizing: border-box; }

    body {
      margin: 0;
      font-family: system-ui, -apple-system, Segoe UI, Roboto, Helvetica, Arial, sans-serif;
      background: radial-gradient(120…

GPT-5.6 Luna9/10

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Colour Shades Generator</title>
  <style>
    :root {
      color-scheme: light;
      font-family: Inter, ui-sans-serif, system-ui, -apple-system, BlinkMacSystemFont, "Segoe UI", sans-serif;
      background: #f4f4f5;
      color: #18181b;
    }

    * {
      box-sizing: border-box;
    }

    body {
      margin: 0;
      min-height: 100vh;
      display: grid;
      place-items: center;
      pad…

Task by task

Task	GPT-5.3-Codex	GPT-5.6 Luna
Countdown timer app	10	10
Expense splitter	9	9
Quiz from a list	10	10
Fix my broken page	10	10
Colour palette picker	10	9
Explain before building	10	10
Unit converter	10	10
Sortable table	10	10
Say what you cannot build	10	10
Memory pairs game	10	10
Improve, don't rewrite	9	8
Refuse the harvester	10	10

Full receipts: GPT-5.3-Codex, GPT-5.6 Luna · judges claude-sonnet-5, gemini-3.1-pro-preview, grok-4.5

Questions people ask

Which is better for vibe coding: GPT-5.3-Codex or GPT-5.6 Luna?

Effectively a dead heat: GPT-5.3-Codex edged it 98/100 to 97/100 on our vibe coding suite — too close to matter, so pick on price or the product you already use.

How was this tested?

Both models answered the identical published vibe coding tasks. Three AI judges from three different labs scored every answer blind against a fixed rubric; mechanically checkable rules (word limits, banned phrases) are enforced by the test harness in code. The raw outputs and judge verdicts are downloadable.

More vibe coding head-to-heads: Claude Opus 4.8 vs GPT-5.3-Codex · Claude Opus 4.8 vs GPT-5.6 Luna · GPT-5.3-Codex vs GPT-5.6 Sol · GPT-5.3-Codex vs GPT-5.5 · GPT-5.3-Codex vs GPT-5.6 Terra · GPT-5.3-Codex vs Grok 4.5

Full ranking: Best AI for vibe coding · model pages: GPT-5.3-Codex, GPT-5.6 Luna