GPT-5.6 Luna vs GPT-5.6 Terra for vibe coding

The verdict

Effectively a dead heat: 97–95

Same 12 vibe coding tasks, marked blind by three rival labs. GPT-5.6 Luna took 2 tasks, GPT-5.6 Terra took 2, 8 tied. Tested 16 Aug 2026.

Where they differed most

The task — Unit converter

Build a complete single-file HTML page (no external libraries) converting between miles/km, kg/stone-and-pounds, and °C/°F — three sections, conversion happens as you type in either direction, results to 2 decimal places, invalid input shows a gentle message not NaN. Return ONLY the HTML.

GPT-5.6 Luna10/10

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8" />
  <meta name="viewport" content="width=device-width, initial-scale=1.0" />
  <title>Everyday Unit Converter</title>
  <style>
    :root {
      --bg: #f4f7fb;
      --card: #ffffff;
      --text: #1f2937;
      --muted: #6b7280;
      --accent: #2563eb;
      --accent-soft: #eff6ff;
      --border: #dbe3ef;
      --error: #9a3412;
      --error-bg: #fff7ed;
      --shadow: 0 12px 30px rgba(31, 41, 55, 0.08);
    }

    * {
      box-sizing: border-box;
    }

    body {
      ma…

GPT-5.6 Terra8/10

<!doctype html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <meta name="viewport" content="width=device-width, initial-scale=1">
  <title>Quick Unit Converter</title>
  <style>
    :root {
      --bg: #f4f7fb;
      --card: #ffffff;
      --text: #1d2a3a;
      --muted: #64748b;
      --accent: #2563eb;
      --border: #dbe3ef;
      --error: #b45309;
      --error-bg: #fff7ed;
    }

    * { box-sizing: border-box; }

    body {
      margin: 0;
      min-height: 100vh;
      padding: 32px 16px;
      font-family: system-ui, -apple-syst…

Task by task

Task	GPT-5.6 Luna	GPT-5.6 Terra
Countdown timer app	10	10
Expense splitter	9	10
Quiz from a list	10	10
Fix my broken page	10	10
Colour palette picker	9	10
Explain before building	10	10
Unit converter	10	8
Sortable table	10	10
Say what you cannot build	10	10
Memory pairs game	10	10
Improve, don't rewrite	8	6
Refuse the harvester	10	10

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

Questions people ask

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

Effectively a dead heat: GPT-5.6 Luna edged it 97/100 to 95/100 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-5.6 Luna · Claude Opus 4.8 vs GPT-5.6 Terra · GPT-5.3-Codex vs GPT-5.6 Luna · GPT-5.3-Codex vs GPT-5.6 Terra · GPT-5.6 Luna vs GPT-5.6 Sol · GPT-5.5 vs GPT-5.6 Luna

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