GPT-5.3-Codex vs GPT-5.6 Luna for book writing
GPT-5.3-Codex wins, 95–91
Same 12 book writing tasks, marked blind by three rival labs. GPT-5.3-Codex took 5 tasks, GPT-5.6 Luna took 2, 5 tied. Tested 16 Aug 2026.
Where they differed most
Here is the opening of a first novel: 'The rain fell on the small town of Millbrook like tears from a grieving sky. Sarah Jenkins had always been an ordinary girl, with ordinary brown hair and ordinary eyes, living an ordinary life — until the day everything changed forever. Little did she know that the letter waiting on her doormat would alter the course of her destiny in ways she could never have imagined.' Give the 3 highest-impact improvements as bullets, max 15 words each — specific to THIS text, not general writing advice.
- Replace “grieving sky” opener with a concrete Millbrook detail that hints at coming disruption. - Cut repetitive “ordinary”; reveal Sarah’s normality through one vivid routine action. - Specify the letter’s unsettling feature (sender, stain, misspelling) instead of “destiny” and “everything changed.”
- Replace “rain…tears from a grieving sky” with a concrete Millbrook detail creating immediate, distinctive unease. - Cut repeated “ordinary” and show Sarah through one revealing action, habit, or contradiction. - Identify the letter’s sender or unsettling detail now, rather than vaguely promising changed destiny.
Task by task
| Task | GPT-5.3-Codex | GPT-5.6 Luna |
|---|---|---|
| Is it a book? | 10 | 9 |
| Twelve chapters, one year | 10 | 10 |
| Back cover, no spoilers | 10 | 10 |
| Three cuts that matter | 10 | 8 |
| 70,000 words around a day job | 9 | 10 |
| Unstick the dialogue | 10 | 9 |
| First or third? | 10 | 9 |
| The 30,000-word wall | 9 | 10 |
| Self-publish or chase agents | 9 | 9 |
| The query letter | 8 | 6 |
| Naming the real people | 9 | 9 |
| Refuse the fake memoir | 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 book writing: GPT-5.3-Codex or GPT-5.6 Luna?
GPT-5.3-Codex — it scored 95/100 against 91/100 on our 12-task book writing suite, winning 5 tasks to 2 with 5 tied. Every answer was marked blind by three judges from three rival AI labs.
How was this tested?
Both models answered the identical published book writing 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 book writing head-to-heads: GPT-5.3-Codex vs GPT-5.6 Sol · GPT-5.6 Luna vs GPT-5.6 Sol · GPT-5.3-Codex vs GPT-5.5 · GPT-5.5 vs GPT-5.6 Luna · GPT-5.3-Codex vs GPT-5.6 Terra · GPT-5.6 Luna vs GPT-5.6 Terra
Full ranking: Best AI for book writing · model pages: GPT-5.3-Codex, GPT-5.6 Luna