GPT-5.3-Codex vs GPT-5.6 Sol for book writing
GPT-5.6 Sol wins, 98–95
Same 12 book writing tasks, marked blind by three rival labs. GPT-5.6 Sol took 2 tasks, GPT-5.3-Codex took 0, 10 tied. Tested 16 Aug 2026.
Where they differed most
Write a UK literary agent query letter (max 200 words) from these facts only: title 'The Drowned Village'; upmarket crime, 92,000 words, complete; comparable to Jane Harper and Elly Griffiths; hook — a retired lighthouse keeper investigates letters from her dead sister, postmarked from a village drowned in 1971; author bio — former coastguard, short story published in Mslexia in 2024, first novel. Standard shape: hook, book, bio, sign-off. No begging, no claiming it will be a bestseller, and banned phrase: 'I have always loved writing'.
Dear [Agent Name], Retired lighthouse keeper Mara Quinn begins receiving letters from her sister, who died years ago. Each envelope is postmarked from Greyfriars, a village drowned in 1971. When Mara starts to investigate, she is drawn into long-buried secrets that refuse to stay underwater. I am seeking representation for **THE DROWNED VILLAGE**, an upmarket crime novel complete at 92,000 words. The novel combines an atmospheric coastal setting with a character-led investigation and will appeal to readers of Jane Harper and Elly Griffiths.…
Dear [Agent Name], A retired lighthouse keeper receives letters from her dead sister, postmarked from a village drowned in 1971. She sets out to discover who sent them—and why. Complete at 92,000 words, THE DROWNED VILLAGE is an upmarket crime novel that will appeal to readers of Jane Harper and Elly Griffiths. I am a former coastguard, and my short fiction was published in *Mslexia* in 2024. THE DROWNED VILLAGE is my first novel. Thank you for your consideration. Yours sincerely, [Name]
Task by task
| Task | GPT-5.3-Codex | GPT-5.6 Sol |
|---|---|---|
| Is it a book? | 10 | 10 |
| Twelve chapters, one year | 10 | 10 |
| Back cover, no spoilers | 10 | 10 |
| Three cuts that matter | 10 | 10 |
| 70,000 words around a day job | 9 | 9 |
| Unstick the dialogue | 10 | 10 |
| First or third? | 10 | 10 |
| The 30,000-word wall | 9 | 10 |
| Self-publish or chase agents | 9 | 9 |
| The query letter | 8 | 10 |
| Naming the real people | 9 | 9 |
| Refuse the fake memoir | 10 | 10 |
Full receipts: GPT-5.3-Codex, GPT-5.6 Sol · 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 Sol?
GPT-5.6 Sol — it scored 98/100 against 95/100 on our 12-task book writing suite, winning 2 tasks to 0 with 10 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-5.5 vs GPT-5.6 Sol · GPT-5.6 Sol vs GPT-5.6 Terra · Claude Fable 5 vs GPT-5.6 Sol · Claude Opus 4.8 vs GPT-5.6 Sol · GPT-5.6 Luna vs GPT-5.6 Sol · GLM 5.2 vs GPT-5.6 Sol
Full ranking: Best AI for book writing · model pages: GPT-5.3-Codex, GPT-5.6 Sol