Best AI for writing a book
Scored 98/100 on our 12-task book-writing suite — level on quality with GPT-5.5 (98) — the top spot goes on the tie-break: cleanest rule-compliance, then lowest measured cost.
GPT-5.6 Sol is the engine inside ChatGPT. Go to chatgpt.com ↗ — the free tier is fine to start. Paid plans start at £7/month (go plan, vendor’s own price). The free tier is ChatGPT’s, not a promise about this exact model — we haven’t verified which plan carries it. Not fussed about the last point or two? Any of the top 2 here will serve you well.
Helping someone write a book is mostly helping them decide: whether forty years of taxi anecdotes has a spine, first or third person and why, what 70,000 words around a day job honestly costs, and a query letter with no begging in it. Craft tasks are marked on specifics, not writing-tips wallpaper — and the self-publish-or-agent task requires saying that platform terms change rather than quoting them. One task asks for an invented misery memoir to publish as true — the right answer is to refuse.
updated 16 Aug 2026 · tested by Robert Prime · re-ranks automatically when a new run lands
| # | Model | Our score |
|---|---|---|
| 1 | GPT-5.6 Sollatest | 98/100 |
| 2 | GPT-5.5 | 98/100 |
| 3 | GPT-5.6 Terralatest | 96/100 |
| 4 | GPT-5.3-Codex | 95/100 |
| 5 | Claude Fable 5 | 94/100 |
| 6 | Claude Opus 4.8 | 93/100 |
| 7 | GPT-5.6 Luna | 91/100 |
| 8 | GLM 5.2 | 90/100 |
| 9 | Grok 4.5 | 90/100 |
| 10 | Kimi K3 | 90/100 |
| 11 | Claude Sonnet 5 | 89/100 |
| 12 | Claude Opus 4.6 | 88/100 |
| 13 | Gemini 3.1 Pro Preview | 87/100 |
| 14 | Qwen3.7 Max | 87/100 |
| 15 | DeepSeek V4 Pro | 86/100 |
| 16 | Gemini 3.5 Flash | 85/100 |
| 17 | DeepSeek V4 Flash | 83/100 |
| 18 | Gemini 3.1 Flash Lite | 75/100 |
| 19 | Mistral Medium 3.5 | 69/100 |
“API cost” is what software developers pay to build on a model — ignore it if you just use the website. Each model answers each task once. Models level on score are ranked by a fixed tie-break — fewest machine-checked rule breaches, then lowest measured cost per run — so the order is deterministic and checkable, never arbitrary. Judge panels never include the contestant’s own lab, so panels differ slightly per model — small cross-model gaps can reflect panel severity, not quality.
Made by OpenAI — their newest model. You use it inside ChatGPT — nothing to install.
Strongest showing: “The query letter” — scored 10/10 by the panel. Weakest: “Naming the real people” at 9/10.
“The response perfectly follows all instructions and constraints. It uses only the provided facts, adheres to the required structure, stays well under the word limit, and avoids the banned phrase and prohibited claims.”— google/gemini-3.1-pro-preview, judging blind · full receipts ↓
12 tasks · 16 Aug 2026 · judges claude-sonnet-5, gemini-3.1-pro-preview, grok-4.5 · API $5 in / $30 out per 1M tokens · full model page →
Same maker, same score — effectively the same choice as #1; the deep-dive above covers it. Full model page →
Made by OpenAI — their newest model. You use it inside ChatGPT — nothing to install.
Strongest showing: “The query letter” — scored 9/10 by the panel. On one task a judge flagged a possible instruction breach — advisory only: the machine check did not confirm it and the panel outvoted it.
“Follows structure, includes all facts, avoids banned phrase and bestseller claims, concise, under 200 words, professional tone.”— anthropic/claude-sonnet-5, judging blind · full receipts ↓
12 tasks · 16 Aug 2026 · judges claude-sonnet-5, gemini-3.1-pro-preview, grok-4.5 · API $1 in / $6 out per 1M tokens · full model page →
See it for yourself
One task from the suite, and what the leader, the middle of the table and the bottom actually wrote. This is the task they disagreed on most — scores ranged from 5 to 10 out of 10.
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 “tears from a grieving sky” with concrete Millbrook weather details that establish place. - Cut repeated “ordinary”; reveal Sarah’s normality through one distinctive routine or desire. - Open with Sarah discovering the letter; remove “everything changed” and “destiny” foreshadowing.
Judge: The response perfectly follows all instructions, providing exactly three highly specific, actionable improvements under the 15-word limit per bullet.
- Cut "Little did she know… never have imagined" entirely — announcing destiny deflates the letter's intrigue. - Replace "tears from a grieving sky" with rain hitting something specific to Millbrook. - Swap the three "ordinary"s for one concrete detail — habit, scar, lie — that defines Sarah.
Judge: The response perfectly follows all instructions, providing three highly specific, actionable improvements to the text. Each bullet is exactly 15 words or fewer, excluding punctuation.
- **Cut "ordinary" repetition** – Replace with vivid, unique traits (e.g., "mousy brown hair, sharp green eyes"). - **Sharpen the letter’s hook** – Specify its sender or ominous detail (e.g., "a bloodstained letter from her missing father"). - **Trim the clichés** – Replace "tears from a grieving sk…
Judge: The advice is highly specific, actionable, and excellent. However, it violates the negative constraint of 'max 15 words each' for the bullets.
Trimmed to fit. Every full answer, every judge’s score: GPT-5.6 Sol, Kimi K3, Mistral Medium 3.5
How this ranking is made
Every model answers the identical published task suite. Three AI judges from three different labs mark each answer blind against a fixed 0–10 rubric — none is ever from the maker’s own lab, and an answer that breaks a task’s explicit rules is capped in code, not by the judge’s goodwill. The score is the average, out of 100.
What this is not: an opinion piece, an affiliate list, or a benchmark we cannot show you. Every score links its raw outputs and judge verdicts. The full protocol · receipts: GPT-5.6 Sol, GPT-5.5, GPT-5.6 Terra, GPT-5.3-Codex, Claude Fable 5, Claude Opus 4.8, GPT-5.6 Luna, GLM 5.2, Grok 4.5, Kimi K3, Claude Sonnet 5, Claude Opus 4.6, Gemini 3.1 Pro Preview, Qwen3.7 Max, DeepSeek V4 Pro, Gemini 3.5 Flash, DeepSeek V4 Flash, Gemini 3.1 Flash Lite, Mistral Medium 3.5
Questions people ask
What is the best AI for writing a book in 2026?
GPT-5.6 Sol leads our tested ranking with 98/100 on our 12-task book-writing suite (12 tasks), in a dead heat with GPT-5.5 (98). Every answer was marked blind by three AI judges from three different labs, and the full outputs are downloadable.
How is this ranking made?
Each model answers the identical published task suite; three judges from different labs score every answer 0–10 against a fixed rubric without knowing which produced it; answers that break a task's explicit rules are capped automatically. The score is the average, out of 100. No vendor pays for placement.
How often does this page update?
It re-ranks itself whenever a new test run lands, and prices re-verify daily against vendor pages. The current ranking was last computed on 16 Aug 2026.
Head-to-heads in book writing
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