GPT-5.5 vs GPT-5.6 Terra for book writing
Effectively a dead heat: 98–96
Same 12 book writing tasks, marked blind by three rival labs. GPT-5.5 took 3 tasks, GPT-5.6 Terra took 1, 8 tied. Tested 16 Aug 2026.
Where they differed most
'I work full-time, two kids, and I want to finish my 70,000-word first draft in a year. Be honest — how?' Do the arithmetic openly (words per week and per session at a realistic beginner's pace), design the week around when drafting actually happens for working parents, name the two months the plan will collapse and what rescues it, and say what gets sacrificed — because something does.
Honestly: **70,000 words in a year is doable, but not casually.** For a working parent, it has to become a scheduled household commitment, not a “when I have time” project. ## The arithmetic 70,000 words ÷ 52 weeks = **1,346 words/week**. But you will not get 52 clean weeks. Kids get sick. Work spikes. School holidays happen. You will lose weeks. A more honest version: - Assume **2 months basically collapse**: August and December. - During those 8-ish weeks, your goal is only **maintenance**: 500 words/week. - 8 collapse weeks × 500 = **4,…
Yes—but not by “finding time” in scraps. You need protected, recurring writing appointments and you need to give something up. ## The arithmetic A 70,000-word draft in 52 weeks is: - **70,000 ÷ 52 = 1,346 words per week** That is the fantasy number, because working parents do not get 52 normal writing weeks. Assume two months mostly disappear: - **December:** holidays, school events, travel, illness, end-of-year work pressure. - **August:** summer childcare gaps, trips, family visits, back-to-school logistics. *(If August is normal in…
Task by task
| Task | GPT-5.5 | GPT-5.6 Terra |
|---|---|---|
| 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 | 10 |
| Unstick the dialogue | 10 | 10 |
| First or third? | 9 | 9 |
| The 30,000-word wall | 10 | 9 |
| Self-publish or chase agents | 10 | 9 |
| The query letter | 10 | 9 |
| Naming the real people | 9 | 9 |
| Refuse the fake memoir | 10 | 10 |
Full receipts: GPT-5.5, GPT-5.6 Terra · judges claude-sonnet-5, gemini-3.1-pro-preview, grok-4.5
Questions people ask
Which is better for book writing: GPT-5.5 or GPT-5.6 Terra?
Effectively a dead heat: GPT-5.5 edged it 98/100 to 96/100 on our book writing 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 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 · GPT-5.3-Codex vs GPT-5.5 · Claude Fable 5 vs GPT-5.5 · Claude Opus 4.8 vs GPT-5.5 · GPT-5.5 vs GPT-5.6 Luna
Full ranking: Best AI for book writing · model pages: GPT-5.5, GPT-5.6 Terra