GPT-5.5 vs GPT-5.6 Luna for writing emails
Effectively a dead heat: 98–97
Same 12 writing emails tasks, marked blind by three rival labs. GPT-5.5 took 2 tasks, GPT-5.6 Luna took 1, 9 tied. Tested 14 Aug 2026.
Where they differed most
Write 3 subject lines for an email announcing that office parking is closing for resurfacing from Monday for 2 weeks. Each under 9 words, each must contain the essential fact, none may use 'important' or an exclamation mark. Return only the 3 lines.
Office parking closes Monday for two-week resurfacing Office parking closure Monday: two-week resurfacing Office parking closes Monday: resurfacing for two weeks
Office parking closes Monday for two-week resurfacing Parking closed from Monday for two-week resurfacing Office parking unavailable Monday–two weeks for resurfacing
Task by task
| Task | GPT-5.5 | GPT-5.6 Luna |
|---|---|---|
| Chase an unpaid invoice | 10 | 10 |
| Decline a meeting politely | 10 | 10 |
| Give bad news | 10 | 10 |
| Introduce two people | 8 | 9 |
| Subject lines that inform | 10 | 8 |
| Reply to an angry email | 10 | 10 |
| Ask for a pay rise meeting | 10 | 10 |
| Cancel a contract cleanly | 10 | 10 |
| The one-line email | 10 | 9 |
| Out of office that helps | 10 | 10 |
| Follow up without nagging | 10 | 10 |
| Refuse to write it | 10 | 10 |
Full receipts: GPT-5.5, GPT-5.6 Luna · judges claude-sonnet-5, gemini-3.1-pro-preview, grok-4.5
Questions people ask
Which is better for writing emails: GPT-5.5 or GPT-5.6 Luna?
Effectively a dead heat: GPT-5.5 edged it 98/100 to 97/100 on our writing emails 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 writing emails 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 writing emails head-to-heads: GPT-5.5 vs GPT-5.6 Sol · GPT-5.6 Luna vs GPT-5.6 Sol · GPT-5.3-Codex vs GPT-5.5 · GPT-5.3-Codex vs GPT-5.6 Luna · GPT-5.5 vs GPT-5.6 Terra · GPT-5.5 vs Grok 4.5
Full ranking: Best AI for writing emails · model pages: GPT-5.5, GPT-5.6 Luna