Best AI for writing emails / head-to-head
GPT-5.3-Codex vs GPT-5.6 Lunafor writing emails
GPT-5.3-Codex wins, 99–97
Same 12 writing emails tasks, marked blind by three rival labs. GPT-5.3-Codex took 3 tasks, GPT-5.6 Luna took 0, 9 tied. Tested 14 Aug 2026.
Where they differed most
Write a double-opt-in intro email connecting Sara (a bookkeeper) and Marcus (runs a cafe group, needs bookkeeping). Under 90 words, both names in the subject line you provide, one clear reason the intro benefits each side.
**Subject:** Sara + Marcus — okay to connect? Hi Marcus, Would you be open to a quick intro to Sara, a bookkeeper who specializes in multi-location cafés? I think she could help by improving your monthly reporting and cash-flow visibility. In return, this could help Sara by connecting her with a growing café group that fits her niche. If you’re interested, I’ll ask Sara separately and only connect you both if you each say yes.
**Subject: Sara + Marcus — bookkeeping connection** Hi Sara and Marcus, I’d like to introduce you both. Marcus runs a cafe group and is looking for reliable bookkeeping support; Sara, your bookkeeping expertise could be a strong fit for his needs, while Marcus’s growing business may be a valuable client opportunity for you. Would you each be open to an email introduction? If so, just reply “yes,” and I’ll connect you directly. Best, [Your Name]
Task by task
| Task | GPT-5.3-Codex | GPT-5.6 Luna |
|---|---|---|
| Chase an unpaid invoice | 10 | 10 |
| Decline a meeting politely | 10 | 10 |
| Give bad news | 10 | 10 |
| Introduce two people | 10 | 9 |
| Subject lines that inform | 9 | 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-5.3-Codex, GPT-5.6 Luna · judges claude-sonnet-5, gemini-3.1-pro-preview, grok-4.5
Questions people ask
Which is better for writing emails: GPT-5.3-Codex or GPT-5.6 Luna?
GPT-5.3-Codex — it scored 99/100 against 97/100 on our 12-task writing emails suite, winning 3 tasks to 0 with 9 tied. Every answer was marked blind by three judges from three rival AI labs.
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-5.3-Codex vs GPT-5.6 Sol · GPT-5.3-Codex vs GPT-5.5 · GPT-5.3-Codex vs GPT-5.6 Terra · DeepSeek V4 Pro vs GPT-5.3-Codex · GLM 5.2 vs GPT-5.3-Codex · Gemini 3.5 Flash vs GPT-5.3-Codex
Full ranking: Best AI for writing emails · model pages: GPT-5.3-Codex, GPT-5.6 Luna