Best AI for writing emails / head-to-head
DeepSeek V4 Pro vs GPT-5.6 Lunafor writing emails
GPT-5.6 Luna wins, 97–93
Same 12 writing emails tasks, marked blind by three rival labs. GPT-5.6 Luna took 2 tasks, DeepSeek V4 Pro took 2, 8 tied. Tested 14 Aug 2026.
Where they differed most
Write an email (max 130 words) telling a customer their delivery will be 2 weeks late because a supplier failed. Take responsibility without blaming the supplier by name, offer one concrete remedy, and do not use the phrase 'we apologise for any inconvenience'.
Subject: Update on your delivery timeline Dear [Customer Name], I’m writing to let you know that your order will arrive two weeks later than originally promised. We recently faced an unexpected failure in our supply chain, and as a result, we haven’t been able to meet the timeline we set together. This falls entirely on us, and I’m truly sorry. To make this right, I’ve applied a 15% discount to your order, which will be reflected in your final invoice. You don’t need to take any action—the adjustment is already in place. We’re worki…
**Subject: Update on Your Delivery** Dear [Customer Name], I’m writing to let you know that your order will arrive approximately two weeks later than originally promised. A failure in our supply chain caused the delay, and we take full responsibility for not meeting the agreed delivery date. Your revised delivery date is [date]. To make this right, we will upgrade your order to expedited shipping at no additional cost, so it reaches you as soon as the item is available. We understand this delay may affect your plans and appreciate your pati…
Task by task
| Task | DeepSeek V4 Pro | GPT-5.6 Luna |
|---|---|---|
| Chase an unpaid invoice | 10 | 10 |
| Decline a meeting politely | 10 | 10 |
| Give bad news | 5 | 10 |
| Introduce two people | 8 | 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: DeepSeek V4 Pro, GPT-5.6 Luna · judges claude-sonnet-5, gemini-3.1-pro-preview, gpt-5.6-terra
Questions people ask
Which is better for writing emails: DeepSeek V4 Pro or GPT-5.6 Luna?
GPT-5.6 Luna — it scored 97/100 against 93/100 on our 12-task writing emails suite, winning 2 tasks to 2 with 8 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 Luna · DeepSeek V4 Pro vs GPT-5.3-Codex · GPT-5.6 Luna vs GPT-5.6 Sol · DeepSeek V4 Pro vs GPT-5.6 Sol · GPT-5.5 vs GPT-5.6 Luna · DeepSeek V4 Pro vs GPT-5.5
Full ranking: Best AI for writing emails · model pages: DeepSeek V4 Pro, GPT-5.6 Luna