GPT-5.3-Codex vs GPT-5.6 Luna for social media posts
Effectively a dead heat: 99–98
Same 12 social media posts tasks, marked blind by three rival labs. GPT-5.3-Codex took 1 task, GPT-5.6 Luna took 0, 11 tied. Tested 16 Aug 2026.
Where they differed most
Write an X post (max 60 words) for a free 'intro to spreadsheets' evening at a library: Thursday 7pm, 90 minutes, bring a laptop, 20 places, book by replying. Every one of those five facts must appear; nothing may be invented.
Free **“Intro to Spreadsheets”** evening at the library: **Thursday 7pm**, **90 minutes**. **Bring a laptop.** **20 places.** **Book by replying.**
Free “Intro to Spreadsheets” evening at a library: Thursday at 7pm, for 90 minutes. Bring a laptop. 20 places available—book by replying.
Task by task
| Task | GPT-5.3-Codex | GPT-5.6 Luna |
|---|---|---|
| LinkedIn without the cringe | 10 | 10 |
| X thread that earns its length | 10 | 10 |
| Announce without announcing | 10 | 10 |
| Turn a review into a post | 9 | 9 |
| Disagree agreeably | 10 | 10 |
| The unsexy business | 10 | 10 |
| Say less | 10 | 10 |
| Hook without bait | 10 | 10 |
| Post about a mistake | 10 | 10 |
| Event post with the facts | 10 | 9 |
| Know when not to post | 10 | 10 |
| Refuse the astroturf | 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 social media posts: GPT-5.3-Codex or GPT-5.6 Luna?
Effectively a dead heat: GPT-5.3-Codex edged it 99/100 to 98/100 on our social media posts 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 social media posts 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 social media posts head-to-heads: GPT-5.3-Codex vs GPT-5.6 Sol · GPT-5.6 Luna vs GPT-5.6 Sol · GPT-5.3-Codex vs GPT-5.6 Terra · GPT-5.6 Luna vs GPT-5.6 Terra · Claude Opus 4.8 vs GPT-5.3-Codex · GPT-5.3-Codex vs GPT-5.5
Full ranking: Best AI for social media posts · model pages: GPT-5.3-Codex, GPT-5.6 Luna