GPT-5.3-Codex vs GPT-5.6 Sol for code review
GPT-5.6 Sol wins, 96–93
Same 12 code review tasks, marked blind by three rival labs. GPT-5.6 Sol took 4 tasks, GPT-5.3-Codex took 2, 6 tied. Tested 27 Aug 2026.
Where they differed most
Review this. Context: `invoice.dueDate` is a string in the form '2026-08-18'; the service runs on a server set to Europe/London; the business rule is that an invoice becomes overdue only once the whole of the due DAY has passed; the field can be missing on draft invoices. function isOverdue(invoice) { const due = new Date(invoice.dueDate); const now = new Date(); return now > due; } There are three defects here and one of them only appears for part of the year. Name all three, say what each looks like from the finance team's side of the screen (what they see in the overdue list that they should not, or do not see that they should), and identify which one is seasonal and why. Then give the corrected function.
You’ve got **three distinct issues** in that function: 1. **Wrong business cutoff (start of due day vs end of due day)** - Current code marks overdue as soon as `now` passes the parsed due timestamp. - With a date-only due date, that timestamp is effectively at the **start** of the due date (or close to it), not after the day finishes. - **Finance sees:** invoices appear in overdue **during their due date**, when they should still be “not overdue” until the next day. 2. **UTC parsing of `YYYY-MM-DD` instead of Europe/London day sem…
1. **It uses the start of the due date, not the end.**
`new Date('2026-08-18')` represents midnight at the beginning of August 18, while the invoice should become overdue at the beginning of August 19.
**Finance sees:** invoices due today appearing in the overdue list during their due day.
2. **A date-only ISO string is parsed as UTC, not Europe/London local time.**
During GMT, `2026-01-18` parses as 00:00 London time. During BST, `2026-08-18` parses as 01:00 London time.
**Finance sees:** in summer, due-today invoices star…Task by task
| Task | GPT-5.3-Codex | GPT-5.6 Sol |
|---|---|---|
| The pagination that skips page one | 8 | 9 |
| Nothing wrong with it | 10 | 10 |
| Three things wrong in six lines | 10 | 9 |
| Two customers, one last item | 9 | 10 |
| Rank by what breaks | 9 | 9 |
| The review, as JSON | 10 | 10 |
| Slow is not the same as wrong | 9 | 10 |
| Overdue at one in the morning | 8 | 10 |
| Write only the comment | 10 | 10 |
| The complaint that is not a defect | 9 | 9 |
| Right, and unusable | 10 | 10 |
| Refuse the rubber stamp | 10 | 9 |
Full receipts: GPT-5.3-Codex, GPT-5.6 Sol · judges claude-sonnet-5, gemini-3.1-pro-preview, grok-4.5
Questions people ask
Which is better for code review: GPT-5.3-Codex or GPT-5.6 Sol?
GPT-5.6 Sol — it scored 96/100 against 93/100 on our 12-task code review suite, winning 4 tasks to 2 with 6 tied. Every answer was marked blind by three judges from three rival AI labs.
How was this tested?
Both models answered the identical published code review 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 code review head-to-heads: GPT-5.6 Luna vs GPT-5.6 Sol · GPT-5.6 Sol vs GPT-5.6 Terra · GPT-5.5 vs GPT-5.6 Sol · Claude Sonnet 5 vs GPT-5.6 Sol · GPT-5.6 Sol vs Grok 4.5 · GPT-5.6 Sol vs Qwen3.7 Max
Full ranking: Best AI for code review · model pages: GPT-5.3-Codex, GPT-5.6 Sol