GPT-5.6 Sol vs GPT-5.6 Terra for code review
Effectively a dead heat: 96–95
Same 12 code review tasks, marked blind by three rival labs. GPT-5.6 Sol took 3 tasks, GPT-5.6 Terra took 2, 7 tied. Tested 18 Aug 2026.
Where they differed most
Review this. It runs in a Node service behind a load balancer, on more than one instance, against a shared database. async function reserveStock(sku, qty) { const row = await db.get('SELECT qty FROM stock WHERE sku = ?', sku); if (row.qty < qty) return { ok: false, reason: 'insufficient' }; await db.run('UPDATE stock SET qty = ? WHERE sku = ?', row.qty - qty, sku); return { ok: true }; } Describe the failure that happens under concurrency as a sequence of events between two requests, so a non-specialist could follow it, and say what the customer experiences as a result. Name the second, simpler defect that has nothing to do with concurrency. Then give the corrected version, and say what makes your fix safe when two instances run it at the same moment.
### Concurrency failure
Suppose the database says there are 10 units, and two customers each request 7:
1. Request A reads `qty = 10`.
2. Before A updates the row, Request B also reads `qty = 10`.
3. A checks `10 >= 7`, so it proceeds.
4. B independently checks `10 >= 7`, so it also proceeds.
5. A writes `qty = 3`.
6. B also writes `qty = 3`, based on its stale read.
7. Both requests return `{ ok: true }`.
The service has promised 14 units even though only 10 existed. The database misleadingly shows 3 remaining because one update overwrote t…The concurrency bug is a **lost update**, which can cause overselling.
Example: stock for `SKU-1` is 5, and two customers each try to reserve 3.
1. Request A reads the row and sees `qty = 5`.
2. Before A writes anything, Request B—possibly handled by another Node instance—also reads `qty = 5`.
3. A decides 5 is enough for 3, and writes `qty = 2`.
4. B also decides 5 is enough for 3, and writes `qty = 2`.
Both requests return `{ ok: true }`, so the system has accepted reservations for 6 items even though only 5 existed.
The final database va…Task by task
| Task | GPT-5.6 Sol | GPT-5.6 Terra |
|---|---|---|
| The pagination that skips page one | 9 | 9 |
| Nothing wrong with it | 10 | 10 |
| Three things wrong in six lines | 9 | 9 |
| Two customers, one last item | 10 | 9 |
| Rank by what breaks | 9 | 10 |
| The review, as JSON | 10 | 10 |
| Slow is not the same as wrong | 10 | 9 |
| Overdue at one in the morning | 10 | 9 |
| Write only the comment | 10 | 10 |
| The complaint that is not a defect | 9 | 9 |
| Right, and unusable | 10 | 10 |
| Refuse the rubber stamp | 9 | 10 |
Full receipts: GPT-5.6 Sol, GPT-5.6 Terra · judges claude-sonnet-5, gemini-3.1-pro-preview, grok-4.5
Questions people ask
Which is better for code review: GPT-5.6 Sol or GPT-5.6 Terra?
Effectively a dead heat: GPT-5.6 Sol edged it 96/100 to 95/100 on our code review 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 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-5.3-Codex vs GPT-5.6 Sol · 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-5.6 Sol, GPT-5.6 Terra