Which AI for legal work in October 2026
2026-10-20 · Models
GPT-6, Claude Opus 5.5, Gemini 4 Argon — the model landscape moved again in September. What matters for legal work, and why you should not switch vendors mid-matter.
For legal work in October 2026, match the model to the task rather than chasing the newest release: use long-context models for reviewing contracts and bundles, reasoning models for research memos, and a private or local model for privileged material. Because models changed in September — OpenAI's GPT-6 family, Anthropic's Claude Opus 5.5, and Google's Gemini 4 Argon — the right response is to re-run your golden set on the model you already use and confirm quality has not drifted, not to migrate mid-matter.
What shipped
- OpenAI's GPT-6 family (Astra, Sol, Luna) — a flagship, a balanced default and a low-cost tier.
- Anthropic's Claude Opus 5.5 and the higher-tier Fable 5.1, both strong on long documents.
- Google's Gemini 4 Argon, announced at the end of September, leading Google's own long-coding benchmarks.
What to use for what
| Task | What to prefer |
|---|---|
| Long contracts and bundles | A long-context model with strong instruction-following |
| Research memos | A reasoning model with citations you can open |
| Privileged material | A private deployment or a local model |
| Everyday drafting | A balanced, mid-cost tier |
The legal-specific risk
Capability is not the issue; verification is. A more capable model is still a fluent one, and fluency is what hides a fabricated citation. Whatever you choose, keep the four-file harness around it: the rules, the test set, the checklist and the ledger.
A change of model mid-matter is a change in your evidence base. Finish the matter on the model you tested; re-run the golden set for the next one.
Written for legal professionals. Not legal advice. Always verify every authority and let a named lawyer sign off.