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# GPT 5.1 led CaseLaw v2 legal research at 73.4% in early 2026, 3.5 points ahead of GPT 4.1
- URL: https://aiadoption.org/data/caselaw-legal-research-accuracy/
- Published: 2026-10-19T16:06:17.000Z
- Updated: 2026-10-21T06:00:49.000Z
- Description: On CaseLaw v2, built from recent US and Canadian court decisions, GPT 5.1 scored 73.4% in early 2026 and GPT 4.1 69.9%. The other 13 of the top 15 models scored between 62.1% and 68.5%.
- Author: Manu
- Tags: Data, AI Benchmarks, Large Language Models, #chart, #ai-index-2026, #hub-benchmarks, #industry-legal

73.4%

#1

CaseLaw v2 accuracy of GPT 5.1, the highest of the top 15; GPT 4.1 scored 69.9%

GPT 5.173.4%

GPT 4.169.9%

Claude Opus 4.6 (Thinking)62.1%

## What the chart shows

CaseLaw v2 tests models on litigation and legal research tasks built from recent US and Canadian court decisions, mostly dated after models’ training cut-offs, so they must reason over the documents supplied. GPT 5.1 led at 73.4%, followed by GPT 4.1 (69.9%) and GPT 5 Mini (68.5%).

The rest of the top 15 fall between 62.1% and 66.5%, with Claude Opus 4.6 (Thinking) lowest. The AI Index notes a recurring weakness: models lean on general legal knowledge instead of the supplied documents, even when told not to.

## Key numbers

| Measure                          | Value                  |
| -------------------------------- | ---------------------- |
| GPT 5.1                          | 73.42%                 |
| GPT 4.1                          | 69.88%                 |
| GPT 5 Mini                       | 68.49%                 |
| GPT 5                            | 66.45%                 |
| Claude Opus 4.6 (Thinking), 15th | 62.06%                 |
| Gap, GPT 5.1 to GPT 4.1          | 3.54 percentage points |

## How to read it

- The benchmark has 300 validation and 104 test questions across seven kinds of legal reasoning, per the AI Index.
- Scores are from Vals.ai’s CaseLaw v2 leaderboard; the AI Index shows the top 15 models.

**Source:** [Vals.ai](https://aiadoption.org/sources/#vals-ai), 2026, via the AI Index Report 2026 (Stanford HAI), figure 2.5.12: *CaseLaw v2: accuracy*. Chart and text by AI Adoption.  
Numbers checked against the published data on 20 October 2026.  
[Download the data (CSV)](https://charts.aiadoption.org/ai-index/csv/caselaw-legal-research-accuracy.csv?ref=aiadoption.org) · [Cite this chart](#source)