The gap between the best open-weight models and the frontier of closed proprietary systems has a new number attached to it. Epoch AI's latest data insight finds that since January 2026, the most capable open-weight models have lagged frontier closed models by an average of four months in the Epoch Capabilities Index (ECI), with an average ECI gap of 8 points, similar to the gap between GPT-5 and GPT-5.5.

That is a modest widening from their previous estimate. The October 2025 analysis found that open models lagged by an average of three months between January 2023 and October 2025. The headline shift from three to four months is small, but the framing matters: the frontier has been moving fast, and open-weight releases are roughly holding their relative position rather than collapsing behind.

What ECI is actually measuring

The Epoch Capabilities Index is the yardstick doing the work here, so it is worth unpacking. The ECI combines scores from many different AI benchmarks into a single general capability scale, allowing comparisons between models even over timespans long enough for single benchmarks to reach saturation. The general ECI uses scores from 40+ distinct benchmarks to generate a single, general capability scale, stitching them together by determining their relative difficulty wherever models are evaluated on multiple benchmarks. Models obtain higher ECI scores if they perform better on harder benchmarks.