Cognition launched Fusion, a lead-and-sidekick model pairing, in Devin Desktop and CLI

On September 11, 2026, Cognition introduced Fusion in Devin Desktop and Devin CLI, a harness that pairs a frontier lead model with a cheaper sidekick and reported lower cost per task on several benchmarks.

Archive entry: this covers an event from September 11, 2026, and we published it on November 8, 2026. The details come from sources dated at the time and may have changed since.

On September 11, 2026, Cognition introduced Fusion in Devin Desktop and Devin CLI, as described in Cognition's Fusion announcement. Fusion was a harness that paired a frontier lead model with a cheaper sidekick model; the two ran in parallel, each with its own persistent context and tool access.

What changed

The lead planned, resolved ambiguity and reviewed work. The sidekick investigated code, applied edits and ran tests. Users always interacted with the lead.
Named leads were Opus 5.5, Fable 5.1 and Astra. Named sidekicks were SWE-2 and GPT-6 Luna.
Cognition claimed efficiency gains of up to 39% over other harnesses on well-known coding benchmarks.
On the Artificial Analysis Coding Agent Index v1.5, Fusion with Opus 5.5 and GPT-6 Luna scored 68.1 at $9.01, versus Claude Code with Opus 5.5 (max) at 66 and $13.04.
With Astra and SWE-2, Fusion scored 58.9 at $4.54, versus Codex with Astra (max) at 61.6 and $7.47.
Across ten benchmark comparisons using SWE-2 as sidekick, Fusion cost 11% to 46% less, with scores mixed.

What it means for builders

Developers paying per token could let an expensive model plan and review while a cheaper model applied the edits and ran tests. The scores were mixed: Fusion scored higher than Claude Code with Opus 5.5 in one comparison and lower than Codex with Astra in another, at a lower cost in both.

Availability

In Devin CLI, users selected Fusion and set one model as the lead and another as the sidekick. The post did not state plan or pricing requirements. Devin Desktop version 3.10.23, listed in the Devin Desktop changelog on September 10, 2026, carried a banner announcing Fusion. According to a chart footnote, the post was updated on October 19, 2026.

Sources

Introducing Fusion in Devin Desktop & CLI (Cognition, September 11, 2026, official)
v3.10.23 (Cognition, September 10, 2026, official)

Written with AI assistance, strictly from the sources listed above. Before it went live, automated checks verified its dates, sources and every language edition.