Cline migrates 11 million extension users to an SDK harness

• All 11 million Cline extension users have moved to the SDK harness, according to Cline's migration announcement.
• Cline reported its task mistake rate fell from 6.34% to 0.62%, a 5.72 percentage-point reduction, in its migration post.
• The Marketplace deployment used a special slow rollout over the prior month, Cline's rollout follow-up said.
• Every Cline product now runs on the new runtime, which Cline built to improve open-weight model performance, in its SDK milestone post.
• LongCat-2.0 is now a free Cline model option, and Cline's LongCat post describes it as a 1.6T open-weights MoE with a 1M-token context window.

The SDK documentation proposes embedding the runtime in CI/CD pipelines, workers, cron jobs, internal tools, and desktop apps. Cline's SDK site says streaming events can connect that runtime to a custom UI.

The user-facing extension update was deliberately invisible. Cline called it its largest update, but said it landed entirely under the hood.

The reported rate change works out to roughly a 90% relative reduction. Cline's post does not define the task set or measurement method behind its “task mistake rate.”

The distribution constraint was VS Marketplace. A Cline rollout follow-up said the team used a special slow-rollout process for the preceding month and needed “some magic” to make it work there.

Cline had already pulled the agent harness out of its extension-centric architecture, which its SDK announcement says had become difficult to maintain, extend, and reuse across products.

The resulting design has three notable pieces:

• A stateless core surrounded by a durable runtime, intended to support long-running agents and sessions that move between surfaces.
• TypeScript packages that expose checkpoints, web fetch, MCPs, cron jobs, and subagents, as listed in the SDK docs.
• Plugins that can wrap tools, providers, message rewriters, automations, and hooks. Cline's plugin guide says the same plugin can run in the CLI, VS Code, JetBrains, and the SDK.

The migration changes the SDK from a public foundation into the runtime beneath Cline's shipped products.

Cline framed the completed migration around open weights, then made Meituan's LongCat-2.0 available at no charge through its /model selector. Its attached Terminal-Bench 2.1 chart places LongCat at 70.8, 0.9 points behind Claude Opus 4.7 and 0.1 ahead of Gemini 3.1 Pro.