How We Migrated 11 Million Users to Cline's Biggest Harness Upgrade

Cline migrated its VS Code extension from a 76,000-line proprietary agent core to the modular Cline SDK, aiming to unify its extension, CLI, JetBrains plugin, and SDK around a runtime better suited to modern AI coding models. After an initial migration failed and was rolled back, the company developed a custom deployment approach to overcome the VS Code Marketplace’s lack of staged rollouts by packaging legacy and SDK-based versions together, using a loader and feature flag to select a version, automatically falling back to the legacy version if the new one failed, and preserving shared user state. Cline also created a combined manifest and comparable telemetry across both versions to support a gradual A/B rollout, ultimately observing that the rate of tasks reaching the agent’s three-consecutive-mistakes limit fell from 6.34% under the old harness to 0.62% under the new one. The company attributes the improvement to replacing older XML-based tool-use mechanisms designed for 2024-era models with a simplified harness using native tool calling, revised prompts, context management, error handling, and tool definitions. As of August 23, the rollout reached 100%, placing the extension on the same engine as Cline’s other products, although some less-used features were deprecated during the transition.

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