How We Migrated 11 Million Users to Cline's Biggest Refactor

Cline rebuilt its VS Code extension around the shared Cline SDK to replace a roughly 76,000-line legacy agent harness that had become difficult to update as models, tools, and product surfaces evolved. After an initial migration failed and was rolled back, the team created a custom rollout system because the VS Code Marketplace lacks staged deployments or immediate reversions: a small loader packaged both legacy and SDK-based extensions, used a feature flag to assign users gradually, automatically fell back to the legacy version if activation failed, and preserved shared settings and task data. The two versions were kept compatible through a generated union manifest and build-time checks, while comparable telemetry enabled a controlled A/B evaluation. During a near-even traffic split, the SDK harness reduced tasks reaching Cline’s three-consecutive-mistakes limit from 6.34% to 0.62%, with particularly large improvements across both proprietary and open-weight models. By August 23, the rollout had reached 100%, consolidating the extension, CLI, and SDK on the same runtime and allowing new agent improvements, provider integrations, and model capabilities to be delivered more consistently, although some less-used features were deprecated.

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