Vercel's Workflow SDK now sends writes over WebSockets by default

Vercel says the Workflow SDK now sends event writes and stream writes to Vercel Workflows over a WebSocket by default. Previously each write, or each group of writes for streams, went out as its own HTTP request. Upgrading to workflow@5.2.0 or later is the only step needed, and existing streaming code keeps working as is.

New runs use the upgrade once the deployment is live, while in-flight runs finish on the deployment they started on. Every step start and completion is an event write, so workflows with many steps make fewer HTTP requests. The queue handler opens the connection for every delivery.

In a benchmark replaying an AI agent's token stream, v5.2.0 delivered the full response 4.2 times faster than v4.8.12, and no chunk was delayed by more than a second. Vercel also says the change improves performance and reduces costs. Users on 4.x are pointed to the upgrade guide first, since v5 is a major release.

Agents that stream tokens or run workflows with many steps should make fewer HTTP calls after upgrading, without changing their code. Teams on 4.x should read the upgrade guide before moving to v5, because it is a major release. Runs already in flight keep their original deployment, so plan rollouts with that in mind.