Vercel Open-Sources durable-harness for Crash-Proof Coding Agents

This package wraps existing harnesses, including Pi, OpenCode, Claude Code, and Codex, without requiring modifications to them. Each session is stored in a durable log, ensuring that a dead function no longer results in lost work, which addresses a significant gap for teams moving agents from laptops to the cloud.

The durable-harness package functions as a set of adapters that store a harness session within a Workflow SDK World, which bundles storage, a queue, and a stream. This system utilizes the same infrastructure that powers Vercel’s Workflow SDK.

Coding agents often lose work in the cloud because harnesses like Pi and Claude Code were originally designed for long-lived local processes that store sessions in memory. In the cloud, a VM with persistent storage does not track completed calls, meaning a failed turn or network error can lead to lost work.

durable-harness operates by writing each step of a turn to a durable log. Before a model or tool call is executed, an intent is recorded. Once the call is complete, its outcome is logged.

Each turn is supported by a message on Vercel Queues. If a function fails, the queue redirects the turn to a new function, which then reads the log to resume the process. This mechanism ensures that finished model and tool calls are retrieved from the log and are not re-run or billed twice.