Versioning — how to modify Flow code

Running Flows continue after you deploy new Worker code. Change code carefully so in-flight executions do not observe incompatible Step graphs.

Principles

1. Prefer additive changes — new Steps, new RPCs, new optional Attributes
2. Do not remove or rename Step types still referenced by open Flows
3. Keep WaitFor/Execute contracts stable for existing Step types (inputs/outputs)
4. Use new Step types for incompatible logic; route only new executions there
5. When a bug leaves Flows stuck, fix forward then reset from a safe point

Safe patterns

• Add a new Step and branch to it only for new Attribute/flag values
• Keep old Step implementations until all old runs complete
• Version RPC names rather than silently changing request schemas

Unsafe patterns

• Deleting a Step type that open Flows still move to
• Changing the meaning of an Attribute without migration
• Assuming no long-running executions exist

Continues and resets

Very long Flows may continue-as-new. Resets jump execution to an earlier Step after a code fix. Combine with monitoring from Application operations.