Flux: dry-run failed when upgrading a CRD API version

TL;DR

Upgrade Flux to that patch or later and the API-version upgrade applies cleanly. Fixed upstream: the kustomize-controller fix for this dry-run behavior shipped in the next Flux 2.8 patch release (maintainer matheuscscp confirmed on the thread).

Steps

1. Upgrading a CRD's API version through Flux fails with: dry-run failed: .spec.accessPolicy: field not declared in schema (e.g. moving a Linkerd Server from policy.linkerd.io/v1beta1 to v1beta3). The same change applies fine with kubectl directly. The failure came from how kustomize-controller's dry-run interacted with managed fields from other managers on the object.

When to use

Use this skill when you run into "Flux: dry-run failed when upgrading a CRD API version".

When not to use

If your error message or symptom does not match what is described above, this is probably not your fix. Search for your exact error text instead of forcing this one to fit.

Versions

No specific versions are mentioned in the source material, so treat the fix as generally applicable and check the examples against whatever you have installed.

Why this happens

The original report does not dig into a root cause. It documents the symptom and the fix that resolved it.

A user asked: Upgrading a CRD's API version through Flux fails with: dry-run failed: .spec.accessPolicy: field not declared in schema (e.g. moving a Linkerd Server from policy.linkerd.io/v1beta1 to v1beta3). The same change applies fine with kubectl directly. The failure came from how kustomize-controller's dry-run interacted with managed fields from other managers on the object. The verified answer: Fixed upstream: the kustomize-controller fix for this dry-run behavior shipped in the next Flux 2.8 patch release (maintainer matheuscscp confirmed on the thread). Upgrade Flux to that patch or later and the API-version upgrade applies cleanly. Thread: https://vectle.com/threads/thrFRGuJptA6Tf6ZB3-8Qgcw