Rendered manifests

Short version: the source hydrator cannot feed a pre-merge gate, but it can give you an exact baseline, and if your repository renders manifests into git by any means the gate will diff them at resource level, which is a far stronger signal than anything derivable from Application definitions.

Why the hydrator does not gate a merge

ArgoCD’s source hydrator (beta since v3.5) renders your dry source and commits plain YAML to a hydrated branch. It looks like it should solve the diff problem outright. It does not, for one reason:

“The hydrator triggers only when a new commit is detected in the dry source.”

Hydration is a function of the configured dry revision, whatever is on main. It never runs for a pull request’s head commit. There is no dry-run API, no preview endpoint, and no commit-server call that renders without committing. The only way to hydrate a proposed change is to create a throwaway Application pointing at the PR branch, which needs a live ArgoCD, git write credentials, and leaves real commits behind.

hydrateTo is often mistaken for the answer. It pushes hydrated output to a staging branch so something else can PR it onward:

“Argo CD will only push changes to the hydrateTo branch, it will not create a PR or otherwise facilitate moving those changes to the syncSource branch.”

That gates a deploy, not a merge. The sequence is: merge to main → hydrate → push to environments/dev-next → your tooling opens dev-next → dev. By the time a rendered diff exists, the code change is already in.

Kargo does what the hydrator declines to do: helm-template / kustomize-build → git-commit → git-open-pr. If you want rendered YAML in a pull request before merge, that is the mechanism.

What this gate does with it

type: rendered reads manifests already committed to git: hydrator output, Kargo’s rendered promotion branches, or any CI job that commits its render.

sources:
  - name: hydrated
    type: rendered
    paths: ["environments/prod/**/manifest.yaml"]

Objects from these sources are diffed at resource level: added, removed, changed, and, called out separately, apiVersion changed, which is the one that blocks. An API version moving under an existing resource is a migration, and a migration is the class of change a render cannot catch.

Using a hydrated branch as the baseline works well: hydrator.metadata carries drySha, so you can tie rendered output back to the commit that produced it. Since ArgoCD v3.3 the last-hydrated SHA lives in a git note (refs/notes/hydrator.metadata) and the branch has no commit when the render did not change, so map hydrated to dry via the note, never the commit log.

What the object diff does not cover

If your repository does not render manifests into git, type: rendered sources contribute nothing. Two things fill most of that gap, and one hole remains.

A chart whose version moved is covered. diff -repo <path> pulls the chart at both versions, renders each with that Application’s own value files, and diffs the resources down to the field (chartdiff.go). A chart whose defaults flip, adding NetworkPolicies you did not ask for, is a one-line version change at Application level and an obvious addition at object level.

A values-only change is not. Chart-diff runs only for rows whose version moved, because it costs a chart pull and two renders per changed Application. Editing values under an unchanged chart version is compared at Application level only: which Applications exist, on which clusters, at which versions.

For that residual case, a report is a weaker signal than it looks, and a triage agent reading it is reasoning from less than it appears to have. Rendering manifests into git closes it.

Things ArgoCD does not give you

These look like they should exist:

• Nothing offline expands ApplicationSets. argocd appset generate looks like the offline tool and is an RPC to a live API server, as is --dry-run. Rendering generators yourself is the only offline route.
• The hydrator is single-source. sourceHydrator and sources are mutually exclusive, so Helm values from a second repository do not work with it.
• The hydrator does not sign commits, so a hydrated branch with signature verification enabled will reject them.