Argo CD application authoring

For developers and platform teams who publish Argo CD Application and ApplicationSet manifests in git. Targets Argo CD v3.4.x (latest stable v3.4.3, 2026-05-28) and the v3.3.x maintenance line (latest v3.3.11); v3.4 reached GA in early May 2026. Cited file paths in references/ are relative to a local clone of argoproj/argo-cd (e.g. docs/user-guide/best_practices.md). Without a local clone, fetch the same content via gh api repos/argoproj/argo-cd/contents/<path> or read https://github.com/argoproj/argo-cd at the matching path.

Quick decision guide

Critical gotchas (May 2026)

1. CVE-2026-42880 — IncludeMutationWebhook=true leaks Secrets

Patched v3.3.9 / v3.2.11 (per GHSA-3v3m-wc6v-x4x3, advisory published 2026-05-01). ServerSideDiff + argocd.argoproj.io/compare-options: IncludeMutationWebhook=true exfiltrates plaintext Secret data to anyone with applications, get. Clusters that handle Secrets at all MUST be on a patched build.

When immediate upgrade isn't possible: strip IncludeMutationWebhook=true from every Application. Block the value in CI with kubeconform/conftest.

2. Argo CD v3.0 flipped seven defaults — don't write apps as if on v2

In May 2026 assume v3.x semantics:

• Resource tracking is annotation-based by default, not labels. Don't add

app.kubernetes.io/instance overrides expecting them to track ownership.

• `.status` is ignored on all resources by default (was: only CRDs).

ServerSideDiff respects this; mutation-webhook drift in .status no longer shows up as OutOfSync.

• Default `resource.exclusions` ships with the install (Endpoints,

EndpointSlice, Lease, *SubjectAccessReview, TokenReview, CSR, CertificateRequest, Kyverno reports, Cilium endpoints, PolicyReports).

• Logs RBAC is enforced: roles need explicit logs, get to view pod logs.
• `update`/`delete` RBAC no longer inherits to managed resources.
• `spec.preserveUnknownFields: false` on CRDs causes drift now — drop the

field; use x-kubernetes-preserve-unknown-fields: true on schemas.

• In-cluster destination disabled (`cluster.inClusterEnabled: "false"`) hard-blocks new apps to that cluster.

Full list with PRs: references/version-changes.md § v3.0.

3. Skip Argo CD 3.3.0 and 3.3.1 — SSA migration regression

3.3.0 forced the kubectl-client-side-apply → SSA field-manager rewrite, broke many real apps, and required ClientSideApplyMigration=false as a temp workaround. 3.3.2 fixed it (2026-02). Operators should be on 3.3.2+ before publishing on this cluster.

When Argo CD manages itself, the meta-Application needs ServerSideApply=true in syncOptions because the ApplicationSet CRD exceeds the 262 144-byte client-side-apply annotation limit.

4. Source Hydrator is still Alpha in v3.4

Despite three minor versions of feature work, schema can break in v3.5. Don't build long-lived automation on spec.sourceHydrator yet. Cosign-verified manifest signing still requires the GnuPG path (signatureKeys) — Argo CD doesn't ship a first-class cosign integration.

5. The default AppProject is a footgun in any multi-tenant cluster

default permits every repo, every destination, every cluster-scoped resource. Production app authors should never set spec.project: default. Make a project-per-team or project-per-environment, set sourceRepos, destinations, clusterResourceWhitelist, signatureKeys. See references/projects-rbac.md.

6. Templated project: in ApplicationSet bypasses tenancy

spec.template.spec.project: "{{path.basename}}" lets an attacker who controls the source git path elevate to any project they can name. Hard-code the project in the ApplicationSet template. Argo CD warns about this in docs/operator-manual/applicationset/Generators-Git.md lines 5-11.

7. ApplicationSets are recommended over app-of-apps

The official guidance — docs/operator-manual/cluster-bootstrapping.md line 7 — is now ApplicationSet first. App-of-apps still works for the bootstrap (root → ApplicationSets), but for app fan-out, prefer an ApplicationSet with a Git directory generator over a parent Application that points at child Applications.

Canonical Application

Smallest sensible production Application:

apiVersion: argoproj.io/v1alpha1
kind: Application
metadata:
  name: payments-api
  namespace: argocd                         # default; with app-in-any-namespace can live in tenant ns
  finalizers:
    - resources-finalizer.argocd.argoproj.io  # cascade delete child resources when App is deleted
spec:
  project: team-payments                    # NEVER `default` in production
  source:
    repoURL: https://github.com/example/config-repo.git
    targetRevision: main                    # pin to a tag or SHA in production
    path: apps/prod/payments-api
    helm:                                   # or `kustomize:`, `directory:`, `plugin:`
      releaseName: payments-api
      valueFiles:
        - values.yaml
        - values-prod.yaml
  destination:
    name: prod-eu-1                         # XOR with `server:` — prefer `name` (stable)
    namespace: payments-api
  syncPolicy:
    automated:
      enabled: true                         # v3.x: use this, not "delete the automated block"
      prune: true                           # default false; without this, deleted resources stay
      selfHeal: true                        # only if no controllers mutate the spec
      allowEmpty: false                     # never prune everything if the app is empty
    syncOptions:
      - CreateNamespace=true
      - ServerSideApply=true                # required for self-managed Argo CD; recommended otherwise
      - ApplyOutOfSyncOnly=true             # huge win on apps with many objects
      - RespectIgnoreDifferences=true       # makes ignoreDifferences actually skip the field on apply
      - PrunePropagationPolicy=foreground
    retry:
      limit: 5
      backoff:
        duration: 5s
        factor: 2
        maxDuration: 3m
  ignoreDifferences:
    - group: apps
      kind: Deployment
      jsonPointers:
        - /spec/replicas                    # HPA-managed; without this the controller sync-loops
  revisionHistoryLimit: 5
  info:
    - name: Runbook
      value: https://wiki.example.com/payments-api

The single most important field for production hygiene: targetRevision — pin to a SHA or tag, not HEAD / main. Floating revisions cause non-deterministic syncs and break audit. Argo's own best_practices.md makes this explicit (docs/user-guide/best_practices.md lines 57-79).

Canonical ApplicationSet

One Application per directory under apps/<env>/ in the config repo:

apiVersion: argoproj.io/v1alpha1
kind: ApplicationSet
metadata:
  name: prod-apps
  namespace: argocd
spec:
  goTemplate: true                          # ALWAYS set; default is the legacy fasttemplate
  goTemplateOptions:
    - missingkey=error                      # fail-fast on typos in template paths
  generators:
    - git:
        repoURL: https://github.com/example/config-repo.git
        revision: main
        directories:
          - path: apps/prod/*               # one Application per matching subdir
  template:
    metadata:
      name: '{{ .path.basename }}'          # e.g. "payments-api"
      finalizers:
        - resources-finalizer.argocd.argoproj.io
    spec:
      project: team-platform                # HARD-CODED — do not template (security)
      source:
        repoURL: https://github.com/example/config-repo.git
        targetRevision: '{{ .metadata.commitSHA }}'  # available with Git generator + values
        path: '{{ .path.path }}'
      destination:
        name: prod-eu-1
        namespace: '{{ .path.basename }}'
      syncPolicy:
        automated:
          enabled: true
          prune: true
          selfHeal: true
        syncOptions:
          - CreateNamespace=true
          - ServerSideApply=true
          - ApplyOutOfSyncOnly=true
  syncPolicy:
    preserveResourcesOnDeletion: false      # delete generated Applications on AppSet deletion
    applicationsSync: sync                  # create+update+delete (was named `policy:` in older docs)

Hard rules:

1. `goTemplate: true` always. The default is false (legacy fasttemplate),

which silently mishandles nested objects, ranges, and if. The new pattern uses Go templates with .path.basename, .path.path, .cluster.name, etc.

1. `goTemplateOptions: ["missingkey=error"]` always. Without it, typos in

template paths render as the empty string and the bug ships.

1. `project:` is hard-coded, never templated. Templated project lets a git

PR author change spec.template.spec.project to a privileged project.

1. `finalizers:` go in the template, not the AppSet itself. Adding the

finalizer to the AppSet metadata won't propagate to children.

1. `applicationsSync: sync` is what most teams want. create-only /

create-update / create-delete are escape hatches for migrations.

1. *For Git directory generators, `path: <dir>/`** matches one level.

<dir>/** doesn't recurse — use <dir>/*/* for two-deep, etc.

ApplicationSet generator picker, all generator schemas, Progressive Sync (Beta in v3.3), and the preservedFields / templatePatch knobs live in references/applicationset.md.

Sync policy decision shortcut

Default policy for a typical workload:

• automated.enabled: true, prune: true, selfHeal: true when no controller

mutates the spec. When one does (HPA, cert-manager, sidecar injectors), keep selfHeal: false OR add precise ignoreDifferences and RespectIgnoreDifferences=true.

• syncOptions to set on every app:

CreateNamespace=true, ServerSideApply=true, ApplyOutOfSyncOnly=true, RespectIgnoreDifferences=true. PrunePropagationPolicy=foreground for synchronous child-cleanup pruning; background for fast pruning.

• retry defaults are fine; raise limit to 10 only for apps with PreSync

hooks that legitimately flake (DB migrations on shared infra).

• For apps with destructive operations (delete Namespaces, PVCs):

Prune=confirm and/or Delete=confirm (v3.3+) — UI prompts before the op.

For the full syncOptions table, sync waves, hooks, health checks, and the diff strategy reference (Server-Side Diff is Stable since v3.2 but not default), see references/sync.md.

CLI quick reference for app authors

The 12 most-used commands:

# Inspect what's in git vs the live cluster
argocd app diff <name>                    # Argo's view of git vs live
argocd app diff <name> --server-side-generate  # render via repo-server, like a sync would

# Force the controller to re-fetch the source
argocd app get <name> --refresh           # re-evaluate sync status (uses cache)
argocd app get <name> --hard-refresh      # bust the manifest cache; rare, slow

# See what Argo CD WOULD apply
argocd app manifests <name>               # rendered manifests from git
argocd app manifests <name> --source live # what's actually live

# Sync controls
argocd app sync <name>                    # full sync
argocd app sync <name> --resource <gk>:<name>          # selective sync (skips hooks, no history)
argocd app sync <name> --prune --dry-run  # preview a prune

# Termination & deletion
argocd app terminate-op <name>            # cancel an in-progress sync
argocd app delete <name> --cascade        # delete app + its managed resources
argocd app delete <name> --no-cascade     # delete app, leave resources

# ApplicationSets
argocd appset get <name>                  # show generated children
argocd appset create -f appset.yaml --dry-run  # preview before applying

The --core mode (no API server, talk straight to k8s) is useful for CI: argocd --core app diff <name> skips the gRPC server entirely. Set KUBECTL_EXTERNAL_DIFF=delta for nicer output.

Annotations cheat sheet

The argocd.argoproj.io/* annotations app authors use most:

Full table including tracking-id, sync-statuses, finalizer variants: references/sync.md § Annotations.

Reference files — when to load

Adjacent skills

• `helm` for chart authoring questions (Helm 4 SSA, OCI digest install,

values, helpers). This skill assumes Helm chart authoring competence.

• `kubefwd` for accessing Argo CD UI / repo-server services from a

laptop without one-port-at-a-time kubectl port-forward.

• `gh-cli` for fetching release notes, PRs, security advisories from the

argo-cd repo (canonical changelog — CHANGELOG.md in the repo is stale back to 2022).