ignoreDifferences with managedFieldsManagers does not work if field removed from the desired manifest

[madaraszg-tulip]

Checklist:

• I've searched in the docs and FAQ for my answer: https://bit.ly/argocd-faq.
• I've included steps to reproduce the bug.
• I've pasted the output of argocd version.

Describe the bug

ignoreDifferences specifying managedFieldsManagers does not work

To Reproduce

I have an application manifest that includes the following:

ignoreDifferences:
  - group: apps
    kind: Deployment
    managedFieldsManagers:
      - kube-contoller-manager

I don't have server side diff or server side apply turned on.

I have a deployment manifest that includes /spec/replicas in the last-applied-configuration:

apiVersion: apps/v1
kind: Deployment
metadata:
  annotations:
    argocd.argoproj.io/tracking-id: alloy:apps/Deployment:monitoring/alloy-profiles
    deployment.kubernetes.io/revision: '10'
    kubectl.kubernetes.io/last-applied-configuration: >
      {"apiVersion":"apps/v1","kind":"Deployment","metadata":{"annotations":{"argocd.argoproj.io/tracking-id":"alloy:apps/Deployment:monitoring/alloy-profiles"},"labels":{"app.kubernetes.io/instance":"alloy","app.kubernetes.io/managed-by":"Helm","app.kubernetes.io/name":"alloy-profiles","app.kubernetes.io/part-of":"alloy","app.kubernetes.io/version":"v1.10.1","helm.sh/chart":"alloy-profiles-1.2.1"},"name":"alloy-profiles","namespace":"monitoring"},"spec":{"minReadySeconds":10,"replicas":2,...
...
  managedFields:
    - apiVersion: apps/v1
      fieldsType: FieldsV1
      fieldsV1:
        f:spec:
          f:replicas: {}
      manager: kube-controller-manager
      operation: Update
      subresource: scale
...
spec:
  minReadySeconds: 10
  progressDeadlineSeconds: 600
  replicas: 2
...

So /spec/replicas is managed by kube-controller-manager.

The desired manifest does not contain /spec/replicas.

Argocd wants to remove /spec/replicas from the live manifest, practically wants to set it to 1

Expected behavior

I would expect argocd to ignore /spec/replicas in the live manifest.

Screenshots

Version

$ argocd version
argocd: v3.0.12+ed1e239
  BuildDate: 2025-07-25T19:36:41Z
  GitCommit: ed1e2397ef9af6d23b284e39d504308cdda1957b
  GitTreeState: clean
  GoVersion: go1.24.4
  Compiler: gc
  Platform: linux/amd64

Logs

[madaraszg-tulip]

Simple reproducer in a helm chart:

Chart.yaml:

apiVersion: v2
description: Test chart for reproducing argocd bug
name: nginx-test
version: 1.0.0

templates/deployment.yaml:

apiVersion: apps/v1
kind: Deployment
metadata:
  name: nginx-test
spec:
  replicas: 1
  selector:
    matchLabels:
      app: nginx-test
  template:
    metadata:
      labels:
        app: nginx-test
    spec:
      containers:
        - name: nginx-container
          image: nginx:latest
          ports:
            - containerPort: 80
          resources:
            requests:
              cpu: 100m

templates/hpa.yaml:

apiVersion: autoscaling/v2
kind: HorizontalPodAutoscaler
metadata:
  name: nginx-test
spec:
  minReplicas: 2
  maxReplicas: 3
  metrics:
    - resource:
        name: cpu
        target:
          averageUtilization: 80
          type: Utilization
      type: Resource
  scaleTargetRef:
    apiVersion: apps/v1
    kind: Deployment
    name: nginx-test

Create simple argocd app with the following ignoreDifferences:

syncPolicy:
  syncOptions:
    - RespectIgnoreDifferences=true
ignoreDifferences:
  - group: apps
    kind: Deployment
    managedFieldsManagers:
      - kube-controller-manager

Deployment is created with replicas: 1, HPA scales it up to replicas: 2, argocd ignores replicas change, so far so good.

Live manifest:

apiVersion: apps/v1
kind: Deployment
metadata:
  annotations:
    argocd.argoproj.io/tracking-id: nginx-test:apps/Deployment:nginx-test/nginx-test
    deployment.kubernetes.io/revision: '1'
    kubectl.kubernetes.io/last-applied-configuration: >
      {"apiVersion":"apps/v1","kind":"Deployment","metadata":{"annotations":{"argocd.argoproj.io/tracking-id":"nginx-test:apps/Deployment:nginx-test/nginx-test"},"name":"nginx-test","namespace":"nginx-test"},"spec":{"replicas":1,"selector":{"matchLabels":{"app":"nginx-test"}},"template":{"metadata":{"labels":{"app":"nginx-test"}},"spec":{"containers":[{"image":"nginx:latest","name":"nginx-container","ports":[{"containerPort":80}],"resources":{"requests":{"cpu":"100m"}}}]}}}}
  creationTimestamp: '2025-10-29T09:56:47Z'
  generation: 2
  managedFields:
    - apiVersion: apps/v1
      fieldsType: FieldsV1
      fieldsV1:
        f:spec:
          f:replicas: {}
      manager: kube-controller-manager
      operation: Update
      subresource: scale
...
spec:
  progressDeadlineSeconds: 600
  replicas: 2
...

Desired manifest:

apiVersion: apps/v1
kind: Deployment
metadata:
  annotations:
    argocd.argoproj.io/tracking-id: nginx-test:apps/Deployment:nginx-test/nginx-test
  name: nginx-test
  namespace: nginx-test
spec:
  replicas: 1
...

replicas diff is properly ignored.

Now modify deployment template in the helm chart, remove replicas:

apiVersion: apps/v1
kind: Deployment
metadata:
  name: nginx-test
spec:
  selector:
    matchLabels:
      app: nginx-test
  template:
    metadata:
      labels:
        app: nginx-test
    spec:
      containers:
        - name: nginx-container
          image: nginx:latest
          ports:
            - containerPort: 80
          resources:
            requests:
              cpu: 100m

diff shows up: