What happened?

When updating or patching the /scale subresource of a custom or built-in resource, the managed field for spec.replicas is not updated as expected if the object was originally created without specifying a value for the spec.replicas field.

What did you expect to happen?

When updating or patching the /scale subresource, the managed field for specReplicasPath should be updated to reflect the subresource operation and the field manager used.

How can we reproduce it (as minimally and precisely as possible)?

1. Create a CRD with a scale subresource:

apiVersion: apiextensions.k8s.io/v1
kind: CustomResourceDefinition
metadata:
  name: mypods.example.com
spec:
  group: example.com
  versions:
    - name: v1
      served: true
      storage: true
      schema:
        openAPIV3Schema:
          type: object
          properties:
            spec:
              type: object
              properties:
                name:
                  type: string
                replicas:
                  type: integer
            status:
              type: object
              properties:
                replicas:
                  type: integer
      subresources:
        status: {}
        scale:
          specReplicasPath: .spec.replicas
          statusReplicasPath: .status.replicas
  scope: Namespaced
  names:
    plural: mypods
    singular: mypod
    kind: MyPod
    shortNames:
      - mpo

2. Create a custom resource:

kubectl apply --server-side --field-manager m1 -f - <<'EOF'
apiVersion: example.com/v1
kind: MyPod
metadata:
  name: test
spec:
  name: mypod
EOF

3. Update or patch replicas using the /scale subresource:

   a. Update the /scale subresource:

   kubectl scale mypods.example.com test --replicas 2
   # Or via direct API call:
   curl -XPUT -H 'content-type: application/json' \
     'http://localhost:8001/apis/example.com/v1/namespaces/default/mypods/test/scale?fieldManager=m2' \
     -d '{"apiVersion": "autoscaling/v1", "kind": "Scale", "metadata": {"name": "test"}, "spec": {"replicas": 2}}'

   b. Patch the /scale subresource:

   kubectl patch mypods.example.com test --subresource scale --type merge --field-manager m2 -p '{"spec": {"replicas": 2}}'
   # Or via direct API call:
   curl -XPATCH -H 'content-type: application/merge-patch+json' \
     'http://localhost:8001/apis/example.com/v1/namespaces/default/mypods/test/scale?fieldManager=m2' \
     -d '{"spec": {"replicas": 2}}'

4. Observe that spec.replicas is not in the managed fields:

   kubectl get mypods.example.com test --show-managed-fields -o yaml

   Output:

   apiVersion: example.com/v1
   kind: MyPod
   metadata:
     creationTimestamp: "2025-05-22T00:46:21Z"
     generation: 2
     managedFields:
     - apiVersion: example.com/v1
       fieldsType: FieldsV1
       fieldsV1:
         f:spec:
           f:name: {}
       manager: m1
       operation: Apply
       time: "2025-05-22T00:46:21Z"
     name: test
     namespace: default
     resourceVersion: "49104"
     uid: 87e87c4d-8557-4aaf-a9e4-15794ea8ad5d
   spec:
     name: mypod
     replicas: 2

For built-in types please see this comment.

Anything else we need to know?

No response

Kubernetes version

$ kubectl version
Client Version: v1.32.2
Kustomize Version: v5.5.0
Server Version: v1.32.4

Cloud provider

OS version

# On Linux:
$ cat /etc/os-release
# paste output here
$ uname -a
# paste output here

# On Windows:
C:\> wmic os get Caption, Version, BuildNumber, OSArchitecture
# paste output here

Install tools

Container runtime (CRI) and version (if applicable)

Related plugins (CNI, CSI, ...) and versions (if applicable)