Fix rad deploy resolving Radius.Core environment ID in the wrong group

pkg/cli/cmd/deploy/deploy.go

@@ -266,7 +266,14 @@ func (r *Runner) Validate(cmd *cobra.Command, args []string) error {
 			return err
 		}
 		if envResult == nil {
-			return clierrors.Message("The environment %q does not exist in scope %q. Run `rad env create` first. You could also provide the environment ID if the environment exists in a different group.", r.EnvironmentNameOrID, r.Workspace.Scope)
+			// If a full environment ID was provided (or came from the workspace default),
+			// report the scope encoded in that ID rather than the possibly-different
+			// --group scope, since that's the scope that was actually checked.
+			errScope := r.Workspace.Scope
+			if envID, parseErr := resources.Parse(r.EnvironmentNameOrID); parseErr == nil {
+				errScope = envID.RootScope()
+			}
+			return clierrors.Message("The environment %q does not exist in scope %q. Run `rad env create` first. You could also provide the environment ID if the environment exists in a different group.", r.EnvironmentNameOrID, errScope)
 		}
 		r.EnvResult = envResult
 	}
@@ -551,7 +558,7 @@ func (r *Runner) getApplicationsCoreEnvironment(ctx context.Context, id string)
 }
 
 // getRadiusCoreEnvironment retrieves environment using Radius Core client and returns as Applications.Core format
-func (r *Runner) getRadiusCoreEnvironment(ctx context.Context, name string) (*v20250801preview.EnvironmentResource, error) {
+func (r *Runner) getRadiusCoreEnvironment(ctx context.Context, scope, name string) (*v20250801preview.EnvironmentResource, error) {
 	if r.RadiusCoreClientFactory == nil {
 		clientFactory, err := cmd.InitializeRadiusCoreClientFactory(ctx, r.Workspace)
 		if err != nil {
@@ -561,7 +568,7 @@ func (r *Runner) getRadiusCoreEnvironment(ctx context.Context, name string) (*v2
 	}
 
 	environmentClient := r.RadiusCoreClientFactory.NewEnvironmentsClient()
-	env, err := environmentClient.Get(ctx, r.Workspace.Scope, name, nil)
+	env, err := environmentClient.Get(ctx, scope, name, nil)
 	if err != nil {
 		return nil, err
 	}
@@ -629,14 +636,20 @@ func (r *Runner) FetchEnvironment(ctx context.Context, envNameOrID string) (*Env
 		}
 	}
 	if fetchRadiusCoreEnv {
+		// If it's a full ID, look it up in the scope encoded in the ID itself (which may
+		// differ from the workspace/--group scope, e.g. when the environment lives in a
+		// different resource group than the one being deployed into). Otherwise, resolve
+		// the name within the current workspace scope.
 		var radCoreEnvName string
+		radCoreScope := r.Workspace.Scope
 		if isID {
 			radCoreEnvName = envID.Name()
+			radCoreScope = envID.RootScope()
 		} else {
 			radCoreEnvName = envNameOrID
 		}
 
-		radiusCoreEnv, err := r.getRadiusCoreEnvironment(ctx, radCoreEnvName)
+		radiusCoreEnv, err := r.getRadiusCoreEnvironment(ctx, radCoreScope, radCoreEnvName)
 		if err != nil {
 			if !clients.Is404Error(err) {
 				return nil, err

pkg/cli/cmd/deploy/deploy_test.go

@@ -41,6 +41,7 @@ import (
 	"github.com/radius-project/radius/pkg/corerp/api/v20250801preview"
 	corerpfake "github.com/radius-project/radius/pkg/corerp/api/v20250801preview/fake"
 	"github.com/radius-project/radius/pkg/to"
+	"github.com/radius-project/radius/pkg/ucp/resources"
 	"github.com/radius-project/radius/test/radcli"
 	"github.com/spf13/cobra"
 	"github.com/stretchr/testify/require"
@@ -1922,7 +1923,7 @@ func Test_getRadiusCoreEnvironment(t *testing.T) {
 			command:         &cobra.Command{},
 			args:            []string{"template.bicep"},
 			expectedEnv: &v20250801preview.EnvironmentResource{
-				Name: new("/planes/radius/local/resourceGroups/test/providers/Radius.Core/environments/myenv"),
+				Name: new("myenv"),
 			},
 			shouldError: false,
 		},
@@ -1942,7 +1943,15 @@ func Test_getRadiusCoreEnvironment(t *testing.T) {
 				},
 			}
 
-			env, err := runner.getRadiusCoreEnvironment(t.Context(), tc.environmentName)
+			// The Radius Core EnvironmentsClient.Get API takes (scope, environmentName), not a
+			// full resource ID. Callers (e.g. FetchEnvironment) extract scope/name from a full
+			// ID before calling getRadiusCoreEnvironment, so mirror that here.
+			callScope, callName := scope, tc.environmentName
+			if parsedID, parseErr := resources.Parse(tc.environmentName); parseErr == nil {
+				callScope, callName = parsedID.RootScope(), parsedID.Name()
+			}
+
+			env, err := runner.getRadiusCoreEnvironment(t.Context(), callScope, callName)
 
 			if tc.shouldError {
 				require.Error(t, err)
@@ -1961,6 +1970,65 @@ func Test_getRadiusCoreEnvironment(t *testing.T) {
 	}
 }
 
+// Test_FetchEnvironment_RadiusCoreEnvironmentUsesOwnGroup is a regression test for
+// https://github.com/radius-project/radius/issues/12573: a Radius.Core environment
+// referenced by full ID (e.g. the workspace's stored default environment) must be looked
+// up in the resource group encoded in that ID, not in the (possibly different)
+// --group-overridden workspace scope used for the deployment itself.
+func Test_FetchEnvironment_RadiusCoreEnvironmentUsesOwnGroup(t *testing.T) {
+	envScope := "/planes/radius/local/resourceGroups/default"
+	envID := envScope + "/providers/Radius.Core/environments/default"
+	deployScope := "/planes/radius/local/resourceGroups/test"
+
+	// Only succeed when queried in the environment's own scope; 404 for any other scope
+	// (such as the --group-overridden deploy scope) to catch a regression.
+	envServer := corerpfake.EnvironmentsServer{
+		Get: func(
+			_ context.Context,
+			rootScope string,
+			environmentName string,
+			_ *v20250801preview.EnvironmentsClientGetOptions,
+		) (resp azfake.Responder[v20250801preview.EnvironmentsClientGetResponse], errResp azfake.ErrorResponder) {
+			// The fake HTTP transport strips the leading slash from the rootScope path
+			// segment, so compare against the trimmed form.
+			if rootScope != strings.TrimPrefix(envScope, "/") {
+				errResp.SetError(fmt.Errorf("environment not found"))
+				errResp.SetResponseError(404, "Not Found")
+				return
+			}
+			resp.SetResponse(200, v20250801preview.EnvironmentsClientGetResponse{
+				EnvironmentResource: v20250801preview.EnvironmentResource{
+					Name: to.Ptr(environmentName),
+					ID:   to.Ptr(envID),
+				},
+			}, nil)
+			return
+		},
+	}
+
+	factory, err := test_client_factory.NewRadiusCoreTestClientFactory(
+		deployScope,
+		func() corerpfake.EnvironmentsServer { return envServer },
+		nil,
+	)
+	require.NoError(t, err)
+
+	runner := &Runner{
+		RadiusCoreClientFactory: factory,
+		Workspace: &workspaces.Workspace{
+			// Simulates `-g test` overriding the deploy scope while the workspace's
+			// default environment still lives in the "default" resource group.
+			Scope: deployScope,
+		},
+	}
+
+	result, err := runner.FetchEnvironment(t.Context(), envID)
+	require.NoError(t, err)
+	require.NotNil(t, result, "environment should resolve using the ID's own resource group, not the --group-overridden workspace scope")
+	require.False(t, result.UseApplicationsCore)
+	require.Equal(t, envID, runner.EnvironmentNameOrID)
+}
+
 func Test_constructApplicationsCoreEnvironmentID(t *testing.T) {
 	runner := &Runner{
 		Workspace: &workspaces.Workspace{