diff --git a/plugins/azure-functions/deployment/plugin.go b/plugins/azure-functions/deployment/plugin.go
index 4bfe036..1bf09ed 100644
--- a/plugins/azure-functions/deployment/plugin.go
+++ b/plugins/azure-functions/deployment/plugin.go
@@ -16,6 +16,7 @@ package deployment
 
 import (
 	"context"
+	"fmt"
 
 	sdk "github.com/pipe-cd/piped-plugin-sdk-go"
 
@@ -50,7 +51,7 @@ func (p *Plugin) ExecuteStage(ctx context.Context, _ *sdk.ConfigNone, dts []*sdk
 			Status: p.executeAzureFuncRollbackStage(ctx, dts, input),
 		}, nil
 	default:
-		panic("unimplemented stage: " + input.Request.StageName)
+		return nil, fmt.Errorf("unimplemented stage: %q", input.Request.StageName)
 	}
 }
 
@@ -76,7 +77,7 @@ func (p *Plugin) DetermineVersions(ctx context.Context, _ *sdk.ConfigNone, d *sd
 			},
 		}, nil
 	default:
-		panic("not supported kind: " + appCfg.Spec.Kind)
+		return nil, fmt.Errorf("not supported kind: %q", appCfg.Spec.Kind)
 	}
 }
 
diff --git a/plugins/azure-functions/deployment/plugin_test.go b/plugins/azure-functions/deployment/plugin_test.go
new file mode 100644
index 0000000..7b6e601
--- /dev/null
+++ b/plugins/azure-functions/deployment/plugin_test.go
@@ -0,0 +1,101 @@
+// Copyright 2025 The PipeCD Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package deployment
+
+import (
+	"testing"
+
+	sdk "github.com/pipe-cd/piped-plugin-sdk-go"
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+
+	"github.com/pipe-cd/community-plugins/plugins/azure-functions/config"
+)
+
+// Test_ExecuteStage_UnknownStage ensures that an unrecognized stage name
+// results in an error instead of a panic, since the stage name originates
+// from the pipeline configuration and can be malformed or unsupported.
+func Test_ExecuteStage_UnknownStage(t *testing.T) {
+	t.Parallel()
+
+	p := &Plugin{}
+
+	got, err := p.ExecuteStage(t.Context(), nil, nil, &sdk.ExecuteStageInput[config.AzureApplicationSpec]{
+		Request: sdk.ExecuteStageRequest[config.AzureApplicationSpec]{
+			StageName: "UNKNOWN_STAGE",
+		},
+	})
+
+	require.Error(t, err)
+	assert.Nil(t, got)
+	assert.Contains(t, err.Error(), "UNKNOWN_STAGE")
+}
+
+// Test_DetermineVersions_UnsupportedKind ensures that an application spec
+// with an unsupported "kind" results in an error instead of a panic, since
+// the kind comes directly from the user-authored application config file.
+func Test_DetermineVersions_UnsupportedKind(t *testing.T) {
+	t.Parallel()
+
+	p := &Plugin{}
+
+	appCfg := &sdk.ApplicationConfig[config.AzureApplicationSpec]{
+		Spec: &config.AzureApplicationSpec{
+			Kind: "unsupported-kind",
+		},
+	}
+
+	got, err := p.DetermineVersions(t.Context(), nil, &sdk.DetermineVersionsInput[config.AzureApplicationSpec]{
+		Request: sdk.DetermineVersionsRequest[config.AzureApplicationSpec]{
+			DeploymentSource: sdk.DeploymentSource[config.AzureApplicationSpec]{
+				ApplicationConfig: appCfg,
+			},
+		},
+	})
+
+	require.Error(t, err)
+	assert.Nil(t, got)
+	assert.Contains(t, err.Error(), "unsupported-kind")
+}
+
+// Test_DetermineVersions_FunctionKind ensures the existing, supported
+// behavior for the "function" kind is preserved.
+func Test_DetermineVersions_FunctionKind(t *testing.T) {
+	t.Parallel()
+
+	p := &Plugin{}
+
+	appCfg := &sdk.ApplicationConfig[config.AzureApplicationSpec]{
+		Spec: &config.AzureApplicationSpec{
+			Kind: config.FunctionKind,
+			FunctionManifest: &config.FunctionsSpec{
+				PackageUri: "https://example.blob.core.windows.net/pkg.zip",
+			},
+		},
+	}
+
+	got, err := p.DetermineVersions(t.Context(), nil, &sdk.DetermineVersionsInput[config.AzureApplicationSpec]{
+		Request: sdk.DetermineVersionsRequest[config.AzureApplicationSpec]{
+			DeploymentSource: sdk.DeploymentSource[config.AzureApplicationSpec]{
+				ApplicationConfig: appCfg,
+			},
+		},
+	})
+
+	require.NoError(t, err)
+	require.NotNil(t, got)
+	require.Len(t, got.Versions, 1)
+	assert.Equal(t, "https://example.blob.core.windows.net/pkg.zip", got.Versions[0].Version)
+}
diff --git a/plugins/azure-functions/go.mod b/plugins/azure-functions/go.mod
index ad1ef82..07bbe54 100644
--- a/plugins/azure-functions/go.mod
+++ b/plugins/azure-functions/go.mod
@@ -14,6 +14,7 @@ require (
 	github.com/Azure/azure-sdk-for-go/sdk/resourcemanager/storage/armstorage v1.8.0
 	github.com/Azure/azure-sdk-for-go/sdk/storage/azblob v1.6.1
 	github.com/pipe-cd/piped-plugin-sdk-go v0.0.0-20250606012947-e00f415619d0
+	github.com/stretchr/testify v1.10.0
 	go.uber.org/zap v1.27.0
 )
 
@@ -27,6 +28,7 @@ require (
 	github.com/cespare/xxhash/v2 v2.3.0 // indirect
 	github.com/coreos/go-oidc/v3 v3.11.0 // indirect
 	github.com/creasty/defaults v1.6.0 // indirect
+	github.com/davecgh/go-spew v1.1.1 // indirect
 	github.com/envoyproxy/protoc-gen-validate v1.0.4 // indirect
 	github.com/go-jose/go-jose/v4 v4.0.5 // indirect
 	github.com/go-logr/logr v1.4.2 // indirect
@@ -45,6 +47,7 @@ require (
 	github.com/matttproud/golang_protobuf_extensions v1.0.2-0.20181231171920-c182affec369 // indirect
 	github.com/pipe-cd/pipecd v0.52.0 // indirect
 	github.com/pkg/browser v0.0.0-20240102092130-5ac0b6a4141c // indirect
+	github.com/pmezard/go-difflib v1.0.0 // indirect
 	github.com/prometheus/client_golang v1.12.1 // indirect
 	github.com/prometheus/client_model v0.5.0 // indirect
 	github.com/prometheus/common v0.32.1 // indirect
@@ -72,5 +75,6 @@ require (
 	google.golang.org/grpc v1.64.1 // indirect
 	google.golang.org/protobuf v1.34.2 // indirect
 	gopkg.in/yaml.v2 v2.4.0 // indirect
+	gopkg.in/yaml.v3 v3.0.1 // indirect
 	sigs.k8s.io/yaml v1.3.0 // indirect
 )