diff --git a/internal/internal_test.go b/internal/internal_test.go
new file mode 100644
index 0000000..b327f5a
--- /dev/null
+++ b/internal/internal_test.go
@@ -0,0 +1,70 @@
+// Copyright 2024 Google LLC
+
+// 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
+
+//     https://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 internal_test
+
+import (
+	"fmt"
+	"net/http"
+	"net/http/httptest"
+	"testing"
+
+	"github.com/google-gemini/proxy-to-gemini/internal"
+)
+
+func TestErrorHandler(t *testing.T) {
+	tests := []struct {
+		name     string
+		method   string
+		code     int
+		msg      string
+		arg      []interface{}
+		wantBody string
+		wantLog  string
+	}{
+		{
+			name:     "Bad request without args",
+			method:   http.MethodPost,
+			code:     http.StatusBadRequest,
+			msg:      "failed to read request body",
+			wantBody: "failed to read request body\n",
+		},
+		{
+			name:     "Internal server error with args",
+			method:   http.MethodGet,
+			code:     http.StatusInternalServerError,
+			msg:      "failed to generate content: %v",
+			arg:      []interface{}{fmt.Errorf("generic error")},
+			wantBody: "failed to generate content: generic error\n",
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			recorder := httptest.NewRecorder()
+
+			req := httptest.NewRequest(tt.method, "/", nil)
+
+			internal.ErrorHandler(recorder, req, tt.code, tt.msg, tt.arg...)
+
+			if recorder.Code != tt.code {
+				t.Errorf("got status %v, want %v", recorder.Code, tt.code)
+			}
+
+			if recorder.Body.String() != tt.wantBody {
+				t.Errorf("got body %v, want %v", recorder.Body.String(), tt.wantBody)
+			}
+		})
+	}
+}