diff --git a/internal/httputil/httputil_test.go b/internal/httputil/httputil_test.go
new file mode 100644
index 000000000..d24a309ff
--- /dev/null
+++ b/internal/httputil/httputil_test.go
@@ -0,0 +1,140 @@
+package httputil
+
+import (
+	"encoding/json"
+	"net/http"
+	"net/http/httptest"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func TestWriteJSONResponse(t *testing.T) {
+	t.Run("sets content-type to application/json", func(t *testing.T) {
+		rec := httptest.NewRecorder()
+		WriteJSONResponse(rec, http.StatusOK, map[string]string{"key": "value"})
+
+		assert.Equal(t, "application/json", rec.Header().Get("Content-Type"))
+	})
+
+	t.Run("writes the provided status code", func(t *testing.T) {
+		tests := []struct {
+			name       string
+			statusCode int
+		}{
+			{"200 OK", http.StatusOK},
+			{"201 Created", http.StatusCreated},
+			{"400 Bad Request", http.StatusBadRequest},
+			{"404 Not Found", http.StatusNotFound},
+			{"500 Internal Server Error", http.StatusInternalServerError},
+		}
+
+		for _, tt := range tests {
+			t.Run(tt.name, func(t *testing.T) {
+				rec := httptest.NewRecorder()
+				WriteJSONResponse(rec, tt.statusCode, nil)
+
+				assert.Equal(t, tt.statusCode, rec.Code)
+			})
+		}
+	})
+
+	t.Run("encodes body as JSON", func(t *testing.T) {
+		type payload struct {
+			Name  string `json:"name"`
+			Count int    `json:"count"`
+		}
+		rec := httptest.NewRecorder()
+		WriteJSONResponse(rec, http.StatusOK, payload{Name: "test", Count: 42})
+
+		var got payload
+		err := json.NewDecoder(rec.Body).Decode(&got)
+		require.NoError(t, err)
+		assert.Equal(t, "test", got.Name)
+		assert.Equal(t, 42, got.Count)
+	})
+
+	t.Run("encodes map body as JSON", func(t *testing.T) {
+		rec := httptest.NewRecorder()
+		WriteJSONResponse(rec, http.StatusOK, map[string]interface{}{
+			"error":   "not found",
+			"code":    404,
+			"details": []string{"a", "b"},
+		})
+
+		var got map[string]interface{}
+		err := json.NewDecoder(rec.Body).Decode(&got)
+		require.NoError(t, err)
+		assert.Equal(t, "not found", got["error"])
+		assert.Equal(t, float64(404), got["code"])
+		assert.Len(t, got["details"], 2)
+	})
+
+	t.Run("encodes nil body as JSON null", func(t *testing.T) {
+		rec := httptest.NewRecorder()
+		WriteJSONResponse(rec, http.StatusOK, nil)
+
+		assert.Equal(t, http.StatusOK, rec.Code)
+		assert.JSONEq(t, "null", rec.Body.String())
+	})
+
+	t.Run("encodes empty struct as empty JSON object", func(t *testing.T) {
+		rec := httptest.NewRecorder()
+		WriteJSONResponse(rec, http.StatusOK, struct{}{})
+
+		assert.JSONEq(t, "{}", rec.Body.String())
+	})
+
+	t.Run("encodes slice body as JSON array", func(t *testing.T) {
+		rec := httptest.NewRecorder()
+		WriteJSONResponse(rec, http.StatusOK, []string{"alpha", "beta"})
+
+		var got []string
+		err := json.NewDecoder(rec.Body).Decode(&got)
+		require.NoError(t, err)
+		assert.Equal(t, []string{"alpha", "beta"}, got)
+	})
+
+	t.Run("encodes nested structs", func(t *testing.T) {
+		type inner struct {
+			ID int `json:"id"`
+		}
+		type outer struct {
+			Items []inner `json:"items"`
+		}
+		rec := httptest.NewRecorder()
+		WriteJSONResponse(rec, http.StatusOK, outer{Items: []inner{{ID: 1}, {ID: 2}}})
+
+		var got outer
+		err := json.NewDecoder(rec.Body).Decode(&got)
+		require.NoError(t, err)
+		require.Len(t, got.Items, 2)
+		assert.Equal(t, 1, got.Items[0].ID)
+		assert.Equal(t, 2, got.Items[1].ID)
+	})
+
+	t.Run("body with special characters", func(t *testing.T) {
+		rec := httptest.NewRecorder()
+		WriteJSONResponse(rec, http.StatusOK, map[string]string{
+			"msg": `hello "world" & <friends>`,
+		})
+
+		var got map[string]string
+		err := json.NewDecoder(rec.Body).Decode(&got)
+		require.NoError(t, err)
+		assert.Equal(t, `hello "world" & <friends>`, got["msg"])
+	})
+
+	t.Run("body with unicode", func(t *testing.T) {
+		rec := httptest.NewRecorder()
+		WriteJSONResponse(rec, http.StatusOK, map[string]string{
+			"greeting": "こんにちは 🌍",
+		})
+
+		var got map[string]string
+		err := json.NewDecoder(rec.Body).Decode(&got)
+		require.NoError(t, err)
+		assert.Equal(t, "こんにちは 🌍", got["greeting"])
+	})
+}