diff --git a/internal/jsondecoder/jsondecoder_test.go b/internal/jsondecoder/jsondecoder_test.go
new file mode 100644
index 0000000..c1b423b
--- /dev/null
+++ b/internal/jsondecoder/jsondecoder_test.go
@@ -0,0 +1,43 @@
+package jsondecoder_test
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+
+	"github.com/italia/publiccode-validator-api/internal/jsondecoder"
+)
+
+type target struct {
+	Name string `json:"name"`
+}
+
+func TestUnmarshalDisallowUnknownFieldsAcceptsKnownField(t *testing.T) {
+	var v target
+
+	err := jsondecoder.UnmarshalDisallowUnknownFields([]byte(`{"name": "foo"}`), &v)
+
+	assert.NoError(t, err)
+	assert.Equal(t, "foo", v.Name)
+}
+
+func TestUnmarshalDisallowUnknownFieldsRejectsUnknownField(t *testing.T) {
+	tests := []struct {
+		description string
+		data        string
+	}{
+		{"unknown field alone", `{"unknown": true}`},
+		{"unknown field after a known one", `{"name": "foo", "unknown": true}`},
+		{"field differing from a known one", `{"name_": "foo"}`},
+	}
+
+	for _, test := range tests {
+		t.Run(test.description, func(t *testing.T) {
+			var v target
+
+			err := jsondecoder.UnmarshalDisallowUnknownFields([]byte(test.data), &v)
+
+			assert.ErrorIs(t, err, jsondecoder.ErrUnknownField)
+		})
+	}
+}