diff --git a/cmd/ci/writer_test.go b/cmd/ci/writer_test.go
new file mode 100644
index 0000000000..5a64f13004
--- /dev/null
+++ b/cmd/ci/writer_test.go
@@ -0,0 +1,44 @@
+package ci_test
+
+import (
+	"testing"
+
+	"gotest.tools/v3/assert"
+	"knative.dev/func/cmd/ci"
+)
+
+// TestBufferWriter_WriteAndExist exercises the in-memory BufferWriter test double:
+// Exist is false on an empty buffer and true after Write.
+func TestBufferWriter_WriteAndExist(t *testing.T) {
+	bw := ci.NewBufferWriter()
+
+	assert.Assert(t, !bw.Exist("any/path"), "empty buffer should report Exist=false")
+
+	content := []byte("name: deploy")
+	err := bw.Write("some/path", content)
+	assert.NilError(t, err)
+
+	assert.Assert(t, bw.Exist("any/path"), "non-empty buffer should report Exist=true")
+	assert.DeepEqual(t, bw.Buffer.Bytes(), content)
+}
+
+// TestBufferWriter_Write_StoresPath checks that Write records the path that was
+// passed to it.
+func TestBufferWriter_Write_StoresPath(t *testing.T) {
+	bw := ci.NewBufferWriter()
+	path := ".github/workflows/func-deploy.yaml"
+
+	assert.NilError(t, bw.Write(path, []byte("data")))
+	assert.Equal(t, bw.Path, path)
+}
+
+// TestBufferWriter_Write_ResetsBuffer ensures that consecutive Write calls do
+// not accumulate content — each call starts with a fresh buffer.
+func TestBufferWriter_Write_ResetsBuffer(t *testing.T) {
+	bw := ci.NewBufferWriter()
+
+	assert.NilError(t, bw.Write("p", []byte("first")))
+	assert.NilError(t, bw.Write("p", []byte("second")))
+
+	assert.DeepEqual(t, bw.Buffer.Bytes(), []byte("second"))
+}