diff --git a/claudetool/testdata/passing_to_failing.txtar b/claudetool/testdata/passing_to_failing.txtar
new file mode 100644
index 0000000..31a0556
--- /dev/null
+++ b/claudetool/testdata/passing_to_failing.txtar
@@ -0,0 +1,32 @@
+Going from a passing test to a failing test is a regression
+
+-- p_test.go --
+package p
+
+import "testing"
+
+func TestP(t *testing.T) {
+}
+
+-- .commit --
+Initial commit with passing test
+
+-- p_test.go --
+package p
+
+import "testing"
+
+func TestP(t *testing.T) {
+	t.FailNow()
+}
+
+-- .commit --
+Break test
+
+-- .run_test --
+Test regressions detected between initial commit (149798f10aaf06bc4700bb62d46ed6d8d88e35ad) and HEAD:
+
+1: sketch.dev.TestP: Was passing, now failing
+
+
+Please fix before proceeding.