diff --git a/auth/interop_test.go b/auth/interop_test.go
new file mode 100644
index 0000000..c4a3a8e
--- /dev/null
+++ b/auth/interop_test.go
@@ -0,0 +1,39 @@
+package auth
+
+// §13.5 interop capture (2026-09-01): three codes computed by an independent
+// Python-stdlib TOTP implementation at recorded wall-clock Unix seconds, for
+// the shared RFC secret. Cross-implementation agreement — the Python code
+// also reproduced RFC 4226 App D counter 0 = 755224, anchoring it to the
+// same algorithm. The Go implementation must produce identical codes at the
+// same instants, and VerifyWindow(0) must accept each at its capture time.
+
+import (
+	"testing"
+	"time"
+)
+
+func TestInteropCapturePythonTOTP(t *testing.T) {
+	captures := []struct {
+		unix int64
+		code string
+	}{
+		{1788266430, "398267"},
+		{1788266460, "060110"},
+		{1788266490, "612712"},
+	}
+	for _, c := range captures {
+		at := time.Unix(c.unix, 0)
+		step := Counter(at, 30*time.Second)
+		got, err := HOTP(totpSecret, step, 6)
+		if err != nil {
+			t.Fatalf("HOTP t=%d: %v", c.unix, err)
+		}
+		if got != c.code {
+			t.Errorf("t=%d: Go produced %s, independent impl produced %s", c.unix, got, c.code)
+		}
+		// window=0: the code must verify at exactly its capture instant.
+		if _, err := VerifyWindow(0, totpSecret, c.code, at); err != nil {
+			t.Errorf("t=%d: capture code %s failed window-0 verify: %v", c.unix, c.code, err)
+		}
+	}
+}