diff --git a/scripts/ci-main-tree-proof.mjs b/scripts/ci-main-tree-proof.mjs
index 397c8958e8..e6533d8011 100644
--- a/scripts/ci-main-tree-proof.mjs
+++ b/scripts/ci-main-tree-proof.mjs
@@ -494,6 +494,7 @@ function assert(condition, message) {
 }
 
 async function selfTest() {
+  if (process.env.CI_TREE_PROOF_SELF_TEST_FORCE_FAIL === "1") assert(false, "forced failure (test hook)");
   const ok = (name, extra = {}) => ({ name, status: "completed", conclusion: "success", ...extra });
   const full = [
     { ...ok("Change scope"), id: 501 },
@@ -775,7 +776,17 @@ function recordTestedTree({ env = process.env, runGit = git } = {}) {
 }
 
 async function main() {
-  if (process.argv.includes("--self-test")) return selfTest();
+  if (process.argv.includes("--self-test")) {
+    // A self-test failure must fail the process. The fail-open handler below exists for the CI
+    // proof path only; applied here it would print a warning and exit 0, hiding broken guards.
+    try {
+      await selfTest();
+    } catch (error) {
+      console.error(`ci-main-tree-proof self-test FAILED: ${String(error?.stack ?? error)}`);
+      process.exitCode = 1;
+    }
+    return;
+  }
   if (process.argv.includes("--record-tested-tree")) {
     try {
       recordTestedTree();
diff --git a/tests/ci-main-tree-proof.test.ts b/tests/ci-main-tree-proof.test.ts
index 8c82ebe49a..9554e74b5b 100644
--- a/tests/ci-main-tree-proof.test.ts
+++ b/tests/ci-main-tree-proof.test.ts
@@ -48,6 +48,34 @@ describe("main tree proof script", () => {
     expect(result.stdout).toContain("ci-main-tree-proof self-test passed.");
   });
 
+  it("exits non-zero when a self-test assertion fails, writing no GitHub outputs", () => {
+    const outDir = mkdtempSync(path.join(tmpdir(), "tree-proof-self-test-"));
+    const outputFile = path.join(outDir, "output");
+    const summaryFile = path.join(outDir, "summary");
+    writeFileSync(outputFile, "");
+    writeFileSync(summaryFile, "");
+    try {
+      const result = spawnSync(process.execPath, ["scripts/ci-main-tree-proof.mjs", "--self-test"], {
+        encoding: "utf8",
+        env: {
+          ...process.env,
+          CI_TREE_PROOF_SELF_TEST_FORCE_FAIL: "1",
+          GITHUB_OUTPUT: outputFile,
+          GITHUB_STEP_SUMMARY: summaryFile,
+        },
+      });
+      expect(result.status).toBe(1);
+      expect(result.stderr).toContain("self-test FAILED");
+      expect(result.stderr).toMatch(/forced failure \(test hook\)[\r\n]\s+at /);
+      expect(result.stdout).not.toContain("self-test passed");
+      expect(result.stdout).not.toContain("coverage=");
+      expect(readFileSync(outputFile, "utf8")).toBe("");
+      expect(readFileSync(summaryFile, "utf8")).toBe("");
+    } finally {
+      rmSync(outDir, { recursive: true, force: true, maxRetries: 5 });
+    }
+  });
+
   it("names exactly the PR-run jobs each group skips, matching ci.yml job names", () => {
     const names = Object.values(jobs).map((job) => String((job as { name?: string }).name ?? ""));
     for (const name of [