diff --git a/src/tests/headless-triage-uok-gates.test.ts b/src/tests/headless-triage-uok-gates.test.ts
index 2c352c49f..de4687ae2 100644
--- a/src/tests/headless-triage-uok-gates.test.ts
+++ b/src/tests/headless-triage-uok-gates.test.ts
@@ -8,7 +8,14 @@
  * surface=headless, runControl=supervised, permissionProfile=high, traceId=
  * the flowId. The outcome reflects the decision (pass/fail/manual-attention).
  */
-import { mkdirSync, mkdtempSync, readFileSync, readdirSync, rmSync } from "node:fs";
+import {
+	mkdirSync,
+	mkdtempSync,
+	readFileSync,
+	readdirSync,
+	rmSync,
+	writeFileSync,
+} from "node:fs";
 import { tmpdir } from "node:os";
 import { join } from "node:path";
 import { afterEach, beforeEach, describe, expect, test } from "vitest";
@@ -179,4 +186,25 @@ describe("runTriageApply emits gate_run trace events with schema-v2 metadata", (
 		// Review gate must NOT fire because plan validation blocked the flow.
 		expect(reviewEvents).toHaveLength(0);
 	});
+
+	test("trace_write_failure_fails_closed_before_agents_run", async () => {
+		const project = makeProject();
+		writeFileSync(join(project, ".sf", "traces"), "not a directory");
+		let calls = 0;
+
+		const result = await runTriageApply(project, "triage prompt", {
+			candidateCount: 1,
+			allowUntrustedRunner: true,
+			agentRunner: async () => {
+				calls += 1;
+				return { ok: true, output: deciderPlan, exitCode: 0 };
+			},
+		});
+
+		expect(result.ok).toBe(false);
+		expect(result.error).toContain(
+			"UOK gate emission failed for trusted-agent-source-gate",
+		);
+		expect(calls).toBe(0);
+	});
 });