diff --git a/src/infra/restart-handoff.test.ts b/src/infra/restart-handoff.test.ts
index cbcdad97e20..ad138720ef9 100644
--- a/src/infra/restart-handoff.test.ts
+++ b/src/infra/restart-handoff.test.ts
@@ -271,4 +271,26 @@ describe("gateway restart handoff", () => {
       "Recent restart handoff: full-process via launchd; source=plugin-change; reason=plugin source changed; pid=12345; age=2s; expiresIn=57s",
     );
   });
+
+  it("formats restart reasons as a single diagnostic line", () => {
+    expect(
+      formatGatewayRestartHandoffDiagnostic(
+        {
+          kind: GATEWAY_SUPERVISOR_RESTART_HANDOFF_KIND,
+          version: 1,
+          intentId: "intent-1",
+          pid: 12_345,
+          createdAt: 10_000,
+          expiresAt: 70_000,
+          reason: "ok\nFake: bad",
+          source: "operator-restart",
+          restartKind: "full-process",
+          supervisorMode: "external",
+        },
+        12_500,
+      ),
+    ).toBe(
+      "Recent restart handoff: full-process via external; source=operator-restart; reason=ok Fake: bad; pid=12345; age=2s; expiresIn=57s",
+    );
+  });
 });
diff --git a/src/infra/restart-handoff.ts b/src/infra/restart-handoff.ts
index 90c7748af04..c3f5d228b3e 100644
--- a/src/infra/restart-handoff.ts
+++ b/src/infra/restart-handoff.ts
@@ -53,14 +53,22 @@ function formatShortDuration(ms: number): string {
   return remainingSeconds === 0 ? `${minutes}m` : `${minutes}m ${remainingSeconds}s`;
 }
 
+function formatDiagnosticValue(value: string): string {
+  return value
+    .replace(/[\u0000-\u001f\u007f]+/gu, " ")
+    .replace(/\s+/gu, " ")
+    .trim();
+}
+
 export function formatGatewayRestartHandoffDiagnostic(
   handoff: GatewayRestartHandoff,
   now = Date.now(),
 ): string {
+  const reason = handoff.reason ? formatDiagnosticValue(handoff.reason) : undefined;
   const detail = [
     `${handoff.restartKind} via ${handoff.supervisorMode}`,
     `source=${handoff.source}`,
-    handoff.reason ? `reason=${handoff.reason}` : undefined,
+    reason ? `reason=${reason}` : undefined,
     `pid=${handoff.pid}`,
     `age=${formatShortDuration(now - handoff.createdAt)}`,
     `expiresIn=${formatShortDuration(handoff.expiresAt - now)}`,