diff --git a/scripts/check-kpi.mjs b/scripts/check-kpi.mjs
index c9b480879..4cd980971 100755
--- a/scripts/check-kpi.mjs
+++ b/scripts/check-kpi.mjs
@@ -55,11 +55,15 @@ let minTimestampMs = Number.POSITIVE_INFINITY;
 let maxTimestampMs = Number.NEGATIVE_INFINITY;
 let exchangesWithTimestamp = 0;
 
-for (const line of lines) {
+for (const [index, line] of lines.entries()) {
   let record;
   try {
     record = JSON.parse(line);
   } catch {
+    if (/^\s*[\[{]/.test(line)) {
+      console.error(`Malformed JSON in KPI log line ${index + 1}.`);
+      process.exit(1);
+    }
     // Wrangler tail can include non-JSON diagnostic noise; preserve that tolerance.
     continue;
   }
diff --git a/scripts/compute-kpi.mjs b/scripts/compute-kpi.mjs
old mode 100755
new mode 100644
diff --git a/test/check-kpi-input-integrity.test.ts b/test/check-kpi-input-integrity.test.ts
index facb6c2d9..9cbf0e5c6 100644
--- a/test/check-kpi-input-integrity.test.ts
+++ b/test/check-kpi-input-integrity.test.ts
@@ -59,6 +59,25 @@ describe("KPI threshold input integrity", () => {
     }
   });
 
+  it("rejects malformed JSON-looking lines instead of silently dropping threshold evidence", () => {
+    const dir = mkdtempSync(join(tmpdir(), "noema-check-kpi-"));
+    try {
+      const logPath = join(dir, "exchange-30d.ndjson");
+      writeFileSync(
+        logPath,
+        '{"event":"http_request","route":"/exchange","status_code":500,"latency_ms":120\n',
+      );
+
+      const result = runCheckKpi(logPath);
+
+      expect(result.status).toBe(1);
+      expect(result.stderr).toContain("Malformed JSON in KPI log line 1");
+      expect(result.stdout).toBe("");
+    } finally {
+      rmSync(dir, { recursive: true, force: true });
+    }
+  });
+
   it("rejects an impossible calendar timestamp instead of normalizing it into window evidence", () => {
     const dir = mkdtempSync(join(tmpdir(), "noema-check-kpi-time-"));
     try {