diff --git a/bin/gstack-redact b/bin/gstack-redact
index ccb6e48c5..f4ee0583f 100755
--- a/bin/gstack-redact
+++ b/bin/gstack-redact
@@ -160,13 +160,27 @@ function readLines(path: string | undefined): string[] | undefined {
 
 function buildOpts(): ScanOptions {
   const vis = (arg("--repo-visibility") as RepoVisibility) || "unknown";
-  const maxBytes = arg("--max-bytes");
+  const maxBytesRaw = arg("--max-bytes");
+  let maxBytes: number | undefined;
+  if (maxBytesRaw !== undefined) {
+    // Reject a malformed cap loudly. Silently passing NaN/0/negative would
+    // weaken the engine's fail-closed oversize guard (exit 1 = usage error,
+    // distinct from the 0/2/3 finding-tier codes).
+    const parsed = Number(maxBytesRaw);
+    if (!Number.isInteger(parsed) || parsed <= 0) {
+      process.stderr.write(
+        `gstack-redact: --max-bytes must be a positive integer, got: ${maxBytesRaw}\n`,
+      );
+      process.exit(1);
+    }
+    maxBytes = parsed;
+  }
   return {
     repoVisibility: ["public", "private", "unknown"].includes(vis) ? vis : "unknown",
     allowlist: readLines(arg("--allowlist")),
     selfEmail: arg("--self-email"),
     repoPublicEmails: readLines(arg("--repo-public-emails")),
-    ...(maxBytes ? { maxBytes: parseInt(maxBytes, 10) } : {}),
+    ...(maxBytes !== undefined ? { maxBytes } : {}),
   };
 }
 
diff --git a/lib/redact-engine.ts b/lib/redact-engine.ts
index 88149f5d9..c4c19882c 100644
--- a/lib/redact-engine.ts
+++ b/lib/redact-engine.ts
@@ -253,7 +253,15 @@ function emailAllowed(email: string, opts: ScanOptions): boolean {
 
 export function scan(input: string, opts: ScanOptions = {}): ScanResult {
   const repoVisibility: RepoVisibility = opts.repoVisibility ?? "unknown";
-  const maxBytes = opts.maxBytes ?? DEFAULT_MAX_BYTES;
+  // `??` only catches null/undefined, so a NaN (e.g. from a malformed
+  // `--max-bytes` flag) or a non-positive value would slip through and make
+  // `byteLen > maxBytes` always false — silently turning this fail-CLOSED guard
+  // into fail-OPEN. Treat any invalid cap as "use the known-good default".
+  const requestedMax = opts.maxBytes;
+  const maxBytes =
+    typeof requestedMax === "number" && Number.isFinite(requestedMax) && requestedMax > 0
+      ? requestedMax
+      : DEFAULT_MAX_BYTES;
 
   // Fail CLOSED on oversize input. Check byte length BEFORE heavy work.
   const byteLen = Buffer.byteLength(input, "utf8");
diff --git a/test/gstack-redact-cli.test.ts b/test/gstack-redact-cli.test.ts
index 4808ba53b..d37b06051 100644
--- a/test/gstack-redact-cli.test.ts
+++ b/test/gstack-redact-cli.test.ts
@@ -94,4 +94,15 @@ describe("gstack-redact oversize fails closed", () => {
     expect(code).toBe(3);
     expect(stdout).toContain("too large");
   });
+
+  // Regression: a malformed --max-bytes must error loudly (usage exit 1), not
+  // silently pass NaN into the engine where it would disable the fail-closed
+  // guard. Exit 1 is distinct from the 0/2/3 finding-tier codes.
+  for (const bad of ["notanumber", "-5", "0", "10.5"]) {
+    test(`malformed --max-bytes (${bad}) exits 1 with a clear error`, () => {
+      const { code, stderr } = run(["--max-bytes", bad], "a".repeat(500));
+      expect(code).toBe(1);
+      expect(stderr).toContain("--max-bytes must be a positive integer");
+    });
+  }
 });
diff --git a/test/redact-engine.test.ts b/test/redact-engine.test.ts
index 52c119a19..64f3b5a39 100644
--- a/test/redact-engine.test.ts
+++ b/test/redact-engine.test.ts
@@ -239,6 +239,20 @@ describe("oversize fails CLOSED", () => {
     expect(r.findings[0].id).toBe("engine.input_too_large");
     expect(exitCodeFor(r)).toBe(3);
   });
+
+  // Regression: an invalid cap (NaN/0/negative) must NOT disable the guard.
+  // `?? DEFAULT` did not catch NaN, so `byteLen > NaN` was always false and the
+  // fail-CLOSED guard silently failed OPEN. Invalid caps fall back to the
+  // 1 MiB default, so a >1 MiB input still blocks.
+  for (const bad of [NaN, 0, -1]) {
+    test(`invalid maxBytes (${bad}) falls back to the default cap and still blocks >1 MiB`, () => {
+      const big = "a".repeat(1024 * 1024 + 1);
+      const r = scan(big, { maxBytes: bad });
+      expect(r.oversize).toBe(true);
+      expect(r.findings[0].id).toBe("engine.input_too_large");
+      expect(exitCodeFor(r)).toBe(3);
+    });
+  }
 });
 
 describe("validators", () => {