diff --git a/src/__tests__/args.test.ts b/src/__tests__/args.test.ts
index 711df3a..0d5104f 100644
--- a/src/__tests__/args.test.ts
+++ b/src/__tests__/args.test.ts
@@ -78,6 +78,14 @@ describe('parseArgs()', () => {
     });
   });
 
+  // Regression for #11: a typo'd rule ID must fail loudly, not silently filter
+  // the rule set to nothing and report a passing (false-negative) scan.
+  it('rejects an unknown --rule value with exit code 2', () => {
+    const result = parseArgs(['--rule=NOAUTH', './mcp.json']);
+    expect(result).toMatchObject({ kind: 'error', exitCode: 2 });
+    expect((result as { message: string }).message).toContain('NOAUTH');
+  });
+
   it('detects URL targets and routes them to serverUrl', () => {
     for (const url of [
       'http://x.example.com',
diff --git a/src/args.ts b/src/args.ts
index 7388c92..e2ce54b 100644
--- a/src/args.ts
+++ b/src/args.ts
@@ -36,6 +36,10 @@ const SEVERITY_BY_NAME: Record<string, Severity> = {
   info: Severity.INFO,
 };
 
+/** Valid rule IDs, used to reject typo'd `--rule` values rather than silently
+ * filtering the rule set down to nothing (a false "PASSED" security gate). */
+const VALID_RULE_IDS = new Set<string>(Object.values(RuleId));
+
 /**
  * Parse raw CLI arguments into a structured, side-effect-free result.
  */
@@ -80,7 +84,17 @@ export function parseArgs(args: string[]): CliParseResult {
         };
       }
     } else if (arg.startsWith('--rule=')) {
-      rules.push(arg.slice('--rule='.length) as RuleId);
+      const val = arg.slice('--rule='.length);
+      if (!VALID_RULE_IDS.has(val)) {
+        return {
+          kind: 'error',
+          message:
+            `Error: Unknown rule "${val}". Valid rule IDs: ` +
+            `${Object.values(RuleId).join(', ')}.`,
+          exitCode: 2,
+        };
+      }
+      rules.push(val as RuleId);
     } else if (!arg.startsWith('--')) {
       target = arg;
     } else {
diff --git a/src/cli.ts b/src/cli.ts
index 9f0033f..43cbe69 100644
--- a/src/cli.ts
+++ b/src/cli.ts
@@ -16,7 +16,7 @@
 
 import { scan } from './scanner.js';
 import { toSarif } from './sarif.js';
-import { Severity, Finding } from './types.js';
+import { Severity, Finding, RuleId } from './types.js';
 import { parseArgs } from './args.js';
 
 const SEVERITY_ORDER: Record<string, number> = {
@@ -39,6 +39,9 @@ function printHelp(): void {
   console.log('  --rule=RULE_ID              Run only the specified rule (repeatable)');
   console.log('  --help, -h                  Show this help message');
   console.log('');
+  console.log('Valid rule IDs:');
+  console.log(`  ${Object.values(RuleId).join(', ')}`);
+  console.log('');
   console.log('Examples:');
   console.log('  mcp-security-scanner ./mcp-server.json');
   console.log('  mcp-security-scanner --format=table ./mcp-server.json');