diff --git a/cli/engine/cli/main.mjs b/cli/engine/cli/main.mjs
index 9f0b67105..4476d04f1 100644
--- a/cli/engine/cli/main.mjs
+++ b/cli/engine/cli/main.mjs
@@ -105,6 +105,13 @@ function formatFindings(findings, jsonMode) {
 // `optionsFor` maps a local path to scan options carrying that path's own
 // project design system (or base options when null). Falls back to a plain
 // object so direct/legacy callers still work.
+async function detectLocalFile(filePath, options) {
+  if (HTML_EXTENSIONS.has(path.extname(filePath).toLowerCase())) {
+    return detectHtml(filePath, options);
+  }
+  return detectText(fs.readFileSync(filePath, 'utf-8'), filePath, options);
+}
+
 async function handleStdin(optionsFor = () => ({})) {
   const resolve = typeof optionsFor === 'function' ? optionsFor : () => optionsFor;
   const chunks = [];
@@ -114,9 +121,7 @@ async function handleStdin(optionsFor = () => ({})) {
     const parsed = JSON.parse(input);
     const fp = parsed?.tool_input?.file_path;
     if (fp && fs.existsSync(fp)) {
-      const options = resolve(fp);
-      return HTML_EXTENSIONS.has(path.extname(fp).toLowerCase())
-        ? detectHtml(fp, options) : detectText(fs.readFileSync(fp, 'utf-8'), fp, options);
+      return detectLocalFile(fp, resolve(fp));
     }
   } catch { /* not JSON */ }
   return detectText(input, '<stdin>', resolve(null));
@@ -374,16 +379,10 @@ async function detectCli() {
           }
 
           for (const file of files) {
-            const ext = path.extname(file).toLowerCase();
             // Each file resolves its own project design system (cached by root),
             // so a scan spanning sibling projects applies the right rules per file.
             const fileOptions = scanOptionsFor(file);
-            let fileFindings;
-            if (HTML_EXTENSIONS.has(ext)) {
-              fileFindings = await detectHtml(file, fileOptions);
-            } else {
-              fileFindings = detectText(fs.readFileSync(file, 'utf-8'), file, fileOptions);
-            }
+            const fileFindings = await detectLocalFile(file, fileOptions);
             // Annotate findings with import context
             const importers = importedByMap.get(file);
             if (importers && importers.size > 0) {
@@ -396,13 +395,8 @@ async function detectCli() {
           }
         } else if (stat.isFile()) {
           if (shouldIgnoreDetectionFile(resolved, process.cwd(), detectionConfig)) continue;
-          const ext = path.extname(resolved).toLowerCase();
           const fileOptions = scanOptionsFor(resolved);
-          if (HTML_EXTENSIONS.has(ext)) {
-            allFindings.push(...await detectHtml(resolved, fileOptions));
-          } else {
-            allFindings.push(...detectText(fs.readFileSync(resolved, 'utf-8'), resolved, fileOptions));
-          }
+          allFindings.push(...await detectLocalFile(resolved, fileOptions));
         }
       }
     } finally {
diff --git a/scripts/test-suites.mjs b/scripts/test-suites.mjs
index 7f726fa6e..4fba4df6d 100644
--- a/scripts/test-suites.mjs
+++ b/scripts/test-suites.mjs
@@ -107,6 +107,7 @@ export const SUITES = {
           'tests/detect-antipatterns-fixtures.test.mjs',
           'tests/detect-antipatterns-browser.test.mjs',
           'tests/detect-cli-design-contamination.test.mjs',
+          'tests/detect-cli-stdin-dispatch.test.mjs',
         ],
       },
     ],
diff --git a/tests/detect-cli-stdin-dispatch.test.mjs b/tests/detect-cli-stdin-dispatch.test.mjs
new file mode 100644
index 000000000..2481d00f8
--- /dev/null
+++ b/tests/detect-cli-stdin-dispatch.test.mjs
@@ -0,0 +1,65 @@
+import { after, describe, it } from 'node:test';
+import assert from 'node:assert/strict';
+import fs from 'node:fs';
+import os from 'node:os';
+import path from 'node:path';
+import { spawnSync } from 'node:child_process';
+import { fileURLToPath } from 'node:url';
+
+const root = path.resolve(path.dirname(fileURLToPath(import.meta.url)), '..');
+const cli = path.join(root, 'cli', 'bin', 'cli.js');
+const tempDir = fs.mkdtempSync(path.join(os.tmpdir(), 'impeccable-stdin-dispatch-'));
+
+function detectStdinFile(filePath) {
+  const result = spawnSync(
+    process.execPath,
+    [cli, 'detect', '--json', '--no-config', '--no-design-system'],
+    {
+      input: JSON.stringify({ tool_input: { file_path: filePath } }),
+      encoding: 'utf8',
+    },
+  );
+  assert.equal(result.status, 2, result.stderr);
+  return JSON.parse(result.stdout);
+}
+
+after(() => {
+  fs.rmSync(tempDir, { recursive: true, force: true });
+});
+
+describe('detect CLI stdin file dispatch', () => {
+  it('uses the static HTML engine for an HTML tool-input path', () => {
+    const filePath = path.join(tempDir, 'page.html');
+    fs.writeFileSync(
+      path.join(tempDir, 'page.css'),
+      '.notice { border-left: 4px solid blue; border-radius: 12px; }',
+    );
+    fs.writeFileSync(filePath, `
+      <!doctype html>
+      <link rel="stylesheet" href="./page.css">
+      <div class="notice">Notice</div>
+    `);
+
+    const findings = detectStdinFile(filePath);
+    assert.ok(findings.some(
+      (item) => item.file === filePath && item.antipattern === 'side-tab',
+    ));
+  });
+
+  it('uses the text engine for a non-HTML tool-input path', () => {
+    const filePath = path.join(tempDir, 'styles.css');
+    fs.writeFileSync(filePath, `
+      .grid {
+        background-image:
+          linear-gradient(#eee 1px, transparent 1px),
+          linear-gradient(90deg, #eee 1px, transparent 1px);
+        background-size: 24px 24px;
+      }
+    `);
+
+    const findings = detectStdinFile(filePath);
+    assert.ok(findings.some(
+      (item) => item.file === filePath && item.antipattern === 'codex-grid-background',
+    ));
+  });
+});