diff --git a/src/renderer/components/tools/ReplPanel.tsx b/src/renderer/components/tools/ReplPanel.tsx
new file mode 100644
index 0000000..e474e7f
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+++ b/src/renderer/components/tools/ReplPanel.tsx
@@ -0,0 +1,57 @@
+import { useState, useEffect } from 'react';
+import { X } from 'lucide-react';
+import { Textarea } from '@/components/ui/textarea';
+import { Button } from '@/components/ui/button';
+import { useSettingsStore } from '@/stores/settings-store';
+import { renderMarkdown } from '@/lib/markdown';
+
+export function ReplPanel() {
+  const replOpen = useSettingsStore((s) => s.replOpen);
+  const setSetting = useSettingsStore((s) => s.setSetting);
+  const [source, setSource] = useState('# Markdown preview\n\nType here…');
+  const [debouncedSource, setDebouncedSource] = useState(source);
+
+  useEffect(() => {
+    const t = setTimeout(() => setDebouncedSource(source), 300);
+    return () => clearTimeout(t);
+  }, [source]);
+
+  if (!replOpen) return null;
+
+  const html = renderMarkdown(debouncedSource);
+
+  return (
+    <div
+      role="region"
+      aria-label="REPL"
+      className="fixed bottom-0 left-0 right-0 z-40 flex h-[30vh] flex-col border-t border-border bg-card/95 shadow-2xl backdrop-blur"
+      data-testid="repl-panel"
+    >
+      <div className="flex h-8 items-center justify-between border-b border-border bg-card/50 px-3 text-xs">
+        <span className="font-semibold">REPL — Markdown snippet preview</span>
+        <Button
+          variant="ghost"
+          size="icon"
+          aria-label="Close REPL"
+          onClick={() => setSetting('replOpen', false)}
+          className="h-6 w-6"
+        >
+          <X className="h-3 w-3" />
+        </Button>
+      </div>
+      <div className="flex flex-1 overflow-hidden">
+        <Textarea
+          value={source}
+          onChange={(e) => setSource(e.target.value)}
+          className="h-full flex-1 resize-none rounded-none border-r border-border font-mono text-xs"
+          aria-label="Markdown source"
+        />
+        <div
+          className="h-full flex-1 overflow-auto p-3 text-sm"
+          dangerouslySetInnerHTML={{ __html: html }}
+          data-testid="repl-preview"
+        />
+      </div>
+    </div>
+  );
+}
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diff --git a/tests/component/tools/ReplPanel.test.tsx b/tests/component/tools/ReplPanel.test.tsx
new file mode 100644
index 0000000..1230f42
--- /dev/null
+++ b/tests/component/tools/ReplPanel.test.tsx
@@ -0,0 +1,34 @@
+import { describe, it, expect, beforeEach } from 'vitest';
+import { render, screen, waitFor } from '@testing-library/react';
+import userEvent from '@testing-library/user-event';
+import { ReplPanel } from '@/components/tools/ReplPanel';
+import { useSettingsStore } from '@/stores/settings-store';
+
+describe('ReplPanel', () => {
+  beforeEach(() => {
+    localStorage.clear();
+    useSettingsStore.setState({ ...useSettingsStore.getInitialState(), replOpen: true });
+  });
+
+  it('renders the textarea and preview when replOpen is true', () => {
+    render(<ReplPanel />);
+    expect(screen.getByRole('textbox')).toBeInTheDocument();
+    expect(screen.getByText(/repl/i)).toBeInTheDocument();
+  });
+
+  it('does not render when replOpen is false', () => {
+    useSettingsStore.setState({ ...useSettingsStore.getInitialState(), replOpen: false });
+    const { container } = render(<ReplPanel />);
+    expect(container.firstChild).toBeNull();
+  });
+
+  it('typing in the textarea updates the preview', async () => {
+    render(<ReplPanel />);
+    const ta = screen.getByRole('textbox');
+    await userEvent.clear(ta);
+    await userEvent.type(ta, '# Hello');
+    // Wait for 300ms debounce to fire
+    await new Promise((r) => setTimeout(r, 400));
+    expect(screen.getByRole('heading', { name: /hello/i })).toBeInTheDocument();
+  });
+});
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