diff --git a/apps/x/KNOWLEDGE_FILE_VIEWER.md b/apps/x/KNOWLEDGE_FILE_VIEWER.md
new file mode 100644
index 00000000..55d0f688
--- /dev/null
+++ b/apps/x/KNOWLEDGE_FILE_VIEWER.md
@@ -0,0 +1,246 @@
+# Knowledge File Viewer — Research & Implementation Plan
+
+## Current State
+
+The gap is a single `<pre>` fallback in `App.tsx:4523–4527`. The decision tree today:
+
+```
+selectedPath ends in .md  →  MarkdownEditor (full ProseMirror, works great)
+selectedPath is anything else  →  <pre> raw text dump  ← THIS IS THE ENTIRE GAP
+```
+
+Everything else needed already exists:
+
+| What's needed | What exists | Where |
+|---|---|---|
+| Read binary files | `shell:readFileBase64` IPC handler | `apps/main/src/ipc.ts:648–667` |
+| Read text files | `workspace:readFile` with `encoding` param | `packages/shared/src/ipc.ts:55–67` |
+| File type detection | `attachment-presentation.ts` utilities | `renderer/src/lib/attachment-presentation.ts` |
+| Audio player component | `AudioFileCard` (base64 → `<audio>`) | `renderer/src/components/ai-elements/file-path-card.tsx` |
+| Image thumbnail | `SystemFileCard` (base64 → `<img>`) | Same file as above |
+| Navigate to knowledge path | `onOpenKnowledgeFile` context | `renderer/src/contexts/file-card-context.tsx` |
+
+The 10MB cap on `shell:readFileBase64` is the main constraint to watch.
+
+---
+
+## Recommended Architecture
+
+### The Core Idea: `app://` Custom Protocol
+
+**Never use `file://` for serving local content.** In Electron, `file://` has elevated same-origin privileges — an HTML file loaded that way can read other files from the filesystem.
+
+Register a custom scheme **before `app.whenReady()`** in `apps/main/src/main.ts`:
+
+```typescript
+protocol.registerSchemesAsPrivileged([{
+  scheme: 'app',
+  privileges: {
+    standard: true,
+    secure: true,
+    supportFetchAPI: true,
+    stream: true  // CRITICAL for video seeking (byte-range requests)
+  }
+}]);
+```
+
+Then in the handler, resolve paths inside the workspace root and block traversal:
+
+```typescript
+protocol.handle('app', (req) => {
+  const filePath = resolveAndGuard(req.url, WORKSPACE_ROOT);
+  if (!filePath) return new Response('Forbidden', { status: 403 });
+  return net.fetch(pathToFileURL(filePath).toString());
+});
+```
+
+This single protocol handles images, video, DOCX, and HTML all from one place.
+
+---
+
+## File Type Strategy
+
+### Images (PNG, JPG, WEBP, GIF, SVG, AVIF)
+
+**Approach:** Native `<img>` via `app://` protocol.
+
+```tsx
+<img src={`app://local/${encodeURIComponent(relativePath)}`} className="max-w-full" />
+```
+
+- Chromium renders all of these natively. Zero dependencies.
+- HEIC/HEIF is not natively supported on Windows — use `sharp` in main process to convert to JPEG first.
+- Strip EXIF before sending to LLM (GPS data). `sharp` does this automatically on JPEG output.
+
+---
+
+### Video (MP4, WebM, MOV)
+
+**Approach:** Native `<video>` via `app://` protocol with `stream: true`.
+
+```tsx
+<video controls src={`app://local/${encodeURIComponent(relativePath)}`} className="w-full" />
+```
+
+`stream: true` is the only non-obvious requirement — it enables HTTP byte-range requests so scrubbing/seeking works. Without it, the entire file downloads before playback starts.
+
+**Supported formats:** H.264/AAC in MP4, WebM (VP8/VP9/AV1). MKV partially. For WMV/AVI on Windows, fall back to "Open in system."
+
+**Do NOT route through `shell:readFileBase64`** — 10MB cap will silently fail on real video files. The custom protocol streams directly from disk.
+
+---
+
+### PDF
+
+**Approach:** Chromium's built-in PDFium renderer via `<webview>` with `plugins: true`.
+
+```tsx
+<webview
+  src={`app://local/${encodeURIComponent(relativePath)}`}
+  webpreferences="plugins=on,javascript=off,contextIsolation=on"
+  sandbox
+  style={{ width: '100%', height: '100%' }}
+/>
+```
+
+Requires `webviewTag: true` in the parent BrowserWindow's `webPreferences`. Zero bundle size cost — Chromium already ships PDFium. Native zoom, scroll, print.
+
+**Alternative if you need text extraction / annotations:** `pdfjs-dist` in a sandboxed iframe. ~35MB bundle cost, but gives you page events, text selection, and highlight APIs. Overkill unless annotation features are planned.
+
+---
+
+### HTML Files
+
+**Approach:** Sandboxed `<webview>` in an isolated session partition, with **all network blocked**.
+
+```tsx
+<webview
+  src={`app://local/${encodeURIComponent(relativePath)}`}
+  partition="sandbox-html"
+  webpreferences="contextIsolation=on,nodeIntegration=off"
+  sandbox
+/>
+```
+
+In `main.ts`, create the partition and block all outbound network:
+
+```typescript
+const sandboxSession = session.fromPartition('sandbox-html', { cache: false });
+sandboxSession.setPermissionRequestHandler((_, __, cb) => cb(false));
+sandboxSession.webRequest.onBeforeRequest({ urls: ['*://*/*'] }, (_, cb) =>
+  cb({ cancel: true })
+);
+```
+
+Relative assets (`./style.css`, `./images/photo.jpg`) served via the `app://` handler still work. External requests are silently blocked.
+
+---
+
+### DOCX / DOC
+
+**Approach:** `docx-preview` for display, `mammoth.js` for LLM text extraction. They solve different problems — do not use them as alternatives.
+
+- **`docx-preview`** — reproduces Word's visual layout in the DOM (tables, fonts, headings, images as base64). High fidelity for reading.
+- **`mammoth.js`** — converts to clean semantic HTML, strips all visual formatting. For feeding document content to the model.
+
+```typescript
+// display
+import { renderAsync } from 'docx-preview';
+const buffer = await window.api.readFileBytes(filePath); // needs new IPC handler
+await renderAsync(buffer, containerElement);
+
+// LLM extraction
+import mammoth from 'mammoth';
+const { value: html } = await mammoth.convertToHtml({ arrayBuffer: buffer });
+```
+
+A new `read-file-bytes` IPC handler is needed in `main/src/ipc.ts` that returns a raw `Uint8Array` — the existing `shell:readFileBase64` returns a base64 string which would need decoding.
+
+---
+
+## Split-Pane Layout
+
+**Recommended library: `react-resizable-panels`** (Brian Vaughn, React core team alum). Powers `shadcn/ui`'s `<Resizable>` component. Used in production by OpenAI and Adobe.
+
+```tsx
+import { Panel, PanelGroup, PanelResizeHandle } from 'react-resizable-panels';
+
+<PanelGroup direction="horizontal" autoSaveId="knowledge-chat-layout">
+  <Panel defaultSize={55} minSize={30}>
+    <FileViewer path={selectedPath} />
+  </Panel>
+  <PanelResizeHandle className="w-1.5 bg-border hover:bg-primary/50 transition-colors" />
+  <Panel defaultSize={45} minSize={25}>
+    <ChatView />
+  </Panel>
+</PanelGroup>
+```
+
+`autoSaveId` persists the split ratio to `localStorage` automatically across sessions.
+
+**Alternative: `allotment`** — extracted directly from VS Code's C++ split-view code. Pixel-identical to VS Code. Slightly less React-idiomatic API.
+
+---
+
+## Security Model
+
+| Concern | Pattern |
+|---|---|
+| Local file access | Main process only via `ipcMain.handle`. Renderer never reads filesystem directly. |
+| Protocol | Custom `app://` scheme, not `file://`. All local resources routed through validated handler. |
+| Path traversal | Every path resolved to absolute, checked with `startsWith(WORKSPACE_ROOT)`. |
+| Renderer isolation | `contextIsolation: true`, `nodeIntegration: false`, `sandbox: true`. |
+| Untrusted HTML | Separate `session.fromPartition('sandbox-html')` with network blocked. |
+
+---
+
+## Implementation Steps
+
+### Step 1 — Register `app://` protocol in `main.ts`
+Before `app.whenReady()`. One change, covers images, video, PDF, and HTML.
+
+### Step 2 — Add `read-file-bytes` IPC handler in `ipc.ts`
+Returns raw `Uint8Array` for DOCX rendering. Avoids base64 encode/decode overhead for large files.
+
+### Step 3 — Create `KnowledgeFileViewer` component
+`apps/x/apps/renderer/src/components/knowledge-file-viewer.tsx`
+
+Extension routing:
+
+| Extensions | Renderer |
+|---|---|
+| `.png .jpg .jpeg .webp .gif .svg .avif` | `<img>` via `app://` |
+| `.mp4 .mov .webm` | `<video>` via `app://` |
+| `.pdf` | `<webview plugins sandbox>` |
+| `.html .htm` | `<webview partition="sandbox-html">` |
+| `.docx .doc` | `docx-preview` in sandboxed iframe |
+| `.mp3 .wav .m4a` | Reuse existing `AudioFileCard` |
+| everything else | "Open in system" button (`shell.openPath`) |
+
+### Step 4 — Replace `<pre>` fallback in `App.tsx:4522–4527`
+One-line swap. All routing logic lives in `KnowledgeFileViewer`.
+
+### Step 5 — Add split-pane layout
+Install `react-resizable-panels`, wrap knowledge view (file viewer + chat) in `PanelGroup`.
+
+---
+
+## Dependencies to Add
+
+| Package | Purpose | Bundle cost |
+|---|---|---|
+| `react-resizable-panels` | Split pane layout | ~15KB |
+| `docx-preview` | DOCX visual rendering | ~500KB |
+| `mammoth` | DOCX → semantic HTML for LLM | ~300KB |
+| `pdfjs-dist` | PDF with text extraction (optional) | ~35MB — only if PDFium isn't enough |
+
+Images, video, PDF (via PDFium), and HTML have zero additional dependencies.
+
+---
+
+## What to Avoid
+
+- **`<iframe src="file:///...">` for anything** — always use `app://`.
+- **Routing large files through `shell:readFileBase64`** — 10MB cap silently fails.
+- **Using `mammoth` for display** — it strips all formatting. LLM extraction only.
+- **Assuming `webviewTag` is enabled** — check `main.ts` BrowserWindow creation before shipping PDF/HTML webviews.
diff --git a/apps/x/PLAN.md b/apps/x/PLAN.md
new file mode 100644
index 00000000..188c06d2
--- /dev/null
+++ b/apps/x/PLAN.md
@@ -0,0 +1,83 @@
+# HTML File Rendering — Implementation Plan
+
+## Goal
+Replace the `<pre>` raw text fallback in the knowledge view with a proper HTML file renderer using `<iframe srcdoc sandbox="allow-scripts">`.
+
+## Scope
+- Only HTML file rendering for now
+- No layout changes, no split pane
+- No other file types in this PR
+
+---
+
+## Phase 1 — IPC: Read HTML file content and pass to renderer
+
+### What
+Add an IPC handler in the main process that reads a local HTML file and returns its content as a string to the renderer.
+
+### Work
+1. Add `knowledge:readHtmlFile` handler in `apps/main/src/ipc.ts`
+   - Accepts a workspace-relative path
+   - Resolves to absolute path, validates it stays inside workspace root (path traversal guard)
+   - Reads file as UTF-8 string
+   - Returns the HTML string to renderer
+2. Add the channel type to `packages/shared/src/ipc.ts`
+
+### Test ✅
+- Open a `.html` file from the knowledge tree
+- Console log the returned string in the renderer
+- Verify: correct HTML content is returned, no errors
+- Verify: attempting a path like `../../secret.txt` is rejected with an error
+
+---
+
+## Phase 2 — Renderer: Detect `.html` files and render in iframe
+
+### What
+In `App.tsx`, detect when `selectedPath` is an `.html` file and render it in a sandboxed `<iframe srcdoc>` instead of the `<pre>` fallback.
+
+### Work
+1. In the file loading logic (`App.tsx:1284–1357`), when extension is `.html`:
+   - Call `knowledge:readHtmlFile` via IPC
+   - Store the HTML string in state
+2. In the knowledge view render switch (`App.tsx:4522–4527`):
+   - Add a condition: if extension is `.html` → render `<HtmlFileViewer html={htmlContent} />`
+   - Otherwise fall through to existing `<pre>` fallback
+3. Create `apps/renderer/src/components/html-file-viewer.tsx`:
+   - Accepts `html: string` prop
+   - Renders `<iframe srcdoc={html} sandbox="allow-scripts" />` with full width/height, no border
+
+### Test ✅
+- Open a real `.html` file from the knowledge tree
+- Verify: file renders visually in the iframe (not raw text)
+- Verify: a non-html file still shows the `<pre>` fallback (no regression)
+- Verify: an HTML file with a `<script>` tag runs its JS (allow-scripts works)
+- Verify: an HTML file with `<script src="https://evil.com">` — open network tab, confirm no request is made (allow-same-origin is absent, so external scripts are blocked by default CSP)
+
+---
+
+## Phase 3 — Polish: Loading state, error state, empty file handling
+
+### What
+Handle edge cases so the viewer never shows a broken or confusing UI.
+
+### Work
+1. Loading state — show a spinner while the IPC call is in flight
+2. Error state — if `knowledge:readHtmlFile` throws (file deleted, permission error), show a clean error message with the file path
+3. Empty file — if HTML string is empty, show "This file is empty" instead of a blank iframe
+4. Large files — if file is over a reasonable size limit (e.g. 5MB), show "File too large to preview — Open in system" button that calls `shell.openPath`
+
+### Test ✅
+- Open a valid HTML file → renders correctly
+- Delete the file while it's open, trigger a reload → error state shown cleanly
+- Open an empty `.html` file → "This file is empty" message shown
+- Simulate a file over 5MB → "File too large" message with open button shown
+- Verify: no console errors in any of the above scenarios
+
+---
+
+## Out of scope for this PR
+- PDF, DOCX, image, video rendering
+- Split pane / resizable layout
+- Relative asset loading (`./style.css`) — Phase 2 uses `srcdoc` which has no base URL; assets will not load. Acceptable for now, documented as known limitation.
+- `app://` custom protocol — not needed until we handle relative assets
diff --git a/apps/x/apps/renderer/src/App.tsx b/apps/x/apps/renderer/src/App.tsx
index ec54e95b..96ae3742 100644
--- a/apps/x/apps/renderer/src/App.tsx
+++ b/apps/x/apps/renderer/src/App.tsx
@@ -13,6 +13,7 @@ import { ChatInputWithMentions, type StagedAttachment } from './components/chat-
 import { ChatMessageAttachments } from '@/components/chat-message-attachments'
 import { GraphView, type GraphEdge, type GraphNode } from '@/components/graph-view';
 import { BasesView, type BaseConfig, DEFAULT_BASE_CONFIG } from '@/components/bases-view';
+import { HtmlFileViewer } from '@/components/html-file-viewer';
 import { useDebounce } from './hooks/use-debounce';
 import { SidebarContentPanel } from '@/components/sidebar-content';
 import { SuggestedTopicsView } from '@/components/suggested-topics-view';
@@ -1424,6 +1425,11 @@ function App() {
     }
     const requestId = (fileLoadRequestIdRef.current += 1)
     const pathToLoad = selectedPath
+    // For HTML files, clear stale content immediately so the viewer shows
+    // its loading state instead of rendering the previous file's bytes.
+    if (pathToLoad.toLowerCase().endsWith('.html') || pathToLoad.toLowerCase().endsWith('.htm')) {
+      setFileContent('')
+    }
     let cancelled = false
     ;(async () => {
       try {
@@ -4819,6 +4825,10 @@ function App() {
                       />
                     )}
                   </div>
+                ) : selectedPath?.toLowerCase().endsWith('.html') || selectedPath?.toLowerCase().endsWith('.htm') ? (
+                  <div className="flex-1 min-h-0 overflow-hidden">
+                    <HtmlFileViewer html={fileContent} path={selectedPath} />
+                  </div>
                 ) : (
                   <div className="flex-1 overflow-auto p-4">
                     <pre className="text-sm font-mono text-foreground whitespace-pre-wrap">
diff --git a/apps/x/apps/renderer/src/components/html-file-viewer.tsx b/apps/x/apps/renderer/src/components/html-file-viewer.tsx
new file mode 100644
index 00000000..3aceb117
--- /dev/null
+++ b/apps/x/apps/renderer/src/components/html-file-viewer.tsx
@@ -0,0 +1,38 @@
+import { useEffect, useState } from 'react'
+import { Loader2Icon } from 'lucide-react'
+
+interface HtmlFileViewerProps {
+  html: string
+  path: string
+}
+
+export function HtmlFileViewer({ html, path }: HtmlFileViewerProps) {
+  const [iframeLoaded, setIframeLoaded] = useState(false)
+
+  useEffect(() => {
+    setIframeLoaded(false)
+  }, [path, html])
+
+  const showSpinner = !html || !iframeLoaded
+
+  return (
+    <div className="relative h-full w-full">
+      {html && (
+        <iframe
+          key={path}
+          srcDoc={html}
+          sandbox="allow-scripts"
+          className="h-full w-full border-0 bg-white"
+          title="HTML preview"
+          onLoad={() => setIframeLoaded(true)}
+        />
+      )}
+      {showSpinner && (
+        <div className="absolute inset-0 flex flex-col items-center justify-center gap-3 bg-background text-muted-foreground">
+          <Loader2Icon className="size-6 animate-spin" />
+          <p className="text-sm">Rendering preview…</p>
+        </div>
+      )}
+    </div>
+  )
+}
diff --git a/apps/x/test-fixtures/html-viewer-test.html b/apps/x/test-fixtures/html-viewer-test.html
new file mode 100644
index 00000000..7e5eeb6e
--- /dev/null
+++ b/apps/x/test-fixtures/html-viewer-test.html
@@ -0,0 +1,104 @@
+<!DOCTYPE html>
+<html lang="en">
+<head>
+  <meta charset="UTF-8">
+  <title>HTML Viewer Test</title>
+  <style>
+    body {
+      font-family: -apple-system, BlinkMacSystemFont, sans-serif;
+      max-width: 720px;
+      margin: 40px auto;
+      padding: 24px;
+      color: #1a1a1a;
+      background: #fafafa;
+    }
+    h1 { color: #2563eb; border-bottom: 2px solid #2563eb; padding-bottom: 8px; }
+    .card {
+      background: white;
+      padding: 16px;
+      border-radius: 8px;
+      box-shadow: 0 1px 3px rgba(0,0,0,0.1);
+      margin: 16px 0;
+    }
+    button {
+      background: #2563eb;
+      color: white;
+      border: none;
+      padding: 10px 20px;
+      border-radius: 6px;
+      cursor: pointer;
+      font-size: 14px;
+    }
+    button:hover { background: #1d4ed8; }
+    .counter { font-size: 24px; font-weight: bold; color: #2563eb; }
+    .danger { color: #dc2626; font-weight: 600; }
+    .ok { color: #16a34a; font-weight: 600; }
+    table { width: 100%; border-collapse: collapse; margin-top: 12px; }
+    th, td { padding: 8px; text-align: left; border-bottom: 1px solid #e5e7eb; }
+    th { background: #f3f4f6; }
+  </style>
+</head>
+<body>
+  <h1>HTML Viewer Test Page</h1>
+
+  <div class="card">
+    <h2>1. Inline Styles</h2>
+    <p>If this page looks <span class="ok">styled and clean</span>, inline CSS works.</p>
+  </div>
+
+  <div class="card">
+    <h2>2. Interactive JavaScript</h2>
+    <p>Counter: <span class="counter" id="count">0</span></p>
+    <button onclick="document.getElementById('count').textContent = parseInt(document.getElementById('count').textContent) + 1">
+      Click to increment
+    </button>
+    <p style="margin-top:8px;font-size:13px;color:#666;">If clicking increments the counter, allow-scripts works.</p>
+  </div>
+
+  <div class="card">
+    <h2>3. Sandbox Verification</h2>
+    <p>The next button tries to access <code>window.parent</code>:</p>
+    <button onclick="
+      try {
+        const x = window.parent.location.href;
+        document.getElementById('parentResult').innerHTML = '<span class=&quot;danger&quot;>FAIL: parent accessible — ' + x + '</span>';
+      } catch (e) {
+        document.getElementById('parentResult').innerHTML = '<span class=&quot;ok&quot;>OK: parent blocked — ' + e.message + '</span>';
+      }
+    ">Test parent access</button>
+    <p id="parentResult" style="margin-top:8px;">Click to test.</p>
+  </div>
+
+  <div class="card">
+    <h2>4. External Network Request</h2>
+    <p>Tries to fetch from a remote URL (should fail due to no allow-same-origin):</p>
+    <button onclick="
+      fetch('https://api.github.com/zen')
+        .then(r => r.text())
+        .then(t => document.getElementById('netResult').innerHTML = '<span class=&quot;danger&quot;>Network worked: ' + t + '</span>')
+        .catch(e => document.getElementById('netResult').innerHTML = '<span class=&quot;ok&quot;>Blocked or failed: ' + e.message + '</span>')
+    ">Test fetch</button>
+    <p id="netResult" style="margin-top:8px;">Click to test.</p>
+  </div>
+
+  <div class="card">
+    <h2>5. Table Rendering</h2>
+    <table>
+      <thead><tr><th>Feature</th><th>Status</th></tr></thead>
+      <tbody>
+        <tr><td>Inline CSS</td><td class="ok">Working</td></tr>
+        <tr><td>Inline JS</td><td class="ok">Working</td></tr>
+        <tr><td>Sandbox isolation</td><td class="ok">Active</td></tr>
+      </tbody>
+    </table>
+  </div>
+
+  <div class="card">
+    <h2>6. Auto-run script on load</h2>
+    <p id="loaded" style="font-size:13px;color:#666;">Script did not run.</p>
+    <script>
+      document.getElementById('loaded').innerHTML = '<span class="ok">Script ran on load at ' + new Date().toLocaleTimeString() + '</span>';
+    </script>
+  </div>
+</body>
+</html>