diff --git a/docs/HTML_EXTENSIONS.md b/docs/HTML_EXTENSIONS.md
new file mode 100644
index 0000000..9bf5be0
--- /dev/null
+++ b/docs/HTML_EXTENSIONS.md
@@ -0,0 +1,445 @@
+# 🎨 HTML/Frontend Extensions Architecture
+
+## Overview
+This document defines the **client-side extension system** for Chat Switchboard. Extensions are HTML/JS/CSS modules that can hook into the application lifecycle, add UI elements, intercept messages, and extend functionality.
+
+---
+
+## 🔌 Extension Interface
+
+### Extension Manifest Structure
+```javascript
+{
+  "id": "example-extension",
+  "name": "Example Extension",
+  "version": "1.0.0",
+  "author": "Your Name",
+  "description": "Does something cool",
+  "type": "frontend", // or "hybrid" if it has backend components
+  "permissions": ["ui", "messages", "storage"],
+  "entrypoint": "extension.js",
+  "styles": "extension.css",
+  "hooks": {
+    "onLoad": true,
+    "onMessageSend": true,
+    "onMessageReceive": true,
+    "onModelSwitch": true
+  },
+  "ui": {
+    "toolbar": true,
+    "sidebar": false,
+    "settings": true
+  }
+}
+```
+
+---
+
+## 📦 Extension API
+
+### Core Extension Class
+```javascript
+class ChatSwitchboardExtension {
+  constructor(manifest) {
+    this.manifest = manifest;
+    this.enabled = true;
+  }
+  
+  // Lifecycle hooks
+  onLoad() {}
+  onUnload() {}
+  onEnable() {}
+  onDisable() {}
+  
+  // Message hooks
+  async onMessageSend(message, context) {
+    // Modify message before sending
+    // return modified message or null to cancel
+    return message;
+  }
+  
+  async onMessageReceive(message, context) {
+    // Process received message
+    // return modified message or original
+    return message;
+  }
+  
+  async onMessageDisplay(message, element) {
+    // Modify message DOM before display
+    return element;
+  }
+  
+  // UI hooks
+  onModelSwitch(oldModel, newModel) {}
+  onChatSwitch(oldChatId, newChatId) {}
+  onSettingsOpen() {}
+  
+  // Render hooks
+  renderToolbarButton() {
+    // Return HTML string or DOM element
+    return null;
+  }
+  
+  renderSidebarPanel() {
+    return null;
+  }
+  
+  renderSettingsPanel() {
+    return null;
+  }
+  
+  renderMessageAction(message) {
+    // Add custom actions to message bubbles
+    return null;
+  }
+}
+```
+
+---
+
+## 🎯 Extension Categories
+
+### 1. **UI Extensions**
+Add visual elements and interface enhancements.
+
+**Examples:**
+- Syntax highlighter themes
+- Message templates/snippets
+- Custom emoji pickers
+- Voice input buttons
+- Image/file attachments
+
+**Hooks:** `renderToolbarButton`, `renderSidebarPanel`, `renderMessageAction`
+
+---
+
+### 2. **Message Processors**
+Transform messages before/after sending.
+
+**Examples:**
+- Markdown preprocessor
+- Code formatter
+- Translation layer
+- Prompt templates
+- Token counter
+
+**Hooks:** `onMessageSend`, `onMessageReceive`, `onMessageDisplay`
+
+---
+
+### 3. **Model Extensions**
+Enhance model selection and routing.
+
+**Examples:**
+- Model performance stats
+- Cost calculator
+- Context window manager
+- Model recommendation engine
+- Fallback routing (if model fails, try another)
+
+**Hooks:** `onModelSwitch`, access to `State.settings.model`
+
+---
+
+### 4. **Storage Extensions**
+Extend data persistence capabilities.
+
+**Examples:**
+- Cloud sync (S3, Dropbox)
+- Export formats (PDF, DOCX)
+- Search indexing
+- Tagging system
+- Chat organization
+
+**Permissions:** `storage`
+
+---
+
+### 5. **Tool/Function Extensions**
+Enable LLM function calling (critical for your use case).
+
+**Examples:**
+- Web search tool
+- Calculator
+- Code execution sandbox
+- API caller
+- File system access
+
+**Hooks:** `onFunctionCall`, `registerFunction`
+
+---
+
+## 🛠️ Extension Manager
+
+### Core API
+```javascript
+// Global extension registry
+window.ExtensionManager = {
+  extensions: new Map(),
+  
+  // Register an extension
+  register(manifest, ExtensionClass) {
+    const ext = new ExtensionClass(manifest);
+    this.extensions.set(manifest.id, ext);
+    ext.onLoad();
+    return ext;
+  },
+  
+  // Enable/disable
+  enable(id) {},
+  disable(id) {},
+  unload(id) {},
+  
+  // Hook execution
+  async executeHook(hookName, ...args) {
+    const results = [];
+    for (const [id, ext] of this.extensions) {
+      if (ext.enabled && ext[hookName]) {
+        const result = await ext[hookName](...args);
+        results.push({ id, result });
+      }
+    }
+    return results;
+  },
+  
+  // Get all extensions with specific capability
+  getByPermission(permission) {
+    return Array.from(this.extensions.values())
+      .filter(ext => ext.manifest.permissions.includes(permission));
+  }
+};
+```
+
+---
+
+## 📂 Extension File Structure
+
+```
+extensions/
+├── example-extension/
+│   ├── manifest.json
+│   ├── extension.js
+│   ├── extension.css (optional)
+│   ├── README.md
+│   └── assets/
+│       └── icon.svg
+```
+
+### Loading Extensions
+```javascript
+async function loadExtension(path) {
+  const manifest = await fetch(`${path}/manifest.json`).then(r => r.json());
+  
+  // Load JS
+  const module = await import(`${path}/${manifest.entrypoint}`);
+  
+  // Load CSS if present
+  if (manifest.styles) {
+    const link = document.createElement('link');
+    link.rel = 'stylesheet';
+    link.href = `${path}/${manifest.styles}`;
+    document.head.appendChild(link);
+  }
+  
+  // Register
+  ExtensionManager.register(manifest, module.default);
+}
+```
+
+---
+
+## 🔐 Security & Permissions
+
+### Permission System
+```javascript
+const PERMISSIONS = {
+  ui: "Modify user interface",
+  messages: "Read and modify messages",
+  storage: "Access local storage",
+  network: "Make network requests",
+  settings: "Access user settings",
+  clipboard: "Access clipboard",
+  notifications: "Show notifications"
+};
+```
+
+### Sandboxing (Future)
+- Use iframes for untrusted extensions
+- Content Security Policy restrictions
+- API key scoping (extensions can't access main API key)
+
+---
+
+## 🎨 Example Extensions
+
+### 1. Token Counter
+```javascript
+class TokenCounterExtension extends ChatSwitchboardExtension {
+  onLoad() {
+    this.tokenCount = 0;
+  }
+  
+  renderToolbarButton() {
+    return `
+      <button class="btn btn-small" id="tokenCounter">
+        Tokens: <span id="tokenCount">0</span>
+      </button>
+    `;
+  }
+  
+  async onMessageSend(message, context) {
+    // Rough estimation: 1 token ≈ 4 chars
+    this.tokenCount = Math.ceil(message.content.length / 4);
+    document.getElementById('tokenCount').textContent = this.tokenCount;
+    return message;
+  }
+}
+```
+
+### 2. Code Formatter
+```javascript
+class CodeFormatterExtension extends ChatSwitchboardExtension {
+  async onMessageDisplay(message, element) {
+    if (message.role === 'assistant') {
+      // Find code blocks and apply Prism.js highlighting
+      element.querySelectorAll('pre code').forEach(block => {
+        Prism.highlightElement(block);
+      });
+    }
+    return element;
+  }
+  
+  renderMessageAction(message) {
+    if (message.content.includes('```')) {
+      return `
+        <button class="btn btn-small" onclick="copyAllCode()">
+          📋 Copy All Code
+        </button>
+      `;
+    }
+    return null;
+  }
+}
+```
+
+### 3. Model Router (Smart Fallback)
+```javascript
+class ModelRouterExtension extends ChatSwitchboardExtension {
+  async onMessageSend(message, context) {
+    // If message is long, use high-context model
+    if (message.content.length > 5000) {
+      const originalModel = State.settings.model;
+      State.settings.model = 'claude-3-opus-20240229'; // High context
+      
+      // Restore after response
+      context.onComplete = () => {
+        State.settings.model = originalModel;
+      };
+    }
+    return message;
+  }
+}
+```
+
+---
+
+## 🚀 Integration with App
+
+### Modify `app.js` to support hooks:
+```javascript
+async function sendMessage() {
+  // ... existing code ...
+  
+  // Execute hook before sending
+  const hookResults = await ExtensionManager.executeHook(
+    'onMessageSend', 
+    message, 
+    { chatId: State.currentChatId }
+  );
+  
+  // Check if any extension cancelled the send
+  if (hookResults.some(r => r.result === null)) {
+    return;
+  }
+  
+  // Apply transformations
+  for (const { result } of hookResults) {
+    if (result && result !== message) {
+      message = result;
+    }
+  }
+  
+  // ... continue with API call ...
+}
+```
+
+---
+
+## 📋 Extension Discovery
+
+### Extension Store (Future)
+```javascript
+{
+  "extensions": [
+    {
+      "id": "web-search",
+      "name": "Web Search Tool",
+      "description": "Adds web search capability via function calling",
+      "author": "Chat Switchboard Team",
+      "version": "1.0.0",
+      "downloadUrl": "https://extensions.switchboard/web-search.zip",
+      "verified": true,
+      "rating": 4.8,
+      "downloads": 1250
+    }
+  ]
+}
+```
+
+---
+
+## 🎯 Next Steps
+
+1. **Implement ExtensionManager** in `src/js/extensions.js`
+2. **Add hook points** to existing code (app.js, ui.js, api.js)
+3. **Create example extensions** to validate API
+4. **Build extension settings UI** for enable/disable/configure
+5. **Document extension development** with starter template
+
+---
+
+## 🤝 Extension Development Workflow
+
+```bash
+# Create new extension
+mkdir extensions/my-extension
+cd extensions/my-extension
+
+# Generate manifest
+cat > manifest.json << EOF
+{
+  "id": "my-extension",
+  "name": "My Extension",
+  "version": "1.0.0",
+  "entrypoint": "extension.js",
+  "permissions": ["messages"]
+}
+EOF
+
+# Create extension class
+cat > extension.js << EOF
+class MyExtension extends ChatSwitchboardExtension {
+  onLoad() {
+    console.log('My extension loaded!');
+  }
+}
+export default MyExtension;
+EOF
+
+# Test in dev mode
+# Extensions auto-load from /extensions/ directory
+```
+
+---
+
+**This architecture makes Chat Switchboard infinitely extensible while keeping the core lean.**
\ No newline at end of file