diff --git a/source/astatic/cyclus.css_t b/source/astatic/cyclus.css_t
index 834dd6ba..81b50a2b 100644
--- a/source/astatic/cyclus.css_t
+++ b/source/astatic/cyclus.css_t
@@ -128,3 +128,92 @@ div.sphinxsidebar ul li a:hover {
     background: none !important; 
     border-color: transparent !important; 
 }
+
+/* Sidebar toggle button */
+.sidebar-toggle-button {
+    position: absolute;
+    top: 10px;
+    right: 10px;
+    z-index: 1000;
+    width: 25px;
+    height: 25px;
+    padding: 0;
+    margin: 0;
+    border: 2px solid #4b1a07;
+    border-radius: 4px;
+    background-color: #fcf1df;
+    color: #4b1a07;
+    font-size: 12px;
+    font-weight: bold;
+    cursor: pointer;
+    display: flex;
+    align-items: center;
+    justify-content: center;
+    box-shadow: 0 2px 4px rgba(0, 0, 0, 0.2);
+    transition: all 0.2s ease;
+}
+
+.sidebar-toggle-button:hover {
+    background-color: #4b1a07;
+    color: #fcf1df;
+    box-shadow: 0 2px 6px rgba(75, 26, 7, 0.4);
+}
+
+.sidebar-toggle-button:active {
+    transform: scale(0.95);
+}
+
+.sidebar-toggle-button:focus {
+    outline: 2px solid #bb3f3f;
+    outline-offset: 2px;
+}
+
+/* Button when collapsed */
+.sidebar-toggle-button.sidebar-toggle-collapsed {
+    position: absolute !important;
+    display: flex !important;
+    visibility: visible !important;
+    opacity: 1 !important;
+    z-index: 1000 !important;
+    transition: none !important; /* No animation when moving */
+}
+
+/* Custom tooltip */
+.sidebar-toggle-button::after {
+    content: attr(title);
+    position: absolute;
+    bottom: 100%;
+    margin-bottom: 5px;
+    padding: 3px 6px;
+    background-color: #34312e;
+    color: #fcf1df;
+    font-size: 11px;
+    white-space: nowrap;
+    border-radius: 3px;
+    opacity: 0;
+    pointer-events: none;
+    transition: opacity 0.1s ease;
+    z-index: 1001;
+}
+
+/* Tooltip when sidebar is expanded (center-aligned) */
+.sidebar-toggle-button::after {
+    left: 50%;
+    transform: translateX(-50%); /* Center align */
+}
+
+/* Tooltip when sidebar is collapsed (left-aligned) */
+.sidebar-toggle-button.sidebar-toggle-collapsed::after {
+    left: 0;
+    transform: none; /* Remove centering, align to left */
+}
+
+.sidebar-toggle-button:hover::after {
+    opacity: 1;
+}
+
+/* Bodywrapper when sidebar is collapsed */
+body.sidebar-collapsed-body .bodywrapper {
+    background-color: white !important;
+    position: relative;
+}
diff --git a/source/astatic/sidebar-toggle.js b/source/astatic/sidebar-toggle.js
new file mode 100644
index 00000000..5d8c2c3c
--- /dev/null
+++ b/source/astatic/sidebar-toggle.js
@@ -0,0 +1,250 @@
+// Sidebar collapse functionality for Cyclus documentation
+(function() {
+    'use strict';
+    
+    const COLLAPSE_THRESHOLD = 0.55; // Auto-collapse when window width <= 55% of screen width
+    const STORAGE_KEY = 'cyclus-sidebar-collapsed';
+    
+    let sidebar = null;
+    let toggleButton = null;
+    
+    function init() {
+        if (document.readyState === 'loading') {
+            document.addEventListener('DOMContentLoaded', setupSidebarToggle);
+        } else {
+            setupSidebarToggle();
+        }
+    }
+    
+    function setupSidebarToggle() {
+        // Find the sidebar element
+        sidebar = document.querySelector('.sphinxsidebar');
+        if (!sidebar) {
+            console.warn('Sidebar element not found');
+            return;
+        }
+        
+        // Create toggle button
+        toggleButton = document.createElement('button');
+        toggleButton.id = 'sidebar-toggle-btn';
+        toggleButton.className = 'sidebar-toggle-button';
+        toggleButton.setAttribute('aria-label', 'Toggle sidebar');
+        toggleButton.setAttribute('title', 'Collapse sidebar');
+        toggleButton.innerHTML = '◀';
+        
+        // Initially place button in sidebar (top-right)
+        sidebar.style.position = 'relative';
+        sidebar.appendChild(toggleButton);
+        
+        // Check initial state
+        const wasCollapsed = localStorage.getItem(STORAGE_KEY) === 'true';
+        const shouldAutoCollapse = checkAutoCollapse();
+        
+        if (wasCollapsed || shouldAutoCollapse) {
+            collapseSidebar(true);
+        }
+        
+        // Button click handler
+        toggleButton.addEventListener('click', function() {
+            const isCollapsed = sidebar.style.display === 'none';
+            if (isCollapsed) {
+                expandSidebar();
+            } else {
+                collapseSidebar(false);
+            }
+        });
+        
+        // Window resize handler for auto-collapse and button repositioning
+        let resizeTimeout;
+        window.addEventListener('resize', function() {
+            clearTimeout(resizeTimeout);
+            resizeTimeout = setTimeout(function() {
+                const shouldCollapse = checkAutoCollapse();
+                const isCollapsed = sidebar.style.display === 'none';
+                
+                // If sidebar is collapsed, update button position and bodywrapper width
+                if (isCollapsed && toggleButton.classList.contains('sidebar-toggle-collapsed')) {
+                    // Recalculate bodywrapper width to match navbar
+                    const bodyWrapper = document.querySelector('.bodywrapper');
+                    if (bodyWrapper) {
+                        const relatedNav = document.querySelector('.related');
+                        if (relatedNav) {
+                            const navRect = relatedNav.getBoundingClientRect();
+                            const docWrapper = document.querySelector('.documentwrapper');
+                            const docLeft = docWrapper ? docWrapper.getBoundingClientRect().left : 0;
+                            
+                            const relativeNavLeft = navRect.left - docLeft;
+                            const navWidth = navRect.right - navRect.left;
+                            
+                            bodyWrapper.style.marginLeft = relativeNavLeft + 'px';
+                            bodyWrapper.style.width = navWidth + 'px';
+                            bodyWrapper.offsetHeight; // Force reflow
+                        }
+                    }
+                    updateCollapsedButtonPosition();
+                }
+                
+                if (shouldCollapse && !isCollapsed) {
+                    collapseSidebar(true);
+                } else if (!shouldCollapse && isCollapsed && localStorage.getItem(STORAGE_KEY) !== 'true') {
+                    expandSidebar();
+                }
+            }, 150);
+        });
+    }
+    
+    function checkAutoCollapse() {
+        const windowWidth = window.innerWidth || document.documentElement.clientWidth;
+        const screenWidth = screen.width;
+        return windowWidth <= (screenWidth * COLLAPSE_THRESHOLD);
+    }
+    
+    function updateCollapsedButtonPosition(targetTop) {
+        // If targetTop not provided, maintain current vertical position
+        if (targetTop === undefined) {
+            const rect = toggleButton.getBoundingClientRect();
+            targetTop = rect.top;
+            
+            // Ensure button is below navbar
+            const relatedNav = document.querySelector('.related');
+            if (relatedNav) {
+                const navBottom = relatedNav.getBoundingClientRect().bottom;
+                if (targetTop < navBottom + 10) {
+                    targetTop = navBottom + 10;
+                }
+            }
+        }
+        
+        // Get bodywrapper's current position
+        const bodyWrapper = document.querySelector('.bodywrapper');
+        let targetLeft = '10px';
+        if (bodyWrapper) {
+            // Force layout recalculation to get updated position
+            bodyWrapper.offsetHeight;
+            const bodyRect = bodyWrapper.getBoundingClientRect();
+            targetLeft = (bodyRect.left + 10) + 'px'; // 10px padding from left edge
+        }
+        
+        // Disable transition for instant positioning
+        toggleButton.style.transition = 'none';
+        toggleButton.style.top = targetTop + 'px';
+        toggleButton.style.left = targetLeft;
+        
+        // Re-enable transition after a brief moment (for hover effects)
+        setTimeout(function() {
+            toggleButton.style.transition = '';
+        }, 10);
+    }
+    
+    function collapseSidebar(isAutoCollapse) {
+        if (!sidebar || !toggleButton) return;
+        
+        // Hide sidebar
+        sidebar.style.display = 'none';
+        
+        // Move button to body first
+        if (sidebar.contains(toggleButton)) {
+            document.body.appendChild(toggleButton);
+        }
+        
+        toggleButton.classList.add('sidebar-toggle-collapsed');
+        toggleButton.style.position = 'fixed';
+        
+        // Adjust bodywrapper to match navbar width FIRST
+        const bodyWrapper = document.querySelector('.bodywrapper');
+        if (bodyWrapper) {
+            const relatedNav = document.querySelector('.related');
+            if (relatedNav) {
+                const navRect = relatedNav.getBoundingClientRect();
+                const navStyles = window.getComputedStyle(relatedNav);
+                const docWrapper = document.querySelector('.documentwrapper');
+                const docLeft = docWrapper ? docWrapper.getBoundingClientRect().left : 0;
+                
+                // Calculate relative position
+                const relativeNavLeft = navRect.left - docLeft;
+                const navWidth = navRect.right - navRect.left;
+                
+                bodyWrapper.style.marginLeft = relativeNavLeft + 'px';
+                bodyWrapper.style.width = navWidth + 'px';
+                bodyWrapper.style.backgroundColor = 'white';
+                // No padding needed - button is fixed and won't scroll with content
+                bodyWrapper.classList.add('sidebar-collapsed-body');
+                
+                // Force layout recalculation
+                bodyWrapper.offsetHeight;
+            }
+        }
+        
+        // NOW position button AFTER bodywrapper has been repositioned
+        // Get button's current vertical position
+        const rect = toggleButton.getBoundingClientRect();
+        let targetTop = rect.top;
+        
+        // Ensure button is below navbar
+        const relatedNav = document.querySelector('.related');
+        if (relatedNav) {
+            const navBottom = relatedNav.getBoundingClientRect().bottom;
+            if (targetTop < navBottom + 10) {
+                targetTop = navBottom + 10;
+            }
+        }
+        
+        // Position button at bodywrapper's left edge with a bit of breathing room
+        // (Called after bodywrapper is repositioned)
+        updateCollapsedButtonPosition(targetTop);
+        
+        toggleButton.innerHTML = '▶';
+        toggleButton.setAttribute('title', 'Expand sidebar');
+        
+        // Store state
+        if (!isAutoCollapse) {
+            localStorage.setItem(STORAGE_KEY, 'true');
+        }
+        
+        document.body.classList.add('sidebar-collapsed-body');
+    }
+    
+    function expandSidebar() {
+        if (!sidebar || !toggleButton) return;
+        
+        // Show sidebar
+        sidebar.style.display = '';
+        
+        // Move button back to sidebar
+        if (document.body.contains(toggleButton)) {
+            sidebar.appendChild(toggleButton);
+        }
+        
+        toggleButton.classList.remove('sidebar-toggle-collapsed');
+        toggleButton.style.position = 'absolute';
+        
+        // Disable transition for instant positioning
+        toggleButton.style.transition = 'none';
+        toggleButton.style.top = '10px';
+        toggleButton.style.right = '10px';
+        toggleButton.style.left = 'auto';
+        toggleButton.innerHTML = '◀';
+        toggleButton.setAttribute('title', 'Collapse sidebar');
+        
+        // Re-enable transition after a brief moment (for hover effects)
+        setTimeout(function() {
+            toggleButton.style.transition = '';
+        }, 10);
+        
+        // Restore bodywrapper
+        const bodyWrapper = document.querySelector('.bodywrapper');
+        if (bodyWrapper) {
+            bodyWrapper.style.marginLeft = '';
+            bodyWrapper.style.width = '';
+            bodyWrapper.style.backgroundColor = '';
+            bodyWrapper.classList.remove('sidebar-collapsed-body');
+        }
+        
+        // Clear state
+        localStorage.removeItem(STORAGE_KEY);
+        document.body.classList.remove('sidebar-collapsed-body');
+    }
+    
+    init();
+})();
+
diff --git a/source/atemplates/layout.html b/source/atemplates/layout.html
new file mode 100644
index 00000000..4e012719
--- /dev/null
+++ b/source/atemplates/layout.html
@@ -0,0 +1,8 @@
+{# Extend the cloud theme's layout and add sidebar toggle script #}
+{% extends "!layout.html" %}
+
+{% block extrahead %}
+{{ super() }}
+<script type="text/javascript" src="{{ pathto('_static/sidebar-toggle.js', 1) }}"></script>
+{% endblock %}
+