diff --git a/css/game.css b/css/game.css
index 0460a3d..4b3528c 100644
--- a/css/game.css
+++ b/css/game.css
@@ -301,6 +301,32 @@ h2, h3 {
     opacity: 0.5;
 }
 
+.buy-max-btn {
+    display: block;
+    width: 100%;
+    margin-top: 6px;
+    background: var(--accent-purple);
+    color: var(--text-primary);
+    border: none;
+    border-radius: 6px;
+    padding: 5px 12px;
+    cursor: pointer;
+    font-weight: bold;
+    font-size: 0.8rem;
+    transition: all 0.2s ease;
+}
+
+.buy-max-btn:hover:not(:disabled) {
+    background: #9b59b6;
+    transform: scale(1.03);
+}
+
+.buy-max-btn:disabled {
+    background: var(--text-muted);
+    cursor: not-allowed;
+    opacity: 0.4;
+}
+
 /* Side Panel Tabs */
 .tabs {
     display: flex;
diff --git a/dev-journal.md b/dev-journal.md
index 8b6f191..f7763bb 100644
--- a/dev-journal.md
+++ b/dev-journal.md
@@ -266,5 +266,31 @@ firstPrestige achievement, save/load round-trip preserving essence.
   (Requires `pip install py_mini_racer`.)
 
 #### Next goals:
-- Achievement progress tracking in stats panel
 - Consider adding a "next essence at X stardust" progress hint
+
+---
+
+### Session 10 - 2026-08-22 (Morning)
+**Status:** ✅ Buy Max (Bulk-Buy) Feature Added
+
+#### What was done:
+- Added a "Buy Max" bulk-buy mechanic — a staple of idle/clicker games that was previously missing (you could only buy one building at a time, forcing many tedious clicks).
+- Added `Game.getMaxAffordable(buildingId)` which computes how many of a given building the player can afford right now using an iterative cost-scaling loop. It is read-only and does not mutate state.
+- Added `Game.buyMaxBuilding(buildingId)` which loops buying one at a time until unaffordable, deducting the correct total stardust and incrementing counts properly (respects prestige costReduction upgrades since it reuses `getBuildingCost`).
+- Wired a "Buy Max" button onto every building card in `UI.renderBuildings()` with an affordable-count badge (e.g. "5x Max").
+- Updated `UI.updateBuildings()` so the Buy Max badge refreshes its count and enabled/disabled state every game tick, keeping it accurate as stardust accrues.
+- Added CSS styling for `.buy-max-btn` (purple theme, hover scale, disabled state) to match existing building-card layout.
+
+#### Files modified:
+- `js/game.js` - added getMaxAffordable() and buyMaxBuilding() methods
+- `js/ui.js` - rendered Buy Max button per building card + tick refresh in updateBuildings()
+- `css/game.css` - .buy-max-btn styles
+- `dev-journal.md` - appended Session 10 entry
+
+#### Verification:
+Wrote a headless Node.js test (`/opt/data/idle-game-test/buy-max-test.mjs`, lives OUTSIDE the repo). Evaluated the real game.js in-process with DOM/UI stubs. 6/6 checks pass: getMaxAffordable matches manual cost-scaling calc, buyMax deducts correct stardust and increments counts correctly, unaffordable purchase returns 0, getMaxAffordable is idempotent, and notifications fire on purchase.
+
+#### Next goals:
+- Consider adding "Buy Max" to upgrades too (upgrades are one-time so less critical, but a bulk "buy all affordable" could be nice).
+- Potential new building tiers or prestige milestones.
+- Sound variations or background music.
diff --git a/js/game.js b/js/game.js
index cb3983c..4f164d1 100644
--- a/js/game.js
+++ b/js/game.js
@@ -233,7 +233,7 @@ const Game = {
     },
 
     /**
-     * Buy a building
+     * Buy a single building
      */
     buyBuilding(buildingId) {
         const cost = this.getBuildingCost(buildingId);
@@ -261,6 +261,63 @@ const Game = {
         return false;
     },
 
+    /**
+     * Calculate how many of a building the player can currently afford.
+     * Does not modify game state.
+     */
+    getMaxAffordable(buildingId) {
+        const building = BUILDINGS[buildingId];
+        if (!building) return 0;
+
+        let stardust = this.state.stardust;
+        let count = 0;
+        let owned = this.state.buildings[buildingId] || 0;
+
+        while (true) {
+            const cost = Math.floor(building.baseCost * Math.pow(this.config.costScaling, owned + count));
+            if (stardust < cost) break;
+            stardust -= cost;
+            count++;
+        }
+
+        return count;
+    },
+
+    /**
+     * Buy as many of a building as the player can afford at once.
+     * Returns the number purchased.
+     */
+    buyMaxBuilding(buildingId) {
+        const building = BUILDINGS[buildingId];
+        if (!building) return 0;
+
+        let count = 0;
+        while (this.state.stardust >= this.getBuildingCost(buildingId)) {
+            const cost = this.getBuildingCost(buildingId);
+            this.state.stardust -= cost;
+            this.state.buildings[buildingId] = (this.state.buildings[buildingId] || 0) + 1;
+            count++;
+        }
+
+        if (count > 0) {
+            UI.showNotification(`Bought ${count}x ${building.name}!`, 'success');
+
+            // Audio feedback
+            Audio.playPurchase();
+
+            // Particle effect on building purchase
+            const card = document.getElementById(`building-${buildingId}`);
+            if (card) {
+                const rect = card.getBoundingClientRect();
+                Particles.createBuildingEffect(rect.left + rect.width / 2, rect.top + rect.height / 2);
+            }
+
+            UI.update();
+        }
+
+        return count;
+    },
+
     /**
      * Buy an upgrade
      */
diff --git a/js/ui.js b/js/ui.js
index bd90adc..ce083ba 100644
--- a/js/ui.js
+++ b/js/ui.js
@@ -69,6 +69,7 @@ const UI = {
                     <div class="building-cost">${Game.formatNumber(cost)} ✨</div>
                     <div class="building-count">${count}</div>
                     <button class="buy-btn" ${!canAfford || locked ? 'disabled' : ''}>Buy</button>
+                    <button class="buy-max-btn" data-building="${id}" ${locked || count === 0 ? 'title="Buy as many as you can afford"' : 'disabled'}>${Game.formatNumber(Game.getMaxAffordable(id))}x Max</button>
                 </div>
             `;
             
@@ -76,6 +77,12 @@ const UI = {
                 card.querySelector('.buy-btn').addEventListener('click', () => {
                     Game.buyBuilding(id);
                 });
+                const maxBtn = card.querySelector('.buy-max-btn');
+                if (maxBtn) {
+                    maxBtn.addEventListener('click', () => {
+                        Game.buyMaxBuilding(id);
+                    });
+                }
             }
             
             container.appendChild(card);
@@ -103,6 +110,15 @@ const UI = {
                 btn.disabled = !canAfford || locked;
             }
             
+            // Buy Max button - reflect how many can be bought at current stardust
+            const maxBtn = card.querySelector('.buy-max-btn');
+            if (maxBtn) {
+                const affordable = Game.getMaxAffordable(id);
+                maxBtn.textContent = `${Game.formatNumber(affordable)}x Max`;
+                maxBtn.disabled = locked || affordable === 0;
+                maxBtn.title = affordable > 0 ? `Buy up to ${affordable} ${building.name}(s)` : 'Not enough stardust';
+            }
+            
             card.classList.toggle('locked', locked);
         }
     },