diff --git a/src/ui/theme/content/message-components.css b/src/ui/theme/content/message-components.css
index a9d8188..794b8b7 100644
--- a/src/ui/theme/content/message-components.css
+++ b/src/ui/theme/content/message-components.css
@@ -8,6 +8,46 @@
   border-radius: var(--pill-radius);
 }
 
+/* Group header — collapsible summary */
+.pi-tool-group__header {
+  display: flex;
+  align-items: center;
+  gap: 6px;
+  width: 100%;
+  padding: 6px 10px;
+  border: none;
+  background: none;
+  cursor: pointer;
+  font-family: var(--font-sans);
+  font-size: var(--text-sm);
+  color: var(--muted-foreground);
+  text-align: left;
+  transition: color var(--duration-fast);
+}
+
+.pi-tool-group__header:hover {
+  color: var(--foreground);
+}
+
+.pi-tool-group__chevron {
+  font-size: 10px;
+  transition: transform var(--duration-fast);
+  flex-shrink: 0;
+}
+
+.pi-tool-group:not(.pi-tool-group--collapsed) .pi-tool-group__chevron {
+  transform: rotate(90deg);
+}
+
+.pi-tool-group__label {
+  font-weight: 500;
+}
+
+/* Collapsed — hide individual tool cards */
+.pi-tool-group--collapsed > tool-message {
+  display: none;
+}
+
 /* Rows inside group */
 .pi-tool-group > tool-message {
   padding: 1px 6px;
diff --git a/src/ui/tool-grouping.ts b/src/ui/tool-grouping.ts
index 48883e1..6f5a59d 100644
--- a/src/ui/tool-grouping.ts
+++ b/src/ui/tool-grouping.ts
@@ -1,8 +1,61 @@
 /**
  * Tool card grouping — wraps consecutive same-tool calls in a single
- * continuous container element. Groups are always expanded.
+ * collapsible container. Groups of 3+ start collapsed; groups of 2 start
+ * expanded (grouping with stripped card chrome only).
  */
 
+/* ── Group header summary ──────────────────────────────── */
+
+/** Map tool-name → human-readable plural label for the group header. */
+const TOOL_GROUP_LABELS: Record<string, string> = {
+  fill_formula: "fill operations",
+  write_cells: "edits",
+  read_range: "reads",
+  format_cells: "format operations",
+  conditional_format: "conditional formats",
+  view_settings: "view changes",
+  chart: "chart operations",
+  execute_office_js: "script executions",
+};
+
+function describeGroup(toolName: string, count: number): string {
+  const label = TOOL_GROUP_LABELS[toolName] ?? `${toolName} calls`;
+  return `${count} ${label}`;
+}
+
+function buildGroupHeader(toolName: string, count: number, collapsed: boolean): HTMLButtonElement {
+  const btn = document.createElement("button");
+  btn.type = "button";
+  btn.className = "pi-tool-group__header";
+  btn.setAttribute("aria-expanded", collapsed ? "false" : "true");
+
+  const chevron = document.createElement("span");
+  chevron.className = "pi-tool-group__chevron";
+  chevron.textContent = "▸";
+
+  const label = document.createElement("span");
+  label.className = "pi-tool-group__label";
+  label.textContent = describeGroup(toolName, count);
+
+  btn.append(chevron, label);
+
+  btn.addEventListener("click", () => {
+    const wrapper = btn.parentElement;
+    if (!wrapper) return;
+    const isCollapsed = wrapper.classList.toggle("pi-tool-group--collapsed");
+    btn.setAttribute("aria-expanded", isCollapsed ? "false" : "true");
+  });
+
+  return btn;
+}
+
+/* ── Collapse threshold ──────────────────────────────────── */
+
+/** Groups with this many or more items start collapsed. */
+const COLLAPSE_THRESHOLD = 3;
+
+/* ── Main entry point ──────────────────────────────────── */
+
 /**
  * Initialise tool-card grouping on the given root element.
  * Returns a cleanup function that disconnects the observer and removes
@@ -11,12 +64,32 @@
 export function initToolGrouping(root: HTMLElement): () => void {
   let rafId = 0;
 
+  /**
+   * Preserve user toggle state across regrouping passes. Keyed on the
+   * first tool-message element of each group — if the user explicitly
+   * expanded or collapsed a group, we restore that state when the group
+   * is rebuilt rather than using the default.
+   */
+  const userToggleState = new WeakMap<Element, boolean>();
+
   /* ── Unwrap existing groups ────────────────────────────── */
 
   function unwrapAll() {
     for (const wrapper of root.querySelectorAll(".pi-tool-group")) {
       const parent = wrapper.parentNode;
       if (!parent) continue;
+
+      // Snapshot user toggle state before unwrapping.
+      const firstMessage = wrapper.querySelector("tool-message");
+      if (firstMessage) {
+        userToggleState.set(
+          firstMessage,
+          wrapper.classList.contains("pi-tool-group--collapsed"),
+        );
+      }
+
+      // Remove injected header before unwrapping.
+      wrapper.querySelector(".pi-tool-group__header")?.remove();
       while (wrapper.firstChild) parent.insertBefore(wrapper.firstChild, wrapper);
       parent.removeChild(wrapper);
     }
@@ -39,23 +112,26 @@ export function initToolGrouping(root: HTMLElement): () => void {
     }
 
     // Identify runs of 2+ consecutive same-name completed tools.
-    const runs: Element[][] = [];
+    const runs: { elements: Element[]; toolName: string }[] = [];
     let currentRun: Element[] = [];
+    let currentToolName = "";
 
     for (const el of toolMessages) {
       const card = el.querySelector(".pi-tool-card");
       if (!card) {
-        if (currentRun.length >= 2) runs.push(currentRun);
+        if (currentRun.length >= 2) runs.push({ elements: currentRun, toolName: currentToolName });
         currentRun = [];
+        currentToolName = "";
         continue;
       }
 
-      const toolName = card.getAttribute("data-tool-name");
+      const toolName = card.getAttribute("data-tool-name") ?? "";
       const cardState = card.getAttribute("data-state");
 
       if (cardState !== "complete" || !toolName) {
-        if (currentRun.length >= 2) runs.push(currentRun);
+        if (currentRun.length >= 2) runs.push({ elements: currentRun, toolName: currentToolName });
         currentRun = [];
+        currentToolName = "";
         continue;
       }
 
@@ -67,25 +143,36 @@ export function initToolGrouping(root: HTMLElement): () => void {
         if (prevName === toolName && areConsecutiveSiblings(prev, el)) {
           currentRun.push(el);
         } else {
-          if (currentRun.length >= 2) runs.push(currentRun);
+          if (currentRun.length >= 2) runs.push({ elements: currentRun, toolName: currentToolName });
           currentRun = [el];
+          currentToolName = toolName;
         }
       } else {
         currentRun.push(el);
+        currentToolName = toolName;
       }
     }
-    if (currentRun.length >= 2) runs.push(currentRun);
+    if (currentRun.length >= 2) runs.push({ elements: currentRun, toolName: currentToolName });
 
-    // Wrap each run in a container element.
+    // Wrap each run in a container element with a collapsible header.
     for (const run of runs) {
-      const leader = run[0];
-      const members = run.slice(1);
+      const leader = run.elements[0];
+      const members = run.elements.slice(1);
+      const count = run.elements.length;
+
+      // Restore user toggle state if available, otherwise use default.
+      const savedState = userToggleState.get(leader);
+      const collapsed = savedState ?? count >= COLLAPSE_THRESHOLD;
 
       const wrapper = document.createElement("div");
-      wrapper.className = "pi-tool-group";
+      wrapper.className = "pi-tool-group" + (collapsed ? " pi-tool-group--collapsed" : "");
+
+      // Inject group header with summary + expand/collapse toggle.
+      const header = buildGroupHeader(run.toolName, count, collapsed);
+      wrapper.appendChild(header);
 
       if (leader.parentNode) leader.parentNode.insertBefore(wrapper, leader);
-      for (const el of run) wrapper.appendChild(el);
+      for (const el of run.elements) wrapper.appendChild(el);
 
       for (const m of members) m.classList.add("pi-group-member");
     }