diff --git a/src/App.vue b/src/App.vue
index 162bddf..731e8b3 100644
--- a/src/App.vue
+++ b/src/App.vue
@@ -164,6 +164,7 @@ function toggleRun(hash: string) {
 }
 
 function toggleCollapseAll() {
+  if (!built.value.collapsible) return showToast("nothing to fold: no run of 3 or more plain commits");
   collapseAll.value = !collapseAll.value;
   toggled.value = new Set();
 }
@@ -244,7 +245,13 @@ onUnmounted(() => {
       <span v-if="repo" class="path">{{ repo.root }}</span>
       <span class="spacer" />
       <span v-if="repo" class="stats">{{ visibleCommits.length }} commits · {{ visibleRefs.length }}/{{ repo.refs.length }} refs</span>
-      <button class="collapse" :class="{ on: collapseAll }" title="c" @click="toggleCollapseAll">
+      <button
+        class="collapse"
+        :class="{ on: collapseAll && built.collapsible }"
+        :disabled="!built.collapsible"
+        :title="built.collapsible ? 'c' : 'Nothing to fold: no run of 3 or more commits without a branch, tag or merge'"
+        @click="toggleCollapseAll"
+      >
         <svg viewBox="0 0 16 16" width="14" height="14" fill="none" stroke="currentColor" stroke-width="1.6" stroke-linecap="round">
           <circle cx="8" cy="2.5" r="1.6" />
           <circle cx="8" cy="13.5" r="1.6" />
@@ -362,6 +369,7 @@ onUnmounted(() => {
 .spacer { flex: 1; }
 .stats { color: var(--text-dim); font-size: 12px; }
 .collapse { display: inline-flex; align-items: center; gap: 6px; }
+.collapse:disabled { opacity: 0.4; cursor: default; background: none; }
 .collapse.on { border-color: var(--accent); color: var(--accent); background: #b197fc1a; }
 .main { flex: 1; display: flex; min-height: 0; }
 .list { flex: 1; overflow: auto; position: relative; }
diff --git a/src/lib/collapse.test.ts b/src/lib/collapse.test.ts
index 3f00179..d5b041e 100644
--- a/src/lib/collapse.test.ts
+++ b/src/lib/collapse.test.ts
@@ -38,7 +38,14 @@ describe("buildRows", () => {
 
   it("does not collapse runs shorter than three", () => {
     const short = chain("top:p1", "p1:p2", "p2:root", "root");
-    expect(hashes(buildRows(short, new Set(["top"]), true, new Set()))).toHaveLength(4);
+    const r = buildRows(short, new Set(["top"]), true, new Set());
+    expect(hashes(r)).toHaveLength(4);
+    expect(r.collapsible).toBe(false);
+  });
+
+  it("reports when a run can collapse, whether collapsed or not", () => {
+    expect(buildRows(commits, labeled, false, new Set()).collapsible).toBe(true);
+    expect(buildRows(commits, labeled, true, new Set()).collapsible).toBe(true);
   });
 
   it("stops a run at merges, forks and labels", () => {
diff --git a/src/lib/collapse.ts b/src/lib/collapse.ts
index 2429586..ccd20f4 100644
--- a/src/lib/collapse.ts
+++ b/src/lib/collapse.ts
@@ -4,6 +4,8 @@ export interface RowsResult {
   rows: DisplayRow[];
   /** Commit hash → id of the run of plain commits it belongs to. */
   runOf: Map<string, string>;
+  /** Whether any run is long enough to collapse. */
+  collapsible: boolean;
 }
 
 /**
@@ -35,6 +37,7 @@ export function buildRows(
   const hidden = new Set<string>();
   /** First commit of a collapsed run → [last commit, number hidden]. */
   const jump = new Map<string, [string, number]>();
+  let collapsible = false;
 
   for (const c of commits) {
     if (!plain(c.hash) || plain(childOf.get(c.hash))) continue;
@@ -47,6 +50,7 @@ export function buildRows(
     }
     const id = c.hash;
     for (const h of run) runOf.set(h, id);
+    if (run.length >= 3) collapsible = true;
     if (run.length >= 3 && collapseAll !== toggled.has(id)) {
       for (const h of run.slice(1, -1)) hidden.add(h);
       jump.set(c.hash, [run[run.length - 1], run.length - 2]);
@@ -63,5 +67,5 @@ export function buildRows(
         : { commit: c, parents: c.parents.map((hash) => ({ hash, dotted: false })) },
     );
   }
-  return { rows, runOf };
+  return { rows, runOf, collapsible };
 }