diff --git a/packages/dsh-plugin-browserskill/src/client/observation-sidebar.tsx b/packages/dsh-plugin-browserskill/src/client/observation-sidebar.tsx
index a4fea16..465a58e 100644
--- a/packages/dsh-plugin-browserskill/src/client/observation-sidebar.tsx
+++ b/packages/dsh-plugin-browserskill/src/client/observation-sidebar.tsx
@@ -64,7 +64,12 @@ export interface SidebarSplitLike {
 export type SidebarNodeLike = SidebarLeafLike | SidebarSplitLike;
 
 export interface SidebarStateLike {
-  splits: SidebarNodeLike;
+  /**
+   * Right-column split tree. DSH 0.1.5 (better-sidebar 0.19+) moved the right
+   * column to dsh's native Sidebar and no longer emits this field, so it is
+   * optional here; earlier versions always provide it.
+   */
+  splits?: SidebarNodeLike;
   bottomSplits: SidebarNodeLike;
   /** Whether the right panel is expanded (the merged drawer on narrow screens). */
   panelOpen?: boolean;
@@ -170,7 +175,11 @@ function* leafNodes(node: SidebarNodeLike): Generator<SidebarLeafLike> {
 /** Whether a tab of our type is already open in either sidebar workbench. */
 export function observationTabOpen(state: SidebarStateLike | undefined): boolean {
   if (state === undefined) return false;
+  // A root may be absent even when the snapshot exists: `splits` is gone on
+  // DSH 0.1.5+ (better-sidebar 0.19+), where the right column belongs to dsh's
+  // native Sidebar. Guard before walking so we do not dereference undefined.
   for (const root of [state.splits, state.bottomSplits]) {
+    if (root === undefined) continue;
     for (const leaf of leafNodes(root)) {
       if (leaf.tabs.some((tab) => tab.type === OBSERVATION_TAB_TYPE)) return true;
     }
diff --git a/packages/dsh-plugin-browserskill/tests/client/observation-sidebar.test.tsx b/packages/dsh-plugin-browserskill/tests/client/observation-sidebar.test.tsx
index cd95f62..9860017 100644
--- a/packages/dsh-plugin-browserskill/tests/client/observation-sidebar.test.tsx
+++ b/packages/dsh-plugin-browserskill/tests/client/observation-sidebar.test.tsx
@@ -117,7 +117,7 @@ function withTabOpened(state: SidebarStateLike, type: string): SidebarStateLike
     }
     return { ...node, children: node.children.map(walk) };
   };
-  return { ...state, splits: walk(state.splits) };
+  return { ...state, splits: state.splits === undefined ? undefined : walk(state.splits) };
 }
 
 function makeSidebar(
@@ -280,6 +280,16 @@ describe("observationTabOpen", () => {
     };
     expect(observationTabOpen(inBottom)).toBe(true);
   });
+
+  it("does not crash when the right-column tree is absent (DSH 0.1.5+)", () => {
+    // better-sidebar 0.19+ no longer emits `splits` (the right column moved to
+    // dsh's native Sidebar); the walk must skip absent roots, not dereference.
+    const noSplits: SidebarStateLike = {
+      bottomSplits: { kind: "leaf", id: "p2", active: null, tabs: [] },
+    };
+    expect(() => observationTabOpen(noSplits)).not.toThrow();
+    expect(observationTabOpen(noSplits)).toBe(false);
+  });
 });
 
 describe("ObservationSidebarTab", () => {