diff --git a/Termind/App/RootView.swift b/Termind/App/RootView.swift
index b9abcc0d..211349e2 100644
--- a/Termind/App/RootView.swift
+++ b/Termind/App/RootView.swift
@@ -19,6 +19,9 @@ struct RootView: View {
     /// split view's minimum sidebar width are rejected so the tab strip keeps
     /// the user's last visible boundary.
     @State private var pinnedTabStripWidth = Metrics.Sidebar.ideal
+    /// Drives the sidebar's responsive upper bound. Start at the default so the
+    /// first layout pass already has a sensible value before geometry reports.
+    @State private var layoutWidth = Metrics.Window.defaultWidth
 
     /// `nonisolated` because geometry transforms are `Sendable` closures and
     /// this coordinate-space name is immutable, not main-actor state.
@@ -45,6 +48,11 @@ struct RootView: View {
             maxHeight: .infinity
         )
         .coordinateSpace(.named(Self.layoutSpace))
+        .onGeometryChange(for: CGFloat.self) {
+            $0.size.width
+        } action: {
+            layoutWidth = $0
+        }
         .toolbarBackground(WindowChrome.background(for: colorScheme), for: .windowToolbar)
         .toolbarBackground(.visible, for: .windowToolbar)
         // ⌘T and every Connect entry land here. A sheet rather than a page,
@@ -123,8 +131,8 @@ struct RootView: View {
                 SidebarView(detailCatalog: .standard(store: app.hostLibrary))
                     .navigationSplitViewColumnWidth(
                         min: Metrics.Sidebar.min,
-                        ideal: Metrics.Sidebar.ideal,
-                        max: Metrics.Sidebar.max
+                        ideal: min(Metrics.Sidebar.ideal, sidebarMaxWidth),
+                        max: sidebarMaxWidth
                     )
             } detail: {
                 VStack(spacing: 0) {
@@ -153,11 +161,15 @@ struct RootView: View {
         VaultSidebarView()
             .navigationSplitViewColumnWidth(
                 min: Metrics.Sidebar.min,
-                ideal: Metrics.Sidebar.ideal,
-                max: Metrics.Sidebar.max
+                ideal: min(Metrics.Sidebar.ideal, sidebarMaxWidth),
+                max: sidebarMaxWidth
             )
     }
 
+    private var sidebarMaxWidth: CGFloat {
+        Metrics.Window.sidebarMaxWidth(for: layoutWidth)
+    }
+
     private func recordPinnedTabStripWidth(_ measuredDetailLeadingEdge: CGFloat) {
         pinnedTabStripWidth = PinnedTopLevelTabSizing.resolvedStripWidth(
             current: pinnedTabStripWidth,
diff --git a/Termind/Design/DesignTokens.swift b/Termind/Design/DesignTokens.swift
index ee57e383..6d9d808e 100644
--- a/Termind/Design/DesignTokens.swift
+++ b/Termind/Design/DesignTokens.swift
@@ -271,16 +271,30 @@ nonisolated enum Metrics {
         static let detailMin: CGFloat = 320
         static let detailMinHeight: CGFloat = 440
 
+        /// Space to the right of the navigation sidebar in Vault's complete
+        /// working layout: two dividers, the main surface, and an Edit/New
+        /// detail panel.
+        static let trailingContentMinWidth: CGFloat = 1
+            + Metrics.VaultContent.min
+            + 1
+            + Metrics.VaultDetail.min
+
         /// Vault's complete working layout sets the application floor: the
         /// navigation sidebar, main surface, and an Edit/New detail panel, plus
         /// the two dividers between them. Reserving the third column even while
         /// it is closed prevents opening an editor inside an already-too-narrow
         /// window and keeps every host form usable from the moment it appears.
-        static let minWidth: CGFloat = Metrics.Sidebar.min
-            + 1
-            + Metrics.VaultContent.min
-            + 1
-            + Metrics.VaultDetail.min
+        static let minWidth: CGFloat = Metrics.Sidebar.min + trailingContentMinWidth
+
+        /// The sidebar may use only width left after the complete Vault work
+        /// area has been reserved. It grows with the window, but never outside
+        /// its own usability bounds.
+        static func sidebarMaxWidth(for availableWidth: CGFloat) -> CGFloat {
+            min(
+                Metrics.Sidebar.max,
+                max(Metrics.Sidebar.min, availableWidth - trailingContentMinWidth)
+            )
+        }
 
         /// Root content includes the pinned tab strip and its divider above the
         /// detail floor. The macOS title bar is outside this measurement.