diff --git a/packages/app/src/context/layout.tsx b/packages/app/src/context/layout.tsx
index 6d93337d7..328a58fd0 100644
--- a/packages/app/src/context/layout.tsx
+++ b/packages/app/src/context/layout.tsx
@@ -24,6 +24,7 @@ export type { ProjectAvatarVariant }
 const AVATAR_COLOR_KEYS = ["pink", "mint", "orange", "purple", "cyan", "lime"] as const
 const DEFAULT_SIDEBAR_WIDTH = 344
 const DEFAULT_FILE_TREE_WIDTH = 200
+const DEFAULT_PANEL_COLUMN_WIDTH = 400
 const DEFAULT_SESSION_WIDTH = 600
 const DEFAULT_TERMINAL_HEIGHT = 280
 export type AvatarColorKey = (typeof AVATAR_COLOR_KEYS)[number]
@@ -269,6 +270,9 @@ export const { use: useLayout, provider: LayoutProvider } = createSimpleContext(
           width: DEFAULT_FILE_TREE_WIDTH,
           tab: "changes" as "changes" | "all",
         },
+        panelColumn: {
+          width: DEFAULT_PANEL_COLUMN_WIDTH,
+        },
         session: {
           width: DEFAULT_SESSION_WIDTH,
         },
@@ -642,6 +646,14 @@ export const { use: useLayout, provider: LayoutProvider } = createSimpleContext(
           setStore("review", "diffStyle", diffStyle)
         },
       },
+      // the session side panel's tabs column (vault + file tabs) — its OWN
+      // width, so the panel never flex-fills the window (Kate 2026-07-27)
+      panelColumn: {
+        width: createMemo(() => store.panelColumn?.width ?? DEFAULT_PANEL_COLUMN_WIDTH),
+        resize(width: number) {
+          setStore("panelColumn", { width })
+        },
+      },
       fileTree: {
         opened: createMemo(() => store.fileTree?.opened ?? true),
         width: createMemo(() => store.fileTree?.width ?? DEFAULT_FILE_TREE_WIDTH),
diff --git a/packages/app/src/pages/session.tsx b/packages/app/src/pages/session.tsx
index 9eae92fbe..c7da6d240 100644
--- a/packages/app/src/pages/session.tsx
+++ b/packages/app/src/pages/session.tsx
@@ -284,7 +284,7 @@ export default function Page() {
   const desktopSidePanelOpen = createMemo(() => desktopReviewOpen() || desktopFileTreeOpen())
   const sessionPanelWidth = createMemo(() => {
     if (!desktopSidePanelOpen()) return "100%"
-    if (desktopReviewOpen()) return `${layout.session.width()}px`
+    if (desktopReviewOpen()) return `calc(100% - ${layout.panelColumn.width()}px)`
     return `calc(100% - ${layout.fileTree.width()}px)`
   })
   const centered = createMemo(() => isDesktop() && !desktopReviewOpen())
@@ -1838,23 +1838,6 @@ export default function Page() {
             <Show when={params.id || !newSessionDesign()}>{composerRegion("dock")}</Show>
           </div>
 
-          <Show when={desktopReviewOpen()}>
-            <div onPointerDown={() => size.start()}>
-              <ResizeHandle
-                classList={{
-                  "-right-1": settings.general.newLayoutDesigns(),
-                }}
-                direction="horizontal"
-                size={layout.session.width()}
-                min={450}
-                max={typeof window === "undefined" ? 1000 : window.innerWidth * 0.45}
-                onResize={(width) => {
-                  size.touch()
-                  layout.session.resize(width)
-                }}
-              />
-            </div>
-          </Show>
         </div>
 
         <SessionSidePanel size={size} />
diff --git a/packages/app/src/pages/session/session-side-panel.tsx b/packages/app/src/pages/session/session-side-panel.tsx
index 39d522840..35d9f8cc4 100644
--- a/packages/app/src/pages/session/session-side-panel.tsx
+++ b/packages/app/src/pages/session/session-side-panel.tsx
@@ -1,4 +1,4 @@
-import { For, Match, Show, Switch, createEffect, createMemo, onCleanup, type JSX } from "solid-js"
+import { For, Match, Show, Switch, createEffect, createMemo, on, onCleanup, type JSX } from "solid-js"
 import { createStore } from "solid-js/store"
 import { createMediaQuery } from "@solid-primitives/media"
 import { Tabs } from "@opencode-ai/ui/tabs"
@@ -63,9 +63,11 @@ export function SessionSidePanel(props: { size: Sizing }) {
       }),
   )
   const open = createMemo(() => tabsOpen() || fileOpen())
+  // the tabs column owns its width (never flex-fills the window) and can be
+  // dragged much narrower — Kate 2026-07-27
   const panelWidth = createMemo(() => {
     if (!open()) return "0px"
-    if (tabsOpen()) return "auto"
+    if (tabsOpen()) return `${layout.panelColumn.width()}px`
     return `${layout.fileTree.width()}px`
   })
   const treeWidth = createMemo(() => (fileOpen() ? `${layout.fileTree.width()}px` : "0px"))
@@ -107,16 +109,38 @@ export function SessionSidePanel(props: { size: Sizing }) {
   // command, and context-tree deep-links open the store; inside a session THIS
   // is the host, so mirror store state into a "vault" tab (and back on close)
   const vaultOpen = createMemo(() => vaultPanel.opened())
-  createEffect(() => {
-    if (!isDesktop()) return
-    if (vaultPanel.opened()) {
-      if (!view().reviewPanel.opened()) view().reviewPanel.open()
-      tabs().open("vault")
-      tabs().setActive("vault")
-    } else {
-      tabs().close("vault")
-    }
-  })
+  // on() scopes tracking to the STORE signal alone — the tab reads/writes in
+  // the callback are untracked. Tracking them looped: tabs().open() writes
+  // the same store the effect would re-read, and Solid spins the effect
+  // until the stack blows (found via Playwright pageerror stack).
+  createEffect(
+    on(
+      () => vaultPanel.opened(),
+      (openNow) => {
+        if (!isDesktop()) return
+        if (openNow) {
+          if (!view().reviewPanel.opened()) view().reviewPanel.open()
+          if (!tabs().all().includes("vault")) tabs().open("vault")
+          if (tabs().active() !== "vault") tabs().setActive("vault")
+        } else if (tabs().all().includes("vault")) {
+          tabs().close("vault")
+        }
+      },
+    ),
+  )
+  // column closed (panel toggle) → the store must follow, or the titlebar
+  // button's next press toggles an invisible state and "does nothing"
+  createEffect(
+    on(
+      tabsOpen,
+      (openNow, wasOpen) => {
+        if (wasOpen && !openNow && vaultPanel.opened()) vaultPanel.close()
+        // a column with nothing to show fills with the vault by default
+        if (!wasOpen && openNow && tabState.activeTab() === "empty") vaultPanel.open()
+      },
+      { defer: true },
+    ),
+  )
 
   const tabState = createSessionTabs({
     tabs,
@@ -187,10 +211,24 @@ export function SessionSidePanel(props: { size: Sizing }) {
           "transition-[width] duration-[240ms] ease-[cubic-bezier(0.22,1,0.36,1)] will-change-[width] motion-reduce:transition-none":
             !props.size.active(),
           "rounded-[10px] shadow-[var(--v2-elevation-raised)] overflow-hidden": settings.general.newLayoutDesigns(),
-          "flex-1": tabsOpen(),
         }}
         style={{ width: panelWidth() }}
       >
+        <Show when={tabsOpen()}>
+          <div onPointerDown={() => props.size.start()}>
+            <ResizeHandle
+              direction="horizontal"
+              edge="start"
+              size={layout.panelColumn.width()}
+              min={240}
+              max={typeof window === "undefined" ? 900 : window.innerWidth * 0.6}
+              onResize={(width) => {
+                props.size.touch()
+                layout.panelColumn.resize(width)
+              }}
+            />
+          </div>
+        </Show>
         <Show when={open()}>
           <div
             class="size-full flex"
@@ -231,12 +269,19 @@ export function SessionSidePanel(props: { size: Sizing }) {
                                 icon="close-small"
                                 variant="ghost"
                                 class="h-5 w-5"
-                                onClick={() => vaultPanel.close()}
+                                onClick={() => {
+                                  vaultPanel.close()
+                                  // nothing else to show → the column goes too
+                                  if (openedTabs().length === 0 && !contextOpen()) view().reviewPanel.close()
+                                }}
                                 aria-label={language.t("amicode.vault.close")}
                               />
                             }
                             hideCloseButton
-                            onMiddleClick={() => vaultPanel.close()}
+                            onMiddleClick={() => {
+                              vaultPanel.close()
+                              if (openedTabs().length === 0 && !contextOpen()) view().reviewPanel.close()
+                            }}
                           >
                             <div>{language.t("amicode.vault.title")}</div>
                           </Tabs.Trigger>
diff --git a/packages/ui/src/amicode/entity-rail.tsx b/packages/ui/src/amicode/entity-rail.tsx
index e859e30da..61e5b151e 100644
--- a/packages/ui/src/amicode/entity-rail.tsx
+++ b/packages/ui/src/amicode/entity-rail.tsx
@@ -67,6 +67,25 @@ interface RailPart {
 
 const RUN_POLL_MS = 2500
 
+// Pending chips are placeholders, not buttons — say WHY they aren't clickable
+// (Kate 2026-07-27: an unexplained dead chip reads as a bug).
+function pendingHint(kind: string): string {
+  switch (kind) {
+    case "pulse":
+      return "No pulse banked yet — a completed run produces one"
+    case "device_session":
+      return "No device session yet — appears when a pulse targets hardware"
+    case "run":
+      return "No run yet — solve the formulation to create one"
+    case "formulation":
+      return "No formulation yet — amico writes one from the problem"
+    case "system":
+      return "No system picked yet"
+    default:
+      return "Not created yet"
+  }
+}
+
 // One line-icon per entity kind (glyphs live in the shared family — icon.tsx).
 function chipIcon(kind: string) {
   switch (kind) {
@@ -235,6 +254,8 @@ export function AmicodeEntityRail(props: {
                       data-slot="amicode-rail-chip"
                       data-stage={chip.kind}
                       data-pending="true"
+                      title={pendingHint(chip.kind)}
+                      aria-label={`${chip.label} — ${pendingHint(chip.kind)}`}
                     >
                       <Icon name={chipIcon(chip.kind)} size="small" />
                       {chip.label}