diff --git a/src/features/chat/chat-state.ts b/src/features/chat/chat-state.ts
index e2b4b277..baa4ad24 100644
--- a/src/features/chat/chat-state.ts
+++ b/src/features/chat/chat-state.ts
@@ -100,6 +100,7 @@ interface ChatComposerState {
 
 interface ChatUiState {
   openDetails: ReadonlySet<string>;
+  toolbarPanel: "history" | "chat-actions" | "status-panel" | null;
 }
 
 export interface ChatState {
@@ -696,6 +697,7 @@ function initialComposerState(): ChatComposerState {
 function initialUiState(): ChatUiState {
   return {
     openDetails: new Set(),
+    toolbarPanel: null,
   };
 }
 
@@ -731,6 +733,7 @@ function cloneChatState(state: ChatState): ChatState {
     },
     ui: {
       openDetails: new Set(state.ui.openDetails),
+      toolbarPanel: state.ui.toolbarPanel,
     },
   };
 }
@@ -740,18 +743,8 @@ function isTranscriptAction(action: ChatSliceAction): action is TranscriptAction
 }
 
 function setPanelSlice(state: ChatUiState, panel: "history" | "chat-actions" | "status-panel" | null, toggle: boolean): ChatUiState {
-  const currentPanel = state.openDetails.has("history")
-    ? "history"
-    : state.openDetails.has("chat-actions")
-      ? "chat-actions"
-      : state.openDetails.has("status-panel")
-        ? "status-panel"
-        : null;
-  const nextPanel = toggle && currentPanel === panel ? null : panel;
-  const toolbarPanelKeys = new Set(["history", "chat-actions", "status-panel"]);
-  return patchObject(state, {
-    openDetails: nextPanel ? new Set([nextPanel]) : new Set([...state.openDetails].filter((key) => !toolbarPanelKeys.has(key))),
-  });
+  const nextPanel = toggle && state.toolbarPanel === panel ? null : panel;
+  return patchObject(state, { toolbarPanel: nextPanel });
 }
 
 function setDetailOpenSlice(state: ChatUiState, key: string, open: boolean): ChatUiState {
diff --git a/src/features/chat/panel/model/toolbar.ts b/src/features/chat/panel/model/toolbar.ts
index badc3528..1340e7d7 100644
--- a/src/features/chat/panel/model/toolbar.ts
+++ b/src/features/chat/panel/model/toolbar.ts
@@ -7,9 +7,9 @@ import type { ConnectionDiagnosticsModelInput, ToolbarThreadRow, ToolbarViewMode
 export function toolbarViewModel(input: ToolbarViewModelInput): ToolbarViewModel {
   const { state, snapshot } = input;
   const limit = rateLimitSummary(snapshot);
-  const historyOpen = state.ui.openDetails.has("history");
-  const chatActionsOpen = state.ui.openDetails.has("chat-actions");
-  const statusPanelOpen = state.ui.openDetails.has("status-panel");
+  const historyOpen = state.ui.toolbarPanel === "history";
+  const chatActionsOpen = state.ui.toolbarPanel === "chat-actions";
+  const statusPanelOpen = state.ui.toolbarPanel === "status-panel";
   return {
     newChatDisabled: input.turnBusy,
     chatActionsOpen,
diff --git a/src/features/chat/panel/snapshot.ts b/src/features/chat/panel/snapshot.ts
index 2b6c1171..38e6fab3 100644
--- a/src/features/chat/panel/snapshot.ts
+++ b/src/features/chat/panel/snapshot.ts
@@ -37,7 +37,7 @@ export function toolbarSlotSnapshot(state: ChatState, connected: boolean): ChatP
     state.runtime.requestedApprovalsReviewer,
     state.runtime.requestedModel,
     state.runtime.requestedReasoningEffort,
-    openDetailsSignature(state.ui.openDetails),
+    state.ui.toolbarPanel,
     state.threadList.threadsLoaded,
     threadListSignature(state.threadList.listedThreads),
     modelsSignature(state.connection.availableModels),
@@ -131,10 +131,6 @@ function stableSignature(value: unknown): string {
   return JSON.stringify(value);
 }
 
-function openDetailsSignature(openDetails: ReadonlySet<string>): string {
-  return [...openDetails].sort().join("\n");
-}
-
 function messageStreamOpenDetailsSignature(openDetails: ReadonlySet<string>): string {
   return [...openDetails].filter(isMessageStreamOpenDetailKey).sort().join("\n");
 }
diff --git a/src/features/chat/panel/toolbar-controller.ts b/src/features/chat/panel/toolbar-controller.ts
index f0eb0101..8134e0a3 100644
--- a/src/features/chat/panel/toolbar-controller.ts
+++ b/src/features/chat/panel/toolbar-controller.ts
@@ -94,11 +94,7 @@ export class ToolbarPanelController {
   }
 
   private hasOpenPanel(): boolean {
-    return (
-      this.state.ui.openDetails.has("history") ||
-      this.state.ui.openDetails.has("chat-actions") ||
-      this.state.ui.openDetails.has("status-panel")
-    );
+    return this.state.ui.toolbarPanel !== null;
   }
 
   private close(): void {
diff --git a/tests/features/chat/chat-state-reducer.test.ts b/tests/features/chat/chat-state-reducer.test.ts
index 23c4dbe3..e4f6e6c3 100644
--- a/tests/features/chat/chat-state-reducer.test.ts
+++ b/tests/features/chat/chat-state-reducer.test.ts
@@ -163,16 +163,16 @@ describe("chatReducer", () => {
   it("clones map and set backed state when updating turn diffs and open panels", () => {
     const state = createChatState();
     state.transcript.turnDiffs = new Map([["turn", "old"]]);
-    state.ui.openDetails = new Set(["status-panel"]);
+    state.ui.toolbarPanel = "status-panel";
 
     const withDiff = chatReducer(state, { type: "transcript/turn-diff-updated", turnId: "turn", diff: "new" });
     const withHistoryPanel = chatReducer(withDiff, { type: "ui/panel-set", panel: "history" });
 
     expect(withDiff.transcript.turnDiffs).not.toBe(state.transcript.turnDiffs);
     expect(withDiff.transcript.turnDiffs.get("turn")).toBe("new");
-    expect(withHistoryPanel.ui.openDetails).not.toBe(withDiff.ui.openDetails);
-    expect(withHistoryPanel.ui.openDetails.has("history")).toBe(true);
-    expect(withHistoryPanel.ui.openDetails.has("status-panel")).toBe(false);
+    expect(withHistoryPanel.ui).not.toBe(withDiff.ui);
+    expect(withHistoryPanel.ui.toolbarPanel).toBe("history");
+    expect(withHistoryPanel.ui.openDetails).toBe(withDiff.ui.openDetails);
   });
 
   it("returns the same state reference for no-op actions", () => {
@@ -352,15 +352,13 @@ describe("chatReducer", () => {
     let state = createChatState();
 
     state = chatReducer(state, { type: "ui/panel-set", panel: "history" });
-    expect(state.ui.openDetails.has("history")).toBe(true);
+    expect(state.ui.toolbarPanel).toBe("history");
 
     state = chatReducer(state, { type: "ui/panel-set", panel: "chat-actions" });
-    expect(state.ui.openDetails.has("history")).toBe(false);
-    expect(state.ui.openDetails.has("chat-actions")).toBe(true);
+    expect(state.ui.toolbarPanel).toBe("chat-actions");
 
     state = chatReducer(state, { type: "ui/panel-set", panel: "status-panel" });
-    expect(state.ui.openDetails.has("chat-actions")).toBe(false);
-    expect(state.ui.openDetails.has("status-panel")).toBe(true);
+    expect(state.ui.toolbarPanel).toBe("status-panel");
   });
 
   it("updates remembered details and user input drafts through typed UI actions", () => {
diff --git a/tests/features/chat/panel/toolbar-controller.test.ts b/tests/features/chat/panel/toolbar-controller.test.ts
index 0dc20269..67cc6c2a 100644
--- a/tests/features/chat/panel/toolbar-controller.test.ts
+++ b/tests/features/chat/panel/toolbar-controller.test.ts
@@ -36,7 +36,7 @@ describe("ToolbarPanelController", () => {
       scheduleRender,
     });
     controller.toggleHistory();
-    expect(stateStore.getState().ui.openDetails.has("history")).toBe(true);
+    expect(stateStore.getState().ui.toolbarPanel).toBe("history");
 
     controller.closeOnOutsidePointer({
       target: document.createElement("button"),
@@ -45,7 +45,7 @@ describe("ToolbarPanelController", () => {
       renameEditing: false,
     });
 
-    expect(stateStore.getState().ui.openDetails.size).toBe(0);
+    expect(stateStore.getState().ui.toolbarPanel).toBeNull();
     expect(scheduleRender).toHaveBeenCalledWith({ forceSlots: true });
   });
 });
diff --git a/tests/features/chat/session/reconnect-actions.test.ts b/tests/features/chat/session/reconnect-actions.test.ts
index 3b963f60..a73ac62a 100644
--- a/tests/features/chat/session/reconnect-actions.test.ts
+++ b/tests/features/chat/session/reconnect-actions.test.ts
@@ -41,7 +41,7 @@ describe("createChatReconnectActions", () => {
 
     await controller.reconnectPanel();
 
-    expect(stateStore.getState().ui.openDetails.size).toBe(0);
+    expect(stateStore.getState().ui.toolbarPanel).toBeNull();
     expect(host.invalidateConnectionWork).toHaveBeenCalledOnce();
     expect(host.invalidateResumeWork).toHaveBeenCalledOnce();
     expect(host.clearDeferredDiagnostics).toHaveBeenCalledOnce();
diff --git a/tests/features/chat/threads/thread-selection-controller.test.ts b/tests/features/chat/threads/thread-selection-controller.test.ts
index 5f474400..830d8fa7 100644
--- a/tests/features/chat/threads/thread-selection-controller.test.ts
+++ b/tests/features/chat/threads/thread-selection-controller.test.ts
@@ -74,7 +74,7 @@ describe("ThreadSelectionController", () => {
 
     await controller.selectThreadFromToolbar("thread");
 
-    expect(stateStore.getState().ui.openDetails.size).toBe(0);
+    expect(stateStore.getState().ui.toolbarPanel).toBeNull();
     expect(host.closeForThreadSelection).toHaveBeenCalledOnce();
     expect(host.resumeThread).toHaveBeenCalledWith("thread");
   });
@@ -87,7 +87,7 @@ describe("ThreadSelectionController", () => {
 
     await controller.selectThreadFromToolbar("other");
 
-    expect(stateStore.getState().ui.openDetails.has("history")).toBe(true);
+    expect(stateStore.getState().ui.toolbarPanel).toBe("history");
     expect(host.addSystemMessage).not.toHaveBeenCalled();
     expect(host.closeForThreadSelection).not.toHaveBeenCalled();
     expect(host.resumeThread).not.toHaveBeenCalled();
diff --git a/tests/features/chat/turns/plan-implementation-actions.test.ts b/tests/features/chat/turns/plan-implementation-actions.test.ts
index 12b2d377..754a85fd 100644
--- a/tests/features/chat/turns/plan-implementation-actions.test.ts
+++ b/tests/features/chat/turns/plan-implementation-actions.test.ts
@@ -75,7 +75,7 @@ describe("createPlanImplementationActions", () => {
 
     expect(ensureConnected).toHaveBeenCalledOnce();
     expect(stateStore.getState().runtime.selectedCollaborationMode).toBe("default");
-    expect(stateStore.getState().ui.openDetails.has("status-panel")).toBe(false);
+    expect(stateStore.getState().ui.toolbarPanel).toBeNull();
     expect(sendTurnText).toHaveBeenCalledWith("Please implement this plan.");
   });
 
diff --git a/tests/features/chat/view-model.test.ts b/tests/features/chat/view-model.test.ts
index d1c36280..d71c8672 100644
--- a/tests/features/chat/view-model.test.ts
+++ b/tests/features/chat/view-model.test.ts
@@ -26,7 +26,7 @@ describe("chat view model", () => {
     state.activeThread.id = "thread-1";
     state.threadList.listedThreads = [threadFixture("thread-1", "Active"), threadFixture("thread-2", "Other")];
     state.turn.lifecycle = { kind: "running", turnId: "turn" };
-    state.ui.openDetails = new Set(["history"]);
+    state.ui.toolbarPanel = "history";
     state.connection.effectiveConfig = effectiveConfigFixture({ model: "gpt-5.5", model_reasoning_effort: "high" });
     state.connection.appServerDiagnostics = createAppServerDiagnostics();
 
diff --git a/tests/features/chat/view-snapshot.test.ts b/tests/features/chat/view-snapshot.test.ts
index 4647ee92..45a54b4a 100644
--- a/tests/features/chat/view-snapshot.test.ts
+++ b/tests/features/chat/view-snapshot.test.ts
@@ -31,7 +31,8 @@ describe("chat view snapshots", () => {
     const state = createChatState();
     const base = messagesSlotSnapshot(state, "");
 
-    state.ui.openDetails = new Set(["history", "chat-actions", "status-panel", "goal:editor"]);
+    state.ui.toolbarPanel = "history";
+    state.ui.openDetails = new Set(["goal:editor"]);
     expect(messagesSlotSnapshot(state, "")).toBe(base);
 
     state.ui.openDetails = new Set(["message:item:expanded"]);