diff --git a/.storybook/mocks/orpc.ts b/.storybook/mocks/orpc.ts
index a1b5fee1da..1e9064aee2 100644
--- a/.storybook/mocks/orpc.ts
+++ b/.storybook/mocks/orpc.ts
@@ -11,6 +11,34 @@ import type { ChatStats } from "@/common/types/chatStats";
 import { DEFAULT_RUNTIME_CONFIG } from "@/common/constants/workspace";
 import { createAsyncMessageQueue } from "@/common/utils/asyncMessageQueue";
 
+/** Session usage data structure matching SessionUsageFileSchema */
+export interface MockSessionUsage {
+  byModel: Record<
+    string,
+    {
+      input: { tokens: number; cost_usd?: number };
+      cached: { tokens: number; cost_usd?: number };
+      cacheCreate: { tokens: number; cost_usd?: number };
+      output: { tokens: number; cost_usd?: number };
+      reasoning: { tokens: number; cost_usd?: number };
+      model?: string;
+    }
+  >;
+  lastRequest?: {
+    model: string;
+    usage: {
+      input: { tokens: number; cost_usd?: number };
+      cached: { tokens: number; cost_usd?: number };
+      cacheCreate: { tokens: number; cost_usd?: number };
+      output: { tokens: number; cost_usd?: number };
+      reasoning: { tokens: number; cost_usd?: number };
+      model?: string;
+    };
+    timestamp: number;
+  };
+  version: 1;
+}
+
 export interface MockORPCClientOptions {
   projects?: Map<string, ProjectConfig>;
   workspaces?: FrontendWorkspaceMetadata[];
@@ -40,6 +68,8 @@ export interface MockORPCClientOptions {
       exitCode?: number;
     }>
   >;
+  /** Session usage data per workspace (for Costs tab) */
+  sessionUsage?: Map<string, MockSessionUsage>;
 }
 
 /**
@@ -69,6 +99,7 @@ export function createMockORPCClient(options: MockORPCClientOptions = {}): APICl
     providersList = [],
     onProjectRemove,
     backgroundProcesses = new Map(),
+    sessionUsage = new Map(),
   } = options;
 
   const workspaceMap = new Map(workspaces.map((w) => [w.id, w]));
@@ -207,7 +238,7 @@ export function createMockORPCClient(options: MockORPCClientOptions = {}): APICl
         terminate: async () => ({ success: true, data: undefined }),
         sendToBackground: async () => ({ success: true, data: undefined }),
       },
-      getSessionUsage: async () => undefined,
+      getSessionUsage: async (input: { workspaceId: string }) => sessionUsage.get(input.workspaceId),
     },
     window: {
       setTitle: async () => undefined,
diff --git a/src/browser/stories/App.rightsidebar.stories.tsx b/src/browser/stories/App.rightsidebar.stories.tsx
new file mode 100644
index 0000000000..7e3962a8d7
--- /dev/null
+++ b/src/browser/stories/App.rightsidebar.stories.tsx
@@ -0,0 +1,137 @@
+/**
+ * RightSidebar tab stories - testing dynamic tab data display
+ *
+ * Uses wide viewport (1600px) to ensure RightSidebar tabs are visible.
+ */
+
+import { appMeta, AppWithMocks, type AppStory } from "./meta.js";
+import { setupSimpleChatStory } from "./storyHelpers";
+import { createUserMessage, createAssistantMessage } from "./mockFactory";
+import { within, userEvent, waitFor } from "@storybook/test";
+import { RIGHT_SIDEBAR_TAB_KEY } from "@/common/constants/storage";
+import type { ComponentType } from "react";
+import type { MockSessionUsage } from "../../../.storybook/mocks/orpc";
+
+export default {
+  ...appMeta,
+  title: "App/RightSidebar",
+  decorators: [
+    (Story: ComponentType) => (
+      <div style={{ width: 1600, height: "100dvh" }}>
+        <Story />
+      </div>
+    ),
+  ],
+  parameters: {
+    ...appMeta.parameters,
+    chromatic: {
+      modes: {
+        dark: { theme: "dark", viewport: 1600 },
+        light: { theme: "light", viewport: 1600 },
+      },
+    },
+  },
+};
+
+/**
+ * Helper to create session usage data with costs
+ */
+function createSessionUsage(cost: number): MockSessionUsage {
+  const inputCost = cost * 0.6;
+  const outputCost = cost * 0.2;
+  const cachedCost = cost * 0.1;
+  const reasoningCost = cost * 0.1;
+
+  return {
+    byModel: {
+      "claude-sonnet-4-20250514": {
+        input: { tokens: 10000, cost_usd: inputCost },
+        cached: { tokens: 5000, cost_usd: cachedCost },
+        cacheCreate: { tokens: 0, cost_usd: 0 },
+        output: { tokens: 2000, cost_usd: outputCost },
+        reasoning: { tokens: 1000, cost_usd: reasoningCost },
+        model: "claude-sonnet-4-20250514",
+      },
+    },
+    version: 1,
+  };
+}
+
+/**
+ * Costs tab with session cost displayed in tab label ($0.56)
+ */
+export const CostsTab: AppStory = {
+  render: () => (
+    <AppWithMocks
+      setup={() => {
+        localStorage.setItem(RIGHT_SIDEBAR_TAB_KEY, JSON.stringify("costs"));
+
+        return setupSimpleChatStory({
+          workspaceId: "ws-costs",
+          workspaceName: "feature/api",
+          projectName: "my-app",
+          messages: [
+            createUserMessage("msg-1", "Help me build an API", { historySequence: 1 }),
+            createAssistantMessage("msg-2", "I'll help you build a REST API.", {
+              historySequence: 2,
+            }),
+          ],
+          sessionUsage: createSessionUsage(0.56),
+        });
+      }}
+    />
+  ),
+  play: async ({ canvasElement }) => {
+    const canvas = within(canvasElement);
+
+    // Session usage is fetched async via WorkspaceStore; wait to avoid snapshot races.
+    await waitFor(
+      () => {
+        canvas.getByRole("tab", { name: /costs.*\$0\.56/i });
+      },
+      { timeout: 5000 }
+    );
+  },
+};
+
+/**
+ * Review tab selected - click switches from Costs to Review tab
+ */
+export const ReviewTab: AppStory = {
+  render: () => (
+    <AppWithMocks
+      setup={() => {
+        localStorage.setItem(RIGHT_SIDEBAR_TAB_KEY, JSON.stringify("costs"));
+
+        return setupSimpleChatStory({
+          workspaceId: "ws-review",
+          workspaceName: "feature/review",
+          projectName: "my-app",
+          messages: [
+            createUserMessage("msg-1", "Add a new component", { historySequence: 1 }),
+            createAssistantMessage("msg-2", "I've added the component.", { historySequence: 2 }),
+          ],
+          sessionUsage: createSessionUsage(0.42),
+        });
+      }}
+    />
+  ),
+  play: async ({ canvasElement }) => {
+    const canvas = within(canvasElement);
+
+    // Wait for session usage to land (avoid theme/mode snapshots diverging on timing).
+    await waitFor(
+      () => {
+        canvas.getByRole("tab", { name: /costs.*\$0\.42/i });
+      },
+      { timeout: 5000 }
+    );
+
+    const reviewTab = canvas.getByRole("tab", { name: /^review/i });
+    await userEvent.click(reviewTab);
+
+    await waitFor(() => {
+      canvas.getByRole("tab", { name: /^review/i, selected: true });
+    });
+  },
+};
diff --git a/src/browser/stories/storyHelpers.ts b/src/browser/stories/storyHelpers.ts
index 85f55d453f..d965143f71 100644
--- a/src/browser/stories/storyHelpers.ts
+++ b/src/browser/stories/storyHelpers.ts
@@ -26,7 +26,7 @@ import {
   createGitStatusOutput,
   type GitStatusFixture,
 } from "./mockFactory";
-import { createMockORPCClient } from "../../../.storybook/mocks/orpc";
+import { createMockORPCClient, type MockSessionUsage } from "../../../.storybook/mocks/orpc";
 
 // ═══════════════════════════════════════════════════════════════════════════════
 // WORKSPACE SELECTION
@@ -155,6 +155,8 @@ export interface SimpleChatSetupOptions {
   gitStatus?: GitStatusFixture;
   providersConfig?: ProvidersConfigMap;
   backgroundProcesses?: BackgroundProcessFixture[];
+  /** Session usage data for Costs tab */
+  sessionUsage?: MockSessionUsage;
 }
 
 /**
@@ -184,6 +186,11 @@ export function setupSimpleChatStory(opts: SimpleChatSetupOptions): APIClient {
     ? new Map([[workspaceId, opts.backgroundProcesses]])
     : undefined;
 
+  // Set up session usage map
+  const sessionUsageMap = opts.sessionUsage
+    ? new Map([[workspaceId, opts.sessionUsage]])
+    : undefined;
+
   // Return ORPC client
   return createMockORPCClient({
     projects: groupWorkspacesByProject(workspaces),
@@ -192,6 +199,7 @@ export function setupSimpleChatStory(opts: SimpleChatSetupOptions): APIClient {
     executeBash: createGitStatusExecutor(gitStatus),
     providersConfig: opts.providersConfig,
     backgroundProcesses: bgProcesses,
+    sessionUsage: sessionUsageMap,
   });
 }
 
diff --git a/src/browser/styles/globals.css b/src/browser/styles/globals.css
index d2f7aaa272..105b9af9fe 100644
--- a/src/browser/styles/globals.css
+++ b/src/browser/styles/globals.css
@@ -1003,7 +1003,8 @@ body {
 /* Root container */
 html,
 body,
-#root {
+#root,
+#storybook-root {
   height: 100dvh;
   overflow: hidden;
 }
@@ -1079,7 +1080,8 @@ body,
 /* Ensure the app uses the full viewport height accounting for browser chrome */
 html,
 body,
-#root {
+#root,
+#storybook-root {
   /* Use dvh (dynamic viewport height) on supported browsers - this accounts for
      mobile browser chrome and keyboard accessory bars */
   min-height: 100dvh;
@@ -1090,7 +1092,8 @@ body,
 /* Handle safe areas for notched devices and keyboard accessory bars */
 @supports (padding: env(safe-area-inset-top)) {
   /* Apply padding to account for iOS safe areas (notch at top, home indicator at bottom) */
-  #root {
+  #root,
+  #storybook-root {
     padding-top: env(safe-area-inset-top, 0);
     padding-bottom: env(safe-area-inset-bottom, 0);
     padding-left: env(safe-area-inset-left, 0);