diff --git a/apps/web/e2e/tests/task/sidebar-secondary-session-status.spec.ts b/apps/web/e2e/tests/task/sidebar-secondary-session-status.spec.ts
new file mode 100644
index 0000000000..fdfcf2f714
--- /dev/null
+++ b/apps/web/e2e/tests/task/sidebar-secondary-session-status.spec.ts
@@ -0,0 +1,102 @@
+import { test, expect } from "../../fixtures/test-base";
+import { KanbanPage } from "../../pages/kanban-page";
+import { SessionPage } from "../../pages/session-page";
+
+const DONE_STATES = ["COMPLETED", "WAITING_FOR_INPUT"];
+const TASK_TITLE = "Secondary Session Status Task";
+
+/**
+ * Regression: when a task already has a primary session that is idle
+ * ("Turn Finished") and the user opens a second chat tab, the sidebar must
+ * reflect the new session's RUNNING state. Before the fix the sidebar read
+ * the primary session's state only, so secondary tabs running in the
+ * background left the task showing "Turn Finished".
+ */
+test.describe("Sidebar status with secondary session", () => {
+  test("sidebar moves to Running when a non-primary chat tab starts", async ({
+    testPage,
+    apiClient,
+    seedData,
+  }) => {
+    test.setTimeout(120_000);
+
+    // 1. Create task whose first session completes quickly.
+    const task = await apiClient.createTaskWithAgent(
+      seedData.workspaceId,
+      TASK_TITLE,
+      seedData.agentProfileId,
+      {
+        description: "/e2e:simple-message",
+        workflow_id: seedData.workflowId,
+        workflow_step_id: seedData.startStepId,
+        repository_ids: [seedData.repositoryId],
+      },
+    );
+
+    // 2. Wait for the first session to reach Turn Finished.
+    await expect
+      .poll(
+        async () => {
+          const { sessions } = await apiClient.listTaskSessions(task.id);
+          return DONE_STATES.includes(sessions[0]?.state ?? "");
+        },
+        { timeout: 30_000, message: "Waiting for first session to finish" },
+      )
+      .toBe(true);
+
+    // 3. Navigate to the task.
+    const kanban = new KanbanPage(testPage);
+    await kanban.goto();
+
+    const card = kanban.taskCardByTitle(TASK_TITLE);
+    await expect(card).toBeVisible({ timeout: 10_000 });
+    await card.click();
+    await expect(testPage).toHaveURL(/\/t\//, { timeout: 15_000 });
+
+    const session = new SessionPage(testPage);
+    await session.waitForLoad();
+    await expect(session.chat.getByText("simple mock response", { exact: false })).toBeVisible({
+      timeout: 15_000,
+    });
+
+    // 4. Sanity: sidebar shows the task as "Turn Finished" before we start
+    //    the secondary session.
+    await expect(session.taskInSection(TASK_TITLE, "Turn Finished")).toBeVisible({
+      timeout: 15_000,
+    });
+
+    // 5. Open the new session dialog and start a slow second session that
+    //    stays RUNNING long enough to assert the sidebar reaction.
+    await session.openNewSessionDialog();
+    await expect(session.newSessionDialog()).toBeVisible({ timeout: 5_000 });
+    await session
+      .newSessionPromptInput()
+      .fill('e2e:message("starting...")\ne2e:delay(15000)\ne2e:message("done")');
+    await session.newSessionStartButton().click();
+    await expect(session.newSessionDialog()).not.toBeVisible({ timeout: 10_000 });
+
+    // 6. Backend must have created a second session.
+    await expect
+      .poll(
+        async () => {
+          const { sessions } = await apiClient.listTaskSessions(task.id);
+          return sessions.length;
+        },
+        { timeout: 30_000, message: "Waiting for second session to appear" },
+      )
+      .toBe(2);
+
+    // 7. Regression assertion: sidebar moves the task into the "Running"
+    //    bucket while the secondary session is still mid-delay.
+    await expect(session.taskInSection(TASK_TITLE, "Running")).toBeVisible({
+      timeout: 30_000,
+    });
+
+    // 8. After the delay completes, the task should land back in
+    //    "Turn Finished" — proving the fix doesn't break the normal
+    //    completion path either.
+    await expect(session.taskInSection(TASK_TITLE, "Turn Finished")).toBeVisible({
+      timeout: 45_000,
+    });
+  });
+});
diff --git a/apps/web/lib/utils/session-info.test.ts b/apps/web/lib/utils/session-info.test.ts
new file mode 100644
index 0000000000..74e7eba204
--- /dev/null
+++ b/apps/web/lib/utils/session-info.test.ts
@@ -0,0 +1,78 @@
+import { describe, it, expect } from "vitest";
+
+import { getSessionInfoForTask } from "./session-info";
+import type { TaskSession } from "@/lib/types/http";
+
+function session(overrides: Partial<TaskSession>): TaskSession {
+  return {
+    id: overrides.id ?? "s1",
+    task_id: overrides.task_id ?? "t1",
+    state: overrides.state ?? "WAITING_FOR_INPUT",
+    started_at: overrides.started_at ?? "",
+    updated_at: overrides.updated_at ?? "",
+    is_primary: overrides.is_primary ?? false,
+    ...overrides,
+  } as TaskSession;
+}
+
+describe("getSessionInfoForTask", () => {
+  it("returns undefined sessionState when the task has no sessions", () => {
+    const info = getSessionInfoForTask("t1", {}, {});
+    expect(info.sessionState).toBeUndefined();
+  });
+
+  it("returns the primary session's state when it is the only session", () => {
+    const info = getSessionInfoForTask(
+      "t1",
+      { t1: [session({ id: "p", is_primary: true, state: "WAITING_FOR_INPUT" })] },
+      {},
+    );
+    expect(info.sessionState).toBe("WAITING_FOR_INPUT");
+  });
+
+  // Regression: the sidebar derives its "running" badge from this state.
+  // A secondary chat tab that the user opens while the primary is idle must
+  // still surface as "in progress" — without this the task keeps showing
+  // "Turn Finished" while the new agent is working.
+  it("returns RUNNING when any non-primary session is running, even if primary is idle", () => {
+    const info = getSessionInfoForTask(
+      "t1",
+      {
+        t1: [
+          session({ id: "p", is_primary: true, state: "WAITING_FOR_INPUT" }),
+          session({ id: "s", state: "RUNNING" }),
+        ],
+      },
+      {},
+    );
+    expect(info.sessionState).toBe("RUNNING");
+  });
+
+  it("prefers RUNNING over STARTING", () => {
+    const info = getSessionInfoForTask(
+      "t1",
+      {
+        t1: [
+          session({ id: "p", is_primary: true, state: "STARTING" }),
+          session({ id: "s", state: "RUNNING" }),
+        ],
+      },
+      {},
+    );
+    expect(info.sessionState).toBe("RUNNING");
+  });
+
+  it("falls back to the primary's state when no session is more active", () => {
+    const info = getSessionInfoForTask(
+      "t1",
+      {
+        t1: [
+          session({ id: "p", is_primary: true, state: "WAITING_FOR_INPUT" }),
+          session({ id: "s", state: "COMPLETED" }),
+        ],
+      },
+      {},
+    );
+    expect(info.sessionState).toBe("WAITING_FOR_INPUT");
+  });
+});
diff --git a/apps/web/lib/utils/session-info.ts b/apps/web/lib/utils/session-info.ts
index e9e6c016d2..b344ba67df 100644
--- a/apps/web/lib/utils/session-info.ts
+++ b/apps/web/lib/utils/session-info.ts
@@ -34,6 +34,37 @@ function computeDiffStats(
   return additions === 0 && deletions === 0 ? undefined : { additions, deletions };
 }
 
+// Activity ranking for the sidebar's status indicator. The primary session
+// drives diff stats and updatedAt (those reflect the task's "default" view),
+// but the sidebar's state badge should reflect whatever session is most
+// active right now — otherwise a secondary chat tab running in the
+// background leaves the task showing the idle primary's "Turn Finished"
+// badge. Lower index = more active.
+const SESSION_STATE_PRIORITY: TaskSessionState[] = [
+  "RUNNING",
+  "STARTING",
+  "WAITING_FOR_INPUT",
+  "COMPLETED",
+  "FAILED",
+  "CANCELLED",
+  "CREATED",
+];
+
+function priority(state: TaskSessionState | undefined): number {
+  if (!state) return SESSION_STATE_PRIORITY.length;
+  const idx = SESSION_STATE_PRIORITY.indexOf(state);
+  return idx === -1 ? SESSION_STATE_PRIORITY.length : idx;
+}
+
+function pickMostActiveState(sessions: TaskSession[]): TaskSessionState | undefined {
+  let best: TaskSessionState | undefined;
+  for (const s of sessions) {
+    const candidate = s.state as TaskSessionState | undefined;
+    if (priority(candidate) < priority(best)) best = candidate;
+  }
+  return best;
+}
+
 export function getSessionInfoForTask(
   taskId: string,
   sessionsByTaskId: Record<string, TaskSession[]>,
@@ -52,7 +83,7 @@ export function getSessionInfoForTask(
   // Empty string means the session was created from a WS event without timestamps;
   // return undefined so callers fall through to task.updatedAt/createdAt instead.
   const updatedAt = latestSession.updated_at || undefined;
-  const sessionState = latestSession.state as TaskSessionState | undefined;
+  const sessionState = pickMostActiveState(sessions);
   const envKey = environmentIdBySessionId?.[latestSession.id] ?? latestSession.id;
   const gitStatus = gitStatusByEnvId[envKey];
   if (!gitStatus) return { diffStats: undefined, updatedAt, sessionState };