diff --git a/coworker/server/app.py b/coworker/server/app.py
index 55cd155f..8476bd88 100644
--- a/coworker/server/app.py
+++ b/coworker/server/app.py
@@ -1707,6 +1707,10 @@ def _resolve_pending(resolution: str) -> None:
                     "command_trust": manager.workspace_command_trust(
                         str(getattr(engine, "audit_context", {}).get("workspace", ""))
                     ),
+                    # Mid-turn reconnect (#311): the client resets `running` on every
+                    # session switch, and turn_start already fired — seed from the
+                    # manager so Stop / Thinking / WaitingForAgent reappear.
+                    "running": manager.is_running(session_id),
                 },
             }
         )
diff --git a/surfaces/gui/e2e/fixtures.ts b/surfaces/gui/e2e/fixtures.ts
index 037fc617..87a0295a 100644
--- a/surfaces/gui/e2e/fixtures.ts
+++ b/surfaces/gui/e2e/fixtures.ts
@@ -574,10 +574,30 @@ export async function mockApi(page: import("@playwright/test").Page) {
     eventSockets.set(page, ws);
   });
 
+  // Sessions mid-turn keep running after the viewing socket closes (real server
+  // behavior) so a reconnect can seed `ready.running` (#311).
+  const runningSessions = new Set<string>();
   await page.routeWebSocket(/\/ws\/session\//, (ws) => {
-    const send = (type: string, data: Record<string, unknown> = {}) =>
-      ws.send(JSON.stringify({ type, data }));
-    send("ready");
+    const sid = (() => {
+      try {
+        const path = new URL(ws.url()).pathname;
+        return path.split("/").filter(Boolean).pop() || "";
+      } catch {
+        return "";
+      }
+    })();
+    const send = (type: string, data: Record<string, unknown> = {}) => {
+      try {
+        ws.send(JSON.stringify({ type, data }));
+      } catch {
+        /* socket may already be closed after a session switch */
+      }
+    };
+    const endTurn = () => {
+      runningSessions.delete(sid);
+      send("turn_done");
+    };
+    send("ready", { running: runningSessions.has(sid), session_id: sid });
     let pendingTool = "run_shell"; // which proposal the next approval decision resolves
     let epicTimer: ReturnType<typeof setInterval> | null = null; // the slow stream, stoppable via interrupt
     let hadTurn = false; // a user_message landed — set_model is now a mid-session switch
@@ -585,6 +605,7 @@ export async function mockApi(page: import("@playwright/test").Page) {
       const msg = JSON.parse(String(raw));
       if (msg.type === "user_message") {
         hadTurn = true;
+        runningSessions.add(sid);
         send("turn_start", { input: msg.text });
         if (/run a tool/i.test(msg.text)) {
           pendingTool = "run_shell";
@@ -677,7 +698,7 @@ export async function mockApi(page: import("@playwright/test").Page) {
               text: "Decision made.",
               reasoning: thoughts.join(""),
             });
-            send("turn_done");
+            endTurn();
           }, 120);
           return;
         }
@@ -686,13 +707,13 @@ export async function mockApi(page: import("@playwright/test").Page) {
         if (/compact the context/i.test(msg.text)) {
           send("compacted", { text: "Context compacted — earlier turns were summarized" });
           send("assistant_message", { text: "Still on it — continuing where I left off." });
-          send("turn_done");
+          endTurn();
           return;
         }
         // A turn that dies on a provider error; the follow-up {type:"retry"} recovers.
         if (/fail the turn/i.test(msg.text)) {
           send("error", { error: "model unreachable" });
-          send("turn_done");
+          endTurn();
           return;
         }
         // A deliberately SLOW multi-second stream (~40 ticks × 120ms) so specs can
@@ -708,7 +729,7 @@ export async function mockApi(page: import("@playwright/test").Page) {
               clearInterval(epicTimer!);
               epicTimer = null;
               send("assistant_message", { text: ("The epic concludes. " + line).repeat(20) });
-              send("turn_done");
+              endTurn();
             }
           }, 120);
           return;
@@ -729,7 +750,7 @@ export async function mockApi(page: import("@playwright/test").Page) {
             cache_write: 800,
           },
         });
-        send("turn_done");
+        endTurn();
       } else if (msg.type === "approval") {
         if (pendingTool === "run_shell") {
           if (msg.decision === "deny") {
@@ -747,7 +768,7 @@ export async function mockApi(page: import("@playwright/test").Page) {
           // The decision echoes back so specs can pin what rode the wire (e.g. always_task).
           send("assistant_message", { text: `Done via ${pendingTool} [decision=${msg.decision}]` });
         }
-        send("turn_done");
+        endTurn();
       } else if (msg.type === "interrupt") {
         // Stop mid-stream: like the real engine, end the turn with `interrupted` and
         // NO assistant_message — the client owns promoting the partial into the transcript.
@@ -756,7 +777,7 @@ export async function mockApi(page: import("@playwright/test").Page) {
           epicTimer = null;
         }
         send("interrupted", {});
-        send("turn_done");
+        endTurn();
       } else if (msg.type === "set_model") {
         // Mid-session switch: the server applies it and broadcasts the persisted marker.
         // Like the real server, the FIRST bind (fresh session) is silent.
@@ -767,9 +788,10 @@ export async function mockApi(page: import("@playwright/test").Page) {
           });
       } else if (msg.type === "retry") {
         // Like the real engine: re-runs with NO new user message (turn_start input is empty).
+        runningSessions.add(sid);
         send("turn_start", { input: "" });
         send("assistant_message", { text: "Recovered after retry." });
-        send("turn_done");
+        endTurn();
       }
     });
   });
diff --git a/surfaces/gui/e2e/session-switch-running.spec.ts b/surfaces/gui/e2e/session-switch-running.spec.ts
new file mode 100644
index 00000000..19fe094f
--- /dev/null
+++ b/surfaces/gui/e2e/session-switch-running.spec.ts
@@ -0,0 +1,24 @@
+// #311: switching away from a mid-turn session used to leave `running` stuck false on
+// return (turn_start already fired). ready.running must restore Stop / live chrome.
+import { expect } from "@playwright/test";
+import { test } from "./fixtures";
+
+test("switching back to a mid-turn session restores the Stop control", async ({ page }) => {
+  await page.goto("/");
+  await page.getByText("Draft the launch note").first().click();
+
+  // Park on an approval — the turn stays live (no turn_done) so ready.running stays true.
+  const box = page.getByPlaceholder(/Ask the coworker/);
+  await box.fill("run a tool");
+  await box.press("Enter");
+  await expect(page.getByRole("button", { name: "Stop" })).toBeVisible({ timeout: 10_000 });
+  await expect(page.getByText(/wants to run a command/i)).toBeVisible();
+
+  // Leave for another session while the turn is still live.
+  await page.getByText("Weekly plan 1").first().click();
+  await expect(page.getByRole("button", { name: "Stop" })).toHaveCount(0);
+
+  // Return — ready.running seeds the live chrome even though turn_start won't re-fire.
+  await page.getByText("Draft the launch note").first().click();
+  await expect(page.getByRole("button", { name: "Stop" })).toBeVisible({ timeout: 10_000 });
+});
diff --git a/surfaces/gui/src/App.tsx b/surfaces/gui/src/App.tsx
index f814cdb8..2b3716b6 100644
--- a/surfaces/gui/src/App.tsx
+++ b/surfaces/gui/src/App.tsx
@@ -584,6 +584,9 @@ export function App() {
           if (d.command_trust?.required) setWorkspaceTrustRequest(d.command_trust);
           // Cowork: adopt the server-provisioned scratch dir (only when we don't already have one).
           if (d.workspace) setWorkspace((cur) => cur || d.workspace);
+          // Mid-turn reconnect (#311): selectSession clears `running`, and turn_start
+          // won't re-fire for an already-live turn — trust the server's claim.
+          if (typeof d.running === "boolean") setRunning(d.running);
           break;
         case "turn_start":
           setRunning(true);
@@ -957,6 +960,7 @@ export function App() {
     setSurface("session"); // selecting a conversation always returns to the conversation view
     setTodo([]);
     setStreaming("");
+    setReasoningStream("");
     setRunning(false);
     if (ag) setAgent(ag);
     if (!gatesWorkspace(ag)) setShowGate(false);
diff --git a/tests/test_server.py b/tests/test_server.py
index af870c51..b485e0fc 100644
--- a/tests/test_server.py
+++ b/tests/test_server.py
@@ -323,6 +323,23 @@ def test_ws_simple_turn(tmp_path):
         assert "turn_end" in types
 
 
+def test_ws_ready_reports_running_for_mid_turn_reconnect(tmp_path):
+    # #311: a client that reconnects mid-turn must learn the session is still live —
+    # turn_start already fired and won't re-fire for that turn.
+    manager = SessionManager(workspace=tmp_path, provider=ScriptedProvider([]))
+    manager.mark_running("mid-turn")
+    client = TestClient(create_app(manager))
+    with client.websocket_connect("/ws/session/mid-turn") as ws:
+        ready = ws.receive_json()
+        assert ready["type"] == "ready"
+        assert ready["data"]["running"] is True
+    manager.mark_idle("mid-turn")
+    with client.websocket_connect("/ws/session/mid-turn") as ws:
+        ready = ws.receive_json()
+        assert ready["type"] == "ready"
+        assert ready["data"]["running"] is False
+
+
 def test_ws_rejects_oversized_message(tmp_path):
     from coworker.server import app as app_mod
     from coworker.attachments import MAX_ATTACHMENTS