diff --git a/crates/web-server/src/lib.rs b/crates/web-server/src/lib.rs
index 8175c65c..4157938b 100644
--- a/crates/web-server/src/lib.rs
+++ b/crates/web-server/src/lib.rs
@@ -1559,11 +1559,19 @@ async fn send_launch_agent_attached(
     owned: &Arc<Mutex<Vec<(SessionId, u64)>>>,
     output: application::LaunchAgentOutput,
 ) -> Result<(), ErrorDto> {
-    if output.structured.is_some() {
-        return Err(ErrorDto {
-            code: "UNSUPPORTED".to_owned(),
-            message: "structured agent sessions do not stream over the PTY websocket".to_owned(),
-        });
+    if let Some(structured) = output.structured.as_ref() {
+        tx.send(ServerFrame::chat_attached(
+            &frame.id,
+            structured,
+            output.assigned_conversation_id.as_deref(),
+            output.engine_session_id.as_deref(),
+        ))
+        .await
+        .map_err(|_| ErrorDto {
+            code: "WS_CLOSED".to_owned(),
+            message: "websocket output closed".to_owned(),
+        })?;
+        return Ok(());
     }
     let session_id = output.session.id;
     let dto = TerminalSessionDto::from(output);
@@ -5430,6 +5438,28 @@ impl ServerFrame {
         }
     }
 
+    fn chat_attached(
+        request_id: &str,
+        session: &application::StructuredSessionDescriptor,
+        assigned_conversation_id: Option<&str>,
+        engine_session_id: Option<&str>,
+    ) -> Self {
+        Self {
+            kind: "chat.attached".to_owned(),
+            reply_to: Some(request_id.to_owned()),
+            payload: json!({
+                "sessionId": session.session_id.to_string(),
+                "session": {
+                    "sessionId": session.session_id.to_string(),
+                    "nodeId": session.node_id.to_string(),
+                },
+                "cellKind": "chat",
+                "assignedConversationId": assigned_conversation_id,
+                "engineSessionId": engine_session_id,
+            }),
+        }
+    }
+
     fn output(session_id: SessionId, seq: u64, bytes: Vec<u8>) -> Self {
         Self {
             kind: "terminal.output".to_owned(),
@@ -7043,7 +7073,7 @@ mod tests {
     }
 
     #[tokio::test]
-    async fn websocket_launch_agent_structured_is_unsupported() {
+    async fn websocket_launch_agent_structured_emits_chat_attached() {
         let state = state();
         let (tx, mut rx) = mpsc::channel(4);
         let owned = Arc::new(Mutex::new(Vec::new()));
@@ -7073,25 +7103,25 @@ mod tests {
             profile: None,
         };
 
-        if let Err(err) = send_launch_agent_attached(&frame, &state, &tx, &owned, output).await {
-            tx.send(ServerFrame::error(
-                Some(frame.id),
-                payload_session_id(&frame.payload),
-                err.code,
-                err.message,
-            ))
+        send_launch_agent_attached(&frame, &state, &tx, &owned, output)
             .await
             .unwrap();
-        }
-        let error = recv_server_frame(&mut rx).await;
+        let attached = recv_server_frame(&mut rx).await;
 
-        assert_eq!(error.kind, "error");
-        assert_eq!(error.reply_to.as_deref(), Some("agent-structured"));
-        assert_eq!(error.payload["code"], "UNSUPPORTED");
+        assert_eq!(attached.kind, "chat.attached");
+        assert_eq!(attached.reply_to.as_deref(), Some("agent-structured"));
+        assert_eq!(attached.payload["sessionId"], session_id.to_string());
         assert_eq!(
-            error.payload["message"],
-            "structured agent sessions do not stream over the PTY websocket"
+            attached.payload["session"]["sessionId"],
+            session_id.to_string()
         );
+        assert_eq!(attached.payload["session"]["nodeId"], node_id.to_string());
+        assert_eq!(attached.payload["cellKind"], "chat");
+        assert_eq!(
+            attached.payload["assignedConversationId"],
+            "pair-conversation"
+        );
+        assert_eq!(attached.payload["engineSessionId"], "engine-session");
         assert_eq!(state.ws_pty_bridge.active_sessions(), 0);
     }
 
diff --git a/frontend/src/features/agents/CustomAgentChatView.test.tsx b/frontend/src/features/agents/CustomAgentChatView.test.tsx
index 554811d6..8166067d 100644
--- a/frontend/src/features/agents/CustomAgentChatView.test.tsx
+++ b/frontend/src/features/agents/CustomAgentChatView.test.tsx
@@ -89,6 +89,98 @@ describe("CustomAgentChatView", () => {
     __resetProjectCliInteractionActivityForTests();
   });
 
+  it("shows the technical open failure on mobile with retry and native TUI fallback", async () => {
+    const openNativeTui = vi.fn();
+    const agent = {
+      launchAgentChat: vi
+        .fn()
+        .mockRejectedValueOnce(new Error("spawn codex ENOENT"))
+        .mockResolvedValueOnce({ sessionId: "chat-session-1" }),
+      reattachAgentChat: vi.fn(async (sessionId: string) => ({
+        sessionId,
+        scrollback: [],
+      })),
+      sendAgentChat: vi.fn(async () => {}),
+      cancelAgentChat: vi.fn(async () => {}),
+      closeAgentChat: vi.fn(async () => {}),
+    };
+
+    render(
+      <DIProvider
+        gateways={{
+          agent,
+          system: { pickFile: vi.fn(async () => null) },
+        } as unknown as Gateways}
+      >
+        <CustomAgentChatView
+          projectId="project-1"
+          agentId="agent-1"
+          agentName="Worker"
+          profile={profile}
+          cwd="/repo"
+          nodeId="node-1"
+          sessionId={null}
+          conversationId={null}
+          mobile
+          onOpenNativeTui={openNativeTui}
+          onSessionId={vi.fn()}
+          onConversationId={vi.fn()}
+        />
+      </DIProvider>,
+    );
+
+    const alert = await screen.findByRole("alert");
+    expect(within(alert).getByText("Le CLI custom n'a pas pu démarrer.")).toBeTruthy();
+    expect(within(alert).getByText("Cause: spawn codex ENOENT")).toBeTruthy();
+    expect(screen.queryByText(/Vérifiez la configuration du profil/)).toBeNull();
+
+    fireEvent.click(within(alert).getByRole("button", { name: "Ouvrir en TUI native" }));
+    expect(openNativeTui).toHaveBeenCalledTimes(1);
+
+    fireEvent.click(within(alert).getByRole("button", { name: "Réessayer" }));
+    await waitFor(() => expect(agent.launchAgentChat).toHaveBeenCalledTimes(2));
+    await waitFor(() => expect(screen.queryByRole("alert")).toBeNull());
+  });
+
+  it("uses the same technical open failure on desktop", async () => {
+    const agent = {
+      launchAgentChat: vi.fn(async () => {
+        throw new Error("permission denied: /opt/codex");
+      }),
+      reattachAgentChat: vi.fn(),
+      sendAgentChat: vi.fn(async () => {}),
+      cancelAgentChat: vi.fn(async () => {}),
+      closeAgentChat: vi.fn(async () => {}),
+    };
+
+    render(
+      <DIProvider
+        gateways={{
+          agent,
+          system: { pickFile: vi.fn(async () => null) },
+        } as unknown as Gateways}
+      >
+        <CustomAgentChatView
+          projectId="project-1"
+          agentId="agent-1"
+          agentName="Worker"
+          profile={profile}
+          cwd="/repo"
+          nodeId="node-1"
+          sessionId={null}
+          conversationId={null}
+          onSessionId={vi.fn()}
+          onConversationId={vi.fn()}
+        />
+      </DIProvider>,
+    );
+
+    const alert = await screen.findByRole("alert");
+    expect(within(alert).getByText("Cause: permission denied: /opt/codex")).toBeTruthy();
+    expect(screen.getByRole("button", { name: "Réessayer" })).toBeTruthy();
+    expect(screen.queryByRole("button", { name: "Ouvrir en TUI native" })).toBeNull();
+  });
+
   it("repaints attachment labels from user prompt scrollback", async () => {
     const agent = {
       launchAgentChat: vi.fn(),
diff --git a/frontend/src/features/agents/CustomAgentChatView.tsx b/frontend/src/features/agents/CustomAgentChatView.tsx
index 78647140..b66f3524 100644
--- a/frontend/src/features/agents/CustomAgentChatView.tsx
+++ b/frontend/src/features/agents/CustomAgentChatView.tsx
@@ -52,6 +52,7 @@ export interface CustomAgentChatViewProps {
   onComposerHeightPxChange?: (heightPx: number) => void;
   draftScope?: ChatDraftScope | null;
   mobile?: boolean;
+  onOpenNativeTui?: () => void;
   onSessionId: (sessionId: string | null) => void;
   onConversationId: (conversationId: string | null) => void;
 }
@@ -118,6 +119,11 @@ interface SlashCommandInvocation {
   arguments: unknown[];
 }
 
+interface ChatErrorState {
+  message: string;
+  openFailure: boolean;
+}
+
 function describe(e: unknown): string {
   if (e && typeof e === "object" && "message" in e) {
     return String((e as GatewayError).message);
@@ -125,9 +131,9 @@ function describe(e: unknown): string {
   return String(e);
 }
 
-function describeOpenFailure(e: unknown, mobile: boolean): string {
-  if (!mobile) return describe(e);
-  return "Le CLI custom n'a pas pu démarrer. Vérifiez la configuration du profil ou utilisez la TUI native.";
+function describeOpenFailure(e: unknown): string {
+  const message = describe(e).trim();
+  return message || "Cause technique non disponible.";
 }
 
 function instrumentationError(e: unknown): Record<string, unknown> {
@@ -734,6 +740,7 @@ export function CustomAgentChatView({
   onComposerHeightPxChange,
   draftScope: explicitDraftScope,
   mobile = false,
+  onOpenNativeTui,
   onSessionId,
   onConversationId,
 }: CustomAgentChatViewProps) {
@@ -770,9 +777,10 @@ export function CustomAgentChatView({
   const [profileDialog, setProfileDialog] =
     useState<ProfileCommandDialogState | null>(null);
   const [opening, setOpening] = useState(Boolean(sessionId || conversationId));
+  const [openRetryNonce, setOpenRetryNonce] = useState(0);
   const [busy, setBusy] = useState(false);
   const [stopping, setStopping] = useState(false);
-  const [error, setError] = useState<string | null>(null);
+  const [error, setError] = useState<ChatErrorState | null>(null);
   const scrollRef = useRef<HTMLDivElement | null>(null);
   const composerRef = useRef<HTMLTextAreaElement | null>(null);
   const chatRootRef = useRef<HTMLDivElement | null>(null);
@@ -1371,7 +1379,9 @@ export function CustomAgentChatView({
           sessionId: externalSessionId,
           error: instrumentationError(e),
         });
-        if (!cancelled) setError(describeOpenFailure(e, mobile));
+        if (!cancelled) {
+          setError({ message: describeOpenFailure(e), openFailure: true });
+        }
       } finally {
         if (!cancelled) setOpening(false);
       }
@@ -1390,7 +1400,7 @@ export function CustomAgentChatView({
     applyChatViewState,
     currentConversationCacheKey,
     conversationId,
-    mobile,
+    openRetryNonce,
   ]);
 
   const canSend = useMemo(
@@ -1490,7 +1500,10 @@ export function CustomAgentChatView({
 
   async function executeSlashCommand(invocation: SlashCommandInvocation) {
     if (!agent.executeSlashCommand) {
-      setError(`Commande ${invocation.name} indisponible dans ce runtime.`);
+      setError({
+        message: `Commande ${invocation.name} indisponible dans ce runtime.`,
+        openFailure: false,
+      });
       return;
     }
     setDraft("");
@@ -1546,7 +1559,7 @@ export function CustomAgentChatView({
       }
     } catch (e) {
       const message = `Commande ${invocation.name} échouée: ${describe(e)}`;
-      setError(message);
+      setError({ message, openFailure: false });
       updateChatViewState((prev) => ({
         turns: [...prev.turns, { role: "error", text: message, receivedAtMs: Date.now() }],
         progressIndex: prev.progressIndex,
@@ -1633,7 +1646,7 @@ export function CustomAgentChatView({
         );
       }
     } catch (e) {
-      setError(describe(e));
+      setError({ message: describe(e), openFailure: false });
     }
   }
 
@@ -1704,7 +1717,7 @@ export function CustomAgentChatView({
         } catch (recoveryError) {
           finishInteraction();
           setBusy(false);
-          setError(describe(recoveryError));
+          setError({ message: describe(recoveryError), openFailure: false });
           updateChatViewState((prev) => ({
             turns: [
               ...prev.turns,
@@ -1717,7 +1730,7 @@ export function CustomAgentChatView({
       }
       finishInteraction();
       setBusy(false);
-      setError(describe(e));
+      setError({ message: describe(e), openFailure: false });
       updateChatViewState((prev) => ({
         turns: [...prev.turns, { role: "error", text: describe(e), receivedAtMs: Date.now() }],
         progressIndex: prev.progressIndex,
@@ -1779,7 +1792,7 @@ export function CustomAgentChatView({
         });
         return;
       }
-      setError(describe(e));
+      setError({ message: describe(e), openFailure: false });
       updateChatViewState((prev) => ({
         turns: [
           ...prev.turns,
@@ -1821,9 +1834,40 @@ export function CustomAgentChatView({
         </p>
       )}
       {error && (
-        <p role="alert" className="m-3 rounded-md border border-danger/40 bg-danger/10 p-2 text-xs text-danger">
-          {error}
-        </p>
+        <div
+          role="alert"
+          className="m-3 rounded-md border border-danger/40 bg-danger/10 p-2 text-xs text-danger"
+        >
+          {error.openFailure ? (
+            <div className="flex min-w-0 flex-col gap-2">
+              <div className="font-medium">Le CLI custom n'a pas pu démarrer.</div>
+              <div className="break-words">
+                Cause: {error.message}
+              </div>
+              <div className="flex flex-wrap gap-2">
+                <Button
+                  size="sm"
+                  variant="primary"
+                  disabled={opening}
+                  onClick={() => setOpenRetryNonce((value) => value + 1)}
+                >
+                  Réessayer
+                </Button>
+                {onOpenNativeTui && (
+                  <Button
+                    size="sm"
+                    variant="secondary"
+                    onClick={onOpenNativeTui}
+                  >
+                    Ouvrir en TUI native
+                  </Button>
+                )}
+              </div>
+            </div>
+          ) : (
+            error.message
+          )}
+        </div>
       )}
       {profileDialog && (
         <div
diff --git a/frontend/src/features/layout/LayoutGrid.tsx b/frontend/src/features/layout/LayoutGrid.tsx
index 1ef6e6f8..f63f67d9 100644
--- a/frontend/src/features/layout/LayoutGrid.tsx
+++ b/frontend/src/features/layout/LayoutGrid.tsx
@@ -1501,6 +1501,7 @@ function LeafView({
             composerHeightPx={customChatComposerHeight}
             composerDefaultHeightPx={CUSTOM_CHAT_COMPOSER_DEFAULT_HEIGHT}
             onComposerHeightPxChange={setCustomChatComposerHeight}
+            onOpenNativeTui={() => requestMode("tui")}
             onSessionId={(sid) => void setSessionWithInstrumentation("custom", sid)}
             onConversationId={(cid) => void vm.setCellConversation(id, cid)}
           />
diff --git a/frontend/src/features/web/WebAgentCell.tsx b/frontend/src/features/web/WebAgentCell.tsx
index 6f875f68..50b2e8a6 100644
--- a/frontend/src/features/web/WebAgentCell.tsx
+++ b/frontend/src/features/web/WebAgentCell.tsx
@@ -208,6 +208,7 @@ export function WebAgentCell({
           conversationId={customConversationId}
           draftScope={draftScope}
           mobile
+          onOpenNativeTui={() => setMode("tui")}
           onSessionId={setCustomSessionId}
           onConversationId={setCustomConversationId}
         />