feat: agent session retry + follow-up UX

- retry/route.ts: reset failed/stopped session and re-fire agent runner
  with optional continueTask follow-up text
- build/page.tsx: Retry button and Follow up input appear on failed/stopped
  sessions so users can continue without losing context or creating a
  duplicate session; task input hint clarifies each Run = new session

Made-with: Cursor

app/[workspace]/project/[projectId]/build/page.tsx

@@ -278,6 +278,9 @@ function AgentMode({ projectId, appName, appPath }: { projectId: string; appName
   const [approveMsg, setApproveMsg] = useState("");
   const [showApproveInput, setShowApproveInput] = useState(false);
   const [approveResult, setApproveResult] = useState<string | null>(null);
+  const [retrying, setRetrying] = useState(false);
+  const [followUp, setFollowUp] = useState("");
+  const [showFollowUp, setShowFollowUp] = useState(false);
   const outputRef = useCallback((el: HTMLDivElement | null) => {
     if (el) el.scrollTop = el.scrollHeight;
   }, []);
@@ -346,6 +349,24 @@ function AgentMode({ projectId, appName, appPath }: { projectId: string; appName
     setActiveSession(prev => prev ? { ...prev, status: "stopped" } : null);
   };
 
+  const handleRetry = async (continueTask?: string) => {
+    if (!activeSessionId) return;
+    setRetrying(true);
+    try {
+      const r = await fetch(`/api/projects/${projectId}/agent/sessions/${activeSessionId}/retry`, {
+        method: "POST",
+        headers: { "Content-Type": "application/json" },
+        body: JSON.stringify({ continueTask: continueTask?.trim() || undefined }),
+      });
+      const d = await r.json();
+      if (d.sessionId) {
+        setActiveSession(prev => prev ? { ...prev, status: "running", output: [], error: null } : null);
+        setShowFollowUp(false);
+        setFollowUp("");
+      }
+    } finally { setRetrying(false); }
+  };
+
   const handleApprove = async () => {
     if (!activeSessionId || !approveMsg.trim()) return;
     setApproving(true);
@@ -459,6 +480,57 @@ function AgentMode({ projectId, appName, appPath }: { projectId: string; appName
               )}
             </div>
 
+            {/* Retry / follow-up panel for failed or stopped sessions */}
+            {["failed", "stopped"].includes(activeSession.status) && (
+              <div style={{ borderTop: "1px solid #e8e4dc", background: "#fff", padding: "12px 20px", flexShrink: 0 }}>
+                {showFollowUp ? (
+                  <div style={{ display: "flex", flexDirection: "column", gap: 8 }}>
+                    <div style={{ fontSize: "0.72rem", color: "#6b6560", fontFamily: "Outfit, sans-serif" }}>
+                      Add a follow-up instruction (optional) then retry:
+                    </div>
+                    <div style={{ display: "flex", gap: 8, alignItems: "flex-end" }}>
+                      <textarea
+                        autoFocus
+                        value={followUp}
+                        onChange={e => setFollowUp(e.target.value)}
+                        onKeyDown={e => { if (e.key === "Enter" && (e.metaKey || e.ctrlKey)) handleRetry(followUp); if (e.key === "Escape") setShowFollowUp(false); }}
+                        placeholder="e.g. Also update the TypeScript types, or just leave blank to retry as-is…"
+                        rows={2}
+                        style={{ flex: 1, resize: "none", border: "1px solid #e8e4dc", borderRadius: 7, padding: "8px 11px", fontSize: "0.78rem", fontFamily: "Outfit, sans-serif", outline: "none", background: "#faf8f5" }}
+                      />
+                      <div style={{ display: "flex", flexDirection: "column", gap: 6 }}>
+                        <button onClick={() => handleRetry(followUp)} disabled={retrying}
+                          style={{ padding: "8px 14px", background: "#1a1a1a", color: "#fff", border: "none", borderRadius: 7, fontSize: "0.75rem", fontWeight: 600, cursor: "pointer", fontFamily: "Outfit, sans-serif", whiteSpace: "nowrap" }}>
+                          {retrying ? "Retrying…" : "Retry"}
+                        </button>
+                        <button onClick={() => setShowFollowUp(false)}
+                          style={{ padding: "6px 14px", background: "transparent", color: "#a09a90", border: "1px solid #e8e4dc", borderRadius: 7, fontSize: "0.73rem", cursor: "pointer", fontFamily: "Outfit, sans-serif" }}>
+                          Cancel
+                        </button>
+                      </div>
+                    </div>
+                    <div style={{ fontSize: "0.63rem", color: "#b5b0a6", fontFamily: "Outfit, sans-serif" }}>
+                      ⌘↵ to retry · Esc to cancel · Same session, fresh run
+                    </div>
+                  </div>
+                ) : (
+                  <div style={{ display: "flex", gap: 8, alignItems: "center" }}>
+                    <div style={{ fontSize: "0.75rem", color: activeSession.status === "failed" ? "#c62828" : "#a09a90", fontFamily: "Outfit, sans-serif", flex: 1 }}>
+                      {activeSession.status === "failed" ? "Session failed — retry without losing context" : "Session stopped"}
+                    </div>
+                    <button onClick={() => handleRetry()} disabled={retrying}
+                      style={{ padding: "7px 14px", background: "#fef9f0", color: "#92400e", border: "1px solid #fde68a", borderRadius: 7, fontSize: "0.75rem", fontWeight: 600, cursor: "pointer", fontFamily: "Outfit, sans-serif", whiteSpace: "nowrap" }}>
+                      {retrying ? "Retrying…" : "↺ Retry"}
+                    </button>
+                    <button onClick={() => setShowFollowUp(true)}
+                      style={{ padding: "7px 14px", background: "#f0ece4", color: "#1a1a1a", border: "1px solid #e8e4dc", borderRadius: 7, fontSize: "0.75rem", cursor: "pointer", fontFamily: "Outfit, sans-serif", whiteSpace: "nowrap" }}>
+                      Follow up…
+                    </button>
+                  </div>
+                )}
+              </div>
+            )}
+
             {/* Changed files */}
             {activeSession.changed_files.length > 0 && (
               <div style={{ borderTop: "1px solid #e8e4dc", background: "#fff", padding: "12px 20px", flexShrink: 0 }}>
@@ -576,7 +648,7 @@ function AgentMode({ projectId, appName, appPath }: { projectId: string; appName
             </button>
           </div>
           <div style={{ fontSize: "0.65rem", color: "#b5b0a6", marginTop: 5, fontFamily: "Outfit, sans-serif" }}>
-            ⌘↵ to run · Session persists if you close the browser
+            ⌘↵ to start · Each task is a new session · Use "Follow up" on a failed session to continue it
           </div>
         </div>
       </div>