diff --git a/packages/dashboard/src/components/dashboard/project/_use-project-data.ts b/packages/dashboard/src/components/dashboard/project/_use-project-data.ts
index db20247..4eadad2 100644
--- a/packages/dashboard/src/components/dashboard/project/_use-project-data.ts
+++ b/packages/dashboard/src/components/dashboard/project/_use-project-data.ts
@@ -22,8 +22,8 @@ interface UseProjectDataResult {
 
 export function useProjectData(slug: string | null): UseProjectDataResult {
   const [project, setProject] = useState<ProjectDetail | null>(null);
-  const [loading, setLoading] = useState(true);
-  const [error, setError] = useState<ProjectLoadError>(null);
+  const [loading, setLoading] = useState(() => Boolean(slug));
+  const [error, setError] = useState<ProjectLoadError>(() => (slug ? null : "not-found"));
   const [conversations, setConversations] = useState<Conversation[]>([]);
   const [convsLoading, setConvsLoading] = useState(false);
 
@@ -37,7 +37,13 @@ export function useProjectData(slug: string | null): UseProjectDataResult {
   // stale response from a previous slug overwriting the current project's
   // data on rapid navigation, and avoids setState after unmount (#318).
   const load = useCallback(() => {
-    if (!slug) return () => {};
+    if (!slug) {
+      setProject(null);
+      setConversations([]);
+      setLoading(false);
+      setError("not-found");
+      return () => {};
+    }
     setLoading(true);
     setError(null);
     let active = true;
diff --git a/packages/dashboard/src/components/dashboard/project/project-content.tsx b/packages/dashboard/src/components/dashboard/project/project-content.tsx
index 77e80f7..5f88f17 100644
--- a/packages/dashboard/src/components/dashboard/project/project-content.tsx
+++ b/packages/dashboard/src/components/dashboard/project/project-content.tsx
@@ -110,7 +110,9 @@ function ProjectContent() {
     conversations,
   );
 
-  // Early returns after all hooks
+  // Early returns after all hooks. Missing `?slug=` is a not-found, not a
+  // load — otherwise loading stays true and this page spins forever (#407).
+  if (!slug) return <_ProjectErrorState kind="not-found" onRetry={retry} />;
   if (loading) return <_ProjectLoadingState />;
   if (!project) return <_ProjectErrorState kind={error} onRetry={retry} />;