diff --git a/backend/src/test/java/com/dbaagent/service/security/AccessControlServiceTest.java b/backend/src/test/java/com/dbaagent/service/security/AccessControlServiceTest.java
index 5551446..afc149f 100644
--- a/backend/src/test/java/com/dbaagent/service/security/AccessControlServiceTest.java
+++ b/backend/src/test/java/com/dbaagent/service/security/AccessControlServiceTest.java
@@ -359,6 +359,44 @@ void impersonatingDbaAllowsMutateSql() {
         assertFalse(accessControlService.isCurrentUserAdmin());
     }
 
+    /**
+     * An admin viewing as a user who lacks access to a connection sees access denied.
+     *
+     * <p>This is the backend counterpart to the frontend fix: when the admin selects a
+     * connection, then uses "View as" to switch to a user without access to that connection,
+     * the UI should not be able to make API calls against the connection. The UI fix clears
+     * the stale connectionId; this test ensures the backend also correctly denies access.
+     */
+    @Test
+    void impersonatingUserWithoutAccessDeniesConnection() {
+        com.dbaagent.model.User impersonator = new com.dbaagent.model.User();
+        impersonator.setId(1L);
+        impersonator.setUsername("admin");
+        impersonator.setRole("ADMIN");
+        com.dbaagent.model.User target = new com.dbaagent.model.User();
+        target.setId(2L);
+        target.setUsername("mart-viewer");
+        target.setRole("DEVELOPER");
+        com.dbaagent.security.ImpersonationContext.enter(
+            new com.dbaagent.security.ImpersonationContext.State(impersonator, target)
+        );
+        SecurityContextHolder.getContext().setAuthentication(
+            new UsernamePasswordAuthenticationToken("mart-viewer", null, List.of())
+        );
+
+        // The target user has no grant on conn-1
+        when(connectionAccessService.resolveAccess("conn-1", "mart-viewer", false))
+            .thenReturn(resolved("conn-1", EffectiveConnectionAccess.NONE, null));
+
+        // Verify: admin bypass is disabled during impersonation
+        assertFalse(accessControlService.isCurrentUserAdmin());
+
+        // Verify: attempting to access the connection should throw 403
+        ResponseStatusException ex = assertThrows(ResponseStatusException.class,
+            () -> accessControlService.assertCanUseChatEditor("conn-1"));
+        assertEquals(403, ex.getStatusCode().value());
+    }
+
     private ConnectionAccessService.ResolvedConnectionAccess resolved(
         String connectionId,
         EffectiveConnectionAccess effectiveAccess,
diff --git a/src/components/sections/AgentChatSection.jsx b/src/components/sections/AgentChatSection.jsx
index de4fd44..4f08c53 100644
--- a/src/components/sections/AgentChatSection.jsx
+++ b/src/components/sections/AgentChatSection.jsx
@@ -3,10 +3,25 @@ import { useAuth } from '@/hooks/useAuth'
 import AgentChatPanel from '@/components/AgentChat/AgentChatPanel'
 
 export default function AgentChatSection() {
-  const { connectionId, selectedConnection } = useConnectionManager()
+  const { connectionId, selectedConnection, isLoading } = useConnectionManager()
   const { username } = useAuth()
 
-  if (!connectionId) {
+  // Wait for the connection list to load before rendering anything. Without this,
+  // we might render the agent panel with a stale connectionId from before an
+  // impersonation change — the new user's connection list hasn't loaded yet, so
+  // selectedConnection is undefined, but connectionId is still the old value.
+  if (isLoading) {
+    return (
+      <div style={{ padding: 40, color: '#6b7280', fontSize: 14 }}>
+        Loading connections…
+      </div>
+    )
+  }
+
+  // Either no connection selected, or the selected connectionId is not in the
+  // current user's connections (stale after impersonation). Both cases mean the
+  // user needs to pick a valid connection before chatting.
+  if (!connectionId || !selectedConnection) {
     return (
       <div style={{ padding: 40, color: '#6b7280', fontSize: 14 }}>
         Select a database connection to chat with the DeepSQL Agent.
@@ -20,8 +35,8 @@ export default function AgentChatSection() {
     <AgentChatPanel
       key={`${username || 'anon'}:${connectionId}`}
       connectionId={connectionId}
-      connectionName={selectedConnection?.connectionName}
-      canManageContent={Boolean(selectedConnection?.canManageContent)}
+      connectionName={selectedConnection.connectionName}
+      canManageContent={Boolean(selectedConnection.canManageContent)}
     />
   )
 }
diff --git a/src/components/sections/CompanyKnowledgeSection.jsx b/src/components/sections/CompanyKnowledgeSection.jsx
index 9ed0bc2..0b69702 100644
--- a/src/components/sections/CompanyKnowledgeSection.jsx
+++ b/src/components/sections/CompanyKnowledgeSection.jsx
@@ -1,12 +1,22 @@
-import { Building2 } from 'lucide-react'
+import { Building2, Loader2 } from 'lucide-react'
 import { useConnectionManager } from '@/lib/hooks/useConnectionManager'
 import CompanyKnowledgePanel from '@/components/company-knowledge/CompanyKnowledgePanel'
 import styles from './SectionEmpty.module.css'
 
 export default function CompanyKnowledgeSection() {
-  const { connectionId } = useConnectionManager()
+  const { connectionId, selectedConnection, isLoading } = useConnectionManager()
 
-  if (!connectionId) {
+  if (isLoading) {
+    return (
+      <div className={styles.root}>
+        <Loader2 size={24} color="#9ca3af" className={styles.spin} />
+        <p className={styles.subtitle}>Loading connections…</p>
+      </div>
+    )
+  }
+
+  // Either no connection or stale connectionId not in the effective user's list
+  if (!connectionId || !selectedConnection) {
     return (
       <div className={styles.root}>
         <div className={styles.iconWrap}><Building2 size={26} color="#9ca3af" /></div>
diff --git a/src/components/sections/DashboardsSection.jsx b/src/components/sections/DashboardsSection.jsx
index c2476c6..efc770c 100644
--- a/src/components/sections/DashboardsSection.jsx
+++ b/src/components/sections/DashboardsSection.jsx
@@ -1,5 +1,5 @@
 import { useState, useCallback, useEffect } from 'react'
-import { LayoutDashboard } from 'lucide-react'
+import { LayoutDashboard, Loader2 } from 'lucide-react'
 import { useConnectionManager } from '@/lib/hooks/useConnectionManager'
 import { useSetImmersive } from '@/lib/stores/useNavStore'
 import DashboardsHome from './DashboardsHome'
@@ -10,7 +10,7 @@ import emptyStyles from './SectionEmpty.module.css'
 // dashboard, a focused full-bleed builder workspace (sidebar hidden via the
 // nav store's immersive flag).
 export default function DashboardsSection() {
-  const { connectionId } = useConnectionManager()
+  const { connectionId, selectedConnection, isLoading } = useConnectionManager()
   const setImmersive = useSetImmersive()
   const [open_, setOpen] = useState(null) // null = gallery; 'new' | dashboard object = workspace
 
@@ -22,7 +22,17 @@ export default function DashboardsSection() {
   // Safety: never leave the app in immersive mode when this section unmounts.
   useEffect(() => () => setImmersive(false), [setImmersive])
 
-  if (!connectionId) {
+  if (isLoading) {
+    return (
+      <div className={emptyStyles.root}>
+        <Loader2 size={24} color="#9ca3af" className={emptyStyles.spin} />
+        <p className={emptyStyles.subtitle}>Loading connections…</p>
+      </div>
+    )
+  }
+
+  // Either no connection or stale connectionId not in the effective user's list
+  if (!connectionId || !selectedConnection) {
     return (
       <div className={emptyStyles.root}>
         <div className={emptyStyles.iconWrap}><LayoutDashboard size={26} color="#9ca3af" /></div>
diff --git a/src/components/sections/DigestSection.jsx b/src/components/sections/DigestSection.jsx
index e8f89a5..8bdcaf4 100644
--- a/src/components/sections/DigestSection.jsx
+++ b/src/components/sections/DigestSection.jsx
@@ -1,5 +1,5 @@
 import { useState, useEffect, useCallback } from 'react'
-import { Newspaper, RefreshCw, Settings, Check, Clock, AlertCircle, Zap } from 'lucide-react'
+import { Loader2, Newspaper, RefreshCw, Settings, Check, Clock, AlertCircle, Zap } from 'lucide-react'
 import { slackDigestAPI, digestPreferencesAPI } from '@/lib/api/client'
 import { useConnectionManager } from '@/lib/hooks/useConnectionManager'
 import DigestPreferencesPanel from './DigestPreferencesPanel'
@@ -165,7 +165,7 @@ function DigestSection({ section }) {
 const DIGEST_PREFS_AUTOPEN_KEY = 'deepsql.digestPrefs.autoOpened.v1'
 
 export default function DigestFeedSection() {
-  const { connectionId, selectedConnection } = useConnectionManager()
+  const { connectionId, selectedConnection, isLoading: connectionsLoading } = useConnectionManager()
   const [digests, setDigests] = useState([])
   const [loading, setLoading] = useState(false)
   const [triggering, setTriggering] = useState(false)
@@ -257,6 +257,26 @@ export default function DigestFeedSection() {
     }
   }
 
+  // Wait for connection list to load first
+  if (connectionsLoading) {
+    return (
+      <div className={styles.root}>
+        <div className={styles.topBar}>
+          <div className={styles.topBarLeft}>
+            <Newspaper size={17} className={styles.topBarIcon} />
+            <span className={styles.topBarTitle}>DB Digest</span>
+          </div>
+        </div>
+        <div className={styles.feed}>
+          <div className={styles.loadingState}>
+            <Loader2 size={20} className={styles.spinning} color="#9ca3af" />
+            <span>Loading connections…</span>
+          </div>
+        </div>
+      </div>
+    )
+  }
+
   return (
     <div className={styles.root}>
       {/* Top bar */}
@@ -309,7 +329,8 @@ export default function DigestFeedSection() {
           </div>
         )}
 
-        {!error && !loading && !connectionId && (
+        {/* Either no connection or stale connectionId not in the effective user's list */}
+        {!error && !loading && (!connectionId || !selectedConnection) && (
           <div className={styles.emptyState}>
             <Newspaper size={32} color="#d1d5db" />
             <h3>No connection selected</h3>
@@ -317,7 +338,7 @@ export default function DigestFeedSection() {
           </div>
         )}
 
-        {!error && !loading && !!connectionId && digests.length === 0 && (
+        {!error && !loading && !!connectionId && !!selectedConnection && digests.length === 0 && (
           <div className={styles.emptyState}>
             <Newspaper size={32} color="#d1d5db" />
             <h3>No digests yet</h3>
diff --git a/src/components/sections/EditorSection.jsx b/src/components/sections/EditorSection.jsx
index 9229c20..a3e1fee 100644
--- a/src/components/sections/EditorSection.jsx
+++ b/src/components/sections/EditorSection.jsx
@@ -1,12 +1,22 @@
 import { useConnectionManager } from '@/lib/hooks/useConnectionManager'
 import SqlRunnerTab from '@/components/tabs/Core/SqlRunnerTab'
-import { Code2 } from 'lucide-react'
+import { Code2, Loader2 } from 'lucide-react'
 import styles from './SectionEmpty.module.css'
 
 export default function EditorSection() {
-  const { connectionId } = useConnectionManager()
+  const { connectionId, selectedConnection, isLoading } = useConnectionManager()
 
-  if (!connectionId) {
+  if (isLoading) {
+    return (
+      <div className={styles.root}>
+        <Loader2 size={24} color="#9ca3af" className={styles.spin} />
+        <p className={styles.subtitle}>Loading connections…</p>
+      </div>
+    )
+  }
+
+  // Either no connection selected, or stale connectionId not in the current user's list
+  if (!connectionId || !selectedConnection) {
     return (
       <div className={styles.root}>
         <div className={styles.iconWrap}>
diff --git a/src/components/sections/MonitorSection.jsx b/src/components/sections/MonitorSection.jsx
index 61022fa..6fe8685 100644
--- a/src/components/sections/MonitorSection.jsx
+++ b/src/components/sections/MonitorSection.jsx
@@ -1,12 +1,12 @@
 import { useEffect, useState } from 'react'
-import { BarChart2 } from 'lucide-react'
+import { BarChart2, Loader2 } from 'lucide-react'
 import { slowQueriesAPI } from '@/lib/api/client'
 import { useConnectionManager } from '@/lib/hooks/useConnectionManager'
 import AnalyticsTab from '@/components/tabs/Monitoring/AnalyticsTab'
 import styles from './SectionEmpty.module.css'
 
 export default function MonitorSection() {
-  const { connectionId } = useConnectionManager()
+  const { connectionId, selectedConnection, isLoading: connectionsLoading } = useConnectionManager()
   const [hasData, setHasData] = useState(null) // null = loading
   const [loading, setLoading] = useState(true)
 
@@ -27,7 +27,18 @@ export default function MonitorSection() {
       .finally(() => setLoading(false))
   }, [connectionId])
 
-  if (!connectionId) {
+  // Wait for connection list to load first
+  if (connectionsLoading) {
+    return (
+      <div className={styles.root}>
+        <Loader2 size={24} color="#9ca3af" className={styles.spin} />
+        <p className={styles.subtitle}>Loading connections…</p>
+      </div>
+    )
+  }
+
+  // Either no connection or stale connectionId not in the effective user's list
+  if (!connectionId || !selectedConnection) {
     return (
       <div className={styles.root}>
         <div className={styles.iconWrap}>
diff --git a/src/components/sections/SchemaDocsSection.jsx b/src/components/sections/SchemaDocsSection.jsx
index b54deba..19f97a0 100644
--- a/src/components/sections/SchemaDocsSection.jsx
+++ b/src/components/sections/SchemaDocsSection.jsx
@@ -1,4 +1,4 @@
-import { FileText } from 'lucide-react'
+import { FileText, Loader2 } from 'lucide-react'
 import { useConnectionManager } from '@/lib/hooks/useConnectionManager'
 import { useSetActiveSection } from '@/lib/stores/useNavStore'
 import { useCompanyKnowledgeStore } from '@/lib/stores/useCompanyKnowledgeStore'
@@ -7,11 +7,21 @@ import styles from './SectionEmpty.module.css'
 import workspaceStyles from './TopLevelSection.module.css'
 
 export default function SchemaDocsSection() {
-  const { connectionId } = useConnectionManager()
+  const { connectionId, selectedConnection, isLoading } = useConnectionManager()
   const setActiveSection = useSetActiveSection()
   const setLinkedFilters = useCompanyKnowledgeStore((state) => state.setLinkedFilters)
 
-  if (!connectionId) {
+  if (isLoading) {
+    return (
+      <div className={styles.root}>
+        <Loader2 size={24} color="#9ca3af" className={styles.spin} />
+        <p className={styles.subtitle}>Loading connections…</p>
+      </div>
+    )
+  }
+
+  // Either no connection or stale connectionId not in the effective user's list
+  if (!connectionId || !selectedConnection) {
     return (
       <div className={styles.root}>
         <div className={styles.iconWrap}><FileText size={26} color="#9ca3af" /></div>
diff --git a/src/components/sections/SchemaSection.jsx b/src/components/sections/SchemaSection.jsx
index ea310fb..81488d7 100644
--- a/src/components/sections/SchemaSection.jsx
+++ b/src/components/sections/SchemaSection.jsx
@@ -1,12 +1,22 @@
-import { Network } from 'lucide-react'
+import { Network, Loader2 } from 'lucide-react'
 import { useConnectionManager } from '@/lib/hooks/useConnectionManager'
 import BrainWorkspace from '@/components/tabs/Brain/BrainWorkspace'
 import styles from './SectionEmpty.module.css'
 
 export default function SchemaSection() {
-  const { connectionId } = useConnectionManager()
+  const { connectionId, selectedConnection, isLoading } = useConnectionManager()
 
-  if (!connectionId) {
+  if (isLoading) {
+    return (
+      <div className={styles.root}>
+        <Loader2 size={24} color="#9ca3af" className={styles.spin} />
+        <p className={styles.subtitle}>Loading connections…</p>
+      </div>
+    )
+  }
+
+  // Either no connection or stale connectionId not in the effective user's list
+  if (!connectionId || !selectedConnection) {
     return (
       <div className={styles.root}>
         <div className={styles.iconWrap}><Network size={26} color="#9ca3af" /></div>
diff --git a/src/components/sections/SectionEmpty.module.css b/src/components/sections/SectionEmpty.module.css
index 6c9435b..db096c1 100644
--- a/src/components/sections/SectionEmpty.module.css
+++ b/src/components/sections/SectionEmpty.module.css
@@ -72,6 +72,14 @@
   background: #f9fafb;
 }
 
+.spin {
+  animation: sectionSpin 1s linear infinite;
+}
+
+@keyframes sectionSpin {
+  to { transform: rotate(360deg); }
+}
+
 .pills {
   display: flex;
   gap: 8px;
diff --git a/src/components/sections/SlowQueriesSection.jsx b/src/components/sections/SlowQueriesSection.jsx
index 44af4c0..7cfbe58 100644
--- a/src/components/sections/SlowQueriesSection.jsx
+++ b/src/components/sections/SlowQueriesSection.jsx
@@ -1,5 +1,5 @@
 import { useRef, useState } from 'react'
-import { Activity, FileText, LineChart, Settings, Users } from 'lucide-react'
+import { Activity, FileText, LineChart, Loader2, Settings, Users } from 'lucide-react'
 import { useConnectionManager } from '@/lib/hooks/useConnectionManager'
 import { useSlowLogSourceConfig } from '@/lib/hooks/queries'
 import QueryTrendsTab from '@/components/tabs/Performance/QueryTrendsTab'
@@ -31,7 +31,7 @@ const LOG_SOURCE_HELP = {
  * is a single empty state whose CTA opens SlowQuerySourceModal.
  */
 export default function SlowQueriesSection() {
-  const { connectionId, selectedConnection } = useConnectionManager()
+  const { connectionId, selectedConnection, isLoading } = useConnectionManager()
   const [tab, setTab] = useState('trends')
   const tabRefs = useRef({})
 
@@ -59,6 +59,22 @@ export default function SlowQueriesSection() {
   const logSourceQ = useSlowLogSourceConfig(connectionId)
   const hasLogSource = Boolean(logSourceQ.data?.id)
 
+  // Wait for connection list to load before rendering anything
+  if (isLoading) {
+    return (
+      <div className={sectionStyles.page}>
+        <div className={sectionStyles.header}>
+          <div className={sectionStyles.eyebrow}>Performance</div>
+          <h1 className={sectionStyles.title}>Slow queries &amp; workload</h1>
+        </div>
+        <div className={styles.empty}>
+          <Loader2 size={20} className={styles.spinIcon} />
+          Loading connections…
+        </div>
+      </div>
+    )
+  }
+
   return (
     <div className={sectionStyles.page}>
       <div className={sectionStyles.header}>
@@ -70,7 +86,8 @@ export default function SlowQueriesSection() {
         </p>
       </div>
 
-      {!connectionId ? (
+      {/* Either no connection or stale connectionId not in the effective user's list */}
+      {(!connectionId || !selectedConnection) ? (
         <div className={styles.empty}>
           Select a database connection to see performance analytics.
         </div>
diff --git a/src/components/sections/SlowQueriesSection.module.css b/src/components/sections/SlowQueriesSection.module.css
index 3b356ad..3f89f5f 100644
--- a/src/components/sections/SlowQueriesSection.module.css
+++ b/src/components/sections/SlowQueriesSection.module.css
@@ -141,3 +141,14 @@
   border-color: #d1d5db;
   color: #111827;
 }
+
+.spinIcon {
+  animation: spin 1s linear infinite;
+  display: inline-block;
+  vertical-align: middle;
+  margin-right: 8px;
+}
+
+@keyframes spin {
+  to { transform: rotate(360deg); }
+}
diff --git a/src/lib/hooks/useConnectionManager.js b/src/lib/hooks/useConnectionManager.js
index 0f497a0..59a76e2 100644
--- a/src/lib/hooks/useConnectionManager.js
+++ b/src/lib/hooks/useConnectionManager.js
@@ -56,6 +56,11 @@ export function useConnectionManager() {
   // selected and a pin that only ran on a blank slate would never actually be the
   // default — which is the whole feature. It applies once per page load
   // (`pinAppliedThisLoad`), so switching connections mid-session still sticks.
+  //
+  // Critical: if `connectionId` is set but NOT in the connections list (e.g. after
+  // "View as" switches to a user without access to that connection), clear it and
+  // select a valid one. Otherwise the UI keeps a stale connection the effective user
+  // cannot access, and chat/agent calls 403.
   useEffect(() => {
     if (connections.length === 0) return
 
@@ -69,12 +74,23 @@ export function useConnectionManager() {
       }
     }
 
-    if (connectionId) return
+    // Validate the selected connection is actually in the current user's list.
+    // When impersonation changes, the connection list is for the target user, but
+    // connectionId might still be a connection the admin had selected. Clear it
+    // so the user must pick from their own list (or gets auto-selected below).
+    if (connectionId) {
+      const stillValid = connections.some((c) => c.id === connectionId)
+      if (stillValid) return
+      // The selected connection is not in the list — clear it and fall through
+      // to auto-select from the valid options.
+      localStorage.removeItem('selectedConnectionId')
+      setConnectionId(null)
+    }
 
     const savedId = localStorage.getItem('selectedConnectionId')
     const saved = savedId ? connections.find((c) => c.id === savedId) : null
     selectConnection((pinned || saved || connections[0]).id)
-  }, [connections, connectionId, selectConnection])
+  }, [connections, connectionId, selectConnection, setConnectionId])
 
   const changeConnection = useCallback(
     (connId) => {