diff --git a/ui/src/pages/Admin/ClaimMappingRulesModal.tsx b/ui/src/pages/Admin/ClaimMappingRulesModal.tsx
index dde57ec8..37dea3a7 100644
--- a/ui/src/pages/Admin/ClaimMappingRulesModal.tsx
+++ b/ui/src/pages/Admin/ClaimMappingRulesModal.tsx
@@ -10,6 +10,7 @@ import {
 } from '../../api/queries/admin/claim-mappings';
 import type { ClaimMappingRule, TestResponse } from '../../api/queries/admin/claim-mappings';
 import { useRoles, useGroups } from '../../api/queries/admin/rbac';
+import { describeApiError } from '../../api/errors';
 import styles from './ClaimMappingRulesModal.module.css';
 
 const MATCH_OPTIONS = [
@@ -231,8 +232,8 @@ export default function ClaimMappingRulesModal({ open, onClose }: Props) {
 
       toast({ title: 'Rules saved', variant: 'success' });
       handleClose();
-    } catch (e: any) {
-      toast({ title: 'Failed to save rules', description: e.message, variant: 'error', duration: 86_400_000 });
+    } catch (e) {
+      toast({ title: 'Failed to save rules', description: describeApiError(e), variant: 'error', duration: 86_400_000 });
     } finally {
       setApplying(false);
     }
@@ -256,7 +257,7 @@ export default function ClaimMappingRulesModal({ open, onClose }: Props) {
     }));
     testRules.mutate({ rules, claims }, {
       onSuccess: (result) => setTestResult(result),
-      onError: (e) => setTestError(e.message),
+      onError: (e) => setTestError(describeApiError(e)),
     });
   }
 
diff --git a/ui/src/pages/Admin/OidcConfigPage.tsx b/ui/src/pages/Admin/OidcConfigPage.tsx
index 72116eff..69cd971a 100644
--- a/ui/src/pages/Admin/OidcConfigPage.tsx
+++ b/ui/src/pages/Admin/OidcConfigPage.tsx
@@ -8,6 +8,7 @@ import { PageLoader } from '../../components/PageLoader';
 import { adminFetch } from '../../api/queries/admin/admin-api';
 import ClaimMappingRulesModal from './ClaimMappingRulesModal';
 import { useClaimMappingRules } from '../../api/queries/admin/claim-mappings';
+import { describeApiError } from '../../api/errors';
 import styles from './OidcConfigPage.module.css';
 import sectionStyles from '../../styles/section-card.module.css';
 
@@ -114,8 +115,8 @@ export default function OidcConfigPage() {
       setFormDraft(null);
       setEditing(false);
       toast({ title: 'Settings saved', description: 'OIDC configuration updated successfully.', variant: 'success' });
-    } catch (e: any) {
-      toast({ title: 'Failed to save OIDC configuration', description: e.message, variant: 'error', duration: 86_400_000 });
+    } catch (e) {
+      toast({ title: 'Failed to save OIDC configuration', description: describeApiError(e), variant: 'error', duration: 86_400_000 });
     } finally {
       setSaving(false);
     }
@@ -127,8 +128,8 @@ export default function OidcConfigPage() {
     try {
       const result = await adminFetch<{ status: string; authorizationEndpoint?: string }>('/oidc/test', { method: 'POST' });
       toast({ title: 'Connection test', description: `OIDC provider responded: ${result.status}`, variant: 'success' });
-    } catch (e: any) {
-      toast({ title: 'Connection test failed', description: e.message, variant: 'error', duration: 86_400_000 });
+    } catch (e) {
+      toast({ title: 'Connection test failed', description: describeApiError(e), variant: 'error', duration: 86_400_000 });
     } finally {
       setTesting(false);
     }
@@ -142,8 +143,8 @@ export default function OidcConfigPage() {
       setFormDraft(null);
       setEditing(false);
       toast({ title: 'Configuration deleted', description: 'OIDC configuration has been removed.', variant: 'warning' });
-    } catch (e: any) {
-      toast({ title: 'Failed to delete OIDC configuration', description: e.message, variant: 'error', duration: 86_400_000 });
+    } catch (e) {
+      toast({ title: 'Failed to delete OIDC configuration', description: describeApiError(e), variant: 'error', duration: 86_400_000 });
     }
   }
 
diff --git a/ui/src/pages/Admin/OutboundConnectionsPage.tsx b/ui/src/pages/Admin/OutboundConnectionsPage.tsx
index de4c3429..1d34729f 100644
--- a/ui/src/pages/Admin/OutboundConnectionsPage.tsx
+++ b/ui/src/pages/Admin/OutboundConnectionsPage.tsx
@@ -16,7 +16,7 @@ export default function OutboundConnectionsPage() {
   const { toast } = useToast();
 
   if (isLoading) return <PageLoader />;
-  if (error) return <div>Failed to load outbound connections: {String(error)}</div>;
+  if (error) return <div>Failed to load outbound connections: {describeApiError(error)}</div>;
 
   const rows = data ?? [];
 
diff --git a/ui/src/pages/AgentHealth/AgentHealth.tsx b/ui/src/pages/AgentHealth/AgentHealth.tsx
index dc2813ec..eb8c3946 100644
--- a/ui/src/pages/AgentHealth/AgentHealth.tsx
+++ b/ui/src/pages/AgentHealth/AgentHealth.tsx
@@ -23,6 +23,7 @@ import type { ConfigUpdateResponse } from '../../api/queries/commands';
 import type { AgentInstance } from '../../api/types';
 import { timeAgo } from '../../utils/format-utils';
 import { formatUptime, mapLogLevel, eventSeverity, eventIcon } from '../../utils/agent-utils';
+import { describeApiError } from '../../api/errors';
 
 // ── Helpers ──────────────────────────────────────────────────────────────────
 
@@ -506,7 +507,7 @@ export default function AgentHealth() {
                   navigate('/runtime');
                 },
                 onError: (err) => {
-                  toast({ title: 'Dismiss failed', description: err.message, variant: 'error', duration: 86_400_000 });
+                  toast({ title: 'Dismiss failed', description: describeApiError(err), variant: 'error', duration: 86_400_000 });
                 },
               });
             }}
diff --git a/ui/src/pages/Alerts/InboxPage.tsx b/ui/src/pages/Alerts/InboxPage.tsx
index 3c158532..e80eb7e1 100644
--- a/ui/src/pages/Alerts/InboxPage.tsx
+++ b/ui/src/pages/Alerts/InboxPage.tsx
@@ -344,7 +344,7 @@ export default function InboxPage() {
   // ── render ─────────────────────────────────────────────────────────────────
 
   if (isLoading) return <PageLoader />;
-  if (error)     return <div className={css.page}>Failed to load alerts: {String(error)}</div>;
+  if (error)     return <div className={css.page}>Failed to load alerts: {describeApiError(error)}</div>;
 
   const selectedIds = Array.from(selected);
 
diff --git a/ui/src/pages/Alerts/RulesListPage.tsx b/ui/src/pages/Alerts/RulesListPage.tsx
index 7bfe127a..149ad096 100644
--- a/ui/src/pages/Alerts/RulesListPage.tsx
+++ b/ui/src/pages/Alerts/RulesListPage.tsx
@@ -32,7 +32,7 @@ export default function RulesListPage() {
   const [pendingDelete, setPendingDelete] = useState<AlertRuleResponse | null>(null);
 
   if (isLoading) return <PageLoader />;
-  if (error) return <div className={css.page}>Failed to load rules: {String(error)}</div>;
+  if (error) return <div className={css.page}>Failed to load rules: {describeApiError(error)}</div>;
 
   const rows = rules ?? [];
   const otherEnvs = (envs ?? []).filter((e) => e.slug !== env);
diff --git a/ui/src/pages/Alerts/SilencesPage.tsx b/ui/src/pages/Alerts/SilencesPage.tsx
index 75a3819a..dd7572fc 100644
--- a/ui/src/pages/Alerts/SilencesPage.tsx
+++ b/ui/src/pages/Alerts/SilencesPage.tsx
@@ -37,7 +37,7 @@ export default function SilencesPage() {
   }, [searchParams]);
 
   if (isLoading) return <PageLoader />;
-  if (error) return <div className={css.page}>Failed to load silences: {String(error)}</div>;
+  if (error) return <div className={css.page}>Failed to load silences: {describeApiError(error)}</div>;
 
   const rows = data ?? [];
 
diff --git a/ui/src/pages/Exchanges/RouteControlBar.tsx b/ui/src/pages/Exchanges/RouteControlBar.tsx
index 094f2c14..6c1524dd 100644
--- a/ui/src/pages/Exchanges/RouteControlBar.tsx
+++ b/ui/src/pages/Exchanges/RouteControlBar.tsx
@@ -4,6 +4,7 @@ import { useToast, ConfirmDialog } from '@cameleer/design-system';
 import { useSendRouteCommand, useReplayExchange } from '../../api/queries/commands';
 import type { CommandGroupResponse } from '../../api/queries/commands';
 import { useEnvironmentStore } from '../../api/environment-store';
+import { describeApiError } from '../../api/errors';
 import styles from './RouteControlBar.module.css';
 
 interface RouteControlBarProps {
@@ -68,7 +69,7 @@ export function RouteControlBar({ application, routeId, routeState, hasRouteCont
           setSendingAction(null);
         },
         onError: (err) => {
-          toast({ title: `Route ${action} failed`, description: err.message, variant: 'error', duration: 86_400_000 });
+          toast({ title: `Route ${action} failed`, description: describeApiError(err), variant: 'error', duration: 86_400_000 });
           setSendingAction(null);
         },
       },
@@ -92,7 +93,7 @@ export function RouteControlBar({ application, routeId, routeState, hasRouteCont
           setSendingAction(null);
         },
         onError: (err) => {
-          toast({ title: 'Replay failed', description: err.message, variant: 'error', duration: 86_400_000 });
+          toast({ title: 'Replay failed', description: describeApiError(err), variant: 'error', duration: 86_400_000 });
           setSendingAction(null);
         },
       },