diff --git a/CLAUDE.md b/CLAUDE.md
index ccdb66e6..19232aac 100644
--- a/CLAUDE.md
+++ b/CLAUDE.md
@@ -74,6 +74,7 @@ PostgreSQL (Flyway): `cameleer-server-app/src/main/resources/db/migration/`
 - V15 — Discriminate open-instance uniqueness by `context->'exchange'->>'id'` so EXCHANGE_MATCH/PER_EXCHANGE emits one alert_instance per matching exchange; scalar kinds resolve to `''` and keep one-open-per-rule.
 - V16 — Generalise the V15 discriminator to prefer `context->>'_subjectFingerprint'` (falls back to the V15 `exchange.id` expression for legacy rows). Enables AGENT_LIFECYCLE to emit one alert_instance per `(agent, eventType, timestamp)` via a canonical fingerprint in the evaluator firing's context.
 - V17 — Alerts inbox redesign: drop `ACKNOWLEDGED` from `alert_state_enum` (ack is now orthogonal via `acked_at`), add `read_at` + `deleted_at` timestamp columns (global, no per-user tracking), drop `alert_reads` table entirely, rework the V13/V15/V16 open-rule unique index predicate to `state IN ('PENDING','FIRING') AND deleted_at IS NULL` so ack doesn't close the slot and soft-delete frees it.
+- V18 — Add `AGENT_LIFECYCLE` to `condition_kind_enum`. The Java `ConditionKind` enum had shipped with this value since the alerting branch, but no migration ever extended the Postgres type — any insert of a rule with `conditionKind=AGENT_LIFECYCLE` failed with `invalid input value for enum condition_kind_enum`. Single-statement migration because `ALTER TYPE ... ADD VALUE` can't coexist with usage of the new value in the same transaction.
 
 ClickHouse: `cameleer-server-app/src/main/resources/clickhouse/init.sql` (run idempotently on startup)
 
diff --git a/cameleer-server-app/src/main/resources/db/migration/V18__condition_kind_add_agent_lifecycle.sql b/cameleer-server-app/src/main/resources/db/migration/V18__condition_kind_add_agent_lifecycle.sql
new file mode 100644
index 00000000..b9c208a4
--- /dev/null
+++ b/cameleer-server-app/src/main/resources/db/migration/V18__condition_kind_add_agent_lifecycle.sql
@@ -0,0 +1,10 @@
+-- V18 — Add AGENT_LIFECYCLE to condition_kind_enum.
+--
+-- The Java ConditionKind enum shipped AGENT_LIFECYCLE with the alerting
+-- branch, but no migration ever extended the Postgres type. Inserting a
+-- rule with conditionKind=AGENT_LIFECYCLE failed with
+--   ERROR: invalid input value for enum condition_kind_enum: "AGENT_LIFECYCLE"
+-- ALTER TYPE ... ADD VALUE must live alone in its migration — Postgres won't
+-- allow the new value to be referenced in the same transaction that adds it.
+
+ALTER TYPE condition_kind_enum ADD VALUE IF NOT EXISTS 'AGENT_LIFECYCLE' AFTER 'AGENT_STATE';
diff --git a/cameleer-server-app/src/test/java/com/cameleer/server/app/alerting/storage/V18MigrationIT.java b/cameleer-server-app/src/test/java/com/cameleer/server/app/alerting/storage/V18MigrationIT.java
new file mode 100644
index 00000000..1551143d
--- /dev/null
+++ b/cameleer-server-app/src/test/java/com/cameleer/server/app/alerting/storage/V18MigrationIT.java
@@ -0,0 +1,20 @@
+package com.cameleer.server.app.alerting.storage;
+
+import com.cameleer.server.app.AbstractPostgresIT;
+import org.junit.jupiter.api.Test;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+class V18MigrationIT extends AbstractPostgresIT {
+
+    @Test
+    void condition_kind_enum_includes_agent_lifecycle() {
+        var values = jdbcTemplate.queryForList("""
+            SELECT unnest(enum_range(NULL::condition_kind_enum))::text AS v
+            """, String.class);
+        assertThat(values).contains("AGENT_LIFECYCLE");
+        assertThat(values).containsExactlyInAnyOrder(
+            "ROUTE_METRIC", "EXCHANGE_MATCH", "AGENT_STATE", "AGENT_LIFECYCLE",
+            "DEPLOYMENT_STATE", "LOG_PATTERN", "JVM_METRIC");
+    }
+}
diff --git a/ui/src/api/errors.ts b/ui/src/api/errors.ts
new file mode 100644
index 00000000..bffec813
--- /dev/null
+++ b/ui/src/api/errors.ts
@@ -0,0 +1,24 @@
+/**
+ * Turn whatever a fetch/mutation threw into a user-readable string.
+ *
+ * openapi-fetch rethrows the parsed response body as-is (a plain object like
+ * `{ error, message, path, status, timestamp }` for Spring errors), so
+ * `String(e)` renders "[object Object]" in a toast. This helper prefers an
+ * Error.message, falls back to a `message` field on plain objects, then
+ * Spring's `error` field, and finally JSON-stringifies the rest.
+ */
+export function describeApiError(e: unknown): string {
+  if (e instanceof Error) return e.message;
+  if (typeof e === 'string') return e;
+  if (typeof e === 'object' && e !== null) {
+    const obj = e as Record<string, unknown>;
+    if (typeof obj.message === 'string' && obj.message) return obj.message;
+    if (typeof obj.error === 'string' && obj.error) return obj.error;
+    try {
+      return JSON.stringify(e);
+    } catch {
+      return String(e);
+    }
+  }
+  return String(e);
+}
diff --git a/ui/src/pages/Admin/OutboundConnectionEditor.tsx b/ui/src/pages/Admin/OutboundConnectionEditor.tsx
index 92ebd3f2..d6ad4f3f 100644
--- a/ui/src/pages/Admin/OutboundConnectionEditor.tsx
+++ b/ui/src/pages/Admin/OutboundConnectionEditor.tsx
@@ -16,6 +16,7 @@ import {
   type TrustMode,
 } from '../../api/queries/admin/outboundConnections';
 import { useEnvironments } from '../../api/queries/admin/environments';
+import { describeApiError } from '../../api/errors';
 import sectionStyles from '../../styles/section-card.module.css';
 
 // ── Form state ──────────────────────────────────────────────────────────
@@ -145,12 +146,12 @@ export default function OutboundConnectionEditor() {
           toast({ title: 'Created', description: form.name, variant: 'success' });
           navigate('/admin/outbound-connections');
         },
-        onError: (e) => toast({ title: 'Create failed', description: String(e), variant: 'error' }),
+        onError: (e) => toast({ title: 'Create failed', description: describeApiError(e), variant: 'error' }),
       });
     } else {
       updateMut.mutate(payload, {
         onSuccess: () => toast({ title: 'Updated', description: form.name, variant: 'success' }),
-        onError: (e) => toast({ title: 'Update failed', description: String(e), variant: 'error' }),
+        onError: (e) => toast({ title: 'Update failed', description: describeApiError(e), variant: 'error' }),
       });
     }
   };
diff --git a/ui/src/pages/Admin/OutboundConnectionsPage.tsx b/ui/src/pages/Admin/OutboundConnectionsPage.tsx
index 03ec73f7..de4c3429 100644
--- a/ui/src/pages/Admin/OutboundConnectionsPage.tsx
+++ b/ui/src/pages/Admin/OutboundConnectionsPage.tsx
@@ -7,6 +7,7 @@ import {
   type OutboundConnectionDto,
   type TrustMode,
 } from '../../api/queries/admin/outboundConnections';
+import { describeApiError } from '../../api/errors';
 import sectionStyles from '../../styles/section-card.module.css';
 
 export default function OutboundConnectionsPage() {
@@ -23,7 +24,7 @@ export default function OutboundConnectionsPage() {
     if (!confirm(`Delete outbound connection "${c.name}"?`)) return;
     deleteMut.mutate(c.id, {
       onSuccess: () => toast({ title: 'Deleted', description: c.name, variant: 'success' }),
-      onError: (e) => toast({ title: 'Delete failed', description: String(e), variant: 'error' }),
+      onError: (e) => toast({ title: 'Delete failed', description: describeApiError(e), variant: 'error' }),
     });
   };
 
diff --git a/ui/src/pages/Alerts/InboxPage.tsx b/ui/src/pages/Alerts/InboxPage.tsx
index e548018e..3c158532 100644
--- a/ui/src/pages/Alerts/InboxPage.tsx
+++ b/ui/src/pages/Alerts/InboxPage.tsx
@@ -14,6 +14,7 @@ import {
   type AlertDto,
 } from '../../api/queries/alerts';
 import { useCreateSilence } from '../../api/queries/alertSilences';
+import { describeApiError } from '../../api/errors';
 import { useAuthStore } from '../../auth/auth-store';
 import { SilenceRuleMenu } from './SilenceRuleMenu';
 import { severityToAccent } from './severity-utils';
@@ -178,7 +179,7 @@ export default function InboxPage() {
       await ack.mutateAsync(id);
       toast({ title: 'Acknowledged', description: title, variant: 'success' });
     } catch (e) {
-      toast({ title: 'Ack failed', description: String(e), variant: 'error' });
+      toast({ title: 'Ack failed', description: describeApiError(e), variant: 'error' });
     }
   };
 
@@ -187,7 +188,7 @@ export default function InboxPage() {
       await markRead.mutateAsync(id);
       toast({ title: 'Marked as read', variant: 'success' });
     } catch (e) {
-      toast({ title: 'Mark read failed', description: String(e), variant: 'error' });
+      toast({ title: 'Mark read failed', description: describeApiError(e), variant: 'error' });
     }
   };
 
@@ -210,7 +211,7 @@ export default function InboxPage() {
               .mutateAsync(id)
               .then(
                 () => toast({ title: 'Restored', variant: 'success' }),
-                (e: unknown) => toast({ title: 'Undo failed', description: String(e), variant: 'error' }),
+                (e: unknown) => toast({ title: 'Undo failed', description: describeApiError(e), variant: 'error' }),
               )
           }
         >
@@ -219,7 +220,7 @@ export default function InboxPage() {
       ) as unknown as string; // DS description accepts ReactNode at runtime
       toast({ title: 'Deleted', description: undoNode, variant: 'success', duration: 5000 });
     } catch (e) {
-      toast({ title: 'Delete failed', description: String(e), variant: 'error' });
+      toast({ title: 'Delete failed', description: describeApiError(e), variant: 'error' });
     }
   };
 
@@ -230,7 +231,7 @@ export default function InboxPage() {
       setSelected(new Set());
       toast({ title: `Acknowledged ${ids.length} alert${ids.length === 1 ? '' : 's'}`, variant: 'success' });
     } catch (e) {
-      toast({ title: 'Bulk ack failed', description: String(e), variant: 'error' });
+      toast({ title: 'Bulk ack failed', description: describeApiError(e), variant: 'error' });
     }
   };
 
@@ -241,7 +242,7 @@ export default function InboxPage() {
       setSelected(new Set());
       toast({ title: `Marked ${ids.length} as read`, variant: 'success' });
     } catch (e) {
-      toast({ title: 'Bulk read failed', description: String(e), variant: 'error' });
+      toast({ title: 'Bulk read failed', description: describeApiError(e), variant: 'error' });
     }
   };
 
diff --git a/ui/src/pages/Alerts/RuleEditor/NotifyStep.tsx b/ui/src/pages/Alerts/RuleEditor/NotifyStep.tsx
index 7aa81068..a137edfe 100644
--- a/ui/src/pages/Alerts/RuleEditor/NotifyStep.tsx
+++ b/ui/src/pages/Alerts/RuleEditor/NotifyStep.tsx
@@ -5,6 +5,7 @@ import { useUsers, useGroups, useRoles } from '../../../api/queries/admin/rbac';
 import { useOutboundConnections } from '../../../api/queries/admin/outboundConnections';
 import { useSelectedEnv } from '../../../api/queries/alertMeta';
 import { useRenderPreview } from '../../../api/queries/alertRules';
+import { describeApiError } from '../../../api/errors';
 import type { FormState } from './form-state';
 
 type TargetKind = FormState['targets'][number]['kind'];
@@ -41,7 +42,7 @@ export function NotifyStep({
       const res = await preview.mutateAsync({ id: ruleId, req: {} });
       setLastPreview(`TITLE:\n${res.title ?? ''}\n\nMESSAGE:\n${res.message ?? ''}`);
     } catch (e) {
-      toast({ title: 'Preview failed', description: String(e), variant: 'error' });
+      toast({ title: 'Preview failed', description: describeApiError(e), variant: 'error' });
     }
   };
 
diff --git a/ui/src/pages/Alerts/RuleEditor/RuleEditorWizard.tsx b/ui/src/pages/Alerts/RuleEditor/RuleEditorWizard.tsx
index 71f12450..62c502cf 100644
--- a/ui/src/pages/Alerts/RuleEditor/RuleEditorWizard.tsx
+++ b/ui/src/pages/Alerts/RuleEditor/RuleEditorWizard.tsx
@@ -24,6 +24,7 @@ import { prefillFromPromotion, type PrefillWarning } from './promotion-prefill';
 import { useCatalog } from '../../../api/queries/catalog';
 import { useOutboundConnections } from '../../../api/queries/admin/outboundConnections';
 import { useSelectedEnv } from '../../../api/queries/alertMeta';
+import { describeApiError } from '../../../api/errors';
 import sectionStyles from '../../../styles/section-card.module.css';
 import css from './wizard.module.css';
 
@@ -128,7 +129,7 @@ export default function RuleEditorWizard() {
       }
       navigate('/alerts/rules');
     } catch (e) {
-      toast({ title: 'Save failed', description: String(e), variant: 'error' });
+      toast({ title: 'Save failed', description: describeApiError(e), variant: 'error' });
     }
   };
 
diff --git a/ui/src/pages/Alerts/RuleEditor/TriggerStep.tsx b/ui/src/pages/Alerts/RuleEditor/TriggerStep.tsx
index 3ed2cf68..61572e9a 100644
--- a/ui/src/pages/Alerts/RuleEditor/TriggerStep.tsx
+++ b/ui/src/pages/Alerts/RuleEditor/TriggerStep.tsx
@@ -1,6 +1,7 @@
 import { useState } from 'react';
 import { Button, FormField, Input, useToast } from '@cameleer/design-system';
 import { useTestEvaluate } from '../../../api/queries/alertRules';
+import { describeApiError } from '../../../api/errors';
 import type { FormState } from './form-state';
 
 export function TriggerStep({
@@ -25,7 +26,7 @@ export function TriggerStep({
       const result = await testEvaluate.mutateAsync({ id: ruleId, req: {} });
       setLastResult(JSON.stringify(result, null, 2));
     } catch (e) {
-      toast({ title: 'Test-evaluate failed', description: String(e), variant: 'error' });
+      toast({ title: 'Test-evaluate failed', description: describeApiError(e), variant: 'error' });
     }
   };
 
diff --git a/ui/src/pages/Alerts/RulesListPage.tsx b/ui/src/pages/Alerts/RulesListPage.tsx
index 890a087a..7bfe127a 100644
--- a/ui/src/pages/Alerts/RulesListPage.tsx
+++ b/ui/src/pages/Alerts/RulesListPage.tsx
@@ -16,6 +16,7 @@ import {
 } from '../../api/queries/alertRules';
 import { useEnvironments } from '../../api/queries/admin/environments';
 import { useSelectedEnv } from '../../api/queries/alertMeta';
+import { describeApiError } from '../../api/errors';
 import tableStyles from '../../styles/table-section.module.css';
 import css from './alerts-page.module.css';
 
@@ -41,7 +42,7 @@ export default function RulesListPage() {
       await setEnabled.mutateAsync({ id: r.id!, enabled: !r.enabled });
       toast({ title: r.enabled ? 'Disabled' : 'Enabled', description: r.name, variant: 'success' });
     } catch (e) {
-      toast({ title: 'Toggle failed', description: String(e), variant: 'error' });
+      toast({ title: 'Toggle failed', description: describeApiError(e), variant: 'error' });
     }
   };
 
@@ -51,7 +52,7 @@ export default function RulesListPage() {
       await deleteRule.mutateAsync(pendingDelete.id!);
       toast({ title: 'Deleted', description: pendingDelete.name, variant: 'success' });
     } catch (e) {
-      toast({ title: 'Delete failed', description: String(e), variant: 'error' });
+      toast({ title: 'Delete failed', description: describeApiError(e), variant: 'error' });
     } finally {
       setPendingDelete(null);
     }
diff --git a/ui/src/pages/Alerts/SilenceRuleMenu.tsx b/ui/src/pages/Alerts/SilenceRuleMenu.tsx
index d4530835..53dfaa96 100644
--- a/ui/src/pages/Alerts/SilenceRuleMenu.tsx
+++ b/ui/src/pages/Alerts/SilenceRuleMenu.tsx
@@ -3,6 +3,7 @@ import { useNavigate } from 'react-router';
 import { Button, Dropdown, useToast } from '@cameleer/design-system';
 import type { DropdownItem } from '@cameleer/design-system';
 import { useCreateSilence } from '../../api/queries/alertSilences';
+import { describeApiError } from '../../api/errors';
 
 interface Props {
   ruleId: string;
@@ -37,7 +38,7 @@ export function SilenceRuleMenu({ ruleId, ruleTitle, onDone, variant = 'row' }:
       toast({ title: `Silenced for ${hours}h`, variant: 'success' });
       onDone?.();
     } catch (e) {
-      toast({ title: 'Silence failed', description: String(e), variant: 'error' });
+      toast({ title: 'Silence failed', description: describeApiError(e), variant: 'error' });
     }
   };
 
diff --git a/ui/src/pages/Alerts/SilencesPage.tsx b/ui/src/pages/Alerts/SilencesPage.tsx
index 41b649c9..75a3819a 100644
--- a/ui/src/pages/Alerts/SilencesPage.tsx
+++ b/ui/src/pages/Alerts/SilencesPage.tsx
@@ -13,6 +13,7 @@ import {
   useDeleteSilence,
   type AlertSilenceResponse,
 } from '../../api/queries/alertSilences';
+import { describeApiError } from '../../api/errors';
 import sectionStyles from '../../styles/section-card.module.css';
 import tableStyles from '../../styles/table-section.module.css';
 import css from './alerts-page.module.css';
@@ -63,7 +64,7 @@ export default function SilencesPage() {
       setHours(1);
       toast({ title: 'Silence created', variant: 'success' });
     } catch (e) {
-      toast({ title: 'Create failed', description: String(e), variant: 'error' });
+      toast({ title: 'Create failed', description: describeApiError(e), variant: 'error' });
     }
   };
 
@@ -73,7 +74,7 @@ export default function SilencesPage() {
       await remove.mutateAsync(pendingEnd.id!);
       toast({ title: 'Silence removed', variant: 'success' });
     } catch (e) {
-      toast({ title: 'Remove failed', description: String(e), variant: 'error' });
+      toast({ title: 'Remove failed', description: describeApiError(e), variant: 'error' });
     } finally {
       setPendingEnd(null);
     }