diff --git a/src/renderer/components/ActivityPanel.tsx b/src/renderer/components/ActivityPanel.tsx
index 771a659..dad54f5 100644
--- a/src/renderer/components/ActivityPanel.tsx
+++ b/src/renderer/components/ActivityPanel.tsx
@@ -1,5 +1,5 @@
 import { useMemo, type ReactNode } from 'react';
-import { Activity, ArrowRight, BarChart3, CheckCircle2, Clock, Cog, ShieldAlert, Sparkles, Users, Zap } from 'lucide-react';
+import { Activity, AlertTriangle, ArrowRight, BarChart3, CheckCircle2, Clock, Cog, ShieldAlert, Sparkles, Users, Zap } from 'lucide-react';
 
 import {
   buildAgentActivityRows,
@@ -190,6 +190,8 @@ function TurnEventIcon({ kind, phase, success }: { kind: SessionEventKind; phase
       return <Sparkles className={`${base} text-[var(--color-accent-purple)]`} />;
     case 'session-compaction':
       return <CheckCircle2 className={`${base} ${phase === 'start' ? 'animate-pulse text-[var(--color-status-warning)]' : 'text-[var(--color-status-success)]'}`} />;
+    case 'workflow-diagnostic':
+      return <AlertTriangle className={`${base} ${success === false ? 'text-[var(--color-status-error)]' : 'text-[var(--color-status-warning)]'}`} />;
     default:
       return <Zap className={`${base} text-[var(--color-text-muted)]`} />;
   }
diff --git a/src/renderer/lib/sessionActivity.ts b/src/renderer/lib/sessionActivity.ts
index 6a3f489..8111a74 100644
--- a/src/renderer/lib/sessionActivity.ts
+++ b/src/renderer/lib/sessionActivity.ts
@@ -1,6 +1,6 @@
 import type { PatternDefinition } from '@shared/domain/pattern';
 import type { SessionEventRecord } from '@shared/domain/event';
-import type { QuotaSnapshot } from '@shared/contracts/sidecar';
+import type { QuotaSnapshot, WorkflowDiagnosticKind, WorkflowDiagnosticSeverity } from '@shared/contracts/sidecar';
 
 export interface AgentActivityState {
   agentId: string;
@@ -260,6 +260,22 @@ function formatHookType(hookType: string | undefined): string {
   return hookTypeLabels[hookType] ?? hookType;
 }
 
+const diagnosticLabels: Record<WorkflowDiagnosticKind, string> = {
+  'workflow-warning': 'Workflow warning',
+  'workflow-error': 'Workflow error',
+  'executor-failed': 'Executor failed',
+  'subworkflow-warning': 'Subworkflow warning',
+  'subworkflow-error': 'Subworkflow error',
+};
+
+function formatDiagnosticLabel(
+  kind: WorkflowDiagnosticKind | undefined,
+  severity: WorkflowDiagnosticSeverity | undefined,
+): string {
+  if (kind) return diagnosticLabels[kind] ?? kind;
+  return severity === 'error' ? 'Workflow error' : 'Workflow warning';
+}
+
 function formatTurnEventEntry(event: SessionEventRecord): TurnEventEntry | undefined {
   switch (event.kind) {
     case 'subagent':
@@ -300,6 +316,21 @@ function formatTurnEventEntry(event: SessionEventRecord): TurnEventEntry | undef
         phase: event.compactionPhase,
         success: event.compactionSuccess,
       };
+    case 'workflow-diagnostic': {
+      const label = formatDiagnosticLabel(event.diagnosticKind, event.diagnosticSeverity);
+      const detailParts: string[] = [];
+      if (event.executorId) detailParts.push(event.executorId);
+      if (event.subworkflowId) detailParts.push(event.subworkflowId);
+      if (event.exceptionType) detailParts.push(event.exceptionType);
+      if (event.diagnosticMessage) detailParts.push(event.diagnosticMessage);
+      return {
+        kind: event.kind,
+        occurredAt: event.occurredAt,
+        label,
+        detail: detailParts.length > 0 ? detailParts.join(' · ') : undefined,
+        success: event.diagnosticSeverity === 'error' ? false : undefined,
+      };
+    }
     default:
       return undefined;
   }
diff --git a/tests/renderer/sessionActivity.test.ts b/tests/renderer/sessionActivity.test.ts
index 00aaef5..3be8a6b 100644
--- a/tests/renderer/sessionActivity.test.ts
+++ b/tests/renderer/sessionActivity.test.ts
@@ -3,6 +3,7 @@ import { describe, expect, test } from 'bun:test';
 import {
   applySessionEventActivity,
   applyAssistantUsageEvent,
+  applyTurnEventLog,
   buildAgentActivityRows,
   formatAgentActivityLabel,
   formatDuration,
@@ -527,3 +528,79 @@ describe('usage formatting helpers', () => {
     expect(formatDuration(150_000)).toBe('2.5m');
   });
 });
+
+describe('workflow diagnostic turn events', () => {
+  function makeDiagnosticEvent(overrides: Partial<SessionEventRecord> = {}): SessionEventRecord {
+    return {
+      sessionId: 'session-1',
+      kind: 'workflow-diagnostic',
+      occurredAt: '2026-03-23T00:00:00.000Z',
+      diagnosticSeverity: 'error',
+      diagnosticKind: 'executor-failed',
+      diagnosticMessage: 'Tool crashed.',
+      ...overrides,
+    };
+  }
+
+  test('formats executor-failed with full metadata', () => {
+    const result = applyTurnEventLog({}, makeDiagnosticEvent({
+      executorId: 'Primary',
+      exceptionType: 'InvalidOperationException',
+    }));
+    const entries = result['session-1']!;
+    expect(entries).toHaveLength(1);
+    expect(entries[0].label).toBe('Executor failed');
+    expect(entries[0].detail).toBe('Primary · InvalidOperationException · Tool crashed.');
+    expect(entries[0].success).toBe(false);
+  });
+
+  test('formats workflow-warning with message only', () => {
+    const result = applyTurnEventLog({}, makeDiagnosticEvent({
+      diagnosticSeverity: 'warning',
+      diagnosticKind: 'workflow-warning',
+      diagnosticMessage: 'Token budget is nearly exhausted.',
+      executorId: undefined,
+      exceptionType: undefined,
+    }));
+    const entries = result['session-1']!;
+    expect(entries[0].label).toBe('Workflow warning');
+    expect(entries[0].detail).toBe('Token budget is nearly exhausted.');
+    expect(entries[0].success).toBeUndefined();
+  });
+
+  test('formats subworkflow-error with subworkflow ID', () => {
+    const result = applyTurnEventLog({}, makeDiagnosticEvent({
+      diagnosticKind: 'subworkflow-error',
+      subworkflowId: 'subworkflow-review',
+      exceptionType: 'InvalidOperationException',
+      diagnosticMessage: 'Reviewer agent failed.',
+    }));
+    const entries = result['session-1']!;
+    expect(entries[0].label).toBe('Subworkflow error');
+    expect(entries[0].detail).toBe('subworkflow-review · InvalidOperationException · Reviewer agent failed.');
+  });
+
+  test('formats workflow-error without optional fields', () => {
+    const result = applyTurnEventLog({}, makeDiagnosticEvent({
+      diagnosticKind: 'workflow-error',
+      diagnosticMessage: 'Workflow terminated unexpectedly.',
+      executorId: undefined,
+      subworkflowId: undefined,
+      exceptionType: undefined,
+    }));
+    const entries = result['session-1']!;
+    expect(entries[0].label).toBe('Workflow error');
+    expect(entries[0].detail).toBe('Workflow terminated unexpectedly.');
+    expect(entries[0].success).toBe(false);
+  });
+
+  test('falls back to severity when diagnosticKind is missing', () => {
+    const result = applyTurnEventLog({}, makeDiagnosticEvent({
+      diagnosticKind: undefined,
+      diagnosticSeverity: 'warning',
+      diagnosticMessage: 'Something odd happened.',
+    }));
+    const entries = result['session-1']!;
+    expect(entries[0].label).toBe('Workflow warning');
+  });
+});