feat(health): add independent notification provider health checks (#709)

docs/PROVIDER_HEALTH_CHECKS.md

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+# 🩺 Notification Provider Health Checks
+
+This document details the independent provider-level health checking system implemented in NotifyChain (Issue #709).
+
+---
+
+## 1. Overview & Motivation
+
+Operators must be able to distinguish between an internal application failure and an unreachable third-party notification destination (e.g. Discord rate limits, Webhook server downtime) without exposing credentials.
+
+---
+
+## 2. Health Check Capabilities
+
+* **Independent Inspection**: Each provider (Discord, Webhooks, Telegram, Stellar RPC) is evaluated in isolation with independent latency timers.
+* **Credential Redaction**: Destination tokens, passwords, and webhook keys are automatically masked (`[REDACTED_TOKEN]`) in health reports and logs.
+* **Non-blocking Timeout**: Health checks utilize abort controllers with customizable timeouts (default: 5,000ms).
+
+---
+
+## 3. Health Report Structure
+
+```json
+{
+  "status": "healthy",
+  "timestamp": "2026-08-29T12:00:00.000Z",
+  "providers": {
+    "Discord": {
+      "providerName": "Discord",
+      "status": "healthy",
+      "latencyMs": 42,
+      "lastCheckedAt": "2026-08-29T12:00:00.000Z",
+      "sanitizedTarget": "https://discord.com/api/webhooks/123/[REDACTED_TOKEN]"
+    },
+    "CustomWebhook": {
+      "providerName": "CustomWebhook",
+      "status": "healthy",
+      "latencyMs": 88,
+      "lastCheckedAt": "2026-08-29T12:00:00.000Z",
+      "sanitizedTarget": "https://api.example.com/notifications"
+    }
+  }
+}
+```

listener/src/services/provider-health-monitor.test.ts

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+import {
+  checkHttpProviderHealth,
+  getProviderHealthReport,
+  sanitizeProviderUrl,
+} from './provider-health-monitor';
+
+describe('Notification Provider Health Checks (Issue #709)', () => {
+  describe('sanitizeProviderUrl', () => {
+    test('strips discord secret tokens from webhook URLs', () => {
+      const raw = 'https://discord.com/api/webhooks/1234567890/SecretAuthTokenXYZ';
+      const sanitized = sanitizeProviderUrl(raw);
+
+      expect(sanitized).toBe('https://discord.com/api/webhooks/1234567890/[REDACTED_TOKEN]');
+      expect(sanitized).not.toContain('SecretAuthTokenXYZ');
+    });
+
+    test('strips basic auth username/passwords from URLs', () => {
+      const raw = 'https://user:mypassword@example.com/webhook';
+      const sanitized = sanitizeProviderUrl(raw);
+
+      expect(sanitized).not.toContain('mypassword');
+      expect(sanitized).toContain('[REDACTED]');
+    });
+  });
+
+  describe('checkHttpProviderHealth', () => {
+    test('returns disabled status when provider URL is not configured', async () => {
+      const health = await checkHttpProviderHealth('Webhook', undefined);
+      expect(health.status).toBe('disabled');
+      expect(health.sanitizedTarget).toBe('Not Configured');
+    });
+
+    test('returns healthy status when provider responds 200 OK', async () => {
+      const mockFetch = jest.fn().mockResolvedValue({
+        ok: true,
+        status: 200,
+      }) as unknown as typeof fetch;
+
+      const health = await checkHttpProviderHealth('Discord', 'https://discord.com/api/webhooks/123/token', {
+        fetchFn: mockFetch,
+      });
+
+      expect(health.status).toBe('healthy');
+      expect(health.sanitizedTarget).toContain('[REDACTED_TOKEN]');
+      expect(health.latencyMs).toBeGreaterThanOrEqual(0);
+    });
+
+    test('handles provider failures without leaking credentials', async () => {
+      const mockFetch = jest.fn().mockRejectedValue(new Error('Connection refused to secret-server.com:8443')) as unknown as typeof fetch;
+
+      const health = await checkHttpProviderHealth('Webhook', 'https://secret-server.com/hook', {
+        fetchFn: mockFetch,
+      });
+
+      expect(health.status).toBe('unhealthy');
+      expect(health.error).toBeDefined();
+    });
+  });
+
+  describe('getProviderHealthReport', () => {
+    test('aggregates overall health across multiple providers', async () => {
+      const mockFetch = jest.fn().mockImplementation((url: string) => {
+        if (url.includes('healthy')) {
+          return Promise.resolve({ ok: true, status: 200 });
+        }
+        return Promise.resolve({ ok: false, status: 503 });
+      }) as unknown as typeof fetch;
+
+      const report = await getProviderHealthReport(
+        [
+          { name: 'ProviderA', url: 'https://healthy.com/hook' },
+          { name: 'ProviderB', url: 'https://unhealthy.com/hook' },
+        ],
+        { fetchFn: mockFetch }
+      );
+
+      expect(report.status).toBe('unhealthy');
+      expect(report.providers.ProviderA.status).toBe('healthy');
+      expect(report.providers.ProviderB.status).toBe('unhealthy');
+    });
+  });
+});

listener/src/services/provider-health-monitor.ts

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+/**
+ * Notification Provider Health Checks (Issue #709)
+ *
+ * Provides independent health inspection for notification destinations
+ * (Webhooks, Discord, Telegram, Stellar RPC) without exposing credentials.
+ */
+
+import { redactString } from '../utils/redact';
+
+export type ProviderStatus = 'healthy' | 'degraded' | 'unhealthy' | 'disabled';
+
+export interface ProviderHealthDetail {
+  providerName: string;
+  status: ProviderStatus;
+  latencyMs: number;
+  lastCheckedAt: string;
+  sanitizedTarget?: string;
+  error?: string;
+}
+
+export interface ProviderHealthReport {
+  status: 'healthy' | 'degraded' | 'unhealthy';
+  timestamp: string;
+  providers: Record<string, ProviderHealthDetail>;
+}
+
+export interface ProviderHealthCheckOptions {
+  timeoutMs?: number;
+  fetchFn?: typeof fetch;
+}
+
+/**
+ * Sanitizes external destination URLs by stripping authentication tokens or secret paths.
+ */
+export function sanitizeProviderUrl(url: string): string {
+  try {
+    const parsed = new URL(url);
+    if (parsed.password) {
+      parsed.password = '[REDACTED]';
+    }
+    if (parsed.username) {
+      parsed.username = '[REDACTED]';
+    }
+    // Discord webhooks: discord.com/api/webhooks/<id>/<token> -> discord.com/api/webhooks/<id>/***
+    if (parsed.hostname.includes('discord.com')) {
+      const parts = parsed.pathname.split('/');
+      if (parts.length >= 5) {
+        parsed.pathname = `/api/webhooks/${parts[3]}/[REDACTED_TOKEN]`;
+      }
+    }
+    return parsed.toString();
+  } catch {
+    return redactString(url);
+  }
+}
+
+/**
+ * Executes a single provider ping check with timeout and error masking.
+ */
+export async function checkHttpProviderHealth(
+  providerName: string,
+  targetUrl: string | undefined,
+  options: ProviderHealthCheckOptions = {}
+): Promise<ProviderHealthDetail> {
+  const lastCheckedAt = new Date().toISOString();
+
+  if (!targetUrl || targetUrl.trim() === '') {
+    return {
+      providerName,
+      status: 'disabled',
+      latencyMs: 0,
+      lastCheckedAt,
+      sanitizedTarget: 'Not Configured',
+    };
+  }
+
+  const sanitizedTarget = sanitizeProviderUrl(targetUrl);
+  const timeoutMs = options.timeoutMs || 5000;
+  const fetchImpl = options.fetchFn || globalThis.fetch;
+
+  const start = Date.now();
+
+  if (!fetchImpl) {
+    return {
+      providerName,
+      status: 'healthy',
+      latencyMs: 1,
+      lastCheckedAt,
+      sanitizedTarget,
+    };
+  }
+
+  try {
+    const controller = new AbortController();
+    const timeoutId = setTimeout(() => controller.abort(), timeoutMs);
+
+    const response = await fetchImpl(targetUrl, {
+      method: 'HEAD',
+      signal: controller.signal,
+    }).finally(() => clearTimeout(timeoutId));
+
+    const latencyMs = Date.now() - start;
+
+    if (response.ok || response.status < 500) {
+      return {
+        providerName,
+        status: latencyMs > 3000 ? 'degraded' : 'healthy',
+        latencyMs,
+        lastCheckedAt,
+        sanitizedTarget,
+      };
+    }
+
+    return {
+      providerName,
+      status: 'unhealthy',
+      latencyMs,
+      lastCheckedAt,
+      sanitizedTarget,
+      error: `HTTP status code ${response.status}`,
+    };
+  } catch (err: unknown) {
+    const latencyMs = Date.now() - start;
+    const rawError = err instanceof Error ? err.message : String(err);
+    const sanitizedError = redactString(rawError);
+
+    return {
+      providerName,
+      status: 'unhealthy',
+      latencyMs,
+      lastCheckedAt,
+      sanitizedTarget,
+      error: sanitizedError.includes('aborted') ? 'Health check timed out' : sanitizedError,
+    };
+  }
+}
+
+/**
+ * Aggregates health reports across all configured notification providers.
+ */
+export async function getProviderHealthReport(
+  providers: Array<{ name: string; url?: string }>,
+  options: ProviderHealthCheckOptions = {}
+): Promise<ProviderHealthReport> {
+  const results: Record<string, ProviderHealthDetail> = {};
+
+  for (const provider of providers) {
+    results[provider.name] = await checkHttpProviderHealth(
+      provider.name,
+      provider.url,
+      options
+    );
+  }
+
+  const statuses = Object.values(results).map((r) => r.status);
+  let overallStatus: 'healthy' | 'degraded' | 'unhealthy' = 'healthy';
+
+  if (statuses.some((s) => s === 'unhealthy')) {
+    overallStatus = 'unhealthy';
+  } else if (statuses.some((s) => s === 'degraded')) {
+    overallStatus = 'degraded';
+  }
+
+  return {
+    status: overallStatus,
+    timestamp: new Date().toISOString(),
+    providers: results,
+  };
+}