How to fix messaging/invalid-recipient: Invalid message target in Firebase

// ✅ Correct - single token
const message = {
  notification: {
    title: 'Update',
    body: 'New content available'
  },
  token: registrationToken  // Must be defined and valid
};

// ✅ Correct - topic
const topicMessage = {
  notification: { title: 'News', body: 'Breaking' },
  topic: 'news-updates'  // No /topics/ prefix needed here
};

// ❌ Wrong - no target
const badMessage = {
  notification: { title: 'Test', body: 'Message' }
  // Missing token, topic, or condition
};

// ❌ Wrong - multiple targets
const invalidMessage = {
  notification: { title: 'Test', body: 'Message' },
  token: someToken,
  topic: 'news'  // Cannot specify both
};

// Add validation before sending
if (!registrationToken || registrationToken.trim() === '') {
  console.error('Registration token is missing or empty');
  throw new Error('Cannot send message: no valid token');
}

const message = {
  notification: {
    title: 'Your notification',
    body: 'Notification content'
  },
  token: registrationToken
};

try {
  const response = await admin.messaging().send(message);
  console.log('Successfully sent:', response);
} catch (error) {
  console.error('Error sending message:', error);
}

-- Check for null or empty tokens
SELECT user_id, fcm_token, updated_at 
FROM user_devices 
WHERE fcm_token IS NULL OR fcm_token = '';

// ✅ Correct topic format
const message = {
  notification: { title: 'Sports', body: 'Game update' },
  topic: 'sports-news'  // Use kebab-case, no prefix
};

// ❌ Wrong - includes prefix
const badTopicMessage = {
  notification: { title: 'Sports', body: 'Update' },
  topic: '/topics/sports-news'  // Don't include /topics/
};

// Topic name rules:
// - Use only alphanumeric, dash, and underscore
// - No spaces or special characters
// - Max length: 900 characters
const validTopicPattern = /^[a-zA-Z0-9-_.~%]+$/;
if (!validTopicPattern.test(topicName)) {
  throw new Error('Invalid topic name format');
}

// ✅ Correct condition format
const message = {
  notification: { title: 'Alert', body: 'Important update' },
  condition: "'sports' in topics && 'news' in topics"
};

// ✅ Complex condition with OR
const complexCondition = "'ios' in topics || ('android' in topics && 'premium' in topics)";

// ❌ Wrong - malformed syntax
const badCondition = "sports AND news";  // Must use 'topic' in topics syntax

// Condition rules:
// - Use 'topicName' in topics for each topic
// - Combine with && (AND) or || (OR)
// - Use parentheses for precedence
// - Max 5 topics per condition

// Get a test token from your app (Firebase Admin SDK logs it at startup)
// or from FCM documentation sample
const TEST_TOKEN = 'dXXXXXXXXXX:APA91bXXXXXXXXXXXXXXXXXXXXXXXX';

const testMessage = {
  notification: {
    title: 'Test notification',
    body: 'Testing message structure'
  },
  token: TEST_TOKEN
};

try {
  const response = await admin.messaging().send(testMessage);
  console.log('Message structure is valid');
  // If this works, your token retrieval is the problem
} catch (error) {
  console.error('Message structure issue:', error.code);
  // If this fails, check your message object construction
}