CapabilitiesComments

Comments (LUD-12)

Comments let senders attach a message to their payment. This is one of the most widely supported Lightning Address capabilities.

Why It Matters

Without comments, payments are just numbers. With comments, they become conversations:

  • Tips with gratitude — "Loved your podcast episode!"
  • Invoices with context — "Payment for logo design"
  • Donations with purpose — "For the open source fund"
  • Payments with instructions — "Table 5, extra sauce"

How It Works

Server Response

When your server responds to the initial LNURL request, include commentAllowed:

{
  "callback": "https://domain.com/lnurlp/user/callback",
  "minSendable": 1000,
  "maxSendable": 100000000000,
  "metadata": "[[\"text/plain\",\"Pay user\"]]",
  "tag": "payRequest",
  "commentAllowed": 255
}

The commentAllowed field specifies the maximum comment length in characters.

Callback Request

When a sender includes a comment, it's passed as a query parameter:

GET /callback?amount=10000&comment=Thanks%20for%20the%20great%20content!

Handling Comments

Your server receives the comment and can:

  • Store it with the payment record
  • Display it in your dashboard
  • Forward it to a notification system
  • Include it in the invoice description

Implementation Example

// Express.js example
app.get('/lnurlp/:username/callback', async (req, res) => {
  const { amount, comment } = req.query;
  const username = req.params.username;

  // Validate comment length
  const maxCommentLength = 255;
  if (comment && comment.length > maxCommentLength) {
    return res.status(400).json({
      status: 'ERROR',
      reason: `Comment too long (max ${maxCommentLength} chars)`
    });
  }

  // Generate invoice with comment in description
  const description = comment
    ? `Payment to ${username}: ${comment}`
    : `Payment to ${username}`;

  const invoice = await generateInvoice({
    amount: parseInt(amount),
    description
  });

  res.json({ pr: invoice });
});

Best Practices

  1. Set reasonable limits — 255 characters is common, but adjust based on your needs
  2. Sanitize input — Don't trust comment content blindly
  3. Show comments — Make it easy for recipients to see messages
  4. Handle encoding — Comments are URL-encoded

Wallet Support

Most Lightning wallets support sending comments. The sender typically sees a text field when the recipient supports comments.