Linear Webhooks Skill

When to Use This Skill

• Setting up Linear webhook handlers
• Debugging Linear signature verification failures
• Validating the Linear-Signature HMAC-SHA256 header
• Handling Linear Issue, Comment, Project, Cycle, IssueLabel, or IssueSLA events
• Reacting to create, update, and remove actions on Linear entities
• Rejecting stale webhook deliveries via the webhookTimestamp field

Essential Code (USE THIS)

Linear Signature Verification (JavaScript)

Linear signs each webhook with HMAC-SHA256 over the raw request body, hex-encoded, sent in the Linear-Signature header. Linear has no first-party Node SDK helper for verifying webhooks, so manual verification is the recommended approach.

const crypto = require('crypto');

function verifyLinearWebhook(rawBody, signatureHeader, secret) {
  if (!signatureHeader || !secret) return false;

  // HMAC-SHA256(rawBody, secret) → hex
  const expected = crypto
    .createHmac('sha256', secret)
    .update(rawBody)
    .digest('hex');

  try {
    return crypto.timingSafeEqual(
      Buffer.from(signatureHeader, 'hex'),
      Buffer.from(expected, 'hex')
    );
  } catch {
    return false;
  }
}

// Reject deliveries older than 1 minute (replay protection)
function isFreshTimestamp(webhookTimestamp) {
  if (typeof webhookTimestamp !== 'number') return false;
  const skewMs = Math.abs(Date.now() - webhookTimestamp);
  return skewMs <= 60 * 1000;
}

Express Webhook Handler

const express = require('express');
const app = express();

// CRITICAL: Use express.raw() - Linear signs the raw body
app.post('/webhooks/linear',
  express.raw({ type: 'application/json' }),
  (req, res) => {
    const signature = req.headers['linear-signature'];
    const event = req.headers['linear-event'];      // e.g. "Issue", "Comment"
    const delivery = req.headers['linear-delivery']; // UUID for idempotency

    if (!verifyLinearWebhook(req.body, signature, process.env.LINEAR_WEBHOOK_SECRET)) {
      return res.status(400).send('Invalid signature');
    }

    const payload = JSON.parse(req.body.toString());

    // Linear requires rejecting deliveries older than 1 minute
    if (!isFreshTimestamp(payload.webhookTimestamp)) {
      return res.status(400).send('Stale webhook');
    }

    console.log(`Linear ${event} ${payload.action} (delivery: ${delivery})`);

    switch (event) {
      case 'Issue':
        console.log(`Issue ${payload.action}:`, payload.data?.title);
        break;
      case 'Comment':
        console.log(`Comment ${payload.action} on issue ${payload.data?.issueId}`);
        break;
      case 'Project':
        console.log(`Project ${payload.action}:`, payload.data?.name);
        break;
      case 'IssueSLA':
        console.log(`SLA event on issue ${payload.issueData?.id}`);
        break;
      default:
        console.log(`Unhandled Linear event: ${event}`);
    }

    res.status(200).send('OK');
  }
);

Python Signature Verification (FastAPI)

import hmac
import hashlib
import time

def verify_linear_webhook(raw_body: bytes, signature_header: str, secret: str) -> bool:
    if not signature_header or not secret:
        return False
    expected = hmac.new(secret.encode("utf-8"), raw_body, hashlib.sha256).hexdigest()
    return hmac.compare_digest(signature_header, expected)

def is_fresh_timestamp(webhook_timestamp_ms: int) -> bool:
    if not isinstance(webhook_timestamp_ms, int):
        return False
    now_ms = int(time.time() * 1000)
    return abs(now_ms - webhook_timestamp_ms) <= 60_000

For complete working examples with tests, see:

• examples/express/ - Full Express implementation
• examples/nextjs/ - Next.js App Router implementation
• examples/fastapi/ - Python FastAPI implementation

Common Linear-Event Header Values

For the full event reference, see Linear's webhook documentation.

Common Action Values

Data change events (Issue, Comment, Project, …) send one of:

IssueSLA and OAuthAppRevoked use event-specific actions (e.g. set, highRisk, breached).

Important Headers

Environment Variables

LINEAR_WEBHOOK_SECRET=your_webhook_secret   # Shown once when the webhook is created in Linear

Local Development

# Start tunnel (no account needed)
npx hookdeck-cli listen 3000 linear --path /webhooks/linear

Use the printed Hookdeck URL as the webhook URL when creating the webhook in Linear's API settings.

Reference Materials

• references/overview.md - Linear webhook concepts and event types
• references/setup.md - Configuring a webhook in Linear
• references/verification.md - Signature verification details and gotchas