Language
TypeScriptgithub.com
Language of the whole monorepo.
UI library
Reactgithub.com
Builds the booking pages and the app interface.
Full-stack framework
Next.jsgithub.com
Framework for the main web app.
Database
PostgreSQLgithub.com
Main database.
Login
NextAuth.jsgithub.com
Handles login in the web app.
Payments
Stripegithub.com
Payments in the web app and the API.
Styling and UI kit
Tailwind CSSgithub.com
Styles the interface.
Build and runtime
Turborepogithub.com
Orchestrates builds, dev servers and tests across the monorepo.
ORM
Prismagithub.com
Defines the schema and handles database migrations and queries.
Data fetching
tRPCgithub.com
Type-safe calls between the web app and its back end.
Redisgithub.com
Backs the API v2 job queues through Bull and ioredis.
Backend framework
NestJSgithub.com
Framework for the public API v2.
Trigger.devtrigger.dev
Runs booking side effects, calendar syncs and fraud checks as durable background tasks.
Why: Developer experience: clear docs, an easy dashboard and a simple fit with their Prisma monorepo.
Testing
Playwrightgithub.com
End-to-end tests, with Vitest for unit tests.
Error monitoring
Sentrygithub.com
Error tracking in the web app and the API.
WhatsApp and SMS
Dailygithub.com
Video calls for meetings booked through Cal.com.
The public code lists its core as Next.js, tRPC, React, Tailwind CSS and Prisma on top of PostgreSQL, organized as a Yarn and Turborepo monorepo.
The main web app talks to the back end through tRPC, while the newer public API (v2) is a separate NestJS app that uses Redis and Bull for queues.
Sources:github.com github.com github.com
Since 2024, Cal.com runs millions of background tasks a month on Trigger.dev: everything that happens after a booking, calendar syncs, and phishing and fraud checks.
A Cal.com engineer said developer experience was the deciding factor: clean docs, an easy dashboard and a simple fit with their Prisma monorepo, with production workflows live within days.
Sources:trigger.dev
In April 2026, Cal.com moved its production codebase to a private repository. The company argued that AI tools now make it cheap to scan public code for vulnerabilities, putting customer data at risk.
The public repo became Cal.diy, an MIT-licensed community edition without the enterprise features. Cal.com says production has since diverged, with major rewrites of systems like authentication and data handling, so the stack below reflects the open code and may differ from what runs today.
Sources:cal.com github.com
Found on the homepage with the open Wappalyzer rules (HTML, scripts and headers). It shows what runs in the browser, not the whole backend.
Checked on 2026-10-29. Every tool links to the public source that shows it.
