Carl is available for hire
Hire CarlCarl Barrdahl
Verified Expert in Engineering
Full-stack Developer
Gothenburg, Sweden
Toptal member since August 15, 2022
Bio
Carl is a senior full-stack engineer who owns problems end-to-end — from the front end a user touches to the AI or Ethereum protocol underneath. His current work spans applied AI (SMS/voice systems on real-time telephony that answer calls, qualify leads, and route decisions) and protocol engineering. He works best with teams that have a hard problem and need someone who thinks in systems and doesn't need hand-holding.
Portfolio
Svarar Alltid
TypeScript, Next.js, React, Node.js, Large Language Models (LLMs), SIP/VoIP...
Zephyr Valley AB
React, Node.js, JavaScript, Ethereum, Chakra UI, Solidity, Next.js, REST APIs...
allo.capital
Solidity, Ethereum, TypeScript, React, Next.js, Smart Contracts
Experience
- React - 12 years
- Node.js - 10 years
- Next.js - 8 years
- SQL - 8 years
- Solidity - 4 years
- Shadcn - 2 years
- AI Agents - 2 years
- Agentic AI - 2 years
Preferred Environment
React, Node.js, Next.js, SQL, Ethereum, Prisma, TypeScript, Claude, Shadcn UI, Mastra
The most amazing...
...thing I've built is an AI receptionist that turns missed calls into qualified, prioritized leads — answered, triaged, and summarized to the owner automatically.
Work Experience
Founder & Full-stack Engineer
2026 - PRESENT
Svarar Alltid
- Founded and built an AI-receptionist platform that captures missed calls for Swedish tradespeople via SMS, qualifies leads, and delivers call summaries to the business owner.
- Architected a real-time voice pipeline using a custom SIP-to-WebSocket gateway connected to large language models (LLMs) for natural, low-latency phone conversations.
- Developed the full stack in TypeScript, Next.js, and React, covering telephony, lead-qualification logic, and the business-owner dashboard.
- Ran a 3-month pilot across 4 tradespeople businesses, handling 189 conversations from 134 unique callers and capturing 58 qualified leads auto-sorted by urgency—all real traffic that would otherwise have been missed calls.
Technologies: TypeScript, Next.js, React, Node.js, Large Language Models (LLMs), SIP/VoIP, WebSockets, Mastra, Full-stack Development
Founder & Senior Full-stack Engineer
2020 - PRESENT
Zephyr Valley AB
- Built a website for a decentralized autonomous organization (DAO) with Next.js and Chakra UI from Figma designs, and connected blockchain wallets via RainbowKit and Wagmi.
- Integrated Ethereum smart contracts into Next.js (React) front ends, handling wallet connection, transactions, and on-chain state.
- Built full-stack MVPs and prototypes with Next.js, tRPC, Prisma, and Supabase, exploring GraphQL, RPC, PostgreSQL, and Vercel.
- Developed fintech prototypes integrating Stripe and DocuSign for payments, invoicing, and e-signature document workflows.
- Built AI-powered voice products pairing LLMs with real-time SIP/WebSocket telephony for conversational phone agents.
Technologies: React, Node.js, JavaScript, Ethereum, Chakra UI, Solidity, Next.js, REST APIs, Vercel, Prisma, Minimum Viable Product (MVP), TypeScript, Full-stack, Stripe, Full-stack Development, APIs, DocuSign, tRPC, RainbowKit, Wagmi, Large Language Models (LLMs), SIP/VoIP, Supabase, PostgreSQL, GraphQL
Full-stack & Protocol Engineer
2024 - 2025
allo.capital
- Contributed to R&D and engineering for a permissionless capital-allocation protocol built on Ethereum.
- Developed Solidity smart contracts and the supporting TypeScript and React tooling around them.
- Supported business development and partner integrations alongside core protocol engineering.
Technologies: Solidity, Ethereum, TypeScript, React, Next.js, Smart Contracts
Full-stack Web3 Developer
2023 - 2024
Gitcoin
- Won an Optimism RFP to build the front end for Optimism's RetroFunding program, then turned those learnings into EasyRetroPGF—an open-source retroactive public-goods funding platform any EVM chain could run.
- Acted as engineering lead for EasyRetroPGF, enabling Celo and Filecoin to distribute approximately $3.5 million in retroactive funding across 290 grants, including 250,000 CELO to 89 projects and 1.05 million FIL to 200 projects.
- Built the platform end-to-end: Solidity smart contracts, on-chain indexing, and the full React/Next.js/TypeScript front end powering allocation, voting, and payouts.
Technologies: Solidity, Ethereum, TypeScript, React, Next.js
Full-stack Developer
2018 - 2019
Adfenix
- Developed a brand new front end for a company in the real estate business. Worked closely with one graphic designer, one UX designer, and a back-end team of five in an Agile environment using Next.js, Apollo, GraphQL, and Jira for project management.
- Created an embeddable widget for customer websites to collect insights from user behavior to target ads.
- Built a marketing campaign configurator with multiple steps and forms.
Technologies: Next.js, React, Bulma, Node.js, GraphQL, Facebook Pixel, REST APIs, JavaScript, Full-stack, Full-stack Development, Front-end, APIs
Full-stack Developer
2018 - 2019
Evolve Technologies
- Worked with two developers and one UI/UX designer to build an e-health platform for video calls with a mobile app and admin dashboard using Serverless on AWS, MongoDB, and BankID.
- Created a dashboard for an emergency response center showing the vehicles on a map in real-time using React and WebSockets.
- Built a progressive web app for IKEA Family to be used to sign in attendees for their internal events with a QR scanner to read tickets and offline support.
Technologies: React, MongoDB, GraphQL, JavaScript, REST APIs, Node.js, Minimum Viable Product (MVP), TypeScript, Full-stack, Full-stack Development, Front-end, APIs
Front-end Developer
2018 - 2018
Forsman & Bodenfors
- Built a progressive web app for IKEA Family to sign in attendees at internal events, styled to look and feel like a native app.
- Implemented offline support with a request queue that syncs automatically once a connection is re-established.
- Integrated device-camera barcode scanning to read IKEA Family cards, with responsive layouts and CI/CD deploying to AWS on each commit.
Technologies: React, Service Worker, JavaScript, CI/CD Pipelines, AWS IoT, Progressive Web Applications (PWAs)
Full-stack Developer
2017 - 2018
Annevo
- Created real-time components in React and Redux used to manage orders from the different stations in the restaurant. Integrated push notifications for customer communications in the app.
- Created an IoT platform with real-time data visualization components in React and Redux. Deployed to AWS for high availability and low latency real-time communications and integrated with Elasticsearch for advanced search queries.
- Developed an online real-time multiplayer mobile game in React Native and GameSparks as back end, push notifications to increase player engagement and payment using Swish (Swedish mobile bank app) integration.
- Built a React Native app for tracking the quality of motocross tracks using Google Cloud and Firebase on the back end.
Technologies: React, Next.js, Elasticsearch, Redux, AWS IoT, Push Notifications, Firebase, GameSparks, Bootstrap, REST APIs, JavaScript, Node.js, Minimum Viable Product (MVP), Full-stack, Full-stack Development, Front-end, APIs
Front-end Developer
2015 - 2016
Telenor Connexion
- Created high-quality data visualizing components in React and Redux.
- Deployed to AWS for high availability and low latency real-time communications.
- Integrated with Elasticsearch for advanced search queries.
Technologies: React, AWS IoT, Material UI, REST APIs, JavaScript, Node.js, Front-end, APIs
Experience
Svarar Alltid
https://svararalltid.seSvarar Alltid is an AI receptionist for Swedish tradespeople. When a call is missed, the service answers via SMS within seconds, around the clock. It qualifies the lead with follow-up questions, then delivers a categorized summary to the business owner, sorted by urgency. I built it for small firms of 1-15 people: plumbers, electricians, builders, and carpenters. It pairs LLMs with a custom SIP-to-WebSocket voice gateway for real-time telephony.
Fleet Tracking & Trip Logging Platform (in Development)
A fleet-tracking and digital driving-journal platform for small vehicle fleets. A TypeScript TCP gateway ingests GPS data from telematics hardware (Teltonika protocol), stores positions in PostgreSQL/PostGIS, and segments them into trips with start and end times per driver. Trips are matched against geofenced places and automatically classified as business or private based on configured work hours, then served through a typed API to a live map dashboard built with React and MapLibre.
Memosay
https://memosay.appMemosay is a meeting note-taking app for teams. It lets users capture notes right after client visits, with real-time transcription that displays text as they speak and allows them to edit before saving. I built it as a collaborative, organization-based workspace with Google and Microsoft sign-in, designed for the Swedish market. Since launching in June 2026, Memosay has seen early adoption across three company domains, including a first multi-seat team pilot with two users in one organization, both actively capturing notes.
Curator Studio (support.eth)
https://curate-fund-docs.vercel.appCurator Studio is a permissionless capital-allocation protocol. Curators create weighted funding strategies, donors fund those strategies, and recipients claim their allocations on-chain, routing money to public goods and grantees without a trusted intermediary. The system spans smart contracts, a TypeScript SDK, and indexing infrastructure. It builds on ERC-8185/8186, Ethereum standards authored for off-chain entity resolution and claimable escrow.
SipStream
https://github.com/carlbarrdahl/sipstreamOpen-source SIP/RTP-to-WebSocket gateway in TypeScript. Streams live phone-call audio bidirectionally to any back end, with a built-in zero-transcode bridge to OpenAI's Realtime API, tenant routing by dialed number, and around 100 concurrent calls per instance.
happyQ
A food truck point-of-sale system where customers can discover nearby food trucks and place orders for pickup.
It was built with Next.js and Chakra-UI. The back end is connected to tRPC and Prisma to connect to a Postgres database hosted on Supabase.
Stripe is used to connect merchants and Mapbox to display the trucks on a map with a detailed information overlay.
It was built with Next.js and Chakra-UI. The back end is connected to tRPC and Prisma to connect to a Postgres database hosted on Supabase.
Stripe is used to connect merchants and Mapbox to display the trucks on a map with a detailed information overlay.
Lizard Pay
https://github.com/carlbarrdahl/lizard-payPay Stripe invoices with USDC.
Built with React, Next.js, Stripe, RainbowKit, and Wagmi and deployed to Vercel. The customer connects their Stripe account, and their invoices are listed, and a payment link can be copied to share with clients to connect their wallet and transfer to the specified wallet address.
A dashboard was built to connect USDC transfers to matching invoices.
Built with React, Next.js, Stripe, RainbowKit, and Wagmi and deployed to Vercel. The customer connects their Stripe account, and their invoices are listed, and a payment link can be copied to share with clients to connect their wallet and transfer to the specified wallet address.
A dashboard was built to connect USDC transfers to matching invoices.
Skills
Libraries/APIs
React, Node.js, Stripe, Wagmi, React Query, Stripe API, REST APIs, Web3.js, Stripe Connect, Stripe Connect API, iCal
Tools
Prisma, Stripe Checkout, Shadcn, Figma, FullCalendar, Facebook Pixel, GameSparks, Claude, Shadcn UI
Languages
JavaScript, GraphQL, TypeScript, HTML, CSS, HTML5, SQL, Solidity
Frameworks
Next.js, Tailwind CSS, tRPC, OAuth 2, Bulma, Redux, Bootstrap, Material UI
Storage
JSON, MongoDB, SQLite, PostgreSQL, Elasticsearch
Platforms
Blockchain, Ethereum, Vercel, Firebase, Mapbox, AWS IoT, Twilio
Other
Chakra UI, User Interface (UI), Minimum Viable Product (MVP), Full-stack, Full-stack Development, Front-end, APIs, API Integration, Back-end, Webhooks, Back-end Development, Supabase, Ethers.js, Gatsby, AI Agents, Agentic AI, Geolocation, Architecture, Content Management Systems (CMS), iCalendar, Cryptocurrency Wallets, Lottie, Push Notifications, Mastra, DocuSign, RainbowKit, Large Language Models (LLMs), SIP/VoIP, WebSockets, Smart Contracts, Service Worker, CI/CD Pipelines, Progressive Web Applications (PWAs), Better Auth, AI Voice Agents, SIP
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