Cirkit Designer runs unmodified ESP32 and ESP32-S3 firmware in the browser with its own Rust emulator

Cirkit Designer, a browser-based tool for drawing circuits, writing firmware and simulating them, now emulates the original ESP32 and the ESP32-S3 at the instruction level. Its founder, Austin Small, described how it works in a post on Espressif's developer portal on 26 October 2026. The point for ESP32 tinkerers: you can test a sketch with a display, sensors and buttons before you have the parts on your desk, and the simulation is free.

How it works

The emulator was written from scratch in Rust and compiled to WebAssembly, so it runs on your own computer inside the browser tab. It executes the same Xtensa instructions as the real chip, including Espressif's ROM code and the second-stage bootloader, and then FreeRTOS and your application. Peripherals are modelled at the register level instead of faking API calls, so your drivers talk to emulated GPIO, UART, I2C, SPI, ADC, timers, LEDC PWM and RMT (used for NeoPixels).

Because everything runs locally, a button press or a display update doesn't wait for a server round trip. Code is compiled with the Arduino CLI on Cirkit's backend, or you can upload a compatible precompiled binary. AI requests and access to external internet services also go through the backend. Network projects can use HTTP, MQTT, WebSocket and UDP through Cirkit's gateway. Building ESP-IDF projects directly is listed as "coming soon".

The team says the emulator took a small group roughly eight months, with ROM compatibility, Xtensa register windows and performance as the hardest parts. They looked at running QEMU on servers instead but rejected it because of network latency and the cost of emulating every session server-side.

The AI agent

Cirkit also has an AI agent that wires the circuit, writes and edits firmware and helps with debugging. In the example, a one-line prompt produced an ESP32-S3 tracker for the International Space Station that fetches live positions over Wi-Fi and draws them on a simulated ILI9341 display. If a part isn't supported yet, the agent can write a simulation model for it. The post shows a 16x2 I2C LCD built that way.

What's next

Support for the ESP32-C3 and ESP32-C6 is "coming soon", which matters for smart home work, since the C6 is the chip with Thread and Zigbee. The team also wants the agent to run the firmware itself, watch the result and fix problems in a loop.

Cirkit isn't the only browser ESP32 simulator: the same portal presented the open-source Velxio in July. Cirkit's take is to keep the whole emulator on the user's machine and fold circuit design, firmware and simulation into one workspace.