---
url: https://incajs.tom96da.com/guide/hmr.md
description: >-
  Hot module replacement under inca dev --experimental-hmr — how to enable it,
  what it does differently, and its known limitations.
---

# Hot Module Replacement 

> \[!WARNING]
> HMR is experimental and opt-in. Plain `inca dev` (no flag) always does
> a full reload.

## Enabling it

```sh
inca dev --experimental-hmr
```

or:

```sh
INCA_EXPERIMENTAL_HMR=1 inca dev
```

## What it does differently

A full reload re-evaluates the whole bundle and remounts the app from
scratch. With experimental HMR enabled, `inca dev` instead reloads only
the changed module:

* A template-only edit preserves the component's state.
* A script edit resets state — the component remounts.

## Known limitations

* The window can briefly open at the wrong size on first launch, then
  snap to the correct size moments later. The host doesn't wait on
  the module round-trip before opening the window.
* A style-only change to an already-mounted node doesn't repaint. A
  structural edit (adding, removing, or changing text) repaints
  correctly, and a full reload repaints a style-only change too.
* HMR can't recover from a source file that was already broken when the
  session started.