Reflex Frontend Exception when re-compiling after code changes

[thorsmann]

Describe the bug
Follow up to this one: #5519

Reflex frontend errors are printed to command line when code is re-compiled after code changes. Killing and restaring process fixes the issue. Just happens during development when code changes are applied directly.

[Reflex Frontend Exception]
 Error:  No module update found for route routes/_index
    at http://localhost:3000/@id/__x00__virtual:react-router/hmr-runtime:476:15

To Reproduce

import reflex as rx


class State(rx.State):
    count: int = 0

    @rx.var
    def count_plus_one(self) -> int:
        """Return the count plus one."""
        return self.count + 1
    
    @rx.event
    def increment(self) -> int:
        self.count += 1    


def index():
    return rx.vstack(
        rx.text("Hey"),
        rx.text(f"Hello there! - {State.count}", font_size=".75em"),
        rx.button("Test", on_click=State.increment)
    )


def dynamic():
    return rx.vstack(
        "dynamic contet",
        # launch app - than activate/deactivate below code to trigger react error
        # rx.cond(
        #     State.count_plus_one,
        #     rx.text(":)")
        # ),
    )


app = rx.App()
app.add_page(index)
app.add_page(dynamic, route="/dynamic")

You may have to comment in/out the code block in the dynamic() function a few times to trigger the exception. I was not able to find a fully deterministic way to provoke the issue.

Specifics (please complete the following information):

• Python Version: 3.12
• Reflex Version: 0.8.1
• OS: OSX
• Browser (Optional): Chrome

Additional context
Not every change triggers the problem. Some do but it is hard to narrow down deterministically. Issue was introduced in 0.8.x release. Did not occur in 0.7.14

[linear]

ENG-6614 Reflex Frontend Exception when re-compiling after code changes

[adhami3310]

i'm not hitting this right now :/

[thorsmann]

Okay, another try. slightly modified code

import reflex as rx

class State(rx.State):
    count: int = 0

    @rx.var
    def count_plus_one(self) -> int:
        """Return the count plus one."""
        return self.count + 1

    @rx.event
    def increment(self) -> int:
        self.count += 1


def index():
    return rx.vstack(
        rx.text(f"Counter at {State.count}"),
        rx.button("Increment", on_click=State.increment),
        rx.button("Redirect", on_click=lambda: rx.redirect("/dynamic")),
    )

def render_more() -> rx.Component:
    return rx.hstack(rx.text("render_more()...", font_size="0.75em"))

def dynamic():
    return rx.vstack(
        rx.cond(State.count == 0, rx.text("Counter is at zero")),
        rx.text("hi"),
        rx.button("back to /", on_click=lambda: rx.redirect("/")),
        render_more()
    )


app = rx.App()
app.add_page(index, route="/")
app.add_page(dynamic, route="/dynamic")

my pyproject.toml

[project]
name = "mwe"
version = "0.1.0"
description = "Add your description here"
readme = "README.md"
requires-python = ">=3.12"
dependencies = [
    "reflex>=0.8.1",
]

1. Start the application, I do uv run reflex run
2. Click on the redirect button
3. Modify this line of code

def render_more() -> rx.Component:
    return rx.hstack(rx.text("render_more()...", font_size="0.75em"))

Modify the font_size parameter to 1.75em and save / trigger re-compile.

For me, this change triggers the issue most of the time. Latest after changing it back to 0.75em and saving to trigger a re-compile I get the exception on the console.

I am on OSX using zsh to start the application/reflex. Is this now reproducable for you?

[adhami3310]

i'm still not reproducing it, do you have node globally? and what version is it node --version?

[thorsmann]

My global node version was v23.11.0. I just updated to the latest 24.4.0. This seems to fix it. Thank you!

[kesevone]

+1

C:\Users\user>node --version
v24.4.1

reflex==0.8.0

Tried to get rid of the error, but honestly I’m kinda stuck. At first I thought it was because I was passing Optional values from my Pydantic model into params that aren’t marked as optional — made sense at first, since the error looked like it was about that. But nope, it still pops up even when that’s not the case.

Then I tried adding a dummy page for the / route that just returns rx.text(), gave it a different name (not index), just in case... but yeah, didn’t help either. Which makes sense, 'cause why would the function name matter? Still, the error kept showing up and disappearing randomly.

I’ll keep digging, but it’s kinda hard to track down since it comes and goes randomly. Feels super weird honestly.

[adhami3310]

we had some fixes we know about that came in 0.8.1 btw, so 0.8.0 might have some predictable ones of this kind

[kesevone]

we had some fixes we know about that came in 0.8.1 btw, so 0.8.0 might have some predictable ones of this kind

Oh, thanks for the info — just updated, I’ll keep an eye on it.

[thorsmann]

The update of node reduced the occurrence quite drastically but I still get this issue. I cannot really make much sense why some changes still cause this error and some don't.

[adhami3310]

generally, if a reflex compile updates multiple files, including ones where the developer is not viewing, that typically happens, but it's more frustrating when reflex compiles a file in-deterministically causing random saves to change things that weren't intended

if anyone has small enough of a repro, feel free to paste it here, i will try investigating any we can reproduce

[masenf]

Closing due to inactivity — we were unable to reproduce this deterministically and there's been no update since August 2025. Hot-reload has seen several improvements since 0.8.1. If you're still hitting this on a recent version of Reflex, please reopen with steps to reproduce.