Not necessarily a critical bug.
Your koa server listens to 0.0.0.0 or :: by default, not 127.0.0.1 (which is the Vite default).
As Node.js sets all sockets with the SO_REUSEADDR flag (explanation: https://medium.com/@eplawless/node-js-is-a-liar-sometimes-8a28196d56b6), it allows other sockets to reuse the same port as long as it doesn't listen to the exact same address.

The problem here is that Vite chooses to display localhost while the actually bound address is 127.0.0.1; this could be misleading in some rare edge cases, like this one.

Take my network environment as an example (may not be very accurate, I'm not a network expert):

• In my /etc/hosts, I have localhost bound to both 127.0.0.1 and ::1, meaning that localhost resolves to ::1 when IPv6 is available;
• My ISP and router have IPv6 configured;
• When I start a Vite server, it listens to 127.0.0.1:3000 by default;
• When I start the koa server, it listens to the unspecified IPv6 address ::, which also accepts connections from ::1;
• So, when I visit localhost:3000, it's equivalent to ::1:3000 as I'm in an IPv6 environment. So I end up hitting the koa server;
• When I visit 127.0.0.1:3000, I can still visit the Vite server.

So the solution could be either of the following:

• Let the server host default to:
  • Both 127.0.0.1 and ::1 (most intuitive, but I don't know how to implement that)
  • Or ::1 when IPv6 is available, and 127.0.0.1 otherwise (most conventional, but would still surprise people sometimes)
• Or, we don't prettify the output to display localhost when it's actually 127.0.0.1

This problem is solved，set server.host = 0.0.0.0，example config:

server: {
    host: '0.0.0.0',
  },

@sodatea This issues should closed