"Cannot connect pipelines with different fluids" does not show if it's a building's output [2.1.8]

[zeldrakef]

When you are mixing two different fluids, "Cannot connect pipelines with different fluids" is supposed to tell you that you can't do that:

Screenshot_20260630_190744.png (3.29 MiB) Viewed 212 times

But no such error appears if the pipe is being placed directly on a building's output, despite the building's internal buffer being full of that fluid:

Screenshot_20260630_190749-1.png (3.28 MiB) Viewed 212 times

And if the pipes are super-forced-placed anyway (eg: by robots), no "Mixing fluid error" will show on the pipes that are right next to a building's output, even though those sure look like mixing fluids to me!

Screenshot_20260630_190805.png (3.37 MiB) Viewed 212 times

What I expected: Consistent behaviour if a pipe is placed next to a building, or next to a pipe is immediately taking a building's output: why do zero tiles of pipe cause it to not complain?

Save game:

mixing-fluid-output-no-error.zip

(2.71 MiB) Downloaded 30 times

and log:

factorio-current.log

(6.37 KiB) Downloaded 19 times

[boskid]

As far as i remember design decisions about fluid mixing checks, this is working as intended to allow for smoothie pipes.

https://www.factorio.com/blog/post/fff-442 - "Fluid mixing prevention"

Notably, fluid mixing only occurs on bidirectional connections; you can still create smoothie pipes by connecting pumps or unidirectional machine outputs with different fluids in them, and the game will not get in your way.

[zeldrakef]

Ah, so this quirk is limited to one-directional connections: like a undirectional-output machine, which won't let fluid inside it. I see! Thanks for the explanation.