A papery, cardboard-like flatness has three common causes and they are easy to separate.
Stale beans
The most likely. As coffee ages, the aromatic compounds go first and what remains reads as flat and faintly cardboard-like. Dark roasts add a stale-oil edge on top.
Check the roast date. Over two months and this is your answer.
An unrinsed paper filter
Paper contributes a genuine papery taste, and it is most noticeable in light roasts and clean brewers where nothing masks it.
Rinse the filter thoroughly with hot water before brewing and discard the rinse water. Chemex papers, being thickest, need the most rinsing.
This takes ten seconds and eliminates the problem entirely.
A dirty brewer or kettle
Old coffee oils in a French press mesh, a stale AeroPress seal, or a kettle with scale and residue all contribute flat, musty notes.
Wash the brewer properly, and descale the kettle — people filter their water and then boil it in something unpleasant.
Less common
Old water. Water that has sat in a jug for days tastes flat and passes it on.
Reboiled water, which loses dissolved oxygen. The effect is real and small.
The order to check
Roast date, then rinse the filter, then clean everything.
Last reviewed 8 August 2026