I can never keep straight in my head all the situations when an X value is and isn't preserved in Slay the Spire. One such situation is when a card with an X-Cost Reinforced Body triggers Echo Form. My guess is that the Value of X would be preserved when the second copy of the card is played.
However, I recently came across a situation where if the value of X wasn't preserved it would have meant losing the run. So before deciding what to play I tried searching online for confirmation of how Echo Form works with X-Cost cards, but I couldn't find an answer. Since experimenting in that situation may have resulted in losing the run, I elected not to let me X-Cost trigger Echo Form.
For next time, though, I'm hoping to get a definite answer to this question that I can easily reference when my memory fails me.
When activating X-Cost cards with Echo Form, is the value of X preserved?