What can be done to keep a CW transmitter from chirping?

"Chirp" in a CW transmitter is typically caused by instability in the oscillator's power supply voltage when the transmitter is rapidly keyed on and off. When the transmitter is keyed to send a dit or dah, it draws more current, which can cause the power supply voltage to sag. If this voltage drop affects the oscillator circuit, the output frequency will momentarily shift, resulting in a chirp. To prevent this, the power supply voltage must be highly regulated and kept steady under varying loads. Adding filters like low-pass or key-click filters will not solve chirp, as those deal with harmonics and keying envelopes rather than voltage-induced oscillator instability.