I opened a channel for 5000 sats. Why is my available balance to send only 4000 sats?

Question

Using Lightning Dev Kit (LDK). Intuitively, it seems like the entire balance should be send-able.

Answer 1

This is an effect of the channel reserve, see: https://docs.rs/lightning/latest/lightning/util/config/struct.ChannelHandshakeConfig.html#structfield.their_channel_reserve_proportional_millionths.

The channel reserve is enforced to ensure each counterparty always has something to lose, if they were to try to cheat. By default, the channel reserve is 1% of the total channel capacity, and LDK sets a hard minimum of 1000sats.