The following are known Action Replay Codes for Boktai: The Sun Is in Your Hand on Nintendo Game Boy Advance (GBA).
These codes were also known as GameShark Codes and in most cases will work on both devices.

These codes are for the U.S. version of the game.

Master Code – Must Be On
3e0727c9
860390aa
918820c2
0fa2447f

Infinite Health
f91b245a
12db248c

Max Health Bar
8ba94826
75c7625e

Infinite Sun
d1d63a15
966406fc

Max Gun Energy
54bb303f
ed470839

Gun Never Overheats
85ca4c9b
7a4201af

Infinite of All Items
5d297163
e9dc86d6
bbc7cae8
fc99817e
00238e02
cc4e4ca9
58eda272
ee5e4e87
2d576869
d6029e1a

Have All Lenses
2c9bd747
8880d812

Have All Frames
2dc19205
78bad96e
91e08c74
dc30f105

Have All Batteries
815b2f53
73fe12cd

Note: This code does not include the Astro Battery as one of the batteries.

Infinite of All Grenades
0e806dbe
6ba4041f
a3312e96
b31e768a