$ hg init repo
$ cd repo
$ touch a.html b.html c.py d.py
$ cat > frontend.sparse << EOF
> [include]
> *.html
> EOF
$ hg -q commit -A -m initial
$ echo 1 > a.html
$ echo 1 > c.py
$ hg commit -m 'commit 1'
Enable sparse profile
$ cat .hg/requires
dotencode
fncache
generaldelta
revlogv1
store
$ hg debugsparse --config extensions.sparse= --enable-profile frontend.sparse
$ ls
a.html
b.html
Requirement for sparse added when sparse is enabled
$ cat .hg/requires
dotencode
exp-sparse
fncache
generaldelta
revlogv1
store
Client without sparse enabled reacts properly
$ hg files
abort: repository is using sparse feature but sparse is not enabled; enable the "sparse" extensions to access!
[255]
Requirement for sparse is removed when sparse is disabled
$ hg debugsparse --reset --config extensions.sparse=
$ cat .hg/requires
dotencode
fncache
generaldelta
revlogv1
store
And client without sparse can access
$ hg files
a.html
b.html
c.py
d.py
frontend.sparse