Setup:

  $ . $TESTDIR/setup.sh
  $ mkdir pipecontext_dir
  $ printf "a\nb\nc\n" > pipecontext_test.txt
  $ cd pipecontext_dir

Do not use parallel flag, which disables stream input:

  $ unalias ag
  $ alias ag="$TESTDIR/../ag --nocolor --workers=1"

B flag on pipe:

  $ cat ../pipecontext_test.txt | ag --numbers -B1 b
  1-a
  2:b

C flag on pipe:

  $ cat ../pipecontext_test.txt | ag --numbers -C1 b
  1-a
  2:b
  3-c

Just match last line:

  $ cat ../pipecontext_test.txt | ag --numbers c
  3:c