From $ crystal play -v i see that the instrumented code for

def foo(x)
  yield x
end
foo(3) do |a|
  a
end

is

def foo(x)
  yield _p.i(2) do
    x
  end
end
_p.i(4) do
  foo(3) do |a|
    _p.i(5) do
      a
    end
  end
end

which does not compile. The to_s of the code uses begin; x; end instead of { x } for the block.

So there is actually a bug in the Crystal::ToSVisitor for printing yields with calls with block. begin/end should be avoided and { ... } should be used to preserve semantics OR a begin/end should wrap the call:

def foo(x)
  yield begin
    _p.i(3) do
      x
    end
  end
end

I would rather not remove the instrumentation for single argument yields. The best thing would be to fix the ToSVisitor. The second best is to change the agent_instrumentor_transformer.cr to explicitly wrap the instrument(...) with a begin/end.