Not really. All you need to be able to do is to produce the first byte of whatever would have been produced in the "fallback case", i.e. when the reader does not handle format negotiation. Then, when the writer sees that the reader supports format negotiation it will need to signal that the alternative format was chosen. I do this by sending a zero byte (which should never appear in the fallback text format). Then the two first bytes are skipped as part of the negotiation framework when a non-fallback format was chosen.