ffmpeg: how to avoid transcoding from a lossy format?

[Luke M]

Is there an option or method with ffmpeg to avoid transcoding lossy audio, only using lossless sources? (This would be for a batch job with a mix of audio).

[pandy]

i think your question is contradictory... - some operation can be performed with lossy sources and outputs by using copy instead processing however mixing imply signal processing thus signal need to be decoded, processed and encoded. If you are using lossless codecs then operations can be performed losslessly (but signal processing may alter signal itself thus it may introduce loss of quality).
If i misunderstand your question then sorry, perhaps you need to rephrase question.

[Luke M]

By "mix" I meant heterogeneous formats. Nothing is being mixed together. I want to copy when the source is lossy and compress when the source is lossless.

[pandy]

Seem such conditional behaviour must implemented by you as two step process, first analyse step where you detecting lossy vs. lossless secondly trig particular ffmpeg behaviour.