Wasn't there a deprecation of the X- prefix in protocol headers? Other thoughts: You could probably make this play better with HTTP libraries by returning the

Wasn't there a deprecation of the X- prefix in protocol headers? Other thoughts: You could probably make this play better with HTTP libraries by returning the header on an existing HTTP message, instead of changing the overall reply semantics. It's a little less efficient, but probably much more broadly compatible. You could also have min-est-completion-time and max-est-completion-time rather than a percentage (partly to address people's concerns about the uncertainty or unknowability of overall progress as a percentage). Perhaps these could customarily be described as 90% intervals (or they could explicitly include a confidence estimate! like "90% of requests at this approximate degree of completion were completed within 7 and 32 seconds").