Thanks @Eduard Lebedyuk , that was helpful. As you describe, headers are not re-used from a previous call. On closer inspection, our confusion arose from the fact that if you provide a set of credentials as a setting on the operation which is implemented by EnsLib.HTTP.OutboundAdapter, even if you don't explicitly set the Basic Authentication headers yourself, they are created for you by the adapter using the provide credentials - we didn't realise this, and so when we removed our own explicit setting of the headers (for testing) we were confused by the headers still arriving in the downstream system!!

Update: this was answered by @Brett Saviano on the GitHub discussion forum for the VS Code extension: Should the extension change line feeds on compilation? · intersystems-community/vscode-objectscript · Discussion #1495