XO1504: a std API the native runtime does not support on this target yet

The LLVM backend’s runtime is ported to Windows in stages (decision 0197). Stage 1 runs programs that compute, print, read standard input, use tasks, channels, timers, faults, files, sockets, HTTP, and TLS; stage 2 (2026-10-12) added processes, so no std API stops a Windows build today. The build stops with this diagnostic, before compiling, when the program uses a part that is not ported to the build’s target (as processes were before stage 2). The message names the parts. The Go backend builds the same program for Windows.

Example

> xo build --backend=llvm .
XO1504: the LLVM backend does not support processes (Proc) on windows yet (decision 0197); build with the Go backend (the default) instead

Fix

# build with the Go backend, which supports every std API on Windows:
xo build .

Run xo explain XO1504 for this text in a terminal, or see section 10.6 of the specification.