Cstddef file not found when porting custom target

Mbed OS
,
1

I’m porting Mbed OS to a custom board using GeehySemi/mbed-os as a reference. My project fails to compile with:
[Fatal Error] platform.h@21,10: ‘cstddef’ file not found
The reference Geehy project compiles successfully with the same toolchain and headers.
Questions:

  1. Why does cstddef fail only in my project?
  2. Beyond targets.json , index.json ,what critical configurations might be missing?
  3. Are there common pitfalls for beginners porting custom targets?

Thanks in advance for any guidance!