The constant LLVM_BLAKE3_OUT_LEN provides the default output length, 32 bytes, which is recommended for most callers.
Declared in <llvm/Support/BLAKE3.h>
template<size_t NumBytes = 32>
using BLAKE3Result = std::array<uint8_t, NumBytes>;Outputs shorter than the default length of 32 bytes (256 bits) provide less security. An N-bit BLAKE3 output is intended to provide N bits of first and second preimage resistance and N/2 bits of collision resistance, for any N up to 256. Longer outputs don't provide any additional security.
Shorter BLAKE3 outputs are prefixes of longer ones. Explicitly requesting a short output is equivalent to truncating the default-length output.