Solo5 is a sandboxed execution environment for unikernels, providing a minimal, legacy-free interface between unikernel applications and host systems. It supports multiple backends: hvt (hardware virtualization via KVM, FreeBSD/OpenBSD/DragonFly vmm), spt (Linux seccomp sandbox), virtio (QEMU/bhyve), Xen, and Muen. The architecture consists of guest-side bindings (implementing the solo5_* API) and host-side tenders (loaders/runtime components). Key entry points include the guest's _start() in each binding's start.c and the tender's main (e.g., hvt_main.c, spt_main.c). Data flows are synchronous: unikernels communicate via hypercalls (hvt), seccomp-allowed syscalls (spt), or Xen hypercalls, with shared-memory ring buffers for I/O. External dependencies include libseccomp (spt), KVM/BSD hypervisor APIs, and vendored Xen public headers. The project originated from IBM Research and is used by MirageOS and other unikernel frameworks.