On Runtime, you pay for the vCPU time your code uses, not for the vCPUs a sandbox holds: $0.025 per active vCPU-hour. A 2 vCPU, 4 GiB sandbox costs $0.03125 an hour while it waits on a model. It costs $0.08 an hour with both vCPUs busy. A paid sandbox can have up to 16 vCPUs (pricing).
Thread, core and vCPU
A physical core can run two instruction streams at the same time. This is called
simultaneous multithreading (SMT). Intel calls it Hyper-Threading. Each stream
is a thread. A hypervisor gives threads to guests as vCPUs. Amazon's EC2
documentation states it plainly: with SMT, "each thread is represented as a
virtual CPU (vCPU) on the instance". So an m5.xlarge with two cores and two
threads per core has four vCPUs.
Two vCPUs are therefore not always two cores. They can be two threads of one core, competing for its execution units. This is why some providers price "physical cores" separately. It is also why a benchmark should state which one it measured.
How providers bill a vCPU
| Basis | What you pay for | Who does it |
|---|---|---|
| Allocated | Every vCPU the machine holds, for as long as it runs | E2B, Daytona, Northflank, most VM clouds |
| Measured | CPU time the code actually used | Runtime, Vercel Sandbox, Fly Sprites |
| Physical core | Whole cores, or the larger of request and use | Modal |
| Included | No CPU line; the memory or machine price covers it | Blaxel, fixed-size machines |
For an agent that spends most of a run waiting on a model, the billing basis matters more than the rate. An allocated vCPU is paid for while it sits idle. On the standard comparison job, 1,000 one-minute runs with 20 CPU-seconds of work cost $2.76 on E2B and $0.64 on Runtime (the comparison).
On Runtime
A sandbox's vCPUs are shared by default. The sandbox is guaranteed a floor of
a twentieth of a vCPU, unless you ask for more. It can burst up to all of its
vCPUs when its code needs them (CPU floor).
cpu: "reserved" keeps every vCPU for that sandbox alone. Reserved vCPUs are
billed as always busy (shared vs reserved CPU).
Inside the sandbox, nproc reports the vCPUs it asked for. The servers are AMD
EPYC, with AVX, AVX2 and FMA guaranteed
(the sandbox environment).
Related
Sources
Checked 25 October 2026.