Every coding agent shipped this year needs a safe place to run the code it writes, and that compute has become one of the fastest-growing lines on an engineering bill. The largest AI labs already run AI-generated code inside sandboxes, and every team shipping a coding or data agent now needs the same thing.
For those teams, sandbox pricing is now a design decision. Solari bills sandboxes by size, per second while they run, from one credit balance that also covers browsers and desktops. Modal bills its sandboxes per second of CPU and memory at a premium over its standard rates, plus a monthly plan fee. This guide explains each line on a Modal bill, what the same sandbox costs on Solari, and how to choose.
Why Does Sandbox Pricing Matter Now?
Serverless computing made per-second billing normal: you pay for compute while it runs and nothing while it sits unused. Agent sandboxes push that model hard, because one agent opens hundreds of short-lived machines a day, each sized for a job it has not seen yet.
The old way was a fixed fleet of servers sized for peak load. The new way is one sandbox per task, created by API and gone when the task ends. The cost question moves from how many servers you need to what one sandbox-hour costs and what else lands on the invoice.
How Does Modal Pricing Work?
Modal charges sandboxes per second for CPU and memory, at a higher rate than its regular functions. Modal says sandboxes are billed at non-preemptible rates of 3x list price, with CPU priced per physical core per second, memory per GiB per second, and one physical core counted as two vCPUs.
Plans sit on top of compute. Modal's Starter plan is $0 plus compute with $30 a month in free credits and up to 100 containers; its Team plan is $250 a month plus compute, with $100 in monthly credits and 5,000 containers.
The rate is only part of the bill. These are the five things to check when you estimate a Modal sandbox:
What you reserve, not only what you use. Modal bills sandboxes at "max(request, actual)". A sandbox that requests four cores and works on one still pays for four, so size each request to the job.
How long a sandbox lives. Modal says a sandbox has a default maximum lifetime of 5 minutes, extendable to 24 hours. Long-running agents need a timeout on every create, or a snapshot and a fresh sandbox to keep going.
Where it runs. Modal charges a multiplier on base prices when you pick a region. A team pinned to one region for data residency should price that multiplier in from day one.
What isolation you get. Modal runs sandboxes on gVisor, a userspace kernel from Google, or on a VM runtime with its own Linux kernel. The runtime decides what the code inside can do, such as running Docker.
Storage and network. Modal bills volumes per GiB per month and network egress per GiB, with some egress included in each plan. Agents that pull large datasets or write big artifacts add to the bill outside compute.
What Does the Same Sandbox Cost on Solari?
Solari bills a sandbox by its size, per vCPU-hour and per GB-hour, per second while it runs. Solari's lowest published rate is $0.0175 per vCPU-hour and $0.0055 per GB-hour on the Team plan, and every plan draws from one credit balance.
At list prices, a 4 vCPU, 8 GB sandbox costs $0.761 per hour on Modal against $0.114 per hour on Solari's Team plan. On the self-serve Professional plan, a 4 vCPU, 16 GB Solari sandbox costs $0.221 per hour.
Solari's Free plan is $0 with $3 in monthly credits. Starter is $20 a month and Professional $200 a month, and each fee comes back as the same amount of usage. Credits refill every month and Solari never bills past your balance, so there is no end-of-month overage charge.
What Role Does Solari Play?
Solari is the infrastructure an agent runs on: sandboxes, cloud browsers and desktops from one API and one bill. For a team weighing a Modal invoice, these are the parts that change the total:
A microVM of your own. Each Solari sandbox runs in a Cloud Hypervisor microVM with a dedicated kernel by default. Code your agent writes stays behind hardware virtualization, and the hardened tier blocks all outbound network.
Pause instead of paying to wait. Pause saves memory and processes, and resume brings the sandbox back exactly as it was. An idle sandbox pauses on its own, so a sandbox waiting on a person or a model is not left running.
Snapshots and forks. Snapshot a running sandbox and fork it to try several fixes in parallel. Snapshot storage costs $0.05 per GB-month, with the first 10 GB free.
A shorter lifecycle. A full sandbox lifecycle runs 8.2 s on Solari against 11.9 s on Modal. Each short task finishes sooner, and your agent moves to the next one.
Browsers and desktops on the same bill. Solari browsers cost $0.07 per hour on Professional, and Solari desktops give computer use models a full screen for $0.02 per hour on top of compute. An agent that runs code and operates the web needs one vendor, not two.
Modal is the more established vendor, with a wide GPU and inference platform around its sandboxes; Solari is the younger company and moves faster on speed and price. The Solari launch covers how the three surfaces fit together.
Where Is Sandbox Pricing Going Next?
Per-second billing is already standard. Over the next year, the comparison shifts in four ways:
Rates over plan fees. Teams will compare the hourly rate of a sized machine, not the headline plan price, because agent workloads swing too much for a fee to predict the bill.
Waiting gets cheaper. Pause, resume and snapshots will matter as much as the rate, because agents spend much of their time waiting on models and people.
Fleets, not single machines. Pricing will be judged at thousands of sandboxes at once, and Solari supports up to 100,000 concurrent sandboxes on Enterprise.
One bill for every surface. Teams will price code, browsers and desktops as one agent runtime instead of three vendors.
What stays human is judgment: which jobs deserve a sandbox, how large it should be, and what an agent is allowed to touch. The teams that keep costs down will size each sandbox to its job and let the infrastructure handle the rest.
Get Started With Solari
Start with one job, such as the sandbox your coding agent runs most often, and run a week of it on Solari next to your current bill. Solari is the fastest and cheapest way to run cloud browsers and VMs for coding agents. Get started with a free account and start running your agents today.
