Best Agent Sandboxes in 2026: Cold Start, Per-Second Pricing, and Network Policy Across E2B, Daytona, Modal, Cloudflare, and Vercel

Every agent that writes code needs somewhere to run it. That “somewhere” is now a product category with at least a dozen vendors, four incompatible billing models, and marketing pages that quote cold starts measured under conditions nobody publishes. This comparison fixes the units. It covers the five platforms most teams shortlist — E2B, Daytona, Modal Sandboxes, Cloudflare Sandbox SDK, and Vercel Sandbox — along with Runloop, Fly.io Sprites, and Northflank where they change the answer. Feature matrices for this category are mostly noise. Four properties change architecture, and everything else is a preference: Cold start under concurrency: An agent loop that creates a sandbox per tool call pays this tax thousands of times a day. Filesystem persistence between turns: Does turn 2 see the pip install from turn 1, or does the agent rebuild its world? Egress policy: Can the sandbox reach the internet, can you turn that off, and can you change your mind mid-session? Idle billing: Agents spend most of their wall-clock waiting on a model. Somebody is paying for those seconds. 1. Cold start: what the numbers actually say The vendor claims are not comparable to each other. Daytona’s pricing…