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

Agents that write code need somewhere to execute it, creating a product category with multiple vendors, incompatible billing models, and inconsistent performance claims. This comparison establishes standardized measurement units across five major platforms, examining cold start times under concurrent load, filesystem persistence between operations, internet access policies, and idle billing charges. The analysis demonstrates that vendor-quoted performance numbers measure different conditions and that tail latency matters more than median speeds for real-world agent workloads. Cost comparisons show that billing structure dramatically affects total expenses, with idle time scenarios producing 3 to 7 times higher costs than short burst scenarios on the same infrastructure.