21st launches Claude Code agent SDK

This is a strong pitch at the emerging “agent infrastructure” layer: less about model novelty, more about making agentized apps feel like ordinary product engineering. If 21st executes, it could save teams from stitching together half a dozen SDKs, sandboxes, and UI kits just to launch a usable in-app agent.

• –The SDK is explicitly code-first, with agents defined in TypeScript and deployed via CLI, which will appeal to product teams that want versioned infra instead of no-code abstractions
• –Built-in UI, token exchange, streaming, chat history, spend limits, and observability target the operational headaches that usually slow agent features down after the demo works
• –The platform leans hard on Claude Code today while also signaling OpenAI support, which makes it easier for teams to adopt without fully locking into one runtime story
• –E2B sandboxing and per-session execution are important differentiators because serious agent products need isolation, not just API wrappers and a pretty chat box