Almost every harness has an SDK now, it seems part of the "mvp" at this point, both open and closed source

https://learn.chatgpt.com/docs/codex-sdk

https://code.claude.com/docs/en/agent-sdk/overview

https://opencode.ai/docs/sdk/

https://pi.dev/docs/latest/sdk

or if you want SDK first, my recommendation is https://adk.dev/