Updated April 29, 2026
21st.dev is the largest React component registry for the agentic internet — an open-source, community-driven marketplace of shadcn/ui-based React + Tailwind components, blocks, and hooks. 1.4M developers (200K MAU), pivoting into a full agent deployment SDK.
Instructor is a popular open-source library for getting structured outputs from LLMs using Pydantic models.
Core capabilities each platform advertises.
What each tool does well, and the limitations to keep in mind.
Pros
Cons
Pros
Cons
Saves so much time — broad set of high-quality, free components, including 3D. Smooth React and Next.js integration.
Read full reviewChoose 21st.dev if you wantChoose if you want
Choose Instructor if you wantChoose if you want
Respan lets you trace LLM and agent calls across any model or framework, A/B test prompts on production traffic, and route requests across 500+ models through one gateway.
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