Thesys

Best Thesys Alternatives in 2025

2 alternatives found

Overview of Thesys

Thesys (C1) is a Generative UI API that augments LLMs to respond with interactive UI like charts, forms, cards, and more in real-time, instead of plain text. It integrates seamlessly with any LLM, framework, or MCP in just 2 lines of code, promising to ship rich, interactive AI apps 10x faster with 80% less UI overhead.

Why Look for Alternatives

While Thesys offers a powerful drop-in solution for adding generative UI to AI applications, it may not suit every team. Some reasons to explore alternatives include:

  • Vendor lock-in: Thesys is a proprietary API, which may concern teams wanting open-source control.
  • Ecosystem maturity: Thesys is relatively new, and its ecosystem may lack community-driven extensions.
  • Specific use cases: If you need a fully open protocol, multi-platform rendering, or a complete agent infrastructure beyond UI generation, alternatives may be better.
  • Cost and scalability: Depending on pricing, a self-hosted or open-source alternative could be more cost-effective at scale.

Top Alternatives

1. A2UI (Score: 75/100)

A2UI is an open protocol (Apache 2.0) for agent-driven UIs, designed to be secure, framework-agnostic, and community-driven. It renders natively on Angular, Flutter, React, mobile, and more, with progressive streaming for real-time updates. Backed by Google and the community, it offers transparency and no vendor lock-in. However, it is less mature than Thesys, requires more setup, and lacks built-in support for specific UI types like charts or forms out of the box. Choose A2UI if you prioritize an open, secure, and flexible protocol for multi-platform environments.

2. 21st Agents SDK (Score: 35/100)

21st Agents SDK provides a complete production-ready agent infrastructure including sandboxing, auth, observability, session management, and billing. It offers a drop-in React chat UI component but focuses on text-based agent chat rather than rich generative UI components like charts or forms. It requires deploying agents on their platform and is more opinionated about the runtime. Choose 21st Agents SDK if you need a full-stack agent platform with built-in infrastructure and your primary interaction is text-based chat.

How to Choose

When evaluating alternatives to Thesys, consider:

  • UI complexity: Do you need rich interactive components (charts, forms, cards) or is text-based chat sufficient?
  • Control and openness: Do you prefer an open protocol (A2UI) or a ready-to-use API (Thesys)?
  • Integration effort: How quickly do you need to ship? Thesys offers 2-line integration, while A2UI requires more setup.
  • Platform support: Do you need multi-platform rendering (A2UI) or are you focused on a single framework?
  • Infrastructure needs: Do you need built-in auth, sandboxing, and billing (21st Agents SDK) or just UI generation?
  • Community and ecosystem: Is community support and long-term viability important?

For most teams seeking a quick, rich UI generation API, Thesys remains a strong choice. But if you value openness, security, and multi-platform support, A2UI is a compelling alternative. For a full agent infrastructure with text-based chat, consider 21st Agents SDK.

Alternatives

A2UI
1. A2UI

A2UI is an open protocol by Google enabling agents to generate rich, interactive UIs. Instead of risky code execution, agents send declarative JSON that clients render natively (Flutter/Web/Mobile). Secure, framework-agnostic, and designed for LLMs.

Pros

  • + Open protocol with Apache 2.0 license, fostering community contributions and transparency.
  • + Secure by design: declarative JSON only, no code execution, reducing UI injection risks.
  • + Framework-agnostic: renders natively on Angular, Flutter, React, mobile, etc.
  • + Progressive rendering: streams UI updates in real-time for a responsive user experience.
  • + Backed by Google and community, with active development and multiple renderer implementations.

Cons

  • - Less mature and smaller ecosystem compared to Thesys's ready-to-use API.
  • - Requires more setup and integration effort to adopt the protocol and build renderers.
  • - No built-in support for specific UI types like charts, forms, or dashboards out of the box.
  • - May have a steeper learning curve for teams wanting a quick drop-in solution.

Choose A2UI over Thesys if you need an open, secure, and framework-agnostic protocol for agent-driven UIs, especially in multi-platform environments where you want full control over rendering and avoid vendor lock-in.

21st Agents SDK
2. 21st Agents SDK

21st Agents SDK is the fastest way to add an AI agent to your app. Define your agent in TypeScript, deploy in one command, and embed a production-ready chat UI with Built-in streaming, session management, usage billing, and observability — so you can focus on what makes your agent unique, not infrastructure. Backed by Y Combinator (W26).

Pros

  • + Provides a complete production-ready agent infrastructure including sandboxing, auth, and observability out of the box
  • + Offers a drop-in React chat UI component for quick integration
  • + Handles session management, usage billing, and tenant isolation automatically
  • + Supports TypeScript-first agent definition with Zod schemas for type safety

Cons

  • - Focuses on text-based agent chat rather than generating rich interactive UI components like charts, forms, or dashboards
  • - Requires deploying agents on their platform rather than integrating with any LLM or framework in 2 lines of code
  • - Does not provide a Generative UI API for rendering dynamic visual elements in real-time
  • - More opinionated about the agent runtime and deployment model compared to Thesys's flexible API approach

Choose 21st Agents SDK over Thesys when you need to quickly ship a production-grade AI agent with built-in infrastructure for sandboxing, auth, and observability, and your primary interaction is text-based chat rather than rich generative UI components.

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