Pydantic AI vs Vercel AI SDK
The Vercel AI SDK adds model calls, tool loops and streaming chat to a TypeScript app, with useChat hooks and a broad provider registry; Eve, a separate Apache-2.0 package, adds an agent harness on top. Pydantic AI is the Python back end for that UI: a typed Agent, the Harness SDK, and a Vercel AI stream adapter so useChat renders our agents.
Pydantic AI is one part of a stack: the Harness SDK for capabilities and complete agents, Pydantic Evals for evaluation, Pydantic Graph for graph-based workflows, Pydantic Logfire for observability, and Pydantic itself for validation. The tables below cover the whole of it.
Already built on the Vercel AI SDK? The migrating-vercel-ai-sdk-and-eve-to-pydantic-ai skill ports an existing application to Pydantic AI one working path at a time, preserving behavior rather than translating API names.
| Vercel AI SDK | Pydantic AI and Harness SDK | |
|---|---|---|
| Language | TypeScript | Python |
| License | Apache-2.0 | MIT |
| Model providers | Many | Many |
| Extensibility | Middleware, tools | Capabilities and toolsets; 50+ with the Harness SDK |
| Harnesses | Eve, a separate package; adapters drive external harnesses | Built-in Coder and Researcher, or compose your own |
| Observability | OpenTelemetry | OpenTelemetry, including Pydantic Logfire |
| Durable execution | Yes | Seven integrations |
| Interfaces | React chat UI, stream protocol, GitHub Action | CLI, web chat, AG-UI, Vercel AI, ACP (experimental), GitHub Agentic Workflows |
| Realtime voice | Yes (experimental) | Realtime |
| Evals | No | Pydantic Evals |
| Image generation | Yes | Image Generation |
| Vercel AI SDK | Pydantic AI and Harness SDK | |
|---|---|---|
| Multi-agent | Yes (Eve) | Subagents, delegation, or pydantic-graph |
| Planning | Yes (Eve) | Planning |
| Skills | Yes (Eve); provider-hosted in the SDK | Skills |
| Memory | No | Memory |
| Compaction | Yes (Eve); pruneMessages in the SDK | Compaction |
| Guardrails | Yes | Guardrails |
| Code sandboxes | Yes (experimental) | Execution environments |
| Browser use | No | Web & research |
These are not exclusive. Keep your Vercel AI SDK frontend and run a Pydantic AI backend behind it: VercelAIAdapter speaks the Vercel AI Data Stream Protocol, so useChat and AI Elements render a Python agent’s runs without frontend changes. AG-UI is the same deal for any AG-UI frontend.