pydantic-ai-harness
Your agent's favorite harness, built on Pydantic AI
Pydantic AI Harness is the official capability and harness library for Pydantic AI, providing over 50 pre-built agent building blocks and complete agent implementations. It extends simple Pydantic AI agents with production-grade capabilities including workspace management, planning systems, persistent memory, sub-agent delegation, context management, and durable execution for long-running tasks. Developers can use complete agents like Coder or Researcher out-of-the-box, or compose custom agents from individual capability blocks added to any Pydantic AI agent's capabilities list. All capabilities are model-agnostic and work with any LLM provider supported by Pydantic AI.
Key Features
Use Cases
- 01Building coding agents that fix bugs, refactor code, or implement features with file editing and test execution capabilities
- 02Creating research agents that search the web, read pages, delegate sub-questions, and compile cited findings
- 03Developing long-running autonomous agents that maintain plans, manage context across hours of execution, and survive restarts
- 04Adding persistent memory to agents so they remember facts and context across multiple sessions
- 05Implementing safe shell command execution with allowlists and credential-stripping for development automation
- 06Composing custom agent stacks by mixing capabilities like web search, file access, sub-agent delegation, and guardrails
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pydantic-ai-harness — FAQ
What is pydantic-ai-harness?+
Pydantic AI Harness is the official capability library for Pydantic AI that provides 50+ pre-built agent components (capabilities) and complete agent implementations. It extends basic Pydantic AI agents with production features like file access, shell execution, memory, planning, sub-agents, and context management for complex, long-running tasks.
How do I install pydantic-ai-harness?+
Install with uv using 'uv add pydantic-ai-harness' or with a provider extra like 'uv add "pydantic-ai-harness[anthropic]"'. This automatically installs pydantic-ai-slim, so you don't need to install Pydantic AI separately. Some capabilities require their own extras documented on their individual pages.
Which AI models and clients work with pydantic-ai-harness?+
All capabilities work with any LLM provider supported by Pydantic AI (Anthropic, OpenAI, Gemini, Groq, Mistral, etc.) since they are model-agnostic. Agents can run in terminals via the CLI, in browsers with the web UI, as chat interfaces, via the Agent Client Protocol (ACP) for editors like Zed, or embedded in custom applications.
Do I need API keys or other prerequisites?+
You need an API key for your chosen LLM provider (e.g., Anthropic, OpenAI). Some optional capabilities require additional API keys: Exa Search needs an Exa API key, You.com capabilities need a You.com API key, StackOne requires StackOne credentials, and web search/fetch may use provider-native implementations or local fallbacks.
Is pydantic-ai-harness free to use?+
Yes, pydantic-ai-harness is open source under the MIT license and free to use. However, you pay for LLM API usage from your chosen provider (Anthropic, OpenAI, etc.) and any third-party services like Exa or You.com if you use those capabilities.
How do I use the Coder or Researcher agents?+
Import and instantiate them as capabilities: 'from pydantic_ai_harness import Coder; agent = Agent("anthropic:claude-fable-5", capabilities=[Coder()])'. You can run them with agent.run_sync(), open them as CLI chat with agent.to_cli_sync(), or run pre-exported agents directly with 'uvx --with pydantic-ai-harness clai -a pydantic_ai_harness.coder:coder_agent'.
How do I install pydantic-ai-harness?+
Open the source repository on GitHub and follow its README. pydantic-ai-harness is a agent — MCP Agents Market links you directly to the official repo.
Is pydantic-ai-harness free?+
pydantic-ai-harness is an open-source project hosted on GitHub. Check the repository for its license and any usage requirements.