Marvin

Your MCP Server is Bad (and you should feel bad) - Jeremiah Lowin, Prefect

Jan 12, 2026 · 54:33

Jeremiah Lowin, CEO of Prefect and creator of FastMCP, argues that most MCP servers are poorly designed because they are simply REST API wrappers, leading to confused LLMs and wasted tokens. He presents a framework for agent-native product design based on three dimensions—discovery, iteration, and context—and offers five actionable principles: outcomes over operations, flatten arguments, treat instructions as context, respect the token budget, and curate ruthlessly. Lowin emphasizes that agents fail when given too many tools or complex arguments, citing a hard limit of 50 tools per agent for reliable performance. He shares real-world examples, including a company with 800 endpoints that could not be effectively exposed, and critiques client implementations like Claude Desktop for not respecting spec-compliant features. The central takeaway: MCP servers should be designed as user interfaces for agents, not as developer APIs.

The AI Pivot: With Chris White of Prefect & Bryan Bischof of Hex

Nov 7, 2023 · 35:16

Chris White (CTO of Prefect) and Bryan Bischof (Head of AI at Hex) detail how their non-AI startups successfully pivoted to integrate AI, arguing that ruthless prioritization and deep product integration are key. White explains Prefect built the open-source Marvin project to learn from LLM experimenters, while adding AI features like error summaries to its core orchestration product, but had to restrain over-enthusiasm from engineers. Bischof describes Hex's Magic feature as an augmentation, not a separate product, and explains they built their own evaluation system but chose not to build a vector database, and killed a promising feature called Crystal Ball to avoid splitting the product experience. They both emphasize machine-to-machine interfaces and typed outputs, and caution that the work is often tedious data engineering. Their hot takes: White says stop building chat interfaces, AI is a tool; Bischof warns that the journey is boring but worth it.

Pydantic is all you need: Jason Liu

Nov 1, 2023 · 17:55

Jason Liu argues that Pydantic is the key to building reliable LLM applications by using structured prompting with OpenAI function calling. He introduces Instructor, a library that patches OpenAI's API to return Pydantic objects instead of raw JSON, ensuring type safety and validation. Liu demonstrates how Pydantic's 70 million downloads make it a trusted tool for defining data models with type hints, field validators, and even LLM-powered validators that catch errors like "don't say mean things" and retry via max retries. He shows concrete examples: decomposing user queries into structured search objects for RAG, generating a query plan DAG with parallel dependencies, extracting knowledge graphs for visualization, and verifying facts by requiring substring quotes from source text. Liu emphasizes that structuring prompts as code moves development from string manipulation to domain modeling, enabling cleaner, more maintainable systems that integrate easily with existing software.