Show HN: What the Harness? LLM agent inconsistencies in MCP implementation

Category: devtools

Tags: mcp, llm-agents, testing, compatibility, protocol

Score: 5.7/10 (Innovation: 7, Technical: 5, Documentation: 6, Utility: 5)

What the Harness? is an MCP compatibility field guide and testing harness that probes how different LLM agent clients (harnesses) implement the Model Context Protocol, checking for issues like duplicate tool outputs, protocol version mismatches, URL elicitation support, and resource reading. It provides a public MCP server and standardized tests so developers can verify their harness behavior and compare results across clients.

Target audience: backend developers, AI engineers, LLM application developers

Repository: https://what-the-harness.polytomic.com

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