What Is a “Tool” in MCP (And Why It Matters More Than You Think)

<p><em>Welcome back to the Harness Engineering series — a 10-part journey from raw language model to production-ready agentic system. Made by builders. For builders.</em></p>

<p>The AI world has a plumbing problem. We have incredible language models, but connecting them to real tools — databases, APIs, file systems, other agents — still feels like

<p>Most API documentation is written for humans.</p> <p>MCP tool descriptions are different.</p> <p>They are read by the model that decides what to call next.</p> <p>That means

<!-- SC_OFF --><div class="md"><p>The basic idea is simple: if you have an OpenAPI REST API, it makes it useable by AI agents with a single command without writing a MCP server

<p>Now I have the freshest 2025–2026 data. Let me write the fully verified, trend-accurate, non-repetitive final version:</p> <h1> Tool Calling in AI Agents: LangChain, LangGraph,