What Nobody Tells You About Building a Multi-Provider MCP Server

Every MCP server tutorial follows the same script: install the SDK, define a tool, return a response. Ship it. That works for a single API. It does not work when you need an agent to reliably choose between, authenticate to, and call 645+ different APIs across 86 categories — and handle everything that goes wrong at 3am with no human. We built Rhumb , an MCP server that proxies hundreds of real APIs for AI agents. Here's what actually broke, why the tutorials don't cover it, and what you need to know if you're building anything beyond hello-world. Bug #1: The slug aliasing problem First surprise: APIs don't have stable identifiers. Brave's search API appears as both brave-search-api and brave-search depending on which documentation page you read. When an agent asks to "search with Brave," your MCP server needs to know these are the same service. This isn't unique to Brave. We found alias collisions in payment providers (same company, multiple API versions with different names), communi