Lessons learned building Fronteira

I designed a harness built on top of dependent types that uses information-flow policies to prevent attacks involving the lethal trifecta. However, expecting users to write liquid types was unrealistic, and creating a custom SDK for every external interface—filesystems, APIs, internet search and MCP tools—was not practical. Following feedback from Lisbon.ai (thanks, Nuno, Fred, Sérgio and David), I started building Fronteira to make these technical ideas accessible to a wider audience. What is Fronteira? Fronteira is an MCP proxy that acts as a behavioral firewall. You log in through the browser and add integrations with GitHub, Gmail and other services through their MCP servers. You authenticate your harnesses with Fronteira and automatically get access to all your configured MCP servers. You also get portability across harnesses, but that is just a side effect. The main advantage of Fronteira is that all MCP interactions go through it, allowing it to track behavior across multiple…