How MCP’s Core Maintainers leverage Val Town to develop the protocol

Paul Carleton, a Core Maintainer of the Model Context Protocol (MCP) from Anthropic's Technical Staff, uses Val Town to develop and test MCP features, including tools for the new MCP specification released on 2026-07-28. He has created dozens of vals, such as an MCP draft spec viewer, a conformance checker, an OAuth debugger, and TypeScript SDK examples, which help the nine-member Core Maintainer team from Anthropic, Google, Microsoft, OpenAI, and Amazon collaborate on protocol changes and enterprise auth solutions.

Code and prose written on val.town /source How MCP’s Core Maintainers leverage Val Town to develop the protocol /mcp-core-maintainers The Model Context Protocol https://modelcontextprotocol.io/ is built by a large open source community. One part of the stewardship of that community is a Core Maintainer team of nine from AI companies like Anthropic, Google, Microsoft, OpenAI, and Amazon. Paul Carleton is one such Core Maintainer from the Technical Staff at Anthropic. With the new MCP specification https://blog.modelcontextprotocol.io/posts/2026-07-28/ released last month, we interviewed Paul about how he relies on Val Town to test and demo MCP features. “It’s so easy to put up a quick MCP or API with Val Town. I’ve tried other platforms and CDN’s, but it’s so much faster and smoother to go with Val Town.” Table of contents Paul’s vals Paul’s work focuses on MCP auth and conformance. He’s written dozens of vals https://www.val.town/u/pcarleton , collaborating with Anthropic teammates and other Core Maintainers to test and demo all things MCP. Developing the new spec The new MCP spec 2026-07-28 includes fundamental changes to the protocol. Paul created an MCP draft spec viewer https://www.val.town/x/pcarleton/mcp-draft-spec-viewer on Val Town for the Core Maintainers to diff the new spec against the old one, leave comments, and view conformance testing coverage for each section. “This one was really handy. The core maintainers all wanted to run through the specification and be able to tell what the diff was between the previous version and this version. There are nine of us from different companies, and so when we need to work on some tool together, it’s nice to be able to deploy something quickly to share it.” Testing conformance To check MCP client conformance against the new spec, Paul also set up an HTTP val that clients can call at mcp-checker-2026-07-28.val.run https://mcp-checker-2026-07-28.val.run . The val itself is an MCP server that evaluates a client’s request and responds with an explanation of what failed and why. “It’s been helpful to be like, is Claude Code ready for the new spec?” Answer: Yes Debugging OAuth for your MCP server If you have an MCP server, you can paste its URL into Paul’s debugger at mcp-oauth-debugger.val.run https://mcp-oauth-debugger.val.run/ to see how it scores on OAuth. “You put in an MCP server, and we’ll check the public endpoints. This one I share a lot.” Demoing the TypeScript SDK Paul’s mcp-v2-examples https://www.val.town/x/pcarleton/mcp-v2-examples val has both MCP client and server examples for the TypeScript SDK that follows the new spec. “The way I build MCP servers most of the time is in Val Town. It’s great. A GET request returns some JSON that renders here on the frontend , and I can also just query the MCP server directly to see what that example server does.” Solving enterprise auth The Core Maintainer team is working on zero-touch OAuth for MCP https://blog.modelcontextprotocol.io/posts/enterprise-managed-auth/ , and Val Town has been the right tool to test many of those new auth flows. Paul has over half a dozen different XAA vals. As he explains: “For auth with MCP, you end up having a lot of different parties involved: you have the client, the authorization server, and the MCP server. We’re trying to make it easier to do single sign-on through MCP through an official extension called “Enterprise Managed Auth” or EMA. That extension is based on a draft RFC called Identity Assertion Authorization Grant or ID-JAG . We needed a server that’s protected by this new form of auth, and with Val Town I can just get an HTTPS URL that has an MCP, and I can tweak that really quickly, and then I can put that MCP into” claude.ai and have our real production backend hit it and test a whole bunch of stuff, which is really nice. Why Val Town Those five examples are just a fraction of the vals Paul and the MCP team use to develop the protocol. Paul uses Claude Code ofc and pushes to Val Town via the API, occasionally jumping into the val.town editor to make changes by hand. The combination of fullstack TypeScript, instant deployments, live HTTPS URLs, and shareable open source code make Val Town a good fit for this kind of work. “I use Val Town for all the things, especially because I work on the open source side. It’s a nice source of truth to help coordinate people in a low friction way. Being able to slot in an MCP server val with an HTTPS URL and very clear, easy-to-get-to logs is really nice.” Business and pleasure At risk of sounding idealistic, we write fairly often about Val Town’s mission to spread the joy of programming. We think Val Town can be both useful and fun /shopify-quick-for-all , so it made us smile to see Paul, smiling, demo one last val: a server to pipe messages from the internet to his flipdots https://flipdots.com/en/products-services/small-7-segment-displays/ display in meatspace. Whenever a friend or coworker submits a message to the .val.run site pictured above, the board behind Paul’s desk makes that ft-t-t-t-t-t-t sound as the dots flip into place. With the new MCP spec out last month, Paul and our team have MCP vals for your remixing pleasure like this simple example https://www.val.town/x/templates/mcp-server . And—for a bit of MCP inception—you can connect to our MCP server https://docs.val.town/guides/prompting/mcp/ to get started, or book a demo https://calendar.app.google/uXdbgkdtz9SLzR25A with our founder Steve to get a feel for Val Town. We're hiring Are you an infra engineer who cares about the joy of programming?