Have we reached peak Markdown? A developer built a sandboxed HTML environment that runs Markdown code blocks directly inside documentation, bundling Three.js, D3 and Mermaid and giving an agent named Astra access over MCP, which produced a playable single-level first-person shooter called "THE LAST FURNACE." The post catalogs the load-bearing security footguns encountered while sandboxing agent-generated code, and the author notes the game would have impressed him a year ago but now only makes him want to apologize to Dan Greenheck. Have we reached peak Markdown? This blog post? Written in Markdown. The below first-person shooter, depicting a possessed furnace, in a style similar to, but legally distinct from: DOOM? Yes. Also Markdown. We thought it would be neat if we could actually run Markdown code blocks in a sandbox environment directly inside your docs. That way, agents could write full-blown apps in html fences inside your design documents or blog posts, and you could illustrate concepts your readers could actually interact with. Or create pointless slop games that surpass my coding ability by 100x, in a span of 5 minutes. We bundled Three.js, D3 and Mermaid into a pre-packaged HTML sandbox environment, and gave the keys to Astra over MCP. It created a single-level game that, if not quite AAA-worthy, would have deeply impressed me a year ago. Today, it just makes me want to apologize to Dan Greenheck https://x.com/dangreenheck/status/2102137910921240807 . Sandboxing is a tricky thing to get right though, and we encountered plenty of load bearing footguns along the way. We present them below for the benefit of the reader. But first, may I present THE LAST FURNACE.