Debugging a black box: 36 renders against Claude, and the part where my own data was wrong

If you've built an MCP App — the HTML widget an MCP server hands a host to render inline — you may have hit this: the tool call succeeds, structuredContent comes back fine, the model announces that a widget rendered, and the user sees nothing. No error. No console output. Just a gap in the conversation.

There's a long issue full of people with this exact symptom, all of them (me included) posting variations of "my server is spec-correct and nothing renders." That's a hard thing to act on. So I built a probe server designed to answer one question at a time and ran it 36 times.

This post is mostly about method — how you experiment on a host whose source you can't read and whose renderer you can't attach a debugger to. The MCP specifics are the worked example. The most useful part is at the end, where my measurements lied to me twice.

Everything behavioural here was measured on 31 July 2026 against claude.ai web. Host behaviour changes; treat the numbers as a snapshot, not a spec.

The finding, up front