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Inline visual replies: sandboxed HTML pages and MCP app UIs in the thread #1525
Description
Replies can render Mermaid (src/components/Markdown.tsx:180), but raw HTML is dropped. MCP servers that return UI resources (ui://, the MCP Apps extension) show nothing. Agents can't show a chart, a prototype or an interactive table inline.
Proposal (M):
- An
htmlfence (or an agent tool that publishes a page) renders in an<iframe sandbox="allow-scripts" srcdoc=…>with no same-origin access, no network by default (strict CSP), a size cap and an "open in browser" escape hatch. - MCP tool results that carry a
ui://resource render through the same frame, with the MCP Apps postMessage bridge so the UI can call back into its server via the host. - Lazy-load it and keep it off the streaming hot path. Run
npx vite buildbefore shipping.
Security review is required: the frame must not reach the preload bridge or the local web server token.
Activity
currentbits commentedon Oct 11, 2026
Implementing on this thread.
Plan
htmlfence (same lazy-chunk pattern as mermaid):iframe sandbox="allow-scripts"+ srcdoc, strict CSP (connect-src 'none', noallow-same-origin), 200KB cap, Open-in-browser as a download of the CSP-wrapped page (never a same-origin blob window — that would inherit the preload bridge).ui:///text/htmlresources from tool results before the 4000-char output truncate (same harvest path as tool images), store onToolCallInfo.html, render through the same frame.event.source === iframe.contentWindow; answerui/initializewithoutserverTools/serverResources(host has no MCP client yet); rejecttools/call/resources/readwith JSON-RPC error.!live) and lazy-load the chunk.npx vite buildbefore shipping.Follow-up (out of scope): a host-side MCP client so the frame can proxy
tools/callback to the server.currentbits commentedon Oct 11, 2026
Implemented on this branch (not merged).
Shipped
```htmlfences render in<iframe sandbox="allow-scripts" srcdoc>with strict CSP (connect-src 'none', noallow-same-origin), 200KB cap, Open-in-browser as a download of the CSP-wrapped page (not a same-origin blob window).HtmlPage(~1.2kB); off the streaming tail (!live).npx vite buildsucceeded.ui:///text/htmlresources harvested before the 4000-char output truncate ontoToolCallInfo.html; same frame +ui/initializehandshake.tools/call/resources/readare refused (no host MCP client).markdownChunksnow ignores<inside fences so a closed html fence can settle while the reply still streams.Security notes for review
window.coder(opaque origin).allow-same-originorconnect-src 'self'.Follow-up suggested: host MCP client so Apps can call tools back.
currentbits commentedon Oct 11, 2026
PR: #1597
Render sandboxed HTML pages and MCP app UIs in the thread (#1525) (#1597
Render sandboxed HTML pages and MCP app UIs in the thread (#1525)