fix: Recover widget structuredContent stripped by Claude Desktop

[jirispilka]

Why

Claude Desktop (Linux + macOS, Claude/1.22209.0 and macOS 1.18286.2 per upstream report) strips structuredContent and _meta from the ui/notifications/tool-result notification delivered to MCP Apps views — the view receives only content/isError. Upstream: modelcontextprotocol/ext-apps#696. Our widgets render exclusively from structuredContent, so on Claude Desktop every widget stayed on loading skeletons forever. claude.ai, ChatGPT and MCP Jam deliver the field intact and are unaffected.

Wire-level evidence from an in-widget tap (Desktop): raw:tool-result[content,isError] — the field is already absent in the postMessage payload, so no client-side parsing tweak can see it.

What changed

Before: widgets set toolResult straight from the notification; missing structuredContent meant nothing to render.

Now mcp-app-context.tsx recovers, in order:

1. Parse content[0] as JSON — the run-widget tools (call-actor-widget, get-actor-run-widget) mirror structuredContent there, so recovery is free and never re-executes anything.
2. Re-call the tool via the host tools/call proxy (Desktop returns the full CallToolResult intact on that path — verified). Only for read-only tools, declared per widget entry via refetchToolForArgs (search-actors-widget, fetch-actor-details-widget); never wired for run-starting tools, so no double runs.

Both paths sit behind if (!structuredContent && !isError) — a no-op wherever the host delivers the field. Temporary by design: when ext-apps#696 is fixed, grep -r "ext-apps#696" src/ finds every line to delete.

Notes for reviewers (human-written)

This was really painful :(

Proof it works

• Claude Desktop (Linux, Claude/1.22209.0), before: all four widgets stuck on skeletons; in-widget trace shows tool-result[sc:MISSING].
• After: search widget trace shows tool-result[sc:MISSING] → refetched-via-tools/call[sc:ok] and actor cards render; run widgets recover via the content[0] JSON mirror. All four widget tools manually verified working on Desktop by @jirispilka.
• Host-strip diagnosis reproduced with a minimal probe server (ext-apps SDK registerAppTool/registerAppResource): notification arrives keys=[content,isError], while the same tool called through the host proxy returns keys=[content,structuredContent].
• Pre-release gate: verify in ChatGPT against staging (?ui=openai) — expect identical rendering and no second tools/call in the server log (the guard makes the recovery inert when structuredContent is delivered).

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