fix(webmcp): survive a host whose modelContext is not an EventTarget

[ribdsp]

The bug

Traces was completely down in ChatGPT Desktop's in-app browser — not degraded, replaced:

Application error: a client-side exception has occurred while loading traces.ribdsp.com

Uncaught TypeError: e.addEventListener is not a function
    at layout-7f4736507f1b2029.js:1:70534

That is the browser the challenge rules name first for judging: "Judges may test WebMCP tools using ChatGPT's in-app browser or Google Chrome with WebMCP enabled."

Root cause

The spec declares interface ModelContext : EventTarget, so src/types/webmcp.d.ts typed addEventListener as present and two components subscribed to toolchange without a guard:

• src/components/ui/tool-status-banner.tsx — mounts first, so this is the one that actually threw
• src/components/ui/webmcp-badge.tsx — never reached, but the same fault

ChatGPT Desktop's document.modelContext has no event methods. Thrown from a client effect, React escalates to the nearest error boundary; in a production build that is Next.js's global one, so a missing optional feature took the whole page.

Why nothing caught it: lib/webmcp/polyfill.ts builds its shim as Object.assign(new EventTarget(), …), so the call is always valid in polyfill mode. The failure is only reachable on a native host — and only on a native host that does not honour that part of the spec.

The fix

One helper, src/lib/webmcp/tool-change.ts, and both call sites go through it. It guards on typeof, wraps the subscribe, and returns a no-op unsubscribe when there was nothing to subscribe to — guarded and wrapped, because the lesson is not "this one method was missing", it is that a draft API on a host we do not control must never be able to take the page down.

Absence is a console.warn, once per page load, and silent in the UI. Both callers already render a real tool list from getTools() or from the registration result; all that is lost is live updates when the surface changes shape mid-session.

The type keeps extends EventTarget. It is what the spec says, the polyfill genuinely is one, and weakening it to Partial<EventTarget> would break shim.dispatchEvent inside our own polyfill. A docstring above the interface points readers at the helper instead.

Tests

Four new cases in tool-change.test.ts:

Case	Asserts
Host with no event methods	no throw, unsubscribe safe to call, listener never fired, warns
Spec-shaped EventTarget host	event delivered, and not delivered after unsubscribe
addEventListener that throws	no throw, warns
Two subscribers, same fault	warns once, not four times under React 19 double-invoke

A version of onToolChange that propagates the original TypeError passes none of them.

Verification

npx tsc --noEmit   clean
npm test           24 files, 293 passed (was 289)

Still to confirm after deploy: whether registerTool and getTools work on that host. The banner throwing first means we never got to read what it said, so the answer is still unknown — this PR is what makes it readable.

Scope

Five files, all inside the WebMCP surface. tailwind.config.ts, globals.css, src/types/domain.ts and next.config.mjs untouched.

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Project	Deployment	Actions	Updated (UTC)
traces	Ready	Preview	Aug 28, 2026 9:03pm