`subscribeRunEvents` has an unsubscribe-function signature but unconditionally throws

[bornajerk]

  subscribeRunEvents(
    _runId: string,
    _onEvent: (event: unknown) => void
  ): () => void {
    // Placeholder for SSE subscription
    // Future implementation will use EventSource for /api/runs/:id/stream
    console.warn("subscribeRunEvents not yet implemented in realClient")

    throw createApiError(
      ApiErrorCode.NOT_IMPLEMENTED,
      "Real-time streaming not yet implemented. Use polling with refetchInterval instead.",
      generateRequestId("real")
    )
  }

(frontend/src/lib/api/realClient.ts:311-325)

The declared return type () => void promises a teardown function. TypeScript accepts the body because a function that always throws is assignable to anything — so the type system offers no warning here at all.

Trigger: the first developer to wire up live run events writes the natural thing:

useEffect(() => api.subscribeRunEvents(runId, onEvent), [runId])

Observed: the effect throws during commit. React unwinds to the nearest boundary — the app-wide ErrorBoundary at App.tsx:49 — and the whole console is replaced by the error card. Not a console warning, not a degraded panel: a full-screen failure, from opting into a method the interface appears to offer.
Expected: either the method does not exist, or calling it is safe.

Two clean options:

// (a) honest no-op — callers keep working, nothing explodes
subscribeRunEvents(): () => void {
  console.warn("SSE not implemented; falling back to polling")
  return () => {}
}

// (b) remove it, and let the absence be a compile error at the call site

I would take (b). This method is not on the ApiClient interface in frontend/src/lib/api/client.ts:15-55, so nothing typed against ApiClient can even see it — it is reachable only by someone who imports RealApiClient concretely and goes looking. That makes it a trap with no upside. Worth noting the codebase already made the right call elsewhere: every panel polls on a 2s interval instead (e.g. IdeaGenerationPanel.tsx:267), which is exactly what the error message recommends.