Function fetchServerSentEvents

packages/ai-client/src/connection-adapters.ts:490–538

(
  url: string | (() => string),
  options:
    | FetchConnectionOptions
    | (() => FetchConnectionOptions | Promise<FetchConnectionOptions>) = {},
)

 * ```
 */
export function fetchServerSentEvents(
  url: string | (() => string),
  options:
    | FetchConnectionOptions
    | (() => FetchConnectionOptions | Promise<FetchConnectionOptions>) = {},
): ConnectConnectionAdapter {
  return {
    async *connect(messages, data, abortSignal, runContext) {
      // Resolve URL and options if they are functions
      const resolvedUrl = typeof url === 'function' ? url() : url
      const resolvedOptions =
        typeof options === 'function' ? await options() : options

      const requestHeaders: Record<string, string> = {
        'Content-Type': 'application/json',
        ...mergeHeaders(resolvedOptions.headers),
      }

      // Build AG-UI RunAgentInput payload.
      //
      // Precedence (later spreads win): static adapter `body` is the base,
      // overridden by `runContext.forwardedProps` (constructor body /
      // forwardedProps options), overridden by per-message `data` passed
      // to `connection.send`. Runtime values win over static config —
      // this matches the documented "forwardedProps wins" semantic.
      const requestBody = buildRunAgentInputBody(
        messages,
        data,
        runContext,
        resolvedOptions,
      )

      const fetchClient = resolvedOptions.fetchClient ?? fetch
      // `RequestInit.signal` is typed `AbortSignal | null` (no `undefined`
      // under `exactOptionalPropertyTypes`), so spread it conditionally
      // rather than passing `undefined` explicitly.
      const signal = abortSignal || resolvedOptions.signal
      const response = await fetchClient(resolvedUrl, {
        method: 'POST',
        headers: requestHeaders,
        body: JSON.stringify(requestBody),
        credentials: resolvedOptions.credentials || 'same-origin',
        ...(signal ? { signal } : {}),
      })

      yield* responseToSSEChunks(response, abortSignal)
    },
  }
}

/**
 * Create an HTTP streaming connection adapter (for raw streaming without SSE format)