withSSEReconnect

Reconnection is opt-in: without this option a failed stream is not retried. Passing undefined clears an earlier setting.

type SSEReconnect = {
  attempts?: number;
  delayMs?: number;
  factor?: number;
  jitter?: number;
  maxDelayMs?: number;
  shouldReconnect?: (context: {
    attempt: number;
    cause?: unknown;
    lastEventId: string;
    method: string;
    open?: {
      response: { status: number; statusText: string; url: string };
      url: string;
    };
  }) => boolean | Promise<boolean>;
};

• attempts is the retry budget for each outage. It defaults to 3 when reconnect is set, and 0 disables retries. A successful handshake resets the budget, callback attempt, and backoff exponent. Omitting it does not give unlimited retries within one outage. Repeated successful opens followed immediately by retryable stream failures can keep reconnecting at the current base delay (subject to jitter and caps), because each open resets the budget; use shouldReconnect to stop them. Normal EOF remains terminal.
• Without shouldReconnect, only GET, HEAD, OPTIONS, PUT, and DELETE reconnect. With it, its result decides for every method, POST included; method in the context tells endpoints apart. Return false to stop for this failure. It runs before the attempts limit is checked.
• A request with a ReadableStream body never reconnects, and shouldReconnect is not called for it.
• delayMs, factor, jitter (0 to 1), and maxDelayMs shape the backoff. factor defaults to 1 and jitter to 0. maxDelayMs caps the final delay after jitter and rounding. Without a configured cap, the platform timer limit of 2_147_483_647 ms still applies. A zero base stays zero; valid-number overflow saturates at the caps.
• delayMs is only the starting base delay (1000 by default). A server retry: field (digits only, in milliseconds) replaces it for later attempts.
• The last event id is sent again as UTF-8 bytes in Last-Event-ID on the next attempt. An empty id: clears it; an ID containing NUL is ignored.
• Reconnect options are validated before interceptors or Fetch: attempts must be a non-negative safe integer; delayMs must be finite and non-negative; maxDelayMs must be a non-negative finite number or positive Infinity, which removes the configured cap but not the platform limit of 2_147_483_647 ms; factor must be finite and positive; jitter must be finite and within 0–1. Invalid values return REQ_OPTIONS_INVALID when the operation runs.