n8n-io/n8n · error · Error

SSE connection failed (${res.status}): ${text}

Error message

SSE connection failed (${res.status}): ${text}

What it means

The SSE client opens a streaming connection to /rest/instance-ai/events/<thread> with cookie auth. If the response is not 2xx, it reads the body and throws with status+text so the caller knows why the stream never opened (typically auth, thread-not-found, or server error).

Source

Thrown at packages/@n8n/instance-ai/evaluations/clients/sse-client.ts:40

 * the response is not a valid SSE stream.
 */
export async function consumeSseStream(
	url: string,
	cookie: string,
	handler: (event: SseEvent) => void,
	signal: AbortSignal,
): Promise<void> {
	const res = await fetch(url, {
		headers: {
			cookie,
			Accept: 'text/event-stream',
		},
		signal,
	});

	if (!res.ok) {
		const text = await res.text();
		throw new Error(`SSE connection failed (${res.status}): ${text}`);
	}

	if (!res.body) {
		throw new Error('SSE response has no body');
	}

	const reader = res.body.pipeThrough(new TextDecoderStream()).getReader();

	// Accumulator for partial lines (SSE data can be split across chunks)
	let buffer = '';

	// Current event being assembled
	let eventId: string | undefined;
	let eventType: string | undefined;
	let dataLines: string[] = [];

	try {
		while (true) {

View on GitHub (pinned to 5ac6606e81)

Solutions

1. Ensure client.login() ran and the cookie is valid before opening the SSE stream.
2. Confirm the threadId exists (await ensureThread) before subscribing to its events.
3. Retry on 5xx with backoff; do not retry on 4xx without fixing the precondition.

if (!client.sessionCookie) await client.login();
await client.ensureThread(threadId); // ensure thread exists before SSE

try { await openSse(url, cookie, signal, onEvent); }
catch (e) {
  if (e instanceof Error && e.message.startsWith('SSE connection failed')) { /* classify status, retry only on 5xx */ }
  else throw e;
}

When it happens

Trigger: Cookie missing/expired (401/403); threadId that doesn't exist (404); instance-ai events endpoint disabled (404); server error (5xx).

Common situations: Reading the cookie before login; race where the thread is created async and SSE starts before it commits; instance-ai feature flag off.

Related errors

• Failed to authenticate with n8n — no session cookie received
• Not authenticated — call login() first
• MCP api-key rotate endpoint returned no apiKey
• MCP api-key rotate endpoint returned a redacted key — cannot
• Invitation accepted but no session cookie received

AI-assisted analysis of n8n-io/n8n@5ac6606e81 (2026-08-12). Data as JSON: /api/errors/dcc0684613061b24. Report an issue: GitHub.