paperclipai/paperclip · error · SyntaxError

MCP SSE response contained no data events

Error message

MCP SSE response contained no data events

What it means

Protocol-shape failure in initializeMcpHttpSession's SSE reader: the HTTP stream from the MCP server produced SSE events but none carried data lines (and no earlier JSON parse succeeded). Thrown as a SyntaxError after all chunks are consumed, meaning the session could not be initialized from this response.

Source

Thrown at server/src/services/mcp-http.ts:83

    const data = dataLines.join("\n");
    let parsed: unknown;
    try {
      parsed = JSON.parse(data) as unknown;
    } catch (error) {
      lastError = error;
      continue;
    }
    if (!sawData) {
      firstParsed = parsed;
      sawData = true;
    }
    if (looksLikeJsonRpcMessage(parsed)) {
      return parsed;
    }
  }
  if (sawData) return firstParsed;
  if (lastError) throw lastError;
  throw new SyntaxError("MCP SSE response contained no data events");
}

View on GitHub (pinned to 120ae5428f)

Solutions

1. Check the MCP server SSE stream; ensure the server emits data events and the connection is not truncated.

When it happens

Trigger: Thrown at server/src/services/mcp-http.ts:83 when the library encounters an invalid state.

Common situations: See trigger scenarios.

AI-assisted analysis of paperclipai/paperclip@120ae5428f (2026-08-18). Data as JSON: /api/errors/a292b1d4b8027761. Report an issue: GitHub.