Add nansen mcp install/verify for local MCP clients (Cursor first) - #511

src/commands/mcp.js — medium

Malformed JSON response from MCP server produces a misleading "endpoint unreachable" error

In verify, the call to parseMcpResponseBody (lines 179–183) lives inside the try block that wraps the fetch. When the server returns a well-formed HTTP 200 but with a body that isn't valid JSON (or isn't valid SSE), parseMcpResponseBody throws a SyntaxError. That SyntaxError is not a NansenError, so the instanceof NansenError guard on line 185 does not re-throw it — instead it falls through to:

const reason = error.name === 'AbortError'
  ? `timed out after ${timeoutMs}ms`
  : (error.cause?.code || error.message);
throw new NansenError(`MCP endpoint unreachable: ${url} (${reason})`, ErrorCode.NETWORK_ERROR);

The result is a message like MCP endpoint unreachable: https://mcp.nansen.ai/ra/mcp (Unexpected token '<', "<!DOCTYPE "... is not valid JSON). The endpoint was reachable — it just returned garbage. This will confuse users (and agents) into diagnosing a network problem when the real issue is a bad server response.

Fix: catch parseMcpResponseBody errors separately, or add a guard before the generic wrap:

} catch (error) {
  if (error instanceof NansenError) throw error;
  if (error instanceof SyntaxError) {
    throw new NansenError(
      `MCP handshake failed: ${url} returned an unparseable response (${error.message})`,
      ErrorCode.UNKNOWN
    );
  }
  const reason = error.name === 'AbortError'
    ? `timed out after ${timeoutMs}ms`
    : (error.cause?.code || error.message);
  throw new NansenError(`MCP endpoint unreachable: ${url} (${reason})`, ErrorCode.NETWORK_ERROR);
}