The Nginx proxy_pass buffer size that truncated streaming payloads

When introducing a Server-Sent Events (SSE) log tailing endpoint, browser clients reported that log events arrived in sporadic batches of 4KB chunks rather than streaming continuously line by line.

location /api/v1/stream {
    proxy_pass http://upstream_backend;
    
    # Disable proxy buffering specifically for SSE connections:
    proxy_buffering off;
    proxy_cache off;
    proxy_set_header Connection '';
    proxy_http_version 1.1;
    chunked_transfer_encoding off;
}

By default Nginx buffers upstream responses until proxy_buffer_size (usually 4KB or 8KB) is filled. Turning off buffering for the streaming path allows PHP output flushes (ob_flush(); flush();) to reach the client with sub-millisecond delivery.