Challenges with Implementing SSE for AI Agent UIs

The article discusses common issues faced when building real-time streaming UIs for AI agents using Server-Sent Events (SSE), including problems with chunk boundaries, excessive React re-renders, unresolved loading states, and ineffective retry logic.

Details

The article explains that teams building AI agent UIs often write their own SSE client implementations, which tend to hit the same four common bugs. The first issue is with the chunk boundary - when network latency causes the 'event:' and 'data:' lines to arrive in separate chunks, the parser resets the currentEvent variable incorrectly, leading to silent event drops. The solution is to make currentEvent a per-stream variable, not per-chunk. The second bug is the high frequency of React re-renders caused by each token update, leading to visible UI jank. Buffering and flushing token updates at a fixed interval can smooth out the rendering. The third bug is the loading state that never resolves when the server crashes mid-stream, leaving the frontend spinner spinning indefinitely. Synthesizing a 'done' event client-side when the connection closes without one can fix this. The fourth bug is with the standard reconnect logic, which retries on both server errors and network drops, making the situation worse in the case of server errors. The article suggests differentiating between the two types of failures and handling them appropriately.