How to Fix Writablestream Error in Next.js

WritableStream Errors in Next.js

Next.js encounters WritableStream issues in streaming SSR, route handlers that return streams, and edge runtime functions.

When It Happens

• Streaming responses in route handlers
• Edge runtime WritableStream differences from Node.js
• Client disconnecting mid-stream during SSR

How to Fix

Handle stream lifecycle properly in route handlers:

// app/api/stream/route.ts
export async function GET() {
  const encoder = new TextEncoder();
  let controller: ReadableStreamDefaultController;

  const stream = new ReadableStream({
    start(c) {
      controller = c;
    },
    cancel() {
      // Client disconnected — clean up
      console.log('Client disconnected');
    }
  });

  // Write data asynchronously
  (async () => {
    try {
      for (let i = 0; i < 10; i++) {
        controller.enqueue(encoder.encode(`data: ${i}\n\n`));
        await new Promise(r => setTimeout(r, 500));
      }
      controller.close();
    } catch (e) {
      // Stream may be cancelled if client disconnected
      if (!String(e).includes('cancelled')) {
        controller.error(e);
      }
    }
  })();

  return new Response(stream, {
    headers: { 'Content-Type': 'text/event-stream' },
  });
}

Always handle the cancel callback and catch errors when writing — the client may disconnect at any time.

Production Hardening

Beyond the immediate fix, consider adding circuit breakers and graceful degradation for this failure mode. Log structured error data so your observability stack can correlate this error with upstream causes. Set up dashboards to track error rates over time and catch regressions early.

Bugsly for Next.js

Bugsly distinguishes between expected stream cancellations (client navigated away) and unexpected stream errors, reducing noise in your error dashboard.