SSE event stream silently drops, leaving clients hanging forever #28729

Bug

The SSE event stream at /event?directory=... can silently drop without sending TCP FIN, causing HTTP clients using ReadableStream (like Node.js fetch().body.getReader()) to hang forever on reader.read() — no error, no {done: true}.

This occurs mid-prompt: OpenCode continues processing internally (tokens generated visible in session API), but events stop flowing to connected SSE clients.

Root Cause

In event.ts, the SSE stream pipeline:

events.pipe(Stream.merge(heartbeat, { haltStrategy: "left" }))

When any error occurs in eventData (JSON.stringify failure on BigInt, circular refs, etc.), Effect-TS interruption mechanism cancels the stream. Bun then cancels the ReadableStream WITHOUT calling controller.close() or sending the final zero-length chunk (0\r\n\r\n) for chunked transfer encoding. The TCP connection stays open with no data flowing. Node.js reader.read() interprets this as idle and blocks forever.

Fix Suggestions

1. Wrap JSON.stringify in try/catch in eventData() — catch serialization errors and emit an error event instead of crashing the pipeline
2. Change haltStrategy from "left" to "either" — so heartbeat continues even if event stream errors
3. Always send [DONE] or proper TCP close on stream termination, error or clean

Reproduction

Run OpenCode with a model that produces large nested tool call results (subagent tasks). After some time (1-5 minutes into a prompt), the SSE stream stops. OpenCode session API shows continued token generation. Client hangs indefinitely.

Environment

• OpenCode 1.15.7 (Bun runtime)
• Client: Node.js fetch() ReadableStream reader
• Connection: localhost, no proxies involved