Bug: /global/event SSE stream loses events on reconnect (no Last-Event-ID support) #25657

Description

Long-running tasks (multi-agent runs emitting frequent events) silently lose UI updates when the SSE stream drops — proxy idle, browser tab backgrounded, network blip. The server keeps working, the client never catches up.

The browser's native EventSource sends Last-Event-ID automatically on reconnect, but /global/event ignores it and resubscribes fresh. Events emitted during the gap are lost. UI ends up out of sync with server state.

Plugins

oh-my-openagent

OpenCode version

dev (HEAD as of 2026-05-03; reproduces on v1.14.33 too — same code path)

Steps to reproduce

1. Open the web UI; start an agent task that emits frequent events (any subagent run will do).
2. Throttle the network in DevTools or background the tab for ~30s.
3. Restore. UI is missing every event from the gap window — agents continued running on the server, the client just doesn't know about it.

Screenshot and/or share link

n/a — server-side behaviour, observable in any browser hitting /global/event.

Operating System

Linux x64 (server) / macOS 15 (client)

Terminal

n/a — browser-based UI

Note re: #19584 — that issue is about an after_seq query parameter and replay-error semantics, on a different surface. This one is specifically about Last-Event-ID (the standard EventSource reconnect header) being ignored on /global/event. I have a PR ready (#25658) that adds a 1024-entry ring buffer + Last-Event-ID honoring. The same pattern can be applied to /event (instance) in a follow-up.