Bug: /event replay semantics are implicit and conflict with SSE backpressure handling

[AndresCdo]

Description

When using the /event endpoint with after_seq, replay failures are not clearly signaled to SSE consumers. This makes reconnect/recovery logic inconsistent, especially when clients are slow or reconnecting.

OpenCode version

v1.3.4

Steps to reproduce

1. Start server (opencode serve).
2. Produce some events.
3. Call /event?after_seq=bad.
4. Call /event?after_seq=0 with replay unavailable/expired cursor.

Expected behavior

SSE consumers should receive explicit replay/backpressure control signals so recovery logic is deterministic.

Screenshot and/or share link

N/A (server-side behavior)

Operating System

Linux x64

Terminal

Ghostty

[github-actions]

This issue doesn't fully meet our contributing guidelines.

What needs to be fixed:

• The issue does not use one of the three required issue templates (Bug Report, Feature Request, or Question). Please resubmit using the appropriate template.
• The description reads as AI-generated: it is highly structured with proposed implementation details, specific internal event names, and design recommendations that go well beyond describing a bug. AI-generated walls of text are not acceptable per our guidelines.
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• The environment section lists v1.3.4 baseline which does not appear to be a real version; please include your actual OpenCode version.

Please edit this issue to address the above within 2 hours, or it will be automatically closed.

---

This issue might be a duplicate of existing issues. Please check:

• [FEATURE]: Durable event export / replay surface for external integrations #19567: Durable event export / replay surface — covers cursor-based replay semantics and SSE replay for external integrations
• SSE event streams leak memory on client disconnect #15645: SSE event streams leak memory on client disconnect — related SSE stream lifecycle issues
• [FEATURE]:Support sessionID Filter for SSE Event Subscription #9650: Support sessionID filter for SSE event subscription — touches the same /event route and SSE stream architecture

If you believe this was flagged incorrectly, please let a maintainer know.

[github-actions]

Thank you for updating the issue! It now follows the contributing guidelines and includes all the necessary information. We appreciate the clear description, reproduction steps, and system details.