Web UI timeout over tail scale

[eytans]

Description

When working through tail scale vpn to connect to the web interface, repeated prompts (2 are enough, as long as the first one is long enough) lead to "failed to fetch" error on prompt send. The prompts wont appear until page is manually refreshed.
The error is given due to a synchronous api that shouldn't have been used already discovered in a previous issue (but that one is an AI text wall, and since I encountered this myself I am recreating an issue).

Related issues: #7990 #12453

Plugins

None

OpenCode version

1.1.53

Steps to reproduce

1. Run web through a "greedy" network stack (tail scale)
2. Run a task or two that run for more then 30 seconds or so.
3. Next attempt to start a task will raise a "failed to fetch" or similar message

Screenshot and/or share link

No response

Operating System

Ubuntu 25.10

Terminal

Web (through android/brave)

[github-actions]

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

• Web UI: "Failed to send prompt" false alarm on mobile browsers — prompt succeeds but response stream breaks #12453: Web UI 'Failed to send prompt' false alarm on mobile browsers — prompt succeeds but response stream breaks (almost identical issue for mobile)
• Connection on a remote server over Tailscale SSH hangs #8415: Connection on a remote server over Tailscale SSH hangs (Tailscale-specific timeout issues)
• iOS Safari/Firefox: session history rolls back after backgrounding until refresh + error toasts on resume #10721: iOS Safari/Firefox session history rollback after backgrounding with error toasts (VPN-like network conditions causing SSE drops)
• OpenCode hangs/freezes mid-stream on Starlink satellite internet connection #8002: OpenCode hangs/freezes mid-stream on Starlink satellite internet connection (unstable network causing mid-stream freezing)
• [WEB-UI] No new session can be started #12627: Web UI no new session can be started with 'failed to fetch' errors (identical symptoms and refresh workaround)
• opencode web: child session created with ?directory= returns empty body on POST /session/{id}/message #12271: opencode web child session returns empty body on POST /session/{id}/message (empty HTTP responses in Web UI)

Also related: #7990 (Poor mobile web experience), #11289 (synchronous API issues in handlers), #10077 (SSE client architecture)

Feel free to ignore if none of these address your specific case.