[BUG]: Free tier (Muse Spark 1.3 Free) fails with 'can only be used from within OpenCode' when using MonoCode frontend with OpenCode backend

Description

Using MonoCode (desktop UI for coding agents, latest release from hardbeat920/monocode) as frontend with OpenCode backend (opencode auth login done, backend working), any request with free-tier model Muse Spark 1.3 Free fails after ~55s with:

Error from provider (Console): OpenCode's free tier can only be used from within OpenCode

Same backend works from OpenCode TUI/Desktop. This looks like the free-tier client gate described in #49433 (User-Agent opencode/<semver> + x-opencode-session: ses_... checks), but for a different third-party client (MonoCode instead of pi). There is no doc stating free tier is official-client-only, and MonoCode is documented as "Works with your OpenCode login - if OpenCode is installed and logged in, MonoCode can run it".

Related but distinct: #49433 (detailed UA/session gate analysis for pi + -git builds), #49579 / #49438 (same error message, no client details).

Request: clarify if free tier is intended to work from third-party UIs that drive the local OpenCode backend, and if so what headers/client contract they must send. If official-client-only, please state it in docs.

Plugins

No response

OpenCode version

1.18.31 (opencode --version)

Steps to reproduce

1. opencode auth login (logged in, free tier available)
2. Install MonoCode latest, select OpenCode as provider/backend
3. Start session with Muse Spark 1.3 Free (Console provider)
4. Send prompt, wait ~55s -> Error from provider (Console): OpenCode's free tier can only be used from within OpenCode

Screenshot and/or share link

Error text from UI (same as screenshot in #49579):
Muse Spark 1.3 Free worked for 55s — Status update — Error from provider (Console): OpenCode's free tier can only be used from within OpenCode

Operating System

Linux CachyOS 7.2.5-1-cachyos x86_64

Terminal

MonoCode desktop app (Tauri) driving local OpenCode backend