[Bug]: OpenCode sessions fail after upgrading or downgrading the OpenCode binary - driver chosen from stale version #27758

Before submitting

• I searched existing issues and did not find a duplicate.
• I included enough detail to reproduce or investigate the problem.

Area

apps/server

Steps to reproduce

1. Install OpenCode 2.x (tested 2.2.4 via curl -fsSL https://opencode.ai/v2/install | bash), log in with opencode auth login (GitHub Copilot).
2. Enable the OpenCode provider in T3 Code settings, leaving Server URL blank so T3 spawns the server. The provider shows "Authenticated", v2.2.4.
3. Start a thread on any OpenCode model (e.g. github-copilot/...) and send a message.

Expected behavior

T3 spawns opencode serve, verifies the version, and the turn runs.

Actual behavior

The run fails after ~7s with "Provider session failed to open".

T3 spawns opencode serve 5 times, and SIGTERMs each ~70ms after it reports server listening. The failing span is opencode.global.health inside verifyOpenCodeServerVersion (called from OpenCodeRuntime.startServerAttempt):

OpenCodeRuntimeError
  at opencode.global.health
  at verifyOpenCodeServerVersion
  at OpenCodeRuntime.startServerAttempt
  at OpenCodeRuntime.startOpenCodeServerProcess
  at OpenCodeAdapterV2.openSession
  [cause]: Error: Request is not supported by this version of OpenCode Server (Server responded with text/html)

OpenCode 2.x has no /global/health route. The request falls through to the web UI and returns 200 text/html, which the SDK response interceptor turns into the error above. I checked 2.0.18, 2.0.32, 2.1.0, 2.1.4, 2.2.0, 2.2.3 and 2.2.4: all return HTML for /global/health. 1.19.5 returns {"healthy":true,"version":"1.19.5"}.

The settings-page probe (probeOpenCodeServer) tries /api/info before /global/health, so the provider reports healthy and authenticated while every session fails. The compatibility manifest marks >=2.0.18 as supported and recommends it, and the settings warning on 1.x says to upgrade to it, which leads users straight into this failure.

Impact

Blocks work completely

Version or commit

T3 Code (Beta) 0.1.5 desktop. OpenCode 2.2.4.

Environment

macOS 26 (Darwin 25.6.0), Apple Silicon. OpenCode installed to ~/.opencode/bin, on PATH via ~/.local/bin symlink. Local spawned server (Server URL blank).

Logs or stack traces

See the stack trace under Actual behavior (from server.trace.ndjson).

Screenshots, recordings, or supporting files

No response

Workaround

Downgrade to OpenCode 1.x (curl -fsSL https://opencode.ai/install | bash). T3 shows a "Limited support" warning, but 1.19.5 passes the /global/health check.