[Bug]: t3 start exits at startup when the welcome thread's model is on a disabled provider #20962

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. Disable Codex and leave no default model set (or set the default model to any disabled instance). A clean data directory with only this settings.json is enough:

   { "providers": { "codex": { "enabled": false } } }

2. From an empty directory, start the server in web mode, the t3 / npx t3 default:

   t3 start --base-dir <that data directory> --no-browser

Expected behavior

The server starts. If the welcome thread cannot use the fallback model, the project is still created and the server starts without a welcome thread, or uses an enabled provider.

Actual behavior

The server binds its port, then exits before it answers a request:

INFO: Listening on http://127.0.0.1:38774
ERROR: server runtime startup failed
  cause: 'ThreadManagementProviderInstanceDisabledError: Provider instance "codex" is disabled for project 39d35635-....
ERROR: ServerRuntimeStartupError: Server runtime startup failed before command readiness.

Web mode turns on autoBootstrapProjectFromCwd by default. When the working directory has no thread yet, resolveAutoBootstrapWelcomeTargets (serverRuntimeStartup.ts) launches a welcome thread with the project's default model, falling back to getAutoBootstrapThreadModelSelection(), which is always the codex instance. ThreadLaunchService.launch rejects a disabled instance, and that failure ends server.startup.

With Codex enabled, the same steps start the server and /.well-known/t3/environment answers 200. A default model on an enabled provider also avoids it, and so does T3CODE_AUTO_BOOTSTRAP_PROJECT_FROM_CWD=false. A default model on a disabled instance fails the same way: with Claude disabled and defaultModelSelection on claudeAgent, startup ends with Provider instance "claudeAgent" is disabled.

Impact

Blocks work completely

Version or commit

main @ 13aa882

Environment

Linux, Node 26.10.0, node apps/server/src/bin.ts start from a worktree of main

Workaround

Enable Codex, set a default model on an enabled provider, or start with T3CODE_AUTO_BOOTSTRAP_PROJECT_FROM_CWD=false.