Session defaults to claude-opus-5[1m] (1M context) which is unavailable to my org, silently falls back and overwrites saved model preference

Environment

• Claude Code version: 2.1.219
• Auth: Claude.ai OAuth (enterprise org), not raw Anthropic API key
• Launch: claude --dangerously-skip-permissions (plain terminal, no --model flag)
• Platform: macOS (Darwin 25.5.0)

What happened

1. Started a new session with no --model flag and no model key set in ~/.claude/settings.json at the time.
2. The session banner reported: "You are powered by the model named Opus 5 (1M context). The exact model ID is claude-opus-5[1m]."
3. On the very first turn, the CLI failed with: "There's an issue with the selected model (claude-opus-5[1m]). It may not exist or you may not have access to it. Run /model to pick a different model."
4. I ran /model, picked Sonnet 5 to unblock, and it was saved as the new default in ~/.claude/settings.json ("model": "sonnet").

Root cause (as far as I can diagnose client-side)

• No env vars (ANTHROPIC_MODEL, CLAUDE_MODEL) and no project/global settings specified a model before this session started.
• No ANTHROPIC_API_KEY present — auth is entirely via the Claude.ai enterprise OAuth session.
• This means the CLI's own default-model resolution logic chose claude-opus-5[1m] (the 1M-context variant of Opus 5) for a brand-new session, without checking whether that specific context-window variant is entitled/available for the authenticated org first.
• The failure then silently persisted "sonnet" as my new global default — overwriting whatever the prior resolved default would have been, as a side effect of an error recovery flow I didn't explicitly opt into.

Expected behavior

• Default model resolution should not select a model/context-variant combination that isn't available to the authenticated account/org, or should validate availability before starting the session.
• If a fallback is required, the CLI should not silently persist that fallback as the user's permanent default in settings.json without an explicit confirmation — it should be scoped to the session, or the user should be asked.

Additional context

Would also like clarity on how the 1M-context Opus 5 variant (claude-opus-5[1m]) is provisioned for enterprise orgs — is it a separate entitlement from base Opus 5, and if so, how can an org admin request/enable it?