/model opusplan fails with "Unsupported model" - worked for months until today

Bug: /model opusplan fails with "Unsupported model" — worked for months until today

Claude Code version: 2.1.260
Platform: Windows 11 Home (10.0.26200), running inside the Claude desktop app "Code" tab
Date issue started: 2026-09-04

Description

I've manually run /model opusplan at the start of chats for months (it was never auto-applied from settings.json's default "model": "opusplan" in this environment — I had to type it each time). As of today, /model opusplan returns Unsupported model, with no code or config change on my end.

Steps to reproduce

1. Open a Claude Code session in the Claude desktop app (Code tab) on Windows, CLI v2.1.260.
2. Run /model opusplan.

Expected behavior

Session switches to the opusplan hybrid alias (Opus during plan mode, Sonnet during execution), as documented at https://code.claude.com/docs/en/model-config and as it has behaved for the past several months.

Actual behavior

Unsupported model

What I've already checked

• ~/.claude/settings.json and ~/.claude.json — no availableModels, disallowedModels, or org/policy restrictions on models.
• No local config changes were made before this started failing.
• Official docs (code.claude.com/docs/en/model-config) still list opusplan as a supported alias.
• Plain aliases (/model opus, /model sonnet) work fine — only the opusplan composite alias is rejected.

Suspected cause

Possibly a regression tied to a recent model-lineup rollout (Opus 5 / Sonnet 5 family) where opusplan's internal resolution still points at a deprecated/removed model id, or a version mismatch between the Claude desktop app's bundled Code harness and the standalone CLI.