[Bug] Sonnet 4.6 sessions: auto mode not available via any entry path (UI, flag, settings), while Opus 4.7 in same install works

Bug Description

In the same Claude Code install, auto mode is available on Opus 4.7 sessions but completely unreachable on Sonnet 4.6 sessions — not just missing from the Shift+Tab cycle, but also ignored when passed via --permission-mode auto or set via permissions.defaultMode: "auto" in settings.json.

Per docs (https://code.claude.com/docs/en/permission-modes) auto mode isn't formally supported on Pro/Max. However issue #49653 confirms Max accounts do receive auto mode through the startup path on Opus. This report is about the model-specific gap: the same Max path that works on Opus does not work on Sonnet.

Environment

• Claude Code: 2.1.117
• OS: macOS 26.1 (Darwin 25.1.0)
• Shell: zsh
• Plan: Max (auto mode working on Opus 4.7 sessions in this install)
• Relevant settings:

``json
{
"permissions": { "defaultMode": "auto" },
"skipAutoPermissionPrompt": true,
"effortLevel": "xhigh"
}
``

Steps to Reproduce

A. Model switch mid-session (Opus → Sonnet)

1. Start session: claude (Opus 4.7, auto mode active ✅)
2. /model sonnet → system emits ## Exited Auto Mode
3. Shift+Tab cycle: auto option is gone

B. Fresh Sonnet session via CLI flag

1. claude --model sonnet --permission-mode auto
2. Auto mode is not in Shift+Tab cycle and indicator does not show "auto mode on" (silently downgrades, same symptom as #42648's Haiku case)

C. Fresh Sonnet session via settings

1. With permissions.defaultMode: "auto" in ~/.claude/settings.json
2. claude --model sonnet
3. Auto mode does not activate on startup (whereas identical config activates it on Opus)

All three paths fail. Restart back to Opus → auto mode returns.

Expected

Either:

• A: Auto mode should work on Sonnet 4.6 in the same paths that succeed on Opus 4.7 (symmetry across supported models per #42648 which states "Sonnet 4.6, Opus 4.6+" as supported).
• B: If Sonnet is intentionally excluded on Max, surface a clear message at startup ("auto mode requires Opus on Max plan" or similar) — current silent downgrade is identical to the UX issue filed in #42648.

Actual

Silent downgrade with no indication of why. User-facing signal is only "auto is missing from Shift+Tab cycle" + one-shot ## Exited Auto Mode message on model switch.

Related

• #49653 — Shift+Tab auto disappears after plan-approval on Opus (similar end-state, different trigger)
• #42648 / #43235 — Auto mode silently unavailable on Haiku (claims Sonnet 4.6 is supported, contradicting this report)
• #46616 — Max plan auto mode gating inconsistency
• #49273 — defaultMode:"auto" not activating at session start (Max + Opus 4.7)

Additional Context

The contradiction with #42648 (which states Sonnet 4.6 is a supported model for auto mode) suggests either (a) a regression on Sonnet 4.6 in recent versions, or (b) Max-plan-specific model gating that's not documented.