Session blocked by "Usage credits required for 1M context" error despite Sonnet model and <50% context usage

Summary

A new Claude Code session becomes completely blocked by the 1M context credits error even though:

• Model is set to Sonnet 4.6 (standard 200k context, no 1M involved)
• Context usage is well under 200k
• /model switching (both session-only and default) does not resolve it

This has happened twice in the same day, in different sessions.

Steps to reproduce

1. Start a new Claude Code session (macOS)
2. Session errors on every message sent

Observed behaviour

Every message returns:

⏺ API Error: Usage credits required for 1M context · run /usage-credits 
  to turn them on, or /model to switch to standard context
✻ Cooked for 0s

First occurrence — /context showed 122k/200k tokens (61%), model Sonnet 4.6.

Second occurrence — /context showed:

Sonnet 4.6
claude-sonnet-4-6          ← two model bars (same model listed twice)
100k/200k tokens (50%)

System prompt:  6.5k tokens
System tools:  14.2k tokens
Memory files:   2.1k tokens
Skills:         1.4k tokens
Messages:      77.9k tokens
Free space:    64.8k (32.4%)
Auto-compact window: 200k tokens

In both cases, /model showed Sonnet 4.6 with ✔. Running /model → Sonnet → Enter ("saved as default") and again with s ("for this session only") both produced the confirmation message but the error persisted on the next message.

Expected behaviour

At 50–61% context usage with Sonnet 4.6 selected, no 1M context upgrade should be attempted and no usage-credits gate should fire.

Additional notes

• The double model bar in /context ("Sonnet 4.6" + "claude-sonnet-4-6") may indicate corrupted session model state
• Error fires at 0s ("Cooked for 0s") — before any inference, suggesting a session initialization issue rather than a mid-turn overflow
• /model switching does not fix the current session — only starting a new session resolves it
• Workaround: kill the session, start a new one

Environment

• Platform: macOS Darwin 25.5.0
• Shell: zsh
• Model configured: "model": "sonnet" in global settings.json