[Bug] /compact fails with "Usage credits required for 1M context" even after /model set to Sonnet 4.6

Bug Description

/compact throws a 1M context error even when the session model is explicitly set to claude-sonnet-4-6. The compaction process internally requests a 1M context model regardless of the user-set session model.

Exact reproduction sequence

/compact   → Error: Usage credits required for 1M context
/login     → Login successful
/compact   → Error: Usage credits required for 1M context
/model     → Set model to Sonnet 4.6
/compact   → Error: Usage credits required for 1M context

Error

Error: Error during compaction: API Error: Usage credits required for 1M context
· run /usage-credits to turn them on, or /model to switch to standard context

Root cause observed

Session transcript shows cache_read_input_tokens reached ~400K, exceeding the standard 200K window. The compaction process apparently tries to use a 1M context model to process this, but the account has service_tier: standard and no usage credits for 1M context. Switching model with /model has no effect because compaction ignores the session model setting.

Expected behavior

/compact should reduce context, not expand it. It should not internally require a 1M context model, or should fall back gracefully to chunked compaction on standard context.

Workaround

/clear — full context loss.

Environment

• Claude Code version: 2.1.156
• Session model: claude-sonnet-4-6 (standard context)
• Trigger: Playwright MCP (large browser snapshots accumulate ~400K cached tokens)
• OS: Linux
• Plan: Pro (standard tier, no usage credits)

Related

• #62063 (1M context triggered without user request)