Set ANTHROPIC_SMALL_FAST_MODEL to Control Background Task Performance

Claude Code quietly uses a secondary, lightweight model for background tasks — things like generating conversation titles, summarising memory, and other housekeeping operations. By default this is Claude Haiku, but you can override it with ANTHROPIC_SMALL_FAST_MODEL.

# Use a specific Haiku version for all background tasks
export ANTHROPIC_SMALL_FAST_MODEL="claude-haiku-4-5-20251001"

# Or set it inline for a single session
ANTHROPIC_SMALL_FAST_MODEL="claude-haiku-4-5-20251001" claude

You can also set it permanently in your shell profile:

# ~/.zshrc or ~/.bashrc
export ANTHROPIC_SMALL_FAST_MODEL="claude-haiku-4-5-20251001"

Why change it? A few reasons:

• Cost control: Pin to the most economical Haiku version to keep background task costs predictable as new models are released
• Speed: If background operations feel slow, a lighter model sharpens them up
• Team consistency: When routing through a team proxy or Bedrock/Vertex, pin a specific model string that your provider has explicitly enabled

Note: this variable only affects background/housekeeping operations. The primary reasoning model is controlled separately via --model, ANTHROPIC_MODEL, or settings.json.

Background tasks should be fast and cheap — pin ANTHROPIC_SMALL_FAST_MODEL so they always are.