CLAUDE_CODE_OPUS_4_6_FAST_MODE_OVERRIDE

Updated October 13, 2026 by Alex Sorokoletov

CLAUDE_CODE_OPUS_4_6_FAST_MODE_OVERRIDE is a Claude Code environment variable: Pin Fast mode to Opus 4.6. Fast mode upgraded to Opus 4.7 by default in v2.1.142. If you're evaluating 4.7 or have a reproducibility requirement — benchmarks, cost comparison — this lets you run the old default explicitly. Remove it once you've validated 4.7 behavior. Added between v2.1.138 and v2.1.143 and still present in v2.1.294. It is listed in the official reference.

How to set it

For one session, export it in the shell that starts Claude Code:

export CLAUDE_CODE_OPUS_4_6_FAST_MODE_OVERRIDE=<value>
claude

To apply it to every session, add it to the env object of a settings file:

// ~/.claude/settings.json
{
  "env": {
    "CLAUDE_CODE_OPUS_4_6_FAST_MODE_OVERRIDE": "<value>"
  }
}

Release notes that mention it

• Claude Code v2.1.143 — 27 New Env Vars, /goal, Agent View, and a Free Model Upgrade
• Claude Code Version Tracker: Undocumented Env Vars, Feature Gates & Models

Related variables

• CLAUDE_CODE_ENABLE_OPUS_4_7_FAST_MODE
• CLAUDE_CODE_DISABLE_FAST_MODE: Disable the /fast toggle that uses the same model with faster output.
• CLAUDE_CODE_SKIP_FAST_MODE_NETWORK_ERRORS: Controls whether Claude Code suppresses or bypasses network errors that occur specifically in fast mode.
• CLAUDE_CODE_SKIP_FAST_MODE_ORG_CHECK: Disables the organization-level eligibility check for fast mode.
• ANTHROPIC_DEFAULT_OPUS_MODEL: Override Opus model ID.
• ANTHROPIC_DEFAULT_OPUS_MODEL_DESCRIPTION
• ANTHROPIC_DEFAULT_OPUS_MODEL_NAME
• ANTHROPIC_DEFAULT_OPUS_MODEL_SUPPORTED_CAPABILITIES

Frequently asked questions

All variables, searchable: Claude Code environment variables.