Fast Opus 4.5 Model Configuration

Issue

The standard Opus 4.5 configuration is limited to 200,000 tokens, causing failures with larger contexts.

Solution: Fast Opus Deployment

Environment Variable Configuration

To use the fast Opus deployment with extended context length, configure Claude Code with:

# Set the Opus alias to use the fast deployment
export ANTHROPIC_DEFAULT_OPUS_MODEL="claude-opus-4-5-20251101-fast"

Model String Updates for Fast Deployment

Platform	Fast Opus 4.5 Model String
Anthropic API (1P)	claude-opus-4-5-20251101-fast
AWS Bedrock	anthropic.claude-opus-4-5-20251101-fast-v1:0
Google Vertex AI	claude-opus-4-5-fast@20251101
Azure AI Foundry	claude-opus-4-5-20251101-fast

Context Length Configuration

The fast deployment supports extended context. Configure the context limit:

# Extended context configuration (up to 1M tokens)
export ANTHROPIC_OPUS_CONTEXT_LIMIT="1000000"

Integration with Migration Tool

When migrating to Opus 4.5 with extended context requirements:

1. Use the fast model strings above instead of standard ones
2. Set the environment variable for Opus alias
3. Configure extended context limit
4. Test with large contexts (>200k tokens)

VSCode Configuration

For VSCode extension users:

{
  "claude-code.environmentVariables": {
    "ANTHROPIC_DEFAULT_OPUS_MODEL": "claude-opus-4-5-20251101-fast",
    "ANTHROPIC_OPUS_CONTEXT_LIMIT": "1000000"
  }
}

Verification

Test the configuration with a prompt that exceeds 200k tokens to ensure the error:

prompt is too long: 282789 tokens > 200000 maximum

is resolved.

When to Use Fast vs Standard

• Use Fast: For large codebases, extensive context analysis, long conversations
• Use Standard: For typical development tasks with smaller context requirements

The fast deployment trades some response speed for much higher context capacity.