promptfoo--promptfoo/examples/anthropic/fable-5-coding/README.md

anthropic/fable-5-coding (Claude Fable 5 Advanced Coding)

This example exercises Claude Fable 5 on hard coding tasks using the xhigh effort level. Fable 5 always uses adaptive thinking, so no thinking configuration is needed.

You can run this example with:

npx promptfoo@latest init --example anthropic/fable-5-coding
cd fable-5-coding

What This Tests

Claude Fable 5 is Anthropic's most powerful model — a new tier above Opus. This example evaluates:

• Distributed-systems debugging with incomplete information
• Production-quality code generation with concurrency concerns
• Security-focused code review with prioritized feedback

Working with Fable 5

• Adaptive thinking is always on. Unlike Opus 4.7/4.8 (where adaptive thinking is opt-in), Fable 5 always thinks adaptively. There is nothing to configure: promptfoo converts a legacy thinking: { type: enabled, budget_tokens: N } config to adaptive and omits thinking: { type: disabled }, because the model rejects both.
• Thinking summaries are opt-in. By default the API omits thinking content (an empty thinking block, which promptfoo excludes from output). Set thinking: { type: adaptive, display: summarized } to include a readable summary.
• Sampling controls are managed for you. Fable 5 rejects temperature, top_p, and top_k at the model level; promptfoo omits them automatically (don't set them in config).
• effort defaults to high; xhigh is available. Start with xhigh for coding and agentic work, and pair high effort with a large max_tokens — thinking consumes output tokens before any visible text, so a tight budget can be spent entirely on thinking and yield an empty response.
• 1M-token context, up to 128K output tokens. Priced at $10/$50 per million input/output tokens.

Running the Example

# Set your API key
export ANTHROPIC_API_KEY=your_api_key_here

# Run the evaluation
npx promptfoo@latest eval

# View results
npx promptfoo@latest view

Other providers

Fable 5 is also reachable through:

• AWS Bedrock — bedrock:global.anthropic.claude-fable-5 (Runtime/Converse; requires the account to opt in to provider data sharing via aws bedrock put-account-data-retention --mode provider_data_share), or bedrock:messages:anthropic.claude-fable-5 for Bedrock's Anthropic-compatible Messages endpoint in us-east-1/eu-north-1
• Google Vertex — vertex:claude-fable-5
• Azure AI Foundry — point anthropic:messages:claude-fable-5 at https://<resource>.services.ai.azure.com/anthropic via apiBaseUrl

Across all providers, promptfoo automatically omits the unsupported sampling parameters and normalizes unsupported thinking configs for Fable 5. Note that Bedrock's regional and geo endpoints and Vertex's regional/multi-region endpoints (everything except global) carry a 10% price premium, which promptfoo includes in cost calculations.

Learn More

• Anthropic documentation
• Promptfoo Anthropic provider docs