AI Model Comparison

GPT-5.6 Luna vs. Claude Fable 5

Compare GPT-5.6 Luna (low) vs Claude Fable 5 (Adaptive Reasoning, Max Effort, Opus 4.8 Fallback) with benchmark results, speed, pricing, and practical workflow guidance.

Best For GPT-5.6 Luna (low)

  • High-speed real-time apps
  • Cost-sensitive workflows
  • High-throughput data tasks

Best For Claude Fable 5 (Adaptive Reasoning, Max Effort, Opus 4.8 Fallback)

  • Complex coding projects
  • Advanced scientific reasoning
  • High-stakes analytical tasks

GPT-5.6 Luna offers high-speed, cost-effective performance, while Claude Fable 5 provides superior reasoning and coding capabilities at a premium price point. Choosing between them depends on whether your priority is rapid deployment or complex, high-stakes analytical tasks.

Quick Take

OpenAI’s GPT-5.6 Luna (low) and Anthropic’s Claude Fable 5 (Adaptive Reasoning, Max Effort, Opus 4.8 Fallback) represent two distinct approaches to AI deployment. GPT-5.6 Luna emphasizes speed and affordability, whereas Claude Fable 5 prioritizes raw intelligence and technical depth.

Benchmark Read

Claude Fable 5 consistently outperforms GPT-5.6 Luna across all shared metrics. In the Intelligence index, Claude Fable 5 scores 59.9 compared to Luna’s 33.3. The gap is even more pronounced in coding, where Claude Fable 5 achieves a 76.5 index score against Luna’s 44.2.

Specific benchmark results further illustrate this divide:

  • GPQA: Claude Fable 5 (0.926) vs. GPT-5.6 Luna (0.835)
  • HLE: Claude Fable 5 (0.533) vs. GPT-5.6 Luna (0.188)
  • SciCode: Claude Fable 5 (0.602) vs. GPT-5.6 Luna (0.456)
  • LCR: Claude Fable 5 (0.7) vs. GPT-5.6 Luna (0.593)

Claude Fable 5 also demonstrates high proficiency in specialized benchmarks like TAU2 (0.985) and TerminalBench Hard (0.628).

Cost and Speed

There is a significant trade-off between performance and efficiency. GPT-5.6 Luna is highly optimized for speed, boasting an output of 232.911 tokens/s and a time-to-first-token of 1.032s. Its blended pricing is $2.25/1M tokens.

Conversely, Claude Fable 5 is built for depth rather than immediate responsiveness. It operates at 60.571 tokens/s with a substantial 54.299s time-to-first-token. Its cost is significantly higher, with a blended price of $20.00/1M tokens, reflecting its status as a high-effort reasoning model.

Best Fit

GPT-5.6 Luna is best suited for high-volume applications, real-time chat interfaces, and tasks where cost-per-token is a primary constraint. Claude Fable 5 is the ideal candidate for complex software engineering, advanced scientific research, and agentic workflows that require high reasoning accuracy regardless of latency.

Benchmark table

Side-by-side scores, speed, and pricing for the selected models.

Metric OpenAI GPT-5.6 Luna (low) Anthropic Claude Fable 5 (Adaptive Reasoning, Max Effort, Opus 4.8 Fallback)
Index Scores
Intelligence Index 33.3 59.9
Coding Index 44.2 76.5
Math Index--
Benchmark Scores
GPQA 83.5 92.6
SciCode 45.6 60.2
IFBench- 63.5
HLE 18.8 53.3
LCR 59.3 70.0
TAU2- 98.5
TerminalBench Hard- 62.9

Verdict

Choose GPT-5.6 Luna if your workflow requires high-throughput, low-latency responses at a budget-friendly price. However, if your projects demand top-tier reasoning, coding precision, and complex problem-solving, Claude Fable 5 is the superior choice. While Claude Fable 5 is significantly more expensive and slower to initiate, its benchmark performance across coding and intelligence metrics makes it the better tool for intensive, high-accuracy development environments.

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