GPT-5.6 Luna vs Claude Fable 5
OpenAI's GPT-5.6 Luna and Anthropic's Claude Fable 5 both target production language workloads, but they price and behave differently. Here is the side-by-side.
Pricing checked against provider documentation on . How we verify
The short answer
GPT-5.6 Luna is the cheaper option — roughly 44.4x less on a blended workload, and it suits high-volume work on a tight budget. Claude Fable 5 justifies its premium when you need long-horizon agentic engineering.
OpenAI • GPT-5.6
$0.20 / $1.20
/ 1M tokens (input / output)
After an 80% price cut in July 2026, Luna became the value outlier among frontier-family models: it outperforms the previous generation's top-end Opus tier on coding evals while costing about a fiftieth of Fable 5 per input token. This is the model to route bulk traffic through in a tiered architecture.
Anthropic • Claude 5
$10.00 / $50.00
/ 1M tokens (input / output)
Anthropic's most capable widely released model, built for agents that run for hours rather than seconds. Adaptive thinking is always on and cannot be disabled, which is part of why it tops the Artificial Analysis Intelligence Index — and why its cost per task runs roughly triple GPT-5.6 Sol's for a one-point intelligence lead.
Input price
Claude Fable 5 is 50.0x the price than GPT-5.6 Luna on input tokens.
Output price
Claude Fable 5 is 41.7x the price than GPT-5.6 Luna on output tokens — usually the side that dominates the bill.
Monthly cost at three workload sizes
Standard (non-batch, non-cached) rates. Reasoning models will exceed these figures because thinking tokens bill as output.
| Workload | GPT-5.6 Luna | Claude Fable 5 | Difference |
|---|---|---|---|
| Light — 1M in / 200K out | $0 | $20 | $20 |
| Moderate — 10M in / 2M out | $4 | $200 | $196 |
| Heavy — 100M in / 20M out | $44 | $2,000 | $1,956 |
Specification comparison
| Attribute | GPT-5.6 Luna | Claude Fable 5 |
|---|---|---|
| Input (/ 1M tokens) | $0.20 | $10.00 |
| Output (/ 1M tokens) | $1.20 | $50.00 |
| Cached input | $0.02 | $1.00 |
| Context window | 1,048,576 tokens | 1,000,000 tokens |
| Max output | 128,000 tokens | 128,000 tokens |
| Native reasoning | Yes | Yes |
| Knowledge cutoff | 2026-02 | 2026-01 |
| Relative latency | low | high |
| Open weights | No | No |
| API model ID | gpt-5.6-luna | claude-fable-5 |
| Status | stable | stable |
GPT-5.6 Luna: Cut 80% from the $1/$6 launch price on 2026-07-30.
Claude Fable 5: 5-minute cache writes $12.50/MTok, 1-hour $20/MTok. Requires 30-day data retention, so not available under zero-data-retention terms.
Choose GPT-5.6 Luna if…
- Classification and content moderation
- High-frequency agent subtasks
- Bulk summarization
- Cost-sensitive chat features
Choose Claude Fable 5 if…
- Multi-hour autonomous coding runs
- Large codebase migrations
- Hard scientific workflows
- High-stakes legal and medical analysis
Frequently asked
Is GPT-5.6 Luna or Claude Fable 5 cheaper?
GPT-5.6 Luna is cheaper. On a blended 3:1 input-to-output workload it costs about 44.4x less than Claude Fable 5.
Which has the larger context window, GPT-5.6 Luna or Claude Fable 5?
GPT-5.6 Luna has the larger window at 1.05M tokens versus 1M.
Should I use GPT-5.6 Luna or Claude Fable 5?
Pick GPT-5.6 Luna for high-volume work on a tight budget. Pick Claude Fable 5 for long-horizon agentic engineering. If cost dominates the decision, GPT-5.6 Luna wins; if you need the capability ceiling, benchmark both on your own evals before committing.