API cost decision in 10 seconds

Claude Opus 5 (Fast) vs GPT-5.6 Luna Pro

Pick GPT-5.6 Luna Pro when budget and context both matter.

Page updated:  Data confirmed:  Prices normalized to USD per 1M tokens Sample workload: 1M input + 500K output

Budget verdict

Pick GPT-5.6 Luna Pro when budget and context both matter.

On the standard 1M input plus 500K output workload, GPT-5.6 Luna Pro is estimated at $0.8 vs $35 for Claude Opus 5 (Fast), saving $34.2 (97.7% lower).

Cost-first pickGPT-5.6 Luna Pro
Context-first pickGPT-5.6 Luna Pro
Sample savings$34.297.7%
10x traffic gap$342

GPT-5.6 Luna Pro is cheaper on the standard workload and also has the larger context window. At 10x that traffic, the same price gap is about $342. Use the calculator below to replace the sample workload with your own token volume.

Cost sensitivity

Workload Sensitivity

Same prices, different token mixes.

GPT-5.6 Luna Pro stays cheaper across input-heavy, balanced, and output-heavy sample workloads.

Workload shapeToken mixBetter pickClaude Opus 5 (Fast)GPT-5.6 Luna Pro
Input-heavy / RAG5M input + 500K outputGPT-5.6 Luna Pro$75$1.6
Balanced workload1M input + 1M outputGPT-5.6 Luna Pro$60$1.4
Output-heavy chatbot1M input + 5M outputGPT-5.6 Luna Pro$260$6.2
Cheaper input GPT-5.6 Luna Pro $10 vs $0.2 / 1M

GPT-5.6 Luna Pro is $9.8 cheaper per 1M input tokens (98% lower; 50x difference).

Cheaper output GPT-5.6 Luna Pro $50 vs $1.2 / 1M

GPT-5.6 Luna Pro is $48.8 cheaper per 1M output tokens (97.6% lower; 41.7x difference).

Larger context GPT-5.6 Luna Pro 1M vs 1.05M

GPT-5.6 Luna Pro has 50K more context (1.05x larger).

Sample workload GPT-5.6 Luna Pro $35 vs $0.8

GPT-5.6 Luna Pro is $34.2 cheaper on the standard workload (97.7% lower).

Estimate your workload cost

Your Workload Cost

Prices are normalized to USD per 1M tokens.
Claude Opus 5 (Fast) Calculating… Estimated API cost
GPT-5.6 Luna Pro Calculating… Estimated API cost
Cheaper for this workload Calculating… Difference: calculating…

This estimate uses normalized public API pricing per 1M tokens. It is a planning aid, not a billing quote. Verify provider pricing, limits, and terms before production use.

Quick Decision

Verdict

GPT-5.6 Luna Pro has the lower input price; GPT-5.6 Luna Pro has the lower output price; GPT-5.6 Luna Pro offers the larger context window. For the 1M input plus 500K output sample, GPT-5.6 Luna Pro is cheaper for the standard workload.

For a 1M input token plus 500K output token workload, the estimated API cost is $35 for Claude Opus 5 (Fast) and $0.8 for GPT-5.6 Luna Pro.

Best Fit

Choose Claude Opus 5 (Fast) when its provider, model quality, latency, or availability is more important than the numeric price/context winner.

Choose GPT-5.6 Luna Pro when you care most about lower input-token price, lower output-token price, and larger context window.

Decision Notes
  • On the standard 1M input plus 500K output workload, GPT-5.6 Luna Pro is estimated at $0.8 vs $35 for Claude Opus 5 (Fast), saving $34.2 (97.7% lower).
  • GPT-5.6 Luna Pro is $34.2 cheaper on the standard workload (97.7% lower).
  • GPT-5.6 Luna Pro is $9.8 cheaper per 1M input tokens (98% lower; 50x difference).
  • GPT-5.6 Luna Pro is $48.8 cheaper per 1M output tokens (97.6% lower; 41.7x difference).
  • GPT-5.6 Luna Pro has 50K more context (1.05x larger).
Head-to-Head Specs
FeatureClaude Opus 5 (Fast)
(Anthropic)		GPT-5.6 Luna Pro
(OpenAI)
Input Price
prompt tokens per 1M		$10$0.2
Completion Price
per 1M tokens		$50$1.2
Sample Workload Cost
1M input + 500K output		$35$0.8
Context Window1M1.05M
Release Date

Use-Case Decision Matrix

Use caseBetter pickWhy
Budget-constrained productionGPT-5.6 Luna ProOn the standard 1M input plus 500K output workload, GPT-5.6 Luna Pro is estimated at $0.8 vs $35 for Claude Opus 5 (Fast), saving $34.2 (97.7% lower).
High-volume input processingGPT-5.6 Luna ProLower prompt-token price matters most when prompts, retrieved passages, or documents dominate the bill.
Long responses and chatbotsGPT-5.6 Luna ProLower output-token price matters most when assistants generate many completion tokens.
RAG or long-document workGPT-5.6 Luna ProA larger context window leaves more room for retrieved passages, conversation history, or source files.

Related Alternatives

Same-provider lower-cost swaps
  • Claude 3 Haiku can replace Claude Opus 5 (Fast) when lower sample workload cost matters most: $0.88.
  • Claude Haiku 4.5 (batch) can replace Claude Opus 5 (Fast) when lower sample workload cost matters most: $1.75.
  • Claude 3.5 Haiku can replace Claude Opus 5 (Fast) when lower sample workload cost matters most: $2.8.
  • Claude Sonnet 5 (batch) can replace Claude Opus 5 (Fast) when lower sample workload cost matters most: $3.5.

Cheaper alternatives

Review low-cost models sorted by a standard 1M input plus 500K output workload.

Open cheapest models

Larger context alternatives

Find models with larger context windows for RAG, long documents, and codebase review.

Open largest context models

Provider catalogs

Compare models within provider hubs before choosing a final API vendor.

Open provider hubs

Anthropic catalog

Review all tracked Anthropic models before deciding whether this matchup is the right shortlist.

Open Anthropic models

OpenAI catalog

Check other OpenAI models with comparable pricing, context, or release timing.

Open OpenAI models
Claude Opus 5 (Fast)

Fast-mode variant of [Opus 5](/anthropic/claude-opus-5) - identical capabilities with higher output speed at 2x pricing relative to regular Opus 5. Learn more in Anthropic's docs: https://platform.claude.com/docs/en/build-with-claude/fast-mode

GPT-5.6 Luna Pro

GPT-5.6 Luna Pro is the same underlying model as [GPT-5.6 Luna](https://openrouter.ai/openai/gpt-5.6-luna), served with `reasoning.mode` set to `pro` for higher-quality responses on complex tasks. Learn more in OpenAI's docs: https://developers.openai.com/api/docs/guides/reasoning#reasoning-mode