BYOK Math: Is Anthropic or OpenAI Cheaper for AI Coding in 2026?
Token-by-token cost comparison for the realistic 8K input + 2K output coding turn, across Sonnet 4.5, Opus 4.5, GPT-5.1, and o3-pro.
Bringing your own key (BYOK) is the cheapest path to serious AI coding — no subscription markup. But which API is actually cheaper for the work you do? Token math, no marketing, just dollars.
The benchmark: one realistic coding turn
We model a single AI-coding round trip as 8,000 input tokens (system prompt + 3-4 file context + your message) and 2,000 output tokens (the model's reply, including a multi-file diff). This matches the median observed in Aider / Continue / Cursor session logs analyzed across 2026.
Per-turn cost, top models
| Model | Input $/M | Output $/M | Cost / turn |
|---|---|---|---|
| Sonnet 4.5 | $3 | $15 | $0.054 |
| Opus 4.5 | $15 | $75 | $0.270 |
| GPT-5.1 | $2.50 | $10 | $0.040 |
| GPT-5.1-mini | $0.25 | $1.00 | $0.0040 |
| o3-pro | $15 | $60 | $0.240 |
Numbers verified against anthropic.com/pricing and openai.com/api/pricing on April 23, 2026.
Translated to monthly bills
If you do 30 AI-assisted turns per workday × 22 workdays = 660 turns/mo.
- Sonnet 4.5 only — ~$36/mo
- GPT-5.1 only — ~$26/mo
- GPT-5.1-mini only — ~$3/mo (yes, three dollars)
- Opus 4.5 only — ~$178/mo (don't)
- 80% mini + 20% Sonnet hybrid — ~$10/mo
Which model for which task
- Code completion / quick fix: GPT-5.1-mini or DeepSeek. Cost is rounding error.
- Writing new code from spec: Sonnet 4.5 or GPT-5.1. Sonnet edges ahead on structured edits across multiple files.
- Hard debugging / architecture: Opus 4.5 or o3-pro. Use sparingly — these are the dollars that add up.
The verdict
For pure coding workloads, OpenAI is cheaper at the entry point (GPT-5.1-mini is unbeatable for autocomplete-grade work). Anthropic is cheaper at the "this is hard" tier (Sonnet beats GPT-5.1 on cost per useful diff). The smartest BYOK strategy is to keep both keys live and route by task — most tools (Continue, Aider, Cline) let you swap models per message without leaving the editor.
Want to compare against subscription plans?
Run your own numbers on the calculator or check the sub-$10 BYOK setup.