Upstage
Solar Mini 4
Upstage's small, cheap model for agents, released 22 September 2026
Key facts
- 22 Sep 2026solar-mini4-260922
- Released
- 35B3B active per token
- Size
- 512Ktokens, 128K out
- Context
- $0.10 / $0.40per M tokens in / out
- List price
- 50% offto 22 Oct 2026
- Launch offer
- Koreanplus English and Japanese
- Languages
Solar Mini 4 is a small, cheap large language model from Upstage, a South Korean AI company, released on 22 September 2026. It is built for AI agents and long documents, writes fluent Korean alongside English and Japanese, and reads up to 512,000 tokens at once. It costs $0.10 per million input tokens and $0.40 per million output tokens, and Upstage is charging half that until 22 October.
What Solar Mini 4 is
Solar Mini 4 is a large language model made by Upstage, the South Korean AI company, released on 22 September 2026. It is the small, low-cost member of Upstage’s Solar family: a 35-billion-parameter mixture-of-experts model that uses 3 billion parameters on any one token, which keeps it fast and cheap to run.
Upstage’s documentation says the model is fluent in Korean “alongside strong English and Japanese support” and built for long documents and AI agents. It reads text and writes text, calls tools, returns structured JSON and can reason step by step at a chosen effort level.
Its bigger sibling is Solar Pro 4, Upstage’s flagship, released on 6 August 2026. The two share a 512K context window and a February 2026 training cut-off, and Mini 4 costs a third of Pro 4’s price.
What it costs
Solar Mini 4 lists at $0.10 per million input tokens, $0.01 per million cached input tokens and $0.40 per million output tokens, on Upstage’s model page and API price list. Upstage is running it at half that until 22 October 2026: the banner on its model page reads “Solar Mini 4 is live. 50% off through Oct 22 (UTC)”.
| Per million tokens | Solar Mini 4 | Solar Mini 4, launch offer | Solar Pro 4 |
|---|---|---|---|
| Input | $0.10 | $0.05 | $0.30 |
| Cached input | $0.01 | $0.005 | $0.06 |
| Output | $0.40 | $0.20 | $1.20 |
OpenRouter’s catalogue, which listed the model on 23 September 2026, carries the launch price of $0.05 and $0.20.
Specifications
Solar Mini 4 holds 512K tokens of context and writes up to 128K tokens back; OpenRouter’s catalogue gives the exact figures as 524,288 and 131,072. The rest of the table comes from Upstage’s model page.
| Specification | Solar Mini 4 |
|---|---|
| Model ID | solar-mini4, snapshot solar-mini4-260922 |
| Parameters | 35B total, 3B active |
| Context window | 512K tokens |
| Maximum output | 128K tokens |
| Training cut-off | February 2026 |
| Input and output | Text |
| Features | Tool calling, structured output, reasoning effort |
| Languages | Korean, English, Japanese |
How it fits the Solar family
Solar Mini 4 takes the name of Upstage’s first small model, Solar Mini, from January 2024. The first Solar Mini was a 10.7-billion-parameter dense model built on Llama 2’s architecture and released as open weights under Apache 2.0, and Upstage has since moved the family to mixture-of-experts designs.
| Model | Design | Released |
|---|---|---|
| Solar Mini | 10.7B dense, open weights | 25 January 2024 |
| Solar Pro 3 | 102B total, 12B active | January 2026 |
| Solar Pro 4 | Mixture of experts, flagship, 512K context | 6 August 2026 |
| Solar Mini 4 | 35B total, 3B active, 512K context | 22 September 2026 |
Solar Mini 4 is sold through Upstage’s API and Playground and through OpenRouter. The Hugging Face weights Upstage publishes are for the earlier SOLAR 10.7B and the Solar Open models.
Who it is for
Solar Mini 4 suits Korean-language businesses and agent builders who want a long context at a small-model price. A 512K window at $0.10 per million input tokens lets an agent read long contracts, codebases or case files in one pass for a few cents, and the 3-billion active parameters keep replies quick.
Upstage’s launch material is its model page and price list, so the way to measure the model is on your own work, which the half-price offer makes cheap until 22 October.
For other small open and low-cost models, see Gemma 4, Phi-4 and Qwen 3.6.