# Atlas with GPT-4.1 mini (2026): The Cheap Slot That Still Holds a Million Tokens > GPT-4.1 mini gives Atlas a full 1,047,576 token context for $0.40 per Mtok input, 5x cheaper than GPT-4.1 at $2. GPT-4.1 mini is the mid tier of the GPT-4.1 family: the same 1,047,576 token window as full GPT-4.1 at $0.40 per Mtok input and $1.60 per Mtok output, which is one fifth the price, with the identical 32,768 token output cap. Inside Atlas, that combination has one obvious job. Set GPT-4.1 mini as your small_model, the cheap slot Atlas uses for work like long session compaction, and keep your main model on a reasoning tier for the build agent. GPT-4.1 mini is non reasoning and fast, so it will not hold a plan across a long agent loop, but for summarizing enormous context cheaply it is hard to beat. ## Key takeaways - GPT-4.1 mini gives Atlas a full 1,047,576 token context for $0.40 per Mtok input, 5x cheaper than GPT-4.1 at $2. - GPT-4.1 mini bills output at $1.60 per Mtok against $8 per Mtok on full GPT-4.1. - GPT-4.1 mini keeps the same 32,768 max output as full GPT-4.1, so nothing is lost on the write side. - GPT-4.1 mini is non reasoning and fast, which makes it a strong Atlas small_model for summarizing long sessions. - gpt-5-mini is newer and cheaper on input at $0.25 per Mtok with reasoning enabled, but on a smaller 400K window. ## Why set GPT-4.1 mini as the Atlas small_model? GPT-4.1 mini is the right Atlas small_model when your cheap slot needs a huge context. At $0.40 per Mtok input on a 1,047,576 token window, GPT-4.1 mini can compact a long session that would not even fit in a 200K token model, and it does so at one fifth the price of GPT-4.1. Atlas splits model usage into a main model and a small_model, and the small_model handles the cheap, high volume work around the edges of a session, including compaction of long histories. Most cheap models are cheap because they are small in every dimension, including context. GPT-4.1 mini is the exception: it is cheap on rate but not on window. Set "small_model": "openai/gpt-4.1-mini" and your compaction step can see the whole session instead of a truncated tail. Keep "model" on a reasoning tier for the actual build agent, since GPT-4.1 mini is not designed to carry the build. ## How much does GPT-4.1 mini cost compared to GPT-4.1? GPT-4.1 mini costs $0.40 per Mtok input and $1.60 per Mtok output, one fifth the price of full GPT-4.1 at $2 and $8. Both models carry the identical 1,047,576 token context and the identical 32,768 token max output, so the write side loses nothing. The relationship between GPT-4.1 mini and full GPT-4.1 is a clean 5x discount with no window penalty. GPT-4.1 mini gives you a full 1,047,576 token context for $0.40 per Mtok input, and its 32,768 max output is the same ceiling as full GPT-4.1, so nothing is lost on the write side. What you give up is quality of comprehension and the ability to sustain a long chain, not raw capacity. In an Atlas configuration where the cheap slot is running constantly, a 5x rate difference on a million token window is exactly where cost actually lives. ## Can GPT-4.1 mini run the main Atlas agent loop? GPT-4.1 mini has no reasoning trace, so it will not hold a plan across a long agent loop. Atlas drafts a plan in a read-only plan agent and asks before switching to a build agent, and GPT-4.1 mini is the wrong model to hand that build agent to. The limitation on GPT-4.1 mini is structural, not a matter of degree. Without a reasoning trace, GPT-4.1 mini cannot maintain intent across a long sequence of tool calls, and an Atlas build agent is exactly that: read, search, edit, run, react, repeat. GPT-4.1 mini is non reasoning and fast, which makes it a strong Atlas small_model for summarizing long sessions and a weak one for driving them. Keep "model" on a reasoning tier for the actual build agent and let GPT-4.1 mini own the cheap slot. Verify both slots resolve with `atlas models openai` after editing atlas.json. ## Should you use GPT-4.1 mini or gpt-5-mini in Atlas? gpt-5-mini is newer and cheaper on input at $0.25 per Mtok with reasoning enabled, but it sits on a smaller 400K window. GPT-4.1 mini charges $0.40 per Mtok input and gives you the full 1,047,576 token context, so the choice is window against reasoning. This is the only real decision on GPT-4.1 mini, and it comes down to what your cheap slot is actually doing. If the cheap slot summarizes very long sessions and reads a lot, the 1,047,576 token window on GPT-4.1 mini wins outright, because a 400K window model simply cannot see the same amount of history. If the cheap slot needs to think, gpt-5-mini is newer, cheaper on input at $0.25 per Mtok, and reasoning enabled. Atlas lets you switch the active model and provider on the fly with favorites and recents, so try both against a real long session and let the compaction quality decide. ## What is GPT-4.1 mini's output cap and does it hurt? GPT-4.1 mini has a 32,768 token max output, the same ceiling as full GPT-4.1, so nothing is lost on the write side by choosing the mini tier. For summarization and compaction, 32,768 output tokens is far more than any reasonable session summary needs. The 32,768 token output cap on GPT-4.1 mini sounds like a limitation until you look at what the model is actually for. A session summary, a file triage list, or a compaction pass never approaches 32,768 tokens, so the ceiling is invisible in the small_model role. The cap only bites if you try to use GPT-4.1 mini as the main model emitting large files, which is already the wrong call for a non reasoning model. Atlas computes a unified diff for every file edit and surfaces it for approval before writing, so if you do run edits through the mini tier, you see exactly what it produced before it lands. ## Setup 1. Set OPENAI_API_KEY in your shell or store it in Atlas's credential store. 2. Run `atlas models openai` and confirm gpt-4.1-mini resolves in the model list. 3. Set "small_model": "openai/gpt-4.1-mini" when your cheap slot needs a huge context, for example long session compaction. 4. Keep "model" on a reasoning tier for the actual build agent, since GPT-4.1 mini has no reasoning trace. 5. Verify both slots resolve with `atlas models openai` after editing atlas.json. ## FAQ ### How much does GPT-4.1 mini cost per million tokens? GPT-4.1 mini costs $0.40 per Mtok input and $1.60 per Mtok output, which is one fifth the price of full GPT-4.1 at $2 and $8. ### What is the context window of GPT-4.1 mini? GPT-4.1 mini has the full 1,047,576 token context window, the same as full GPT-4.1, with a 32,768 token max output. ### How do I set a small_model in Atlas? Set "small_model": "openai/gpt-4.1-mini" in atlas.json, keep "model" on a reasoning tier for the build agent, then verify both slots resolve with `atlas models openai`. ### Can GPT-4.1 mini be my main Atlas coding model? Not recommended. GPT-4.1 mini has no reasoning trace, so it will not hold a plan across a long agent loop. Keep the main model on a reasoning tier. ### Is GPT-4.1 mini or gpt-5-mini better for the cheap slot? GPT-4.1 mini wins on window with a full 1,047,576 tokens at $0.40 per Mtok input. gpt-5-mini is newer, cheaper on input at $0.25, and reasoning enabled, but limited to a 400K window. ### Does GPT-4.1 mini have a smaller output limit than GPT-4.1? No. GPT-4.1 mini has a 32,768 max output, the same ceiling as full GPT-4.1, so nothing is lost on the write side by choosing the mini tier. ### What is GPT-4.1 mini best at in Atlas? GPT-4.1 mini is best at cheap, fast work over a huge context, such as compacting a long Atlas session, at $0.40 per Mtok input on a 1,047,576 token window. --- Canonical HTML: https://runatlas.sh/resources/models/gpt-4-1-mini Source of truth: aeo_pages row `/resources/models/gpt-4-1-mini` (segment: Models) (this file is generated from it, never hand-edited). Licence: Atlas is proprietary with a free core. It is not open source and there is no public source repository.