OpenAI o4-mini launched alongside o3 in April 2025 as the cheap reasoning tier of the newer generation, and in Atlas it is the model you route parallel subagents to when you want each one to actually think rather than pattern match. OpenAI o4-mini costs $1.10 per Mtok input and $4.40 per Mtok output on a 200K token context with a 100K max output, which is the exact same price as o3-mini from a newer base model, and half the input price of base o3 at $2. Its good latency-to-reasoning ratio is what makes it usable in a live Atlas loop rather than only in a batch job.
Why is OpenAI o4-mini a strict upgrade over o3-mini?
OpenAI o4-mini carries the same $1.10 per Mtok input and $4.40 per Mtok output price as o3-mini, but comes from a newer generation released in April 2025 alongside o3. Identical cost plus a newer base model is the definition of a strict upgrade at equal price.
Model choice is usually a tradeoff, and OpenAI o4-mini is the rare case where it is not. o3-mini and o4-mini bill identically at $1.10 per Mtok input and $4.40 per Mtok output, both sit on a 200K token context, and o4-mini is the newer of the two. For Atlas users that removes a decision: if you were going to run the cheap o-series reasoning tier, run o4-mini. Atlas lets you switch the active model and provider on the fly with favorites and recents, so you can put an identical prompt through both in one session and see the difference yourself before you pin anything in atlas.json.
What is OpenAI o4-mini best at inside Atlas?
OpenAI o4-mini is best inside Atlas as the model behind parallel subagents that need to reason rather than summarize. Its good latency-to-reasoning ratio, at $1.10 per Mtok input, means a fan-out of thinking subagents stays both responsive and affordable across a long session.
Atlas fans out work to subagents that can run in the foreground or in parallel background sessions, and the economics of that fan-out depend entirely on the per token price of the model behind it. OpenAI o4-mini at $1.10 per Mtok input makes a wide fan-out cheap, and unlike a pure speed tier it is a real reasoning model, so each subagent produces an argument rather than a restatement. Point one o4-mini subagent at a failing test, one at the call graph, one at the recent git history that Atlas reads directly, and let them come back with conclusions. With 100K max output on a 200K window, each of those conclusions can be as long as the problem deserves.
How does OpenAI o4-mini compare to base o3 on price?
OpenAI o4-mini costs $1.10 per Mtok input, which is half the $2 per Mtok input of base o3, on the same 200K token context with the same 100K max output. Output on o4-mini bills at $4.40 per Mtok against $8 per Mtok on base o3.
Within the o-series, OpenAI o4-mini is the value tier and base o3 is the depth tier, and the window is identical at 200K tokens. Running o4-mini halves your input cost against o3, which matters in Atlas because the agent loop is read heavy: every file read, tool result, and unified diff flows back through input tokens. The right escalation pattern is to run o4-mini by default for cheap reasoning heavy sessions, and escalate to openai/o3 with /models when the mini tier stalls on a hard bug. You pay the $2 per Mtok input only on the turns that need it.
Does the 200K context on OpenAI o4-mini limit large repositories?
OpenAI o4-mini has a 200K token context, which caps how much of a large repository it can hold at once. Atlas works around that cap with AST level retrieval rather than by shrinking the problem, but a genuine whole-monorepo prompt does not fit in 200K tokens.
The 200K token window is the real ceiling on OpenAI o4-mini, and it does not move. What Atlas does about it is make every token count. Atlas indexes code by AST declarations using tree-sitter, not blind line windows, so o4-mini sees whole functions and whole types instead of arbitrary slices that straddle two declarations. Atlas searches code with hybrid semantic and keyword retrieval fused by reciprocal rank fusion, so the declarations that land in the window are the relevant ones. If you truly need a million tokens of context in a single prompt, o4-mini is the wrong model and no amount of retrieval tuning changes that.
When should you pick a different model instead of OpenAI o4-mini?
OpenAI o4-mini is now a niche pick in 2026, because the GPT-5 mini tiers are cheaper, with gpt-5-mini at $0.25 per Mtok input, and newer. Pick o4-mini when you specifically want o-series reasoning at $1.10 in, and escalate to openai/o3 on a hard bug.
Honesty about OpenAI o4-mini in 2026 means naming the squeeze it is in. Below it, gpt-5-mini is cheaper on input at $0.25 per Mtok and newer. Above it, base o3 is a deeper reasoner for $2 per Mtok input. o4-mini's remaining case is a real one: an o-series reasoning tier with a good latency-to-reasoning ratio that you can fan out cheaply, and a 200K context that is often enough. When the mini tier stalls on a hard bug, escalate to openai/o3 with /models rather than grinding. Atlas switching the active model on the fly is what makes that escalation a keystroke instead of a restart.
Setup
- 01Export OPENAI_API_KEY in your shell or store it in Atlas's credential store.
- 02Run `atlas models openai` and confirm o4-mini resolves in the model list.
- 03Pin "model": "openai/o4-mini" in atlas.json for cheap reasoning heavy sessions.
- 04Route parallel Atlas subagents to o4-mini when you want each one to reason rather than pattern match.
- 05Escalate to openai/o3 with /models when the mini tier stalls on a hard bug.
Frequently asked questions
- How much does OpenAI o4-mini cost per million tokens?
- OpenAI o4-mini costs $1.10 per Mtok input and $4.40 per Mtok output. That is half the input price of base o3, which charges $2 per Mtok.
- Is o4-mini better than o3-mini?
- Yes at equal cost. OpenAI o4-mini and o3-mini both charge $1.10 per Mtok input and $4.40 per Mtok output, but o4-mini comes from a newer generation, released April 2025 alongside o3.
- What is the context window of OpenAI o4-mini?
- OpenAI o4-mini has a 200K token context window with a 100K max output. The 200K cap limits how much of a large repository it can hold at once.
- How do I set OpenAI o4-mini as the default in Atlas?
- Export OPENAI_API_KEY, run `atlas models openai` to confirm o4-mini resolves, then pin "model": "openai/o4-mini" in atlas.json for cheap reasoning heavy sessions.
- Is OpenAI o4-mini good for parallel agents?
- Yes. Atlas fans out work to subagents that run in the foreground or in parallel background sessions, and o4-mini's good latency-to-reasoning ratio at $1.10 per Mtok input makes that fan-out both fast and cheap.
- Should I use o4-mini or gpt-5-mini?
- gpt-5-mini is cheaper on input at $0.25 per Mtok and newer, so o4-mini is now a niche pick. Choose o4-mini when you specifically want the o-series reasoning tier.
- When should I escalate from OpenAI o4-mini to o3?
- Escalate to openai/o3 with /models when o4-mini stalls on a hard bug. Base o3 costs $2 per Mtok input against $1.10 on o4-mini, and both share a 200K context.
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