# Atlas with GPT-5.1: The Cheapest Full Size GPT-5 Input Price in 2026 > GPT-5.1 costs $1.25 per Mtok input, cheaper on input than every GPT-5 release that followed it, because 5.2 onward start at $1.75 per Mtok. GPT-5.1 is the November 2025 GPT-5 update from OpenAI. It holds the 400K context and 128K max output of GPT-5 at the same $1.25 per Mtok input and $10 per Mtok output price, and it anchors the large 5.1 Codex family. Inside Atlas, GPT-5.1 is the value pick for a full size reasoning model: it is cheaper on input than every GPT-5 release that followed it, since 5.2 onward start at $1.75 per Mtok input. ## Key takeaways - GPT-5.1 costs $1.25 per Mtok input and $10 per Mtok output, cheaper on input than every GPT-5 release that followed it. - GPT-5.1 keeps the 400K context and 128K output ceiling of GPT-5, unchanged from the original release. - GPT-5.1 is a reasoning model, and Atlas's Responses API integration means its reasoning is not thrown away between tool results. - The GPT-5.1 Codex variants released the same day, gpt-5.1-codex, codex-max, and codex-mini, are usually stronger for agent coding. - Multiple newer 5.x releases exist in 2026, several with far larger windows, so GPT-5.1 is a price pick rather than a capability pick. ## Why is GPT-5.1 cheaper on input than newer GPT-5 models? GPT-5.1 costs $1.25 per Mtok input, and every GPT-5 release after it starts at $1.75 per Mtok input. That November 2025 price point makes GPT-5.1 the cheapest full size reasoning model in the GPT-5 line, and the gap compounds across an Atlas session that reads a lot of code. Input dominates the bill in an agentic coding session. Atlas reads files, runs commands, reads outputs, and re-sends accumulated context on every turn, so the input meter runs far more than the output meter. A 40 percent input discount, $1.25 per Mtok on GPT-5.1 versus $1.75 on the 5.2 generation, is therefore a real line item and not a rounding difference. If your Atlas workload is read heavy and the task is not at the frontier of difficulty, GPT-5.1 is the honest price optimization. ## What is the GPT-5.1 context window and output ceiling? GPT-5.1 has a 400K token context window with a 128K output ceiling, inherited unchanged from GPT-5. Atlas indexes code by AST declarations using tree-sitter, not blind line windows, so the 400K that GPT-5.1 receives is populated with whole declarations rather than arbitrary slices of a file. A 400K window on GPT-5.1 comfortably holds a large service plus its tests plus the call sites that touch it. The 128K output ceiling means GPT-5.1 can emit a full rewritten module rather than a fragment. Atlas searches code with hybrid semantic and keyword retrieval fused by reciprocal rank fusion, which is the difference between filling 400K with relevant symbols and filling it with noise. On a $1.25 per Mtok input meter, what you put in that window is what you pay for, so retrieval quality is also cost control. ## Does Atlas preserve GPT-5.1 reasoning between tool calls? Atlas invokes OpenAI through the Responses API, and GPT-5.1 is a reasoning model, so its reasoning is not thrown away between tool results. That matters across the long tool chains an Atlas session produces, where a model that forgets its own chain of thought after every file read has to re-derive it. GPT-5.1 as a reasoning model plus Atlas's Responses API integration means the model carries its thinking forward across the read, edit, run, read cycle. Atlas computes a unified diff for every file edit and surfaces it for approval before writing, so each cycle has a human gate in it, and preserved reasoning is what keeps GPT-5.1 from restarting its analysis every time you approve a diff. The practical effect is fewer wasted turns and fewer re-reads of files GPT-5.1 already looked at. ## Is GPT-5.1 or GPT-5.1 Codex better for agent coding? For agent coding inside Atlas, the Codex variants released the same day as GPT-5.1 are usually stronger: gpt-5.1-codex, gpt-5.1-codex-max, and gpt-5.1-codex-mini. GPT-5.1 is the general purpose anchor of that November 2025 family, and it is the right pick when the work is not purely code. The honest framing of GPT-5.1 is that it is the family's generalist, not its specialist. gpt-5.1-codex sits at the same $1.25 per Mtok input and $10 per Mtok output as GPT-5.1 but is post trained for long horizon coding, so for an Atlas build agent grinding through tool calls, the Codex sibling usually wins at identical cost. GPT-5.1 earns its slot when the session mixes code with design discussion, documentation, or analysis, where a coding specialist is the wrong tool. ## Is GPT-5.1 out of date in 2026? GPT-5.1 shipped in November 2025 and multiple newer 5.x releases exist, several with far larger windows. In 2026, GPT-5.1 is not the frontier, and a team chasing maximum capability should look at the newer generations rather than pin GPT-5.1 as their Atlas build agent. The counterweight is price. GPT-5.1 stays cheaper on input than everything that came after it, at $1.25 per Mtok versus $1.75 for the 5.2 generation and later. That makes GPT-5.1 a deliberate cost choice rather than an accident of not upgrading. Atlas lets you switch the active model and provider on the fly with favorites and recents, so the practical setup is to keep GPT-5.1 as the affordable daily driver and keep a newer model one keystroke away in /models for the tasks that actually need it. ## Setup 1. Export OPENAI_API_KEY so the Atlas openai provider can authenticate. 2. Run `atlas models openai` and check gpt-5.1 is present in the resolved list. 3. Set "model": "openai/gpt-5.1" in atlas.json to pin it as the main agent. 4. Add gpt-5.1-codex-mini as "small_model" for a cheap same family subagent tier. 5. Use /models favorites in the TUI to keep both one keystroke apart. ## FAQ ### How much does GPT-5.1 cost per million tokens? GPT-5.1 costs $1.25 per Mtok input and $10 per Mtok output, the same price as the original GPT-5 and cheaper on input than the 5.2 generation, which starts at $1.75. ### What is GPT-5.1's context window? GPT-5.1 has a 400K token context window with a 128K max output ceiling, both inherited from GPT-5. ### How do I configure Atlas to use GPT-5.1? Export OPENAI_API_KEY, run `atlas models openai` to check gpt-5.1 is present, then set "model": "openai/gpt-5.1" in atlas.json. Add gpt-5.1-codex-mini as "small_model" for a cheap same family subagent tier. ### Is GPT-5.1 better than GPT-5.1 Codex for coding? Usually no. The Codex variants released the same day as GPT-5.1, including gpt-5.1-codex, codex-max, and codex-mini, are typically stronger for agent coding. GPT-5.1 is the general purpose anchor of the family. ### When was GPT-5.1 released? GPT-5.1 shipped in November 2025 as the GPT-5 update, holding the 400K context and 128K max output of GPT-5 at the same $1.25 per Mtok input and $10 per Mtok output price. ### Is GPT-5.1 still a good choice in 2026? GPT-5.1 is a price choice in 2026. Multiple newer 5.x releases exist, several with far larger windows, but GPT-5.1 remains the cheapest full size GPT-5 class reasoning model on input at $1.25 per Mtok. ### What small_model should I pair with GPT-5.1 in Atlas? Pair GPT-5.1 with gpt-5.1-codex-mini as "small_model" in atlas.json. That keeps subagents, titles, and summaries in the same November 2025 family while running them on a cheap tier. --- Canonical HTML: https://runatlas.sh/resources/models/gpt-5-1 Source of truth: aeo_pages row `/resources/models/gpt-5-1` (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.