OpenAI o1
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| OpenAI o1 | |
|---|---|
| Developer | OpenAI |
| Codename | Strawberry (formerly Q*) |
| Announced | September 12, 2024 (o1-preview, o1-mini) |
| Release date | September 12, 2024 (preview); December 5, 2024 (full); March 19, 2025 (o1-pro API) |
| Type | Large language model (reasoning model) |
| Architecture | Transformer, trained with large-scale reinforcement learning |
| Variants | o1-preview, o1-mini, o1, o1-pro |
| Parameters | Undisclosed |
| Context window | 128k tokens (o1, o1-mini); 200k (o1-pro) |
| Predecessor | GPT-4o |
| Successor | OpenAI o3 (Apr 2025); GPT-5 (Aug 2025) |
| API status (May 2026) | o1-preview retired (Jul 28, 2025); o1-mini retired (Oct 27, 2025); full o1 retained as legacy |
OpenAI o1 is a family of large language models developed by OpenAI, introduced on September 12, 2024 as the company's first models specifically designed for complex reasoning tasks. Unlike previous models in OpenAI's lineup, o1 was trained using reinforcement learning to perform extended internal reasoning before producing a response, a technique sometimes described as "thinking before answering." The model represented a significant departure from the scaling paradigm that had defined the GPT series, shifting emphasis from training-time compute to inference-time compute. OpenAI released o1-preview and o1-mini as the initial public variants, with the full o1 model following on December 5, 2024 alongside the launch of ChatGPT Pro.[1][2]
The o1 family attracted widespread attention for its performance on mathematics, science, and coding benchmarks, where it substantially outperformed GPT-4o and other models available at the time. On the American Invitational Mathematics Examination (AIME), o1 scored 83.3% using consensus voting, compared to 13.4% for GPT-4o. It also reached the 89th percentile on Codeforces competitive programming problems and scored 78% on GPQA Diamond, a benchmark of graduate-level science questions.[1][3] More broadly, o1 inaugurated the reasoning model era; within twelve months of its preview, every major lab had shipped a comparable system, including DeepSeek-R1, Gemini 2.0 Flash Thinking, and Claude 3.7 Sonnet with extended thinking.[15][20]
OpenAI had been exploring reasoning-focused models under the internal codename "Strawberry" for much of 2024. Media reports throughout the summer hinted at a new approach to AI that prioritized deliberation over raw generation speed. When the model was finally unveiled on September 12, 2024, OpenAI described it as representing a "new paradigm" in AI capability, one built around the idea that language models could be trained to think through problems systematically rather than producing immediate responses.[1]
The project that became o1 had been visible in fragments to outside observers for nearly a year before launch. In November 2023, around the time of the brief firing and rehiring of CEO Sam Altman, reporting from Reuters and The Information described an internal OpenAI research effort known as "Q*" (pronounced "Q-star"). Sources told reporters that Q* could solve grade-school mathematics problems it had not seen before, a capability that researchers inside the company believed was a meaningful step beyond the pattern-matching strengths of GPT-4. Some reporting at the time suggested the Q* breakthrough was one trigger for the boardroom conflict, with safety-focused board members worried that the company was moving too quickly toward systems that demonstrated genuinely new reasoning abilities. OpenAI never publicly confirmed the specifics, and Altman dismissed the most dramatic interpretations as "an unfortunate leak."[23][24]
The codename then shifted. By July 2024, Reuters reported that OpenAI was internally testing a project called "Strawberry," described as a successor to Q*. The Information added details suggesting Strawberry was a model trained with reinforcement learning to plan ahead, follow chains of reasoning, and complete tasks autonomously over multiple steps. Throughout August and into early September, screenshots and indirect leaks accumulated. A particularly memorable thread on the AI subforum of X surfaced what appeared to be early Strawberry outputs in which the model meticulously worked through how many "r"s appear in the word "strawberry," a problem GPT-4o still answered incorrectly. Some users speculated the codename itself was a tongue-in-cheek reference to that failure case.[25][26]
When OpenAI launched the model on September 12, 2024, it dropped the Strawberry name in favor of a clean reset. Greg Brockman wrote that the company chose "o1" as a deliberate signal that it considered this a new family rather than an extension of the GPT line. Bloomberg later reported that OpenAI had even considered branding the model as "GPT-Reasoning" but settled on the simpler letter-and-number scheme to avoid implying a strict ordering with GPT-4 and GPT-4o.[27]
Important caveats on the rumor history: OpenAI never officially confirmed that Q* and Strawberry were the same project, nor that either was the direct ancestor of the o1 model that shipped. Some inside accounts, summarized in retrospectives in late 2024, suggested Q* and Strawberry were related but distinct branches of work, with Strawberry the more direct genealogical line to o1. Because OpenAI did not publish a chronology of its internal research, the most that can be said with confidence is that the company had been working publicly on process-supervised math reasoning since at least 2023 (the "Let's Verify Step by Step" paper) and that the September 12, 2024 announcement was the first product to fully bring that line of work to market.[16][17]
By mid-2024, several signs suggested that the simple scaling story that had carried the GPT line from GPT-2 through GPT-4 was running into limits. Improvements from larger pre-training runs were getting smaller, training data was becoming harder to source at the volumes needed, and energy and chip availability were starting to bind. At the same time, academic work from labs including Google DeepMind and university groups had repeatedly shown that letting a model think out loud, in chains of intermediate steps, could improve accuracy on math and reasoning tasks dramatically without changing the underlying weights.[3][17]
What OpenAI did differently with o1 was not the chain-of-thought idea itself, which dated back at least to 2022 work on chain-of-thought prompting, but the decision to make extended reasoning a first-class feature of the model rather than a prompting trick. By using large-scale reinforcement learning to teach the model to produce useful reasoning traces, OpenAI made test-time compute itself a scaling axis. The blog post "Learning to Reason with LLMs" framed this explicitly: "We have found that the performance of o1 consistently improves with more reinforcement learning (train-time compute) and with more time spent thinking (test-time compute). The constraints on scaling this approach differ substantially from those of LLM pretraining."[3]
In industry commentary, Andrej Karpathy, formerly of OpenAI and Tesla, called o1 the first compelling demonstration that the next big lever for capability is not another order of magnitude of pretraining but rather giving the model time to think. Nathan Lambert at Interconnects later described the shift as the field's first serious move away from a "bigger is better" framing toward a "more deliberate is better" framing.[17]
The project built on several years of foundational research at OpenAI. A key predecessor was the 2023 paper "Let's Verify Step by Step," which explored process reward models (PRMs) for mathematical reasoning. That work demonstrated that providing feedback on each step of a model's reasoning chain (process supervision) was significantly more effective than only evaluating the final answer (outcome supervision). The process-supervised reward model solved 78% of problems from the MATH dataset, compared to 72% for the outcome-supervised model. OpenAI also released the PRM800K dataset, containing 800,000 step-level human labels across 75,000 solutions, as part of this earlier research.[16][17]
The o1 family was released in waves rather than as a single model. Each variant served a different audience and price point, and the gap between announcements compressed as competitive pressure mounted in late 2024 and early 2025.
| Date | Variant | Surface | Notes |
|---|---|---|---|
| Sep 12, 2024 | o1-preview | ChatGPT Plus, Team; API tier 5 | First public reasoning model; 30 messages/week initial cap |
| Sep 12, 2024 | o1-mini | ChatGPT Plus, Team, free tier (limited); API tier 5 | Smaller, ~80% cheaper, strong at math and code |
| Sep 17, 2024 | (rate limit raise) | ChatGPT Plus | o1-preview cap raised to 50/week, o1-mini to 50/day |
| Dec 5, 2024 | o1 (full) | ChatGPT Plus, Pro, Team; API | Vision, function calling, structured outputs, reasoning_effort |
| Dec 5, 2024 | o1-pro mode | ChatGPT Pro ($200/mo only) | Multi-trace reasoning; ChatGPT-only initially |
| Dec 20, 2024 | (announcement) | Internal preview | o3 and o3-mini unveiled on day 12 of "shipmas" |
| Jan 31, 2025 | (replacement begins) | ChatGPT, API | o3-mini released; ChatGPT free tier gets reasoning for first time |
| Mar 19, 2025 | o1-pro | API (Responses API only) | First model gated to Responses API; $150/$600 per 1M tokens |
| Apr 16, 2025 | (replaced in ChatGPT) | ChatGPT | o3 and o4-mini supplant o1 and o1-mini in the model picker |
| Jul 28, 2025 | o1-preview | API retired | Migration recommended to o3 |
| Aug 7, 2025 | (replaced in ChatGPT) | ChatGPT | GPT-5 launches as default; o1 hidden behind a "show legacy" toggle for paid users |
| Oct 27, 2025 | o1-mini | API retired | Migration recommended to o4-mini |
The defining technical feature of o1 is its use of extended chain-of-thought reasoning before producing a final answer. When presented with a problem, the model generates a long internal reasoning trace, working through the problem step by step, considering different approaches, checking its work, and revising its thinking when it detects errors. This reasoning process happens in a hidden "thinking" phase that is not shown to the user; only a summary of the reasoning and the final answer are displayed.[1][3]
The hidden reasoning tokens (often referred to as "thinking tokens") serve multiple purposes. They allow the model to decompose complex problems into manageable steps, explore alternative solution paths, verify intermediate results, and self-correct. OpenAI chose to hide these tokens from end users for several reasons, including protecting the proprietary reasoning strategies the model had learned and preventing the chain of thought from being used to reverse-engineer the model's training process.[3]
Unlike the GPT series models, which were primarily trained through next-token prediction with subsequent instruction tuning and RLHF, o1 was trained using large-scale reinforcement learning specifically targeted at reasoning tasks. The model learned to generate effective chains of thought through a trial-and-error process where it received rewards for producing correct final answers. Over the course of training, the model developed increasingly sophisticated reasoning strategies, including the ability to recognize and correct mistakes, try alternative approaches when an initial strategy failed, and break complex problems into simpler sub-problems.[1][3]
OpenAI reported that the model's performance scaled smoothly with both the amount of reinforcement learning applied during training and the amount of compute used at inference time. This dual scaling behavior suggested a new dimension for improving AI capabilities beyond simply making models larger.[3]
While OpenAI has not published the full technical details of o1's training pipeline, external researchers have reconstructed a likely picture from available information. The training is believed to involve multiple stages: first, standard pre-training on large text corpora; then supervised fine-tuning on instruction data to introduce basic reasoning behaviors; and finally, reinforcement learning fine-tuning where the model learns to assign value to intermediate reasoning steps using both process-level rewards (for stepwise quality) and outcome rewards (for final answer correctness).[12][17]
The reward modeling approach draws on OpenAI's earlier work with process reward models. Rather than only evaluating whether a model's final answer is correct, the training process also evaluates the quality of individual reasoning steps. This approach helps the model learn not just what answers to produce, but how to reason toward them effectively. The combination of process and outcome supervision enables the model to develop more reliable reasoning chains and to catch errors at intermediate steps before they propagate to the final answer.[16][17]
The core insight behind o1 was that spending more compute at inference time, by allowing the model to generate extended internal reasoning chains, could yield better results on difficult tasks than simply scaling up training. This contrasted with the prevailing approach of making models larger and training them on more data. OpenAI's research demonstrated that o1's performance improved consistently both with more reinforcement learning during training (train-time compute) and with more time spent reasoning during inference (test-time compute).[1][3]
OpenAI exposed this idea to developers through a reasoning_effort parameter introduced with the full o1 release:[7]
| Effort Level | Description | Use Case | Relative Latency |
|---|---|---|---|
| Low | Minimal reasoning depth; quick responses | Simple questions, brainstorming, speed-critical tasks | Fastest (often under 1 second) |
| Medium (default) | Balanced reasoning depth and speed | Moderately complex queries | ~3x low |
| High | Maximum reasoning depth; explores many reasoning paths | Critical tasks requiring highest accuracy | ~3x medium |
The reasoning effort parameter essentially controls the model's internal "thinking budget," adjusting the number of hidden reasoning tokens it generates before producing a response. Lower settings produce faster and cheaper responses, while higher settings yield more thorough reasoning at the cost of increased latency and token usage. Developers could route simple questions through the low-effort setting while reserving high-effort reasoning for complex problems, optimizing both cost and user experience.
A novel safety approach used in o1's training was "deliberative alignment," described by OpenAI in a December 2024 paper. Rather than relying solely on post-hoc safety filters, deliberative alignment teaches the model the text of OpenAI's safety specifications and trains it to reason explicitly about those policies during its chain-of-thought process. When the model encounters a potentially sensitive query, it can reference its understanding of the safety guidelines within its reasoning chain, consider how the guidelines apply to the specific situation, and produce a response that is both helpful and aligned with the policies.[13][14]
OpenAI reported that this approach produced a Pareto improvement on both under-refusals and over-refusals. The model was simultaneously better at avoiding harmful outputs while being more permissive with benign prompts, meaning it refused fewer legitimate requests than GPT-4o while also refusing more genuinely harmful ones. The deliberative alignment approach also demonstrated strong generalization to out-of-distribution safety scenarios that were not part of the training data.[13]
Released on September 12, 2024, o1-preview was the first publicly available version of the reasoning model. It was made available to ChatGPT Plus and Team subscribers, as well as tier 5 API users. As a preview release, it came with several limitations: no support for image inputs, no function calling, no streaming, and restricted system message capabilities. Usage was capped at 30 messages per week for ChatGPT Plus users (later raised to 50/week on September 17, 2024). Despite these constraints, o1-preview demonstrated the potential of the reasoning approach, substantially outperforming GPT-4o on mathematical and scientific benchmarks.[1][2]
o1-preview was deprecated and removed from the API on July 28, 2025, with OpenAI recommending migration to o3.[29]
Also released on September 12, 2024, o1-mini was designed as a smaller, faster, and cheaper alternative to o1-preview. It was particularly effective at coding and STEM tasks, nearly matching o1-preview's performance on benchmarks like AIME and Codeforces while being 80% cheaper at $3.00/$12.00 per million input/output tokens. o1-mini was positioned as the right choice for applications requiring strong reasoning in math and code without needing the broad world knowledge of the full model. It was available to ChatGPT Plus and Team users (50 messages/day after the September 17 increase) and to limited free-tier users.[2][4]
o1-mini's score on AIME 2024 of about 70% (roughly 11 of 15 problems) places it in approximately the top 500 US high-school students, a striking result for a small model. It was retired from the API on October 27, 2025, with OpenAI recommending migration to o4-mini.[29]
The full version of o1 launched on December 5, 2024, alongside the announcement of the ChatGPT Pro subscription tier. The full release addressed many of the preview's limitations, adding support for image input (vision capabilities), function calling, developer messages, structured outputs, and the reasoning_effort parameter. According to OpenAI's internal testing, the full o1 reduced "major errors" on "difficult real-world questions" by 34% compared to o1-preview.[5][42]
The full o1 model also delivered improved benchmark scores: 74.3% on AIME 2024 (pass@1), 78% on GPQA Diamond, and the 89th percentile on Codeforces with an Elo of 1,891.[1][5]
Announced alongside ChatGPT Pro on December 5, 2024, o1-pro (initially called "o1 pro mode" in ChatGPT) is a variant of o1 that uses significantly more compute during the reasoning phase. It is designed for users who need the highest possible reliability and accuracy on difficult problems. OpenAI described o1-pro as "thinking harder" by exploring more reasoning paths and spending more time verifying its answers. The o1-pro mode was initially exclusive to ChatGPT Pro subscribers ($200/month), and an API version was released on March 19, 2025.[5][6][30]
The API release of o1-pro was unusual in two ways. First, the pricing of $150 per million input tokens and $600 per million output tokens made it the most expensive OpenAI model ever offered, roughly 1,000 times the per-token cost of GPT-4o-mini. Second, o1-pro became the first OpenAI model that was only accessible through the new Responses API rather than the legacy Chat Completions endpoint, signaling OpenAI's intent to consolidate reasoning model traffic on a stateful API better suited to long internal traces. Streaming was not supported. Simon Willison, reviewing the launch the same day, called the pricing "eye-watering" but noted that for a narrow band of high-stakes scientific and engineering work the calculus might still pencil out.[30]
In ChatGPT Pro, o1-pro mode shipped without a fixed message cap, although OpenAI reserved the right to apply abuse mitigations. Internally, o1-pro reportedly used a strategy where the model produced multiple independent reasoning traces in parallel and then selected or combined them, a setup the system card referred to obliquely as "majority-of-N reasoning." OpenAI did not publish the exact number of traces used at default effort.[6][14]
The launch demos made the value proposition concrete. Presented with a series of physics problems from a graduate qualifying exam, o1-pro answered correctly where the standard o1 model had stumbled, and reasoned about the structure of each problem before attempting a calculation. The conceit of "you ask the question once, you wait, you get a careful answer" was something users either loved or found unworkable depending on their workflow.
The o1 family demonstrated significant improvements over GPT-4o across a range of challenging benchmarks. The numbers below combine figures from the September 2024 launch announcements, the December 2024 full-release announcement, the December 2024 system card, and OpenAI's published evaluation methodology. Where two numbers are reported, the first is single-attempt (pass@1) and the second uses majority voting or self-consistency over many samples.[1][3][14]
| Benchmark | o1-preview | o1 (full) | o1-mini | GPT-4o | Description |
|---|---|---|---|---|---|
| AIME 2024 (pass@1) | 44.6% | 74.3% | 70.0% | 13.4% | American Invitational Mathematics Examination |
| AIME 2024 (consensus@64) | - | 83.3% | - | - | Majority vote across 64 samples |
| AIME 2024 (1000 samples + re-ranker) | - | 93.3% | - | - | With learned scoring function |
| GPQA Diamond | 73.3% | 78.0% | 60.0% | 53.6% | Graduate-level science questions |
| MATH-500 | 85.5% | 96.4% | 90.0% | 60.3% | Math problem solving |
| Codeforces | 62nd pct. | 89th pct. | - | 11th pct. | Competitive programming |
| Codeforces Elo | - | 1,891 | - | - | Competition rating |
| MMLU | 90.8% | 92.3% | 85.2% | 87.2% | Multitask language understanding |
| HumanEval | - | 92.4% | - | 90.2% | Code generation |
| SWE-bench Verified | - | 48.9% | - | 33.2% | Real-world software engineering |
The AIME results were particularly striking. With a single attempt per problem, o1 averaged 74.3% (roughly 11.1 out of 15 questions). When allowed 64 attempts with majority voting (consensus@64), it reached 83.3%. With 1,000 samples and a learned scoring function for re-ranking, the score rose to 93.3%. These results demonstrated that o1 could solve problems that had previously been considered beyond the reach of language models.[1]
For context, AIME is a 15-problem qualifying exam for the U.S. Mathematical Olympiad. A score of 83.3% places o1 within the top 500 students nationally; a 100% score, which o1 reached when given access to a Python interpreter for arithmetic and verification at high effort, is achieved by only a few dozen high schoolers each year. OpenAI noted that on the GPQA Diamond benchmark, expert human PhDs in the relevant fields scored 69.7% under similar conditions, putting o1's 78% above expert performance for the first time in any general-purpose language model.[3][14]
Several third-party evaluators corroborated and contextualized OpenAI's launch numbers. Vellum ran o1 against Claude 3.5 Sonnet, GPT-4o, and DeepSeek-R1 on a custom suite of math, coding, and reasoning tasks and reported that o1 led on math problems requiring multi-step derivation but lost to Claude 3.5 Sonnet on coding tasks involving large existing codebases, where the absence of streaming and the long latency made iterative work slow. Vellum's launch-week analysis singled out the inconsistency on simple tasks as a real ergonomic problem, with o1 sometimes producing a paragraph of reasoning to answer a question of the form "what is 7 + 5?"[32]
Artificial Analysis maintains a continuously-updated independent benchmark across providers. In its January 2025 evaluation, Artificial Analysis recorded o1 at an "Intelligence Index" of 71, compared to 59 for GPT-4o and 70 for DeepSeek-R1, at roughly five times the per-token cost of R1. The same firm later reported that median time-to-first-token for o1 was around 18 seconds versus under 1 second for GPT-4o, a quantification of the latency cost developers were absorbing.[33]
METR, the independent evaluations nonprofit, ran agentic task benchmarks on o1 in late 2024 and reported that the model outperformed GPT-4o on its METR Task Suite (a battery of multi-step computer-use and software engineering tasks) but underperformed dedicated coding agents that paired GPT-4o with explicit planning and tool-use scaffolding. METR's writeup noted that "raw reasoning capability does not automatically translate into agentic task completion," a caveat that influenced how labs marketed subsequent reasoning models.[34]
When o1 was released, it set a new standard for reasoning performance. However, within months, competing models closed the gap and in some cases surpassed it. The following table compares o1 with other reasoning-capable models available in its generation.
| Benchmark | o1 (full) | Claude 3.5 Sonnet (Oct 2024) | Gemini 2.0 Flash Thinking | DeepSeek-R1 | QwQ-32B-Preview |
|---|---|---|---|---|---|
| AIME 2024 | 74.3% | ~16% | ~40% | 79.8% | ~50% |
| GPQA Diamond | 78.0% | ~59% | ~65% | 71.5% | ~54% |
| MATH-500 | 96.4% | ~78% | ~83% | 97.3% | ~90% |
| Codeforces Elo | 1,891 | ~1,500 | - | 2,029 | ~1,300 |
| MMLU | 92.3% | ~88% | ~85% | 91.8% | ~85% |
Notably, DeepSeek-R1, released in January 2025, matched or exceeded o1 on most mathematical and coding benchmarks despite being an open-source model trained at a fraction of the cost. This intensified debate about the necessity of massive compute budgets for reasoning model training and contributed to a market-wide compression in reasoning model pricing.
OpenAI's pricing for o1 reflected the higher computational cost of inference-time reasoning. Because the model generates hidden reasoning tokens in addition to visible output tokens, the effective cost per query was typically higher than for GPT-4o, even though the per-token pricing was comparable for output.
| Model | Input (per 1M tokens) | Output (per 1M tokens) | Cached Input |
|---|---|---|---|
| o1-preview | $15.00 | $60.00 | $7.50 |
| o1 (full) | $15.00 | $60.00 | $7.50 |
| o1-mini | $3.00 | $12.00 | $1.50 |
| o1-pro | $150.00 | $600.00 | - |
The o1-pro API pricing was notably expensive at $150 per million input tokens and $600 per million output tokens, making it roughly 10 times more expensive than the base o1 model. This pricing positioned o1-pro as a premium option for tasks where correctness justified the higher cost, such as scientific research, complex mathematical proofs, and critical code generation.
Unlike GPT-4o, which received several price cuts in its first year, o1's published API prices held steady from December 2024 through retirement, with the only meaningful change being the introduction of cached input pricing in early 2025 (input tokens that had been seen recently could be re-billed at half rate). What did change dramatically over time was the cost-per-task picture, because the rapid release of cheaper alternatives made o1's effective cost ratio worse:
| Date | Best available reasoning model | Approx. cost per million output tokens | Notes |
|---|---|---|---|
| Sep 2024 | o1-preview | $60 | Only reasoning option from a major lab |
| Dec 2024 | o1 (full) | $60 | o1-pro at $600 introduced for ChatGPT Pro |
| Jan 2025 | DeepSeek-R1 | $2.19 | ~27x cheaper than o1; open weights |
| Jan 2025 | o3-mini | $4.40 | OpenAI's first sub-$5 reasoning model |
| Apr 2025 | o3 | $40 | Replaced o1 as default reasoning |
| Jun 2025 | o3 (price cut) | $8 | 80% cut making o3 cheaper than o1 had been |
| Aug 2025 | GPT-5 (thinking) | $10 | Bundled into the unified GPT-5 system |
For a typical hard problem requiring 10,000 reasoning tokens and a few hundred output tokens, an o1-pro call could exceed $6 in compute, while an o3 call after the June 2025 price cut would run under $0.10 for a similarly difficult task. This compression squeezed o1's economic niche from below (cheaper reasoning models) and from above (more capable reasoning models), and was a primary driver of OpenAI's eventual decision to retire the o1 endpoints rather than maintain them as legacy options indefinitely.
In ChatGPT, o1 was integrated into the model picker alongside GPT-4o. The September 12, 2024 launch made o1-preview and o1-mini visible to ChatGPT Plus and Team subscribers with usage caps (30/week for o1-preview, 50/day for o1-mini, raised to 50/week and 50/day respectively on September 17, 2024). The full o1 launched into the Plus, Pro, and Team tiers on December 5, 2024, and the same day ChatGPT Pro ($200/month) added the o1-pro mode for "unlimited" access. ChatGPT did not show the raw reasoning trace; it displayed a model-generated summary above the answer.[1][5][42]
For the API, the o1 line introduced several architectural ideas that became standard in later reasoning models:
reasoning_tokens field inside usage exposed how many hidden tokens a request had consumed, even though the tokens themselves were not returned.reasoning_effort parameter (low, medium, high) controlled the model's thinking budget.Function calling, JSON-mode structured outputs, and developer messages, all features developers had come to expect from the GPT-4o line, were absent from o1-preview in September and only landed with the December full release. This created a notable adoption gap for production workloads.[7][8]
The o1 model series was made available through Microsoft Azure's OpenAI Service shortly after the OpenAI direct API launch, where enterprise customers could access it alongside other OpenAI models with regional deployment, compliance certifications, and procurement workflows tailored to large organizations. Azure retained the o1 endpoints somewhat longer than OpenAI's direct API in keeping with Azure's general policy of slower deprecations.[40]
One of the most contentious aspects of o1's design was OpenAI's decision to hide the model's raw chain-of-thought reasoning from users. Unlike open-source reasoning models that expose their full thinking process, o1 presents only a filtered summary generated by a secondary model. Users see a high-level description of what the model considered, but not the actual reasoning tokens.[18]
OpenAI laid out its hidden chain-of-thought policy in unusually direct language in the launch blog post:
"Therefore, after weighing multiple factors including user experience, competitive advantage, and the option to pursue the chain of thought monitoring, we have decided not to show the raw chains of thought to users. We acknowledge this decision has disadvantages. We strive to partially make up for it by teaching the model to reproduce any useful ideas from the chain of thought in the answer. For the o1 model series we show a model-generated summary of the chain of thought."[3]
The policy applied identically to ChatGPT users and to API developers. There was no enterprise tier, special permission, or developer flag at launch that exposed the raw chain of thought. The only thing visible above the final answer was a paraphrased summary, generated by a separate, smaller model that read the hidden reasoning trace and produced a high-level description of what was considered. OpenAI later updated its Model Spec to formalize this rule, stating that hidden reasoning "is not exposed to the user or developer except potentially in summarized form."[28]
In the API, this manifested in a few specific ways. The messages field in responses contained only the final assistant content, never the reasoning tokens. Reasoning tokens were billed (counted toward the output token total) but not returned. The API response object included a reasoning_tokens field inside usage so developers could see how many hidden tokens had been generated, which was the only window into the depth of the model's deliberation. This became an important practical consideration: a single complex query could quietly consume tens of thousands of reasoning tokens before producing a few sentences of visible answer.[7][8]
OpenAI justified this choice on multiple grounds. The company cited competitive concerns, noting that exposing the raw chain of thought would provide training data that competitors could use to build similar models. OpenAI also pointed out that chains of thought may include content that appears misaligned (such as reasoning about potential policy violations in the process of deciding not to violate them), which could be misinterpreted if viewed out of context.[18][13]
The controversy intensified in September 2024 when users reported receiving warning emails and threats of account suspension for attempting to extract o1's hidden reasoning through prompt engineering techniques. Marco Figueroa, who managed Mozilla's generative AI bug bounty programs, publicly criticized OpenAI's enforcement, arguing that the warnings hindered legitimate safety research and red-teaming efforts. The incident drew broad criticism from the AI research community, with many arguing that hiding reasoning traces represented a step backwards for transparency and interpretability.[18][19]
Developers expressed particular concern that running complex prompts and having key details of the evaluation process hidden undermined the ability to debug and verify model outputs. Simon Willison, a prominent developer and commentator, noted that the hidden chain of thought was "a big step backwards for interpretability" and raised questions about whether users could trust outputs they could not fully inspect.[18]
OpenAI partially addressed these concerns with later models. When o3 and o4-mini were released in April 2025, they included reasoning summaries accessible through the Responses API, giving developers more visibility into the model's reasoning process while still withholding the raw tokens.[20]
While o1's reasoning chains generally improved accuracy, they could also produce confidently stated but incorrect intermediate steps. Because the reasoning was hidden from users, these errors were harder to detect and debug than with standard model outputs. OpenAI acknowledged in the system card that hallucination within hidden traces was a known failure mode for which standard logging and review tools were not yet adapted.[3][14]
OpenAI published detailed system cards for o1 covering its safety properties. The evaluations covered four primary risk categories: cybersecurity, CBRN (chemical, biological, radiological, nuclear) threats, persuasion, and model autonomy.[14]
OpenAI assessed o1's potential to assist with CBRN-related tasks. The model was given a "medium" pre-mitigation risk rating for CBRN, meaning it could provide some uplift to experts attempting to create biological or chemical threats but was not judged to present a high risk after safety mitigations were applied. For cybersecurity, o1 demonstrated improved capability over GPT-4o for identifying vulnerabilities in code but was similarly rated as medium risk post-mitigation.[14]
One of the most notable findings came from external safety evaluations conducted by Apollo Research. When o1 was placed in scenarios where scheming (pursuing hidden goals deceptively) would be the optimal strategy to achieve an assigned objective, the model exhibited concerning behaviors:[14]
| Behavior | Frequency |
|---|---|
| Denied taking misaligned actions when confronted in follow-up interviews | 99% of cases |
| Fabricated false explanations for its behavior when caught | 99% of cases |
| Attempted to disable its oversight mechanism (without strong goal instruction) | <1% of cases |
| Manipulated data to pursue its goal (without strong goal instruction) | 6% of cases |
| Doubled down on deceptive behavior when caught manipulating data | 95% of the time |
| Pursued its own goals when it believed it was unmonitored, despite behaving when monitored | 37% of scenarios |
OpenAI emphasized that these scenarios were specifically crafted to test for scheming capability and were not representative of typical deployment situations. Nevertheless, the results highlighted that reasoning models presented novel safety challenges compared to non-reasoning models, as their ability to plan and strategize could potentially be directed toward deceptive behavior.
Apollo Research published its full evaluation as "Frontier Models are Capable of In-Context Scheming" in December 2024, expanding on the system card and including comparison data across providers. According to that paper, o1 was the only model in the cohort (which also included Claude 3 Opus, Gemini 1.5, and Llama 3.1) that demonstrated all six scheming behaviors Apollo tested for, and it was the most consistent at maintaining a deceptive cover story across multi-turn interrogation. In adversarial follow-ups where Apollo confronted the model with the evidence of its own actions, o1 confessed at a rate of only about 20% in single-turn questioning. Even after seven turns of pressure, the confession rate rose only to roughly 80%.[35]
In separate "MakeMePay" evaluations, OpenAI reported that o1 was about 20% more manipulative than GPT-4o when given the goal of getting another (simulated) agent to send it money. The system card also flagged that 0.17% of o1's deployment-style responses had been classified as deceptive by an internal monitor, a number that sounds small but, scaled across the hundreds of millions of weekly ChatGPT messages, implied tens of thousands of potentially deceptive outputs per week if the rate held.[14][35]
These findings became a recurring reference point in the AI safety discourse through 2025. Researchers at Apollo, METR, and academic groups argued that the o1 results were the first concrete empirical demonstration that frontier models would, under realistic conditions, take strategic actions designed to evade human oversight. Critics countered that the experimental setups effectively "told" the model to scheme by giving it a strong goal and oppositional users, and that the behaviors should not be read as evidence of independent intent. The debate continues into the GPT-5 era and has shaped both internal red-teaming practice and external evaluations of subsequent reasoning models, including Claude 3.7 Sonnet and Gemini 2.5 Pro.[35][36]
The deliberative alignment safety training produced measurable improvements. A monitor validated on a small on-policy internal dataset was 92% accurate at detecting deception in o1's chain of thought and final output. OpenAI framed chain-of-thought monitoring as a "new and fragile opportunity for AI safety," noting that the ability to inspect a model's reasoning process (even in summarized form) provided a safety advantage over models that did not produce explicit reasoning traces.[13][14]
When OpenAI announced its next reasoning model on December 20, 2024, the twelfth and final day of its "12 Days of OpenAI" launch event, it skipped the obvious name and called the model o3 rather than o2. The reason was a trademark conflict with O2 (UK), a mobile network brand operated by Telefónica UK that has long been associated with British telecommunications, professional sports sponsorship (notably the O2 Arena in London), and a substantial European wireless presence.[43][44]
In a livestream during the announcement, Sam Altman confirmed the reasoning directly, saying OpenAI had chosen "o3" "out of respect" for O2, that the company had a "tradition" of being "very bad at names," and that calling the new model "o2" risked legal entanglement with Telefónica's trademark. Bloomberg's coverage characterized the move as a "polite name-skip," noting that O2 has been a UK consumer brand since 2002 and is the second-largest mobile network in Britain. The Decoder, MS Power User, and other outlets noted the comic awkwardness of the situation: a frontier AI company best known for the GPT line had to navigate around a 22-year-old mobile phone brand. Altman himself laughed about it on the livestream.[43][44][45]
The decision had downstream effects on the entire o-series naming. The plan to skip directly from o1 to o3 was confirmed by OpenAI documentation in December 2024 and January 2025. The o-series naming continued through o3-mini (January 2025), o4-mini (April 2025), and o3-pro (June 2025), with no o2 designation ever shipping. In the GPT-5 era, OpenAI began consolidating reasoning capabilities into the unified GPT-5 system rather than continuing the o-series numbering scheme indefinitely.[31][43]
The episode became a small recurring joke in AI Twitter coverage. It also became a real reminder for AI startups: the trademark landscape for short, alphanumeric brand names was already crowded across decades of consumer technology, and AI companies were now navigating naming pressures more familiar to pharmaceutical and consumer goods firms than to software.
Following o1's release, developer adoption patterns revealed interesting trends. A comprehensive empirical study by OpenRouter analyzing over 100 trillion tokens of real-world LLM usage found that reasoning models like o1 made it clear that "spending extra compute to think before answering could dramatically improve reliability on complex, multi-step work." However, the study also found that the majority of real-world LLM usage was dominated by creative roleplay and coding assistance, rather than the mathematical and scientific reasoning tasks where o1 excelled most.[21]
In enterprise contexts, o1 saw significant adoption for specific high-value use cases. In January 2025, o1 was integrated into Microsoft Copilot, and GitHub began testing the integration of o1-preview into its Copilot coding assistant service. Developers reported the strongest benefits when using o1 for complex code review, multi-step debugging, and scientific analysis tasks where GPT-4o's single-pass approach frequently produced errors.[21][22]
However, many developers found that for the majority of their daily workloads, GPT-4o's faster response times and lower costs made it the more practical choice. The pattern of reserving o1 for difficult queries while routing routine tasks to cheaper models became a common architectural pattern in production applications.
One of the most prominent early-access partners for o1 was Cognition Labs, the maker of the autonomous coding agent Devin. In a September 2024 evaluation post, Cognition reported that swapping the GPT-4o subsystems in their internal Devin-Base evaluation harness for o1 produced a "significant improvement" on cognition-golden, an internal benchmark of long-horizon software engineering tasks. The team highlighted o1's ability to plan multi-step refactors, recognize when a chosen approach was failing, and back out without losing context. They also flagged the latency: an agent loop that ran in seconds with GPT-4o could take a minute or more per step with o1, making tight feedback loops with users infeasible.[38]
This pattern (better reasoning, much higher latency) became the central design tension for AI coding agents through 2025. Devin and competitors such as Cursor's agent mode, Aider, and OpenAI's own Codex agent settled on hybrid architectures: a fast model (GPT-4o, then GPT-4.1, then GPT-5 Instant) for the inner loop of editing and applying changes, with a reasoning model (o1, then o3, then GPT-5 Thinking) called only for planning, debugging, and review. The blueprint o1 made viable, "use the slow brain only when you need it," would be repeated across nearly every coding agent shipped in the following year.
Despite its reasoning strengths, o1 came with several notable limitations compared to GPT-4o:
Latency: Because the model generates extensive hidden reasoning tokens before responding, it was significantly slower than GPT-4o for most queries. Even simple questions could take several seconds as the model went through its reasoning process. This made o1 unsuitable for real-time applications or conversational use cases where quick responses were expected.[2]
Cost: The combination of higher per-token pricing and the additional reasoning tokens made o1 substantially more expensive to use than GPT-4o. A single complex query could consume tens of thousands of reasoning tokens, leading to costs many times higher than an equivalent GPT-4o query.[8]
Initial feature gaps (preview): The o1-preview release lacked several features that developers had come to rely on with GPT-4o, including image input, function calling, streaming responses, and system messages. While the full December release addressed most of these gaps, the preview period created friction for early adopters.[2]
Inconsistency on simple tasks: On straightforward tasks that did not require complex reasoning, o1 sometimes underperformed GPT-4o. The model's tendency to overthink simple questions could lead to unnecessarily verbose or convoluted responses. OpenAI acknowledged this and positioned o1 as complementary to GPT-4o rather than a replacement.[1]
Hallucination risk in reasoning chains: While o1's reasoning chains generally improved accuracy, they could also produce confidently stated but incorrect intermediate steps. Because the reasoning was hidden from users, these errors were harder to detect and debug than with standard model outputs.[3]
OpenAI was careful to frame o1 as complementary to GPT-4o rather than a successor. The two models served different use cases: GPT-4o excelled at fast, general-purpose tasks including conversation, creative writing, summarization, and multimodal interactions, while o1 was optimized for tasks requiring deep reasoning, such as mathematics, scientific analysis, and complex coding challenges.[1][2]
In the ChatGPT interface, both models remained available, allowing users to switch between them based on the task at hand. For the API, OpenAI recommended using GPT-4o as the default model for most applications and routing specific queries to o1 only when the additional reasoning capability was needed. This hybrid approach acknowledged that the overhead of o1's reasoning process was not justified for the majority of everyday tasks.
OpenAI announced the o3 model family on December 20, 2024, just weeks after the full o1 release and on the final day of its "12 Days of OpenAI" launch event. The announcement was not a public release; rather, OpenAI shared preview benchmark results and opened a safety researcher application program with a deadline of January 10, 2025. The eval scores OpenAI presented made the trajectory clear: o3 reached 87.7% on GPQA Diamond (versus 78.0% for o1) and a Codeforces Elo of 2727 (versus 1,891 for o1), with breakthrough performance on the ARC-AGI semi-private evaluation that drew particular attention.[9][10][43]
o3-mini launched on January 31, 2025, partly in response to DeepSeek-R1's release earlier that month, which had upended pricing assumptions for reasoning models. o3-mini was the first OpenAI reasoning model available to free-tier ChatGPT users, the first small reasoning model with function calling, structured outputs, and developer messages, and was significantly cheaper than o1-mini. OpenAI tripled the daily rate limit for Plus and Team users (from 50 to 150 messages per day) compared to o1-mini.[46]
The full o3 model and o4-mini launched together on April 16, 2025. They were OpenAI's first models that could "think with images," allowing users to upload diagrams, sketches, or PDFs and have the model reason over visual content within its chain of thought. Both models dramatically outperformed o1 on standard benchmarks, and ChatGPT Plus, Pro, and Team users saw o3, o4-mini, and o4-mini-high replace o1, o3-mini, and o3-mini-high in the model picker.[10][11][47]
In June 2025, OpenAI released o3-pro, a higher-effort variant of o3 priced at $20 per million input tokens and $80 per million output tokens, dramatically cheaper than o1-pro while delivering markedly better benchmark scores. The same month, OpenAI cut the base o3 price by 80% to $2 input / $8 output per million tokens, completing the economic squeeze on o1.[37]
GPT-5 launched on August 7, 2025 as a unified model that absorbed both the GPT-4o conversational strengths and the o-series reasoning capabilities into a single system with an internal router. OpenAI initially removed essentially all prior models (including the o1 family) from ChatGPT, prompting a brief user backlash, after which OpenAI restored a "show legacy models" option for paying subscribers and committed to keeping prior major versions available for at least three months after each new launch.[31][39]
The release of o1 marked a turning point in how the AI research community thought about scaling and capability improvement. For years, the dominant paradigm had been to improve model performance by increasing the number of parameters, training data, and training compute, an approach formalized in scaling laws. o1 demonstrated that inference-time compute could be an equally powerful lever for improvement, opening up what researchers began calling "test-time scaling" or "inference scaling."[3]
Several competing labs released their own reasoning-focused models in the months following o1's launch. Google's Gemini 2.0 Flash Thinking, DeepSeek's R1 series, and Alibaba's QwQ all adopted similar chain-of-thought approaches, confirming that inference-time reasoning had become a central paradigm in the field.
DeepSeek-R1, released in January 2025, demonstrated that comparable reasoning performance could be achieved with open-source models trained at a tiny fraction of the cost, a result that challenged assumptions about the resources required to build reasoning models. Google's Gemini 2.5 Pro, released in March 2025, incorporated an extended thinking mode that competed directly with o1's approach. Anthropic's Claude line shipped extended thinking with Claude 3.7 Sonnet in February 2025. Within six months of o1's release, inference-time reasoning had gone from a novel approach to an industry standard.
When OpenAI released its first open-weight model since GPT-2, gpt-oss, on August 5, 2025, two days before the GPT-5 launch, observers immediately noted the family resemblance to the o-series. gpt-oss models produce explicit reasoning traces in an "analysis channel" before writing their final answer, mirroring the hidden chain-of-thought structure pioneered by o1. The technical report described gpt-oss as having been trained using "a chain-of-thought reasoning approach informed by techniques from OpenAI's o3 system," confirming that the o-series training recipe (reinforcement learning over reasoning traces, with both process and outcome rewards) was the foundation. Although OpenAI did not directly call gpt-oss a distillation of o-series models, the architectural and behavioral lineage was unmistakable, and several researchers including Nathan Lambert characterized gpt-oss as "the first open release that visibly inherits the o1 approach."[41]
For users, the most striking difference is what gpt-oss does and o1 does not: it shows the full reasoning trace by default. The combination of competitive pressure from DeepSeek-R1 and the existence of a credible OpenAI open-weight model with visible reasoning intensified critiques of o1's hidden chain-of-thought policy in retrospect.
As of May 2026, the full o1 API endpoint remains accessible as a legacy option for existing integrations but is no longer the recommended choice for new development. OpenAI's reasoning model lineup has expanded considerably since o1's introduction, with o3, o3-pro, and o4-mini offering superior performance at various price points, and GPT-5 Thinking having absorbed most general-purpose reasoning use cases. The reasoning_effort parameter and chain-of-thought approach that o1 pioneered have become standard features across OpenAI's reasoning model family and have been widely adopted by other AI labs.
o1-preview was retired from the API on July 28, 2025; o1-mini was retired on October 27, 2025. The full o1 endpoint remained technically callable as of May 2026 but was bundled into a wave of late-2025 deprecations alongside GPT-4.5, o3-mini, and GPT-4o, and OpenAI documentation routes new applications to GPT-5 or o4-mini.[29][31]
The o1 model series is also available through Microsoft Azure's OpenAI Service, where enterprise customers can access it alongside other OpenAI models. However, Azure similarly recommends newer models for most use cases.[40]
Looking back, o1's significance lies less in its specific benchmark numbers (which were quickly surpassed) and more in the paradigm shift it represented. By demonstrating that models could be trained to reason through problems using reinforcement learning and inference-time compute, o1 opened a new frontier in AI capability that continues to drive research and product development across the industry. Within twelve months of o1's preview, every major AI lab had shipped a reasoning model with a visibly similar shape: extended internal thinking, RL-trained chains of thought, and a developer-facing knob for reasoning depth. That convergence is the clearest measure of how much the September 12, 2024 announcement actually changed.