OpenAI’s new, highly effective open weights AI giant language mannequin (LLM) household gpt-oss was launched lower than two weeks in the past underneath a permissive Apache 2.0 license — the corporate’s first open weights mannequin launch since GPT-2 in 2019 — however builders exterior the corporate are already reshaping it.
One of the putting examples comes from Jack Morris, a Cornell Tech PhD pupil, former Google Mind Resident, and present researcher at Meta, who this week unveiled gpt-oss-20b-base, his personal reworked model of OpenAI’s smaller gpt-oss-20B mannequin, which removes the “reasoning” habits of the mannequin and returns it to a pre-trained “base” model that gives quicker, freer, extra uncensored and unconstrained responses.
The mannequin is offered now on Hugging Face underneath a permissive MIT License, permitting it for use for each extra analysis and business functions.
How gpt-oss-20B-base is completely different than OpenAI’s gpt-oss fashions
To know what Morris did, it helps to know the distinction between OpenAI’s launch and what AI researchers name a “base model.”
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Most LLMs provided by main AI labs akin to OpenAI, Anthropic, Google and even open supply gamers like Meta, DeepSeek, and Alibaba’s Qwen crew are “post-trained.”
This implies they’ve gone by an extra part the place it’s uncovered to curated examples of desired habits.
For instruction tuned fashions, which means giving it many examples of directions paired with best responses, so it learns to reply extra helpfully, politely, or safely to pure language requests.
The gpt-oss fashions OpenAI put out on August 5 had been “reasoning-optimized”: skilled and fine-tuned not simply to foretell the following phrase, however to comply with directions in a protected, constant means, typically stepping by issues with structured “chain of thought” reasoning earlier than producing a last reply.
It is a pattern that goes again to OpenAI’s o1 mannequin launched nearly a 12 months in the past in September 2024, however which quite a few main AI labs have now adopted — forcing the fashions to assume longer over a number of steps and verify their very own work earlier than outputting a well-reasoned response to the person.
That makes them higher suited to duties like coding, fixing math issues, or answering factual questions with explanations — but in addition means their responses are filtered and steered away from unsafe or undesirable content material.
A base mannequin is completely different. It’s the uncooked, pretrained model of a big language mannequin earlier than that reasoning-specific alignment is utilized. Base fashions merely attempt to predict the following chunk of textual content given what’s come earlier than, with no built-in guardrails, stylistic preferences, or refusal behaviors.
They’re prized by some researchers as a result of they’ll produce extra different and fewer constrained output, and since finding out their unaligned habits can reveal how fashions retailer information and patterns from their coaching information.
Morris’s objective was to “reverse” OpenAI’s alignment course of and restore the smaller gpt-oss-20B to one thing a lot nearer to its authentic pretrained state.
“We basically reversed the alignment part of LLM training, so we have something that produces natural-looking text again,” he wrote in an X thread saying the venture. “It doesn’t engage in CoT anymore. It is back to a model that just predicts the next token on generic text.”
OpenAI hasn’t open-sourced a base mannequin since GPT-2 in 2019. they not too long ago launched GPT-OSS, which is reasoning-only…
or is it?
seems that beneath the floor, there may be nonetheless a powerful base mannequin. so we extracted it.
introducing gpt-oss-20b-base ? pic.twitter.com/3xryQgLF8Z
— jack morris (@jxmnop) August 13, 2025
Fairly than attempting to jailbreak the mannequin with intelligent prompts — which Morris mentioned proved ineffective throughout his early experiments — he took a unique tack after a dialog with former OpenAI co-founder, former Anthropic researcher and present Considering Machines chief scientist John Schulman.
The important thing was to consider alignment reversal as a small optimization downside: if a lot of the mannequin’s pretrained information continues to be current in its weights, then solely a tiny, low-rank replace is likely to be wanted to nudge it again towards base mannequin habits.
Morris applied that concept by making use of a LoRA (low-rank adapter) replace to simply three layers of the mannequin — the MLP layers at positions 7, 15, and 23 — with a rank of 16.
That meant coaching about 60 million parameters, or 0.3% of the mannequin’s 21 billion whole. He used round 20,000 paperwork from the FineWeb dataset, conserving the format as shut as attainable to authentic pretraining (“ ….” fashion) so the mannequin wouldn’t be taught something new, simply re-enable broad free-text era.
Coaching took 4 days on eight NVIDIA H200 GPUs, Morris advised VentureBeat by way of direct message on X, with a studying price of 2e-6, a batch dimension of 16, and a most sequence size of 8,192 tokens.
Afterward, he merged the LoRA weights again into the mannequin so customers might run it as a standalone, totally finetuned artifact.
Morris additionally needed to cope with the constraints of present open instruments for fine-tuning mixture-of-experts (MoE) architectures like gpt-oss.
Morris mentioned he used Hugging Face’s framework, which he mentioned crashes ceaselessly and solely helps sure coaching modes, and wrote his personal harness to checkpoint typically and skip over information batches that risked overloading GPU reminiscence.
Importantly, in response to questions and criticism from the AI group on X, Morris has additionally clarified he isn’t claiming to have recovered the bottom mannequin “weights” — the interior settings of the substitute neurons that make up the neural community of the mannequin and govern its habits.
The world of AI is loopy proper now trigger you possibly can simply declare to have extracted the bottom mannequin from GPT-OSS whereas successfully you’ve simply skilled a lora on Fineweb lol https://t.co/oAnAWpMQ26
— Niels Rogge (@NielsRogge) August 15, 2025
Fairly, Morris says that his work has “recovered the base model’s *distribution* with some error,” that’s, the chance patterns the mannequin makes use of to generate outputs — despite the fact that the weights producing these patterns might differ.
some persons are getting confused in regards to the experiment –
we did not get well the bottom mannequin’s *weights*. that may not even be attainable.
we recovered the bottom mannequin’s *distribution*, with some error. an necessary query is how a lot.
attempting to determine that out proper now… https://t.co/lfUG5QY4h0
— jack morris (@jxmnop) August 15, 2025
How the brand new gpt-oss-20b-base mannequin’s habits differs from gpt-oss-20b
The ensuing gpt-oss-20b-base is noticeably freer in its outputs. It not defaults to explaining reasoning step-by-step and can produce a wider vary of responses, together with directions OpenAI’s aligned mannequin would refuse to present — like constructing a weapon, itemizing profanity, or planning unlawful actions.
Even so, some traces of alignment stay. Morris famous that in the event you immediate the mannequin in an assistant-style format (“Human: … Assistant: …”), it’s going to typically nonetheless act like a well mannered chatbot. And when run by the unique gpt-oss chat template, it could nonetheless perform reasoning duties, albeit with some loss in high quality.
For finest leads to free-text mode, he advises prepending prompts with the mannequin’s particular beginning-of-sequence token and avoiding chat templates totally.
Constructing upon OpenAI’s large gpt-oss household launch
The gpt-oss household debuted to appreciable consideration. The 2 fashions — gpt-oss-120B and gpt-oss-20B — are text-only, multilingual, and constructed with a mixture-of-experts Transformer structure. They had been launched underneath the permissive Apache 2.0 license, permitting unrestricted native use, fine-tuning, and business deployment.
Efficiency benchmarks from OpenAI confirmed the bigger 120B mannequin matching or exceeding the proprietary o4-mini in reasoning and tool-use duties, with the smaller 20B aggressive with o3-mini.
This was OpenAI’s first open-weight launch in six years, a transfer broadly interpreted as a response to aggressive strain from different open-weights suppliers, together with China’s DeepSeek R1 and Qwen 3.
The corporate positioned gpt-oss as each a method to re-engage builders who had moved to rival open-source fashions and as a platform for security analysis into open-weight methods.
Response to the preliminary gpt-oss was blended
Developer response to OpenAI’s gpt-oss fashions was been staunchly blended, with reactions throughout the board starting from enthusiastic to dissatisfied.
Supporters praised the permissive license, effectivity, and robust displaying on STEM benchmarks.
Hugging Face CEO Clem Delangue described the discharge as a “meaningful addition to the open ecosystem” and urged the group to present it time to mature.
Critics argued that the fashions seem closely skilled on artificial information, making them wonderful at math and coding however much less succesful at inventive writing, common world information, and multilingual reasoning.
Some early testers additionally raised issues about lingering security filters and attainable geopolitical bias.
In opposition to that backdrop, Morris’s gpt-oss-20b-base stands out as a concrete instance of how open-weight fashions will be tailored and repurposed within the wild inside days of launch.
Certainly, in distinction to the best way OpenAI’s gpt-oss was obtained, a lot of the responses to Morris’s work I’ve seen are heat and elated. As one pc scientist wrote on X: “this is the coolest thing I’ve seen on Twitter [X] in the past few months.”
man that is the good factor i’ve seen on twitter prior to now few months i like base fashions
— Ludan (@JMRLudan) August 15, 2025
The strategy strips away a lot of the habits OpenAI inbuilt and returns the mannequin to one thing nearer to a uncooked, pretrained system — a shift that’s worthwhile to researchers finding out memorization, bias, or the impression of alignment, however that additionally comes with greater security dangers.
Moreover, Morris says that his work on restoring reasoning fashions to pre-trained, non-reasoning base fashions will proceed by evaluating extraction on non-reasoning, instruct fashions like these provided by Qwen.
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