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Open weights means a model’s trained parameters are available for people to obtain and use under the release’s terms. Those parameters are the learned numerical values that help turn an input into an output. Their availability does not, by itself, tell you whether the training data or full training code is available—or what uses the release permits.

To assess a particular model, check the weights, code, data disclosures, documentation, license and any separate usage policy. “Open-weight” and “open source AI” are not automatically interchangeable labels.

What are model weights?

Weights are learned parameters: numerical values produced during training that work with a model’s architecture to generate outputs. The Open Source Initiative (OSI) defines weights as parameters that “overlay the model architecture to produce an output from a given input.” A downloadable set of weights gives you access to a trained artifact; it is not the same thing as receiving every component or record behind its creation.

The OSI describes an AI model as comprising its architecture, parameters—including weights—and inference code, which runs the model. Training code and information about training data also matter to its definition of Open Source AI.

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Does open-weight mean open source AI?

Not necessarily. The OSI’s Open Source AI Definition 1.0 sets requirements for a system’s relevant components and the preferred form for modification. It calls for complete code used to train and run the system and sufficiently detailed information about training data. Publicly downloadable weights alone do not establish that a release meets that definition.

The OSI FAQ explains that its definition does not draw a distinction based on whether people call the relevant system a model, weights or parameters: the standard applies to the components that make up the system. In other words, calling a release “open-weight” describes an aspect of access; calling it “open source AI” makes a broader claim that should name the definition being applied.

A separate, narrower Open Weight Definition focuses on distribution and does not require source materials such as training data. It includes conditions such as free redistribution and usable, non-obfuscated weights. Its page identifies it as version 0.3, last modified January 21, 2025. This is a different standard from the OSI definition, not another name for it.

What should you check before using a specific model?

Evaluate the release itself rather than relying on a label. For a meaningful comparison, use the same questions for each model:

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  1. Can you obtain usable weights? Check whether the actual weights are available and how they can be downloaded or accessed. Confirm whether redistribution is permitted under the release’s terms.
  2. What uses are permitted? Read the license and any separate acceptable-use policy or other conditions. A label or license name alone may not describe every restriction or permission.
  3. What code is included? Look for inference code needed to run the model, and separately check whether complete training code and its configuration are available.
  4. What is disclosed about training data? Find out whether the release provides information about the data, and whether it is sufficiently detailed for the openness standard being claimed.
  5. What does the documentation say? Read the model card and repository metadata, then verify the actual release terms. Hugging Face explains that repositories can declare licenses for code or data and that model cards provide repository documentation and metadata.
  6. How reproducible is the release? Consider the documentation, available code and data information together. These are useful comparison criteria, not proof that any single model is fully reproducible.

Can you use an open-weight model commercially?

That depends on the particular release’s license and any accompanying usage policy—not on the words “open weights” alone. Confirm that the intended activity, including commercial use if relevant, is permitted by the applicable terms. Legal effects can vary by jurisdiction; this overview does not resolve the legal status of a particular license.

For example, OpenAI calls its gpt-oss models “open models” or “open-weight” because their trained weights are publicly available under Apache 2.0 and the gpt-oss usage policy. Its help article says the models can be downloaded, run on a user’s own infrastructure or supported hosted frameworks, and customized or fine-tuned. It also notes that some provider infrastructure or tooling around them may remain proprietary. Those statements describe those named models and terms, not a general rule for all open-weight releases.

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What do you get when you download weights?

You get the trained parameters made available by that release. You do not automatically get the complete training recipe, the training data, inference software, documentation or unrestricted rights to use and redistribute the model. Those items vary by release and must be checked individually.

Use precise wording when describing what you have verified: “the weights are publicly available” is safer than “open source AI” when you have only confirmed access to weights. The OECD’s 2025 report notes that licenses designed for source code do not directly apply to AI model weights, another reason to identify the artifact and read its terms rather than infer rights from a label.

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Why do sources use different meanings of “open”?

There is no single phrase that answers every question about access, disclosure and permission. The OSI definition evaluates a broader set of AI-system components and information needed for modification; the Open Weight Definition takes a distribution-focused approach. A UK government glossary in the International AI Safety Report 2025 also describes open-weight models as having publicly downloadable weights and contrasts them with fully open models, but that glossary page is marked withdrawn. It is corroboration, not current government guidance.

When a vendor or repository calls a model “open,” ask which artifact is available, which standard the label refers to, and what the release’s own terms allow. That keeps a claim about downloadable parameters from being mistaken for a promise of complete transparency or unrestricted use.

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