Antimony 3 Brings Human-Readable Modeling Up to Speed with SBML Level 3

Systems biologists have long faced a frustrating trade-off: the models that describe the intricate dance of biochemical networks are often written in formats that machines love but humans find nearly impossible to read. A new release of an open-source modeling language aims to close that gap. Antimony 3, described in a paper published in PLOS Computational Biology by Adel Heydarabadipour, Lucian P. Smith, Joseph L. Hellerstein, and Herbert M. Sauro, is a comprehensive update to a language that lets scientists write out biological models in plain, human-readable text while remaining fully compatible with the Systems Biology Markup Language, or SBML, the community standard that underpins computational modeling in biology.

Antimony was originally developed by the systems biology community to solve a simple but persistent problem. SBML, the XML-based format used to exchange models between software tools, is precise and machine-friendly, but its nested tags make it cumbersome for researchers to write or inspect by hand. Antimony offered an alternative: a concise syntax in which a scientist can define species, compartments, reactions, and parameters in a few lines of text, then convert seamlessly back and forth between that text and standards-compliant SBML. The idea proved popular, and Antimony became embedded in widely used tools across the field, including the popular simulation environment Tellurium.

But software standards do not stand still. Since Antimony’s original release, SBML itself has evolved significantly, most notably with the introduction of SBML Level 3 and a growing family of extension packages that add capabilities the original specification lacked. Modeling practice has evolved too, with researchers increasingly combining kinetic reaction models with constraint-based approaches, attaching statistical uncertainty to parameters, and demanding publication-quality visualizations of their networks. Antimony’s developers recognized that the language needed a formal, comprehensive update to keep pace, and Antimony 3 is the result: a version that consolidates years of cumulative improvements and extends the language’s reach into the newest corners of the SBML ecosystem.

The headline technical advance in Antimony 3 is its expanded support for SBML Level 3 Core and several of its most important packages. The update covers the Flux Balance Constraints package, known as FBC, which underpins flux balance analysis, a technique that uses stoichiometric constraints to predict metabolic fluxes in genome-scale models. It also supports the Distributions package, which allows modelers to represent parameters as probability distributions rather than fixed numbers, capturing uncertainty in experimental measurements. The Layout and Render packages round out the list, enabling models to carry structured spatial diagrams and visual styling information alongside their mathematical content.

The practical significance of that package support is that Antimony 3 allows modelers to combine capabilities that previously required separate tools or manual file editing. A single plain-text model specification can now mix kinetic reactions with flux balance analysis, represent uncertain parameters using probability distributions, add biological context through annotations, and define publication-ready visualizations, all in one unified format. For researchers who build, share, and reuse models across laboratories and software platforms, this means fewer format conversions, fewer opportunities for error, and a single readable document that captures the full richness of a modern computational model.

Under the hood, Antimony 3 is delivered as a lightweight C/C++ library with a stable C API, a design choice that makes it easy to embed in other applications. The library ships with official bindings for three major scripting ecosystems: Python, Julia, and JavaScript compiled to WebAssembly. The WebAssembly support is particularly notable, because it means Antimony can run directly inside a web browser, opening the door to browser-based modeling tools that require no installation at all. The developers have also released a cross-platform desktop graphical interface, so researchers who prefer point-and-click workflows can use the language without touching a command line.

This multi-platform delivery reflects a broader trend in computational biology, where the same model may need to live in a Jupyter notebook, a desktop application, and a web service. By providing a stable C API at its core and consistent bindings above it, Antimony 3 ensures that a model written once behaves identically across all of these environments. The library is released as open-source software under the permissive BSD 3-Clause License, and the source code is freely available on GitHub, allowing anyone to inspect, modify, or integrate it into their own projects without licensing barriers.

For the systems biology community, the release addresses a growing pain point. As models have grown larger and more sophisticated, the burden of maintaining them in machine-oriented formats has grown with them. Genome-scale metabolic models, for example, can contain thousands of reactions, and constraint-based analyses increasingly sit alongside dynamic kinetic simulations in the same research workflow. A human-readable language that spans both worlds lowers the barrier for experimentalists who want to check what a model actually says, for students learning to build their first networks, and for reviewers who need to verify that a published model matches its description in a paper.

The emphasis on uncertainty representation also speaks to a maturing field. Point estimates for kinetic parameters are often optimistic, since experimental measurements carry noise and biological variability. By supporting the SBML Distributions package, Antimony 3 lets modelers encode that variability directly in the model definition, making uncertainty an explicit part of the shared artifact rather than a footnote in the accompanying text. Combined with annotation support for biological context, the language pushes models toward becoming self-describing documents that carry their evidence, their mathematics, and their visual presentation together.

Antimony 3 is available now at the project’s GitHub repository, and the developers describe it as a formalization of the language’s cumulative evolution rather than a departure from it. Existing users should find their models carry forward, while gaining access to the extended SBML Level 3 features. As computational models take on ever larger roles in drug discovery, metabolic engineering, and basic biological research, tools that make those models easier to write, read, and trust are likely to matter more with every passing year. With this release, the community’s answer to machine-only modeling formats has caught up with the state of the art.

Subject of Research: A human-readable modeling language for SBML Level 3 systems biology models

Article Title: Antimony 3: Extending human-readable model definitions for SBML Level 3 Core and Packages

Article References: Heydarabadipour, A., Smith, L. P., Hellerstein, J. L., & Sauro, H. M. (2026). Antimony 3: Extending human-readable model definitions for SBML Level 3 Core and Packages. PLOS Computational Biology, 22(10), e1014207. https://doi.org/10.1371/journal.pcbi.1014207

Image Credits: AI Generated

DOI: 10.1371/journal.pcbi.1014207

Keywords: Antimony, SBML Level 3, systems biology, flux balance analysis, FBC package, Distributions package, computational modeling, open-source software, biochemical networks, model interchange, WebAssembly, PLOS Computational Biology