As blockchain applications become more complex, there is a strong need for safe and smart contract languages. Many platforms today rely on languages and virtual machines (VMs) that weren't originally designed to handle the unique constraints and requirements of blockchain execution.
It’s no secret that the architectural decisions made in 2014 for Solidity and the EVM now show clear signs of age. Among these is their susceptibility to vulnerabilities that make writing secure Ethereum smart contracts particularly challenging. Some might argue that these issues can be solved with better tooling, but the problem runs deeper than that. The result? Slow performance, security vulnerabilities, and developer headaches.
Enter Sway——a performant, secure-by-design language that's purpose-built for the next generation of blockchain applications. We built Sway to solve these problems from the ground up. Sway catches exploits at compile time, and handles native assets without overhead, all with a Rust-inspired syntax that feels natural to write.
But don't take our word for it. Let's look at a quick example:
Say you’re writing some simple smart contract method.
You want to release some item from another contract when some condition is met. In Sway, this might look something like this:
Looks quite reasonable, but this particular code has a flaw that could potentially be very dangerous. Storage is only updated after transferring ownership, which means that provided the right circumstances, this could allow for a reentrancy attack.
But this is Sway, so the compiler actually warns you about this:
You may have also noticed the #[storage(read, write)] annotation. Sway will not let code touch critical parts of your contract without your explicit permission, which makes it a lot harder to introduce potentially catastrophic problems through small oversights like this.
We borrow this philosophy from Rust, along with its approach to Zero Cost Abstractions.This combined with domain specific optimizations and easy to use integrated tooling like forc and the Sway Language Server Protocol (LSP), make for a very safe and pleasant language for you to write the next generation of smart contracts.
What is Sway?
Sway is a Rust-inspired domain-specific language purpose-built for the Fuel Virtual Machine (FuelVM). The language serves three distinct purposes:
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Smart Contract Development - The foundation of on-chain logic, following the familiar Ethereum-style approach of deploying stored programs that manage state
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Predicate Creation - A groundbreaking feature that allows developers to write stateless validation logic that can enforce complex conditions without deploying or storing any code on-chain, similar to Bitcoin's script system but with far more power and flexibility
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Script Development - Off-chain testing and complex actions without deployment costs
Sway really shines when you're looking to get a smart contract into production from scratch, but want guardrails to prevent costly mistakes without being boxed in by overly restrictive frameworks. What makes it particularly appealing is how quickly you can go from zero to being productive - the comprehensive tooling integration means you're spending less time on setup and more time actually building. The experience is notably smoother compared to the typical smart contract development workflow.
At its core, Sway emphasizes safety, performance, and developer ergonomics. Sway's power comes from what it doesn't do. Unlike general-purpose languages that try to be everything to everyone, Sway zeroes in on blockchain's unique computing environment:
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Programs run transiently but their storage is permanent and extremely costly, requiring careful consideration of state management
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Gas costs and program size often matter more than raw execution speed, demanding an emphasis on bytecode efficiency
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Every transaction competes for limited blockspace, requiring fine-grained control and minimal overhead at every level
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Immutable ledger semantics demanding perfect code from day one
Sway's domain-specific nature enables it to tackle these challenges head-on through specialized features and optimizations. The compiler catches the usual suspects like re-entrancy vulnerabilities and unexpected memory usage patterns, with built-in optimizations for common web3 operations that you'd otherwise have to handle manually (like minting tokens, creating orders, or signing transactions). Some might argue that general-purpose languages can handle these challenges too, but the overhead they bring makes this impractical at scale.
The endgame here is pretty clear: We want to make Sway the go-to solution for on-chain programming, bringing modern language features without taking away user control or performance.
From Solidity to Sway
Solidity has served the ecosystem well, being the go-to language for Ethereum and EVM chains. But after years of production deployments, certain pain points became clear. Solidity has a less expressive type system that catches bugs too late (or never) and is also susceptible to re-entrancy attacks due to the EVM’s architecture. Solidity also has some early design decisions that complicates token standards and safe transfers today. Its lack of native asset types means every token interaction has to go through complex smart contract calls and approval systems.
The whole ecosystem has been looking for something better, something with stronger guarantees. At Fuel Labs, we figured scaling isn't just about having a faster VM, though that's important, it's about having a language that's actually built for where smart contracts are going, not where they've been. That's why we created Sway: a clean-slate approach focused on throughput and safety.
Rust showed us how a language could revolutionize an entire field by prioritizing safety without sacrificing performance, with concepts like ownership, strict memory safety, and zero-cost abstractions. While Rust is powerful, it also carries complexities—like a borrow checker and runtime model—unsuitable for typical on-chain logic, where resources are heavily constrained and execution costs must remain minimal.
Sway borrows Rust’s familiar syntax and developer-friendly features, including curly-brace style, strong static typing, and trait-based polymorphism. The language also maintains Rust's safety-first approach, implementing compile-time checks to eliminate entire categories of runtime errors.
However, it omits or modifies features that don't make sense for blockchain. This means you end up with something that's lighter than Rust but still preserves its strengths, which turns out to be a pretty good fit for what you actually need when you're writing smart contracts. It's not trying to be everything to everyone - it's specifically built for on-chain use cases, and that focus really shows in how it handles common smart contract patterns.
Sway’s Key Features
Predicates
Predicates in Fuel solve a critical challenge in blockchain architecture: the need for complex transaction validation without adding to permanent state bloat. While traditional smart contracts store their logic and state data using an accounts model, requiring full nodes to maintain this information indefinitely and limiting parallelization by requiring synchronization, predicates function as stateless smart contracts that can perform sophisticated validation independently.
Predicates are monotonic ephemeral spending conditions that can verify complex requirements – like multi-sig conditions, time locks, or mathematical validations – while working more like a bouncer checking IDs than a librarian keeping records. Once a predicate validates a transaction by returning true or false, nothing needs to be stored in the blockchain's state, enabling developers to implement advanced conditional logic without contributing to the growing storage burden on full nodes.
Type System & Safety
Smart contract vulnerabilities have cost the industry billions, and many issues stem from type-related bugs and reentrancy attacks that are hard to catch before deployment.
Sway employs a powerful type system that includes Algebraic Data Types (ADTs) for representing domain-specific states and events with clarity, trait-based polymorphism that are similar to Rust's traits, letting you define shared behaviors across types and generics that let you write reusable code without sacrificing type safety. Moreover, the compiler doesn't just check your code, it understands blockchain-specific patterns like CEI (Checks-Effects-Interactions) and catches reentrancy risks before they hit mainnet. This approach is stricter than patterns we see in Solidity, adding a layer of security right out of the box.
Native Assets and Simplicity in Token Transfers
In Solidity, transferring tokens commonly involves calling a contract to adjust balances, checking return values or revert conditions, and verifying approvals. It works, but it's not exactly elegant. We took a different approach. On the Fuel VM, assets are native, not smart contract abstractions. This means you can transfer tokens with a single line of code, no central contract needed. It's a fundamentally simpler approach that eliminates common sources of errors.
Compiler and Development Tools
We believe great code needs great tools but smart contract development often requires juggling multiple disconnected tools, making the development process fragmented and error-prone.
That's why we built forc (Fuel Orchestrator), a powerful CLI that initializes new Sway projects, handles dependencies, compiles code, and deploys to the Fuel network.
The Sway Language Server Protocol (LSP) offers code completion, inline diagnostics, and automatic formatting in popular editors like VS Code. For full stack developers, Fuel Indexer, Rust SDK, and TypeScript SDK are additional tools for off-chain indexing and for interacting with Sway contracts in your application's backend or frontend.
However, when it comes to development tooling, there are some clear gaps that need filling. While the Explorer and debugging capabilities aren't yet where we want them to be, we're making solid progress. The team has recently shipped major updates to the Explorer's transaction visualization and contract interaction features, and there's active work on a Compiler explorer implementation. Runtime visibility, which used to be a significant pain point, is getting better with each release - we've added better error messages, stack traces, and memory inspection tools. There's still work to be done, but the tooling improvements are shipping at a steady pace and the development experience is getting notably better with each update.
Library Ecosystem and Support Infrastructure
Smart contract devs often need to reinvent basic building blocks, wasting time on common functionality instead of focusing on their unique application logic.
Sway is building towards a comprehensive library infrastructure. Looking at ecosystem priorities, there are some pretty obvious gaps that need to be filled. Basic mathematical operations are still a pain point - robust, well-tested libraries are needed for things like bignum, fixed point, floating point, and even just basic signed integer operations. Error handling has been particularly frustrating. Sure, the library ecosystem is tiny compared to Solidity's, but here's the thing: once we get a proper package manager going, it has the potential to be much more developer-friendly than what's out there now.
As for creating a more cohesive development environment, we've got some existing static analysis tools that have been developed externally like the Sway Analyzer tool for example. It might make sense to start actively promoting these instead of reinventing the wheel. These tools could really help shore up Sway’s development infrastructure if leveraged properly.
The State of Smart Contract Languages
Solidity has become the de facto standard for Ethereum and has such widespread adoption and ecosystem support that it will continue to be widely used for the foreseeable future. But it's also unlikely to change very much, and its developer tools, frameworks, and supporting software are spread across many different projects and teams, without a single unified standard or cohesive integration between them. For example, you might need Hardhat for development, Slither for security checks, OpenZeppelin for contracts, and various other tools - all maintained separately and sometimes not working perfectly together. This fragmented state is likely to persist rather than consolidate.
Meanwhile, Rust has shown the industry how a modern language can provide both safety and performance at scale. But as blockchain technology evolves, new challenges emerge that these pioneering languages weren't originally designed to address.
In contrast, newer smart contract languages have emerged with different approaches. Some prioritize simplicity but sacrifice expressiveness, while others offer robust features but come with steep learning curves. Many remain tightly coupled to their specific blockchain implementations, limiting their broader applicability. Sway aims for broader adoption and is less tied to a single blockchain's design. Some languages have even split into multiple incompatible dialects over time, creating additional complexity for developers.
What Makes Sway Stand Out
Sway has its own issues too (notably the language is still much less stable than alternatives) but the tooling is really some of the best available and it's potentially very expressive. There is a good niche that is yet to be filled by a language that has a quality set of easily available libraries with a package manager and enough behavior defined by types and enforced at compile time that you can be reasonably sure your contract isn't going to blow up if it compiles without error.
Sway breaks from tradition in five key ways. First, it uses traits (powerful class-like patterns) to make code more reusable and type-safe. Second, unlike languages that force specific patterns, Sway lets you control exactly how your code behaves on-chain. Third, like we’ve mentioned before, it maintains safety through robust static analysis that catches common bugs at compile time, saving millions in potential exploits. Fourth, it handles both contracts and predicates in one language (imagine writing both smart contracts and unlock conditions with the same tools). Finally, its tooling just works. From editor support to deployment, everything's integrated out of the box.
Getting Started with Sway
The easiest route to installing Sway and the associated Fuel toolchains is through fuelup, which manages versions of the Fuel toolchain (including Sway, forc, Fuel indexer, etc.):
Then, confirm installation by running:
This should list the components installed. You can also configure your environment to use the Sway Language Server in VSCode or any other editor supporting the LSP for auto-completion, syntax highlighting, and error-checking.
For those wanting a zero-install experience, the Fuel Labs team and community are working on a Sway Playground—a browser-based environment where you can experiment with Sway code without installing anything locally. Simply open the Playground, select a template (e.g., a token contract), and tweak to your heart’s content.
The official Sway Applications repository on GitHub includes example token contracts that demonstrate both basic and advanced ERC-20-like functionality for the Fuel network, NFT implementations that showcase efficient handling of collectible assets, and DeFi primitives including lending protocols, DEX examples and more.
Community tutorials also pop up regularly, offering step-by-step guidance for building your first Sway-based dApp. From scripting simple token swaps to orchestrating multi-contract interactions, these guides reduce the learning curve dramatically.
Testing, Auditing, and Deployment
Sway offers built-in testing capabilities. Developers can write tests directly within their project files:
The forc test command compiles and runs these tests on a local Fuel VM environment, helping you catch logic errors before you deploy your code to production.
Once your contract is tested and ready, you can use the Rust SDK which gives you type-safe contract interactions right from your backend, leveraging typed bindings that ensure type-safe calls. Or the TypeScript SDK which makes frontend integration a breeze - perfect for your React, Vue or other Javascript frameworks.
Deployment is straightforward with forc deploy, which sends the compiled contract to a Fuel network node. Just specify your node, set your gas limits, and you're live.
One of Sway’s strengths is that it reduces the surface area for bugs through robust static analysis and compile-time checks. However, professional audits still serve an important function, especially for high-value DeFi projects. Multiple security firms and community-led audit groups are beginning to explore Sway’s codebase to provide specialized tools and services. The ultimate goal is a future where auditing is both simpler and more thorough, thanks to Sway’s language design.
Contributing to Sway
Sway is entirely open-source. Its development thrives on community feedback and contributions. You can:
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Open an Issue: Report bugs, request new features, or propose language improvements.
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Submit Pull Requests: Help fix bugs or implement new features in the compiler, language server, or documentation.
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Join Community Calls: Engage with the Fuel Labs team and other developers in Discord or community forums to stay updated and share ideas.
Just as Ethereum has ERC standards, the Fuel community is developing Sway’s SRC (Sway Request for Comments) standards to define common interfaces (e.g., token operations, NFT structures, or multi-sig protocols). Active participation in developing and implementing these standards ensures compatibility across Sway-based projects and helps shape the best practices for the ecosystem.
Conclusion
Sway represents a next-generation approach to writing secure and performant smart contracts. By drawing inspiration from Rust, focusing on domain-specific blockchain constraints, and offering integrated tooling, Sway lowers the barrier to entry for developers while raising the security bar. It bypasses the historical baggage of EVM-based languages like Solidity and the complexities of general-purpose systems languages like Rust.
As you explore Sway, you’ll discover a language deliberately shaped to simplify your most common on-chain tasks—from token transfers to advanced DeFi logic—while providing robust safeguards against vulnerabilities. Whether you’re a seasoned Solidity developer looking for higher throughput and better safety or a Rust enthusiast intrigued by the next wave of modular blockchains, Sway is worth your attention.
In the broader scheme, Sway stands at the forefront of a movement to rethink how we build decentralized applications. By marrying clarity, security, and performance, it helps usher in a future where smart contract development is as accessible as it is powerful. We invite you to try Sway, join the growing community, and contribute to shaping the evolution of smart contracts on Fuel and beyond.
To dive deeper into Fuel’s philosophy and architecture, as well as what sets us apart from other blockchain solutions, check out the Fuel Book. Check out our documentation for detailed instructions on how to get started in the Fuel ecosystem.