When building or integrating with web services, you'll encounter two foundational approaches that shape how applications communicate: SOAP and REST. While REST has become the standard for modern APIs due to its simplicity and web-native design, SOAP remains critical in enterprise environments where formal contracts and advanced security are non-negotiable.
This guide breaks down the technical differences between these approaches, compares their performance on modern workloads, and provides a clear decision framework for choosing the right protocol for your project in 2025.
When building or integrating with web services, you’ll inevitably encounter two foundational approaches: SOAP and REST. While they both enable applications to communicate over a network, they operate on fundamentally different principles. REST, an architectural style, has become the de facto standard for public and mobile APIs due to its simplicity and alignment with web standards.
However, SOAP, a formal protocol, remains deeply embedded in enterprise, financial, and government systems where strict contracts and advanced security are non-negotiable. Understanding the core differences isn't just an academic exercise; it's a practical necessity for making informed architectural decisions in 2025. This guide breaks down each approach, compares them on modern workloads, and provides a clear framework for choosing the right path for your project.
SOAP protocol fundamentals
SOAP, or Simple Object Access Protocol, is a standardized protocol that defines a strict, XML-based messaging structure. Unlike REST, it's a formal protocol with a rigid specification, which includes built-in rules for advanced security and transactions. This makes it a common choice for enterprise environments where strict contracts are required.
Every SOAP message is an XML document consisting of a few key parts that ensure a standardized, self-describing request. The structure of a SOAP message is called an envelope, which acts as a wrapper for the entire message.
Envelope: The root element of every SOAP message that identifies the XML document as a SOAP message.
Header: An optional element that contains application-specific information, like authentication or routing details. The powerful WS-Security standard, for example, is implemented here.
Body: A mandatory element that contains the actual message being sent, such as the specific function to call and its parameters.
Fault: An optional element within the body that holds information about errors that occurred while processing the message.
This rigid structure is formally defined by a Web Services Description Language (WSDL) file. The WSDL acts as a formal contract, detailing exactly what the web service can do, what data it expects, and what it will return. This contract-first approach allows for strong typing and enables automatic code generation for clients.
REST architectural fundamentals
REST, or Representational State Transfer, is an architectural style, not a protocol. It provides a set of guiding principles for designing networked applications, leveraging the existing standards of the web, primarily HTTP. This makes RESTful APIs intuitive for developers familiar with web technologies.
At its core, REST is resource-oriented. Every piece of content or object is a resource, and each is identified by a unique URL, like /users/123. You interact with these resources using the standard HTTP verbs that browsers already use.
GETto retrieve a resource.POSTto create a new resource.PUTto update an existing resource.DELETEto remove a resource.
A key principle of REST is statelessness. Each request from a client to the server must contain all the information needed to understand and complete the request. The server does not store any client context between requests, which simplifies server design and improves scalability. While REST can technically use XML, its overwhelming preference is for JSON (JavaScript Object Notation), which is more lightweight and easier for both humans and machines to parse.
Protocol comparison for modern workloads
Choosing between SOAP and REST requires understanding their trade-offs in the context of modern application development. Factors like performance, security, and developer experience weigh heavily in the decision.
Message size and bandwidth
SOAP messages are exclusively XML and wrapped in a verbose envelope structure, leading to larger message sizes. This can impact performance, especially in bandwidth-constrained environments like mobile networks.
REST typically uses JSON, which is inherently less verbose than XML. The lack of a required envelope makes REST payloads significantly smaller and faster to parse, resulting in better performance and lower latency.
Security model depth
SOAP has a comprehensive, built-in security standard called WS-Security. It provides enterprise-level features like message-level encryption and digital signatures. This is a major reason for its continued use in banking and government.
REST does not have a prescribed security standard. Instead, it relies on transport-level security, almost always by enforcing HTTPS. Authentication is typically handled with patterns like API keys, JSON Web Tokens (JWT), or OAuth 2.0.
Transport and queue flexibility
A key advantage of SOAP is its transport independence. While it can use HTTP, it's also designed to work over other protocols like SMTP (email) and JMS (Java Message Service). This makes it suitable for asynchronous or distributed enterprise workflows.
REST is almost exclusively tied to HTTP. This simplifies its design and makes it a natural fit for web-based applications but limits its use in scenarios that require a different transport protocol.
Developer workflow impact
The developer experience differs significantly. SOAP's contract-first approach relies on WSDL files, which can be used to automatically generate client-side code. However, debugging can be complex due to the XML verbosity.
REST's code-on-demand style is generally considered more flexible. Developers can easily test endpoints with browser tools or curl. The rise of the OpenAPI Specification provides a way to define REST API contracts, enabling powerful tooling for documentation and automatic SDK generation that feels idiomatic and easy to use. Learning how to create OpenAPI specs depends on your API design complexity and available tools.
Feature | SOAP | REST |
|---|---|---|
Type | Protocol | Architectural Style |
Data Format | XML only | JSON, XML, HTML, etc. |
Transport | HTTP, SMTP, JMS, etc. | Primarily HTTP |
Security | WS-Security (built-in) | HTTPS + OAuth, API Keys |
State | Can be stateful | Stateless |
Contract | WSDL (strict) | OpenAPI (optional) |
Performance | Slower, more overhead | Faster, lightweight |
Decision framework for 2025 projects
Your choice of protocol should be driven by your project's specific requirements. Here’s a simple framework to guide your decision.
Enterprise compliance requirement
You should strongly consider SOAP if your project involves integrating with legacy enterprise systems that only expose SOAP endpoints. It's also the right choice when you have strict regulatory requirements for message-level security and transactional integrity, common in finance and healthcare.
Public API scale target
REST is the default choice for public-facing APIs, mobile applications, and web clients. Its low barrier to entry, excellent performance, and vast ecosystem of tools lead to faster developer onboarding and wider adoption. If your goal is to build a large developer community, REST is the clear winner.
AI agent readiness factor
Modern AI agents are built to consume and interact with the web. RESTful APIs, especially those documented with an OpenAPI spec, are far easier for these agents to use. A well-defined REST API can be automatically converted into a Model Context Protocol (MCP) server, allowing AI models to discover and call your API endpoints using natural language. The ability to generate an MCP server from an OpenAPI spec exposes your API as tools that LLMs can understand and use.
Legacy migration path
You don't always have to choose one or the other. If you're stuck with a legacy SOAP service but need the flexibility of REST, you can implement a façade. This involves building a thin REST API that acts as a translator, accepting modern REST/JSON requests and converting them into SOAP/XML calls to the backend service.
Stainless workflow for both protocols
Automating the developer experience is key, regardless of the underlying protocol. Modern tooling can bridge the gap between legacy systems and modern development practices.
For a native REST API, the workflow is straightforward. You start with an OpenAPI specification, which acts as the source of truth. From there, a robust generator can produce high-quality, idiomatic SDKs in multiple languages, complete with features like automatic retries and authentication helpers. The Stainless SDK generator makes this process available to every developer with a REST API.
This same spec can also generate an MCP server, making your API instantly AI-ready. The transformation from API to MCP requires careful consideration of tool structure, schemas, and workflows.
For a legacy SOAP API, the workflow starts with the façade pattern. Once you have a REST façade in front of your SOAP service, you can generate an OpenAPI spec from it. From that point on, you follow the exact same automated workflow as a native REST API, generating SDKs and other developer tools that hide the underlying complexity from your end-users. Remember that your API isn't finished until the SDK ships—SDKs shape developer experience and reduce support burdens.
Frequently asked questions about SOAP and REST
What is SOAP API used for?
SOAP is primarily used in enterprise environments for B2B integrations, financial services, and government systems that require formal contracts, high security, and transactional reliability.
How can I tell if an API is REST or SOAP?
Look at the endpoint and the data format. If you see a WSDL file and messages are structured in an XML envelope, it's SOAP. If you're interacting with resource-based URLs and receiving JSON data, it's almost certainly REST.
Is SOAP obsolete in 2025?
SOAP is niche, not obsolete. While REST dominates new development, SOAP remains a critical technology for specific enterprise use cases and will be part of the technology landscape for the foreseeable future.
Can I convert SOAP to REST without rewriting everything?
Yes, by using a REST façade. This is an intermediate service that translates RESTful HTTP requests into SOAP requests for a legacy backend, allowing you to offer a modern interface without a complete overhaul.
When should I look at GraphQL or gRPC instead?
Consider GraphQL when you want to give clients the power to request exactly the data they need, avoiding over-fetching with complex data models. Look at gRPC for high-performance communication between internal microservices, where efficiency is more important than human readability.
Ready to provide a first-class developer experience for your API? Get started for free.