The API specification landscape is quickly becoming almost unrecognizable from what it was even five years ago. AI was still on the horizon in 2020, still the territory of sci-fi dreamers and bleeding-edge tech enthusiasts. Now, not only has AI caused an explosion of API usage — the most recent Postman State of the API Report finds that AI usage has caused a 73% increase in API traffic — it’s also causing a tsunami of new specifications to flood the market, as developers, designers, and API engineers race to keep up with the industry’s breakneck pace.

For those paying attention, it can sometimes feel like there’s a new API specification coming out every week. To help you keep track and take stock of all the different API specifications shaping the ecosystem, we’ve put together this guide to the top API specifications of 2025.

1. OpenAPI Specification

The OpenAPI Specification, often referred to simply as OpenAPI, is the most widely used standard for describing HTTP/REST APIs. Originally derived from the Swagger specification, the project was first formalized around 2011 and later renamed in 2016.

OpenAPI describes an API’s endpoints, operations (GET, POST, etc.), parameters, and payloads in a machine- and human-readable format, such as YAML or JSON, so that clients, servers, documentation tools, and testing frameworks can all be used interchangeably. Its popularity comes from the fact that it aligns beautifully with RESTful services and has broad support for tooling like code generation, documentation generation, and mock servers. It allows teams to adopt a contract-first approach: defining the interface, then implementing the logic. Because of this, OpenAPI remains the foundational API specification.

2. Arazzo Specification

The Arazzo Specification is a newer specification developed under the umbrella of the OpenAPI Initiative. Designed for API workflows, Arazzo provides a way to describe sequences of API operations, including the dependencies between calls, and then express those workflows in a machine-readable format. For example, you might describe a sequence like: login → fetch profile → submit order → notify user, and specify the dependencies among them.

A specification for defining workflows and dependencies has become increasingly relevant as microservices and event-driven architectures proliferate, causing applications to coordinate multi-step interactions rather than make single calls to isolated endpoints. Its popularity is also tied to the need for storytelling in API design, making workflows explicit, documented, and executable. Because it complements OpenAPI instead of replacing it, Arazzo is carving out a niche for describing logic flows across APIs.

3. Model Context Protocol (MCP)

The Model Context Protocol (MCP) is one of the hottest new standards, introduced in late 2024 by Anthropic and quickly generating buzz among those interested in AI-agent architectures. While MCP is technically a protocol rather than an API specification, it deserves a spot on this list for how it standardizes the way AI agents discover and call external APIs.

MCP’s purpose differs from traditional API specifications. It enables large language models (LLMs) or AI agents to connect to external tools like data sources, services, and functions in a consistent way. In other words, MCP treats external interfaces as part of the agent’s context-feeding mechanism instead of simply a REST API for human clients. For example, an AI assistant might discover and call tools via MCP rather than having to use bespoke connectors. It supports JSON-RPC 2.0 over transports like stdio or HTTP/SSE while defining the roles of host, client, and server in the interaction.

Because of the prevalence of AI-agent workflows and tool integration, MCP’s popularity has exploded in 2025. It’s clearly one of the most exciting standards to watch this year.

4. gRPC

The gRPC framework and specification was launched by Google around 2015. It defines service methods and message types in a protocol buffer format, uses HTTP/2 as its transport, and supports features such as streaming, bidirectional communication, and efficient binary serialization.

gRPC is primarily used for high-performance microservices communication, where latency, speed, and cross-language interoperability are important. As many modern backend systems adopt microservices or hybrid multicloud architectures, gRPC has become a popular alternative to traditional REST or JSON-over-HTTP.

The gRPC specification enables features such as code generation across multiple languages, efficient network usage, and strong backward compatibility. While it remains somewhat niche, gRPC is increasingly handling mission-critical workloads. Any team working with APIs where performance is key should be familiar with it.

5. AsyncAPI

Event-driven architecture (EDA) is everywhere these days. 85% of organizations recognize the benefits of EDA. The AsyncAPI Specification was created to address this need, defining how to design, describe, and document asynchronous, event-driven APIs, such as those using Kafka, MQTT, WebSockets, message brokers, and other non–request-response architectures. The specification defines channels, messages, operations (publish/subscribe), and bindings for different protocols.

Modern applications like microservices, IoT systems, event-driven platforms, and reactive services have an increasing need to describe event flows and message models. As such, AsyncAPI has become popular for bridging the gap in machine-readable descriptions of asynchronous interfaces, enabling documentation generation, scanning, contract testing for message-based systems, and other tooling similar to what OpenAPI enables for REST APIs.

6. GraphQL

The GraphQL specification defines both a query language and an execution model for APIs, originally developed at Facebook and open-sourced in 2015. Unlike REST, where the server defines the shape of the response, GraphQL allows the client to specify exactly what data it wants, and the server responds accordingly. It supports querying and mutating data and also supports subscriptions for real-time updates. Because many front-end rich applications and mobile apps require flexible data queries and minimal overfetching, GraphQL has become popular in client-centric API design.

The specification has since matured to include strong tooling, introspection capabilities, type systems, and developer-friendly workflows. As APIs increasingly serve rich UIs or multiple consumer types, like mobile, web, or IoT, GraphQL remains an important and popular API specification for designers of data-centric services.

7. JSON:API

JSON:API is a specification for building APIs in JSON. It defines how clients should request resources and how servers should respond in a JSON-based REST API. The specification was first released in 2015, with media type registration in 2013. Its main goal is to reduce the number of HTTP requests, simplify how relationships are expressed, and standardize error handling, metadata, pagination, filtering, inclusion of related resources, and more.

Because REST by itself often leaves many conventions unspecified, leading to inconsistency across APIs, JSON:API offers a convention-over-configuration approach. Teams building many resource-centric endpoints, especially when multiple teams or public APIs are involved, often favor JSON:API for its consistency, discoverability, and predictable developer experience.

8. SOAP/XML Web Services

Before REST and JSON, there was SOAP and XML. SOAP represents an era when APIs were built on rigid contracts and enterprise-grade reliability. It uses XML as its message format, relying on Web Services Description Language (WSDL) to formally define operations, message types, endpoints, and data structures. In contrast to the lightweight REST or event-driven models common today, SOAP was designed for strict structure, interoperability, and guaranteed delivery, the values prioritized by early enterprise and government systems.

SOAP’s ongoing relevance in 2025 comes down to one simple truth: enterprises don’t move fast. Banking, insurance, telecommunications, and healthcare systems still depend on SOAP for vital integrations that require consistency and compliance. Because SOAP supports advanced features like WS-Security, WS-Addressing, WS-ReliableMessaging, and transactional integrity, it remains a fixture in environments that demand strict regulation. Financial institutions still rely on SOAP-based APIs for secure money transfers and credit card processing, for instance, while healthcare systems use it to ensure their medical record transactions comply with HL7.

The Top 8 API Specifications of 2025: A Quick Comparison

Final Thoughts on API Specifications in 2025

The evolution of API specifications in 2025 is the result of not only a technological shift but a whole new paradigm in how developers, designers, and even AI systems think about connectivity. APIs aren’t just about integration between software — they’re the connective tissue of the digital world, the shared language through which humans, systems, and now intelligent agents communicate. The landscape that once revolved around REST and SOAP has exploded into a complex ecosystem of specialized specifications, each addressing a different layer of interaction, intent, or complexity.

OpenAPI is still the backbone of RESTful design, enabling interoperability and rapid development. Meanwhile, specifications like Arazzo and AsyncAPI are pushing beyond single calls, focusing on describing workflows and event-driven interactions that reflect how modern applications behave. gRPC is thriving in distributed systems where performance and streaming matter most, while GraphQL has cemented its position as the go-to for front-end flexibility and efficient data fetching. JSON:API continues to bring order and convention to RESTful chaos, offering clarity where custom implementation was once required.

Perhaps the most transformative addition to the landscape is the rise of MCP, a reflection of the interaction between AI and APIs. MCP doesn’t just describe services for humans, it also describes tools for machines. It allows LLMs and agents to understand, discover, and invoke capabilities autonomously, marking the beginning of APIs designed as interfaces for AI instead of just software. This is a seismic shift that will redefine the API industry in the next decade.

Still, even as new specifications proliferate, SOAP serves as a reminder of where it all started: a time when consistency, reliability, and precision mattered more than speed or flexibility. Its endurance in enterprise environments underscores that API evolution isn’t just about innovation — it’s also about trust, stability, and the cost of change.