SIL: A semantic interface layer for web applications and AI agents The Semantic Interface Layer (SIL) v1.1.0, an open specification for providing AI agents with a direct semantic interface to web applications, is now defined as an extension of the standalone Semantic Text Format (STF) core specification, published at github.com/ais-space/stf. SIL adds a structured textual representation of application state, objects, relationships, navigation, capabilities, and actions, allowing agents to understand an application's structure and meaning without reverse-engineering presentation artifacts, while preserving the human interface. A text-based interface protocol for intelligent agents. SIL Semantic Interface Layer is an open specification for providing AI agents with a direct, semantic interface to web applications. A conventional web application is primarily designed for human interaction. Its interface exposes information through visual layout, navigation, controls, client-side behavior, and other presentation mechanisms. An agent can use that interface, but often has to reconstruct the application's structure and meaning from those presentation artifacts. SIL adds another interface to the same application: a structured textual representation of its application state, available objects, relationships, navigation, capabilities, and actions. The human interface remains. SIL does not replace it. The goal is to give an agent the equivalent of a well-defined interface rather than requiring it to reverse-engineer one. The two interfaces can share the same backend, business logic, data, authentication, authorization, and application state. ┌─────────────────────┐ │ Application │ │ │ │ business logic │ │ data & state │ │ permissions │ └──────────┬──────────┘ │ ┌─────────────┴─────────────┐ │ │ ▼ ▼ Human Interface Semantic Interface HTML SIL / STF │ │ ▼ ▼ Humans Agents This is not necessarily duplication of information. It is a separation of presentation according to the capabilities and requirements of the consumer. Agents can already interact with web pages, APIs, MCP tools, and other interfaces. SIL addresses a different problem: How can an agent understand an application itself — its structure, current state, available actions, navigation, and boundaries — without reconstructing that information from presentation-oriented artifacts? For example, an HTML page may contain a button whose meaning is determined by its surrounding elements, styling, JavaScript handlers, application state, and visual context. A SIL representation can state that meaning directly: DownloadButton Type: Button Role: PrimaryAction Id: btn download Caption: Download Actions: Activate The difference is not that one contains information and the other does not. The difference is where the interpretation has to happen . With a conventional interface, the agent may have to infer semantics from presentation. With SIL, the application can expose those semantics explicitly. SIL defines an application vocabulary and interface protocol for intelligent agents. As of SIL v1.1.0, the underlying syntax and semantic core are no longer defined inline. SIL is now specified as an extension of STF core — a separate, standalone specification Semantic Text Format published independently at github.com/ais-space/stf https://github.com/ais-space/stf . The relationship is now: STF core standalone specification ├── syntax §2 ├── semantic model §3 ├── structural security §4 └── data / serialization model §5, §6 SIL v1.1.0 this specification ├── SIL Vocabulary — application extension of STF core §3 ├── Application Vocabulary §4 ├── SIL Protocol — discovery & interaction §5 ├── Interaction Patterns §6 ├── Security-specific guidance §7 ├── Conformance profiles §8 └── Extensions & Future Directions §9 STF core is referenced normatively, by reference, without version pinning : any SIL document is, by construction, a valid STF document. When STF core evolves, SIL tracks it at the application layer. STF core is intentionally textual and hierarchical, designed to remain readable by humans while being straightforward for language models to interpret. The SIL specification is itself written in STF, making it a self-describing, conformant document rather than merely a document about the format. A SIL-enabled application exposes semantic pages through HTTP. A typical interaction looks like this: Agent SIL Server │ │ │ GET /revizor.sil │ │──────────────────────────── │ │ │ │ STF document │ │<────────────────────────────│ │ │ │ POST /revizor.sil │ │ {"intent":"activate", │ │ "target":"btn download"} │ │──────────────────────────── │ │ │ │ Updated STF document │ │<────────────────────────────│ The returned document can contain: - application context; - site navigation; - page structure; - semantic objects; - object state; - relationships; - available actions; - forms and fields; - validation information; - permissions and interaction constraints; - user data boundaries; - agent-only application spaces. The exact capabilities available depend on the SIL conformance profile implemented by the server. Objects describe their type, role, state, and available capabilities instead of requiring the agent to infer them from presentation. SIL is designed for direct interpretation by language models. It does not require task-specific fine-tuning or a vision model merely to understand the application's basic structure. A SIL response represents the application's current state. It can therefore be used for interaction rather than serving merely as static documentation. Actions are represented explicitly and submitted through the SIL protocol. The agent does not need to synthesize arbitrary JavaScript or reconstruct UI event handlers. SIL distinguishes application-defined knowledge from user-provided data at the structural level. This allows an implementation to establish explicit trust boundaries between information that defines application behavior and information that must be treated as data. SIL does not claim to make an application inherently secure. Authentication, authorization, validation, transport security, business rules, and implementation-specific security remain the responsibility of the application. A SIL interface does not have to correspond one-to-one with an HTML page. An application can expose Agent Spaces that exist specifically for agent interaction and have no human-facing counterpart. SIL defines conformance profiles so that an implementation can start with a limited read-only interface and add interaction capabilities progressively. Current profiles include: Core Core Read Forms Events Agent Spaces For example: This implementation conforms to SIL v1.1.0: Core + Forms + Events. SIL can be discovered through several mechanisms. An implementation may advertise its SIL interface through HTTP Link headers, HTML