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A company that serves people through a website or app and developers through an API is not running two separate businesses. It is presenting one set of capabilities through two interfaces, each designed for a different user. People need an experience that helps them complete a task; developers need a dependable contract they can integrate into another service. Designing both well means researching both groups, setting deliberate capability and access boundaries, and keeping their understanding of the business consistent.
“Two-interface company” is a useful design frame, not a formally standardized term. It does not mean every company needs an API, or that its website and API should work alike. It means treating the machine-facing interface as a product surface—not as a technical afterthought.
What it means to design for people and machines
A human-facing interface might be a website, mobile app, or service portal. Its user is a person trying to do something: find information, make a decision, submit a request, or manage an account. A machine-facing interface, commonly an API, lets software request capabilities or data so a developer can build another service or connect systems.
The people using these interfaces have different jobs. A person may need guidance through a sequence of choices. A developer may need a stable, documented way to make a request, interpret its response, handle errors, and know what can change. One underlying business can serve both without forcing both groups through the same interaction pattern.
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The UK Government Digital Service and Central Digital and Data Office put the starting point plainly in their API technical and data standards: “Before you build an API, you must understand the needs of your users.” The guidance identifies an API user as a developer consuming the API to deliver a service. The W3C’s Web Platform Design Principles likewise calls for understanding and documenting the user need an API addresses.
How the two interfaces differ
Use these dimensions to make design decisions, not as a published scoring system. The right implementation depends on the people and developers being served, the task, and the data involved.
| Design dimension | Human-facing interface | Machine-facing interface |
|---|---|---|
| Primary user and job | A person trying to complete a task, make a decision, or understand a service. | A developer integrating the business’s capabilities into another service or system. |
| Task completion | Guide the person through the decisions and actions needed for their task. | Provide a documented sequence of requests and responses that software can use. |
| Clarity and errors | Make choices, next steps, and recovery from errors understandable to the person. | Define expected inputs, outputs, and error behavior so the integration can respond appropriately. |
| Discoverability | Help people find the service and understand what it lets them do. | Help developers find the API, assess whether it fits, understand its purpose, and integrate it. |
| Accessibility and usability | Account for people’s accessibility needs, context, and familiarity with the task. | Make the contract and supporting documentation clear enough for the intended developer audience to use. |
| Data and access | Decide what information or actions a person can access, and under what conditions. | Define which clients can use each capability, what data they receive, and what authentication and monitoring apply. |
| Change and support | Explain changes that affect the service and provide appropriate help. | Communicate API version, status, support, and changes that could affect integrations. |
| Reuse | Present capabilities in a flow suited to the person’s task. | Expose appropriate capabilities for reuse across consuming services without making internal data structures the contract. |
The comparison is a practical synthesis of API and documentation guidance from GOV.UK and user-need principles from the W3C, rather than an official framework or a promise that one interface will automatically improve the other.
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How to design the two interfaces together
1. Research the users and their jobs separately
Start with the people using the website or app: who they are, what they are trying to accomplish, what decisions they face, and what accessibility or contextual needs shape the experience. Then investigate API consumers as their own user group. A developer may be integrating a capability into another service, and their technical familiarity, constraints, and integration task matter.
Do not infer either group’s needs from your organization chart or assume that internal teams’ data categories match user tasks. The W3C principles emphasize documenting the user need behind an API; GOV.UK’s standards make developer needs the starting point for API design.
2. Define business capabilities before choosing what to expose
Describe what the business enables users to do or learn before mapping those capabilities to screens, endpoints, or stored data. Then decide which capabilities belong in each interface and what information is necessary to support them.
GOV.UK recommends API-first design where appropriate. Its standards say beginning with an API can help keep business logic distinct from the data structures in underlying stores. The practical implication is to design a service contract around stable business capabilities and consumer needs, rather than inadvertently making the organization of a database the product interface. API-first is an approach to consider, not a rule that every project must start with an API.
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3. Choose an API style for the consumers and task
For a REST API, GOV.UK describes OpenAPI as a standardized way to describe the interface. It recommends creating the specification early and evolving it with the design. A specification can help teams check consistency, lint for rules and security issues, and generate reference documentation.
REST is not the only option. GOV.UK’s standards also identify GraphQL and gRPC as alternatives that may fit particular projects; GraphQL can be useful when the views developers need are uncertain. Choose based on consumer needs, system constraints, and the behavior the service must support, rather than treating one style as universally best.
4. Treat documentation as part of the API product
A reference page full of endpoint details may not be enough to get a developer from discovery to a working integration. GOV.UK’s Documenting APIs guidance says documentation should help developers discover an API, decide whether they can use it, understand its purpose, and integrate it.
Provide the materials that answer those questions:
- A short introduction that explains the API’s purpose and intended users.
- A conceptual overview where developers need a mental model of the service.
- A simple walkthrough showing a request and its response.
- Task-based guidance for common integration jobs.
- A full technical reference.
- Version, status, and support information.
Generated reference material can contribute to this set, but it does not by itself explain why the API exists, whether it fits a developer’s job, or how to complete a common integration task.
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For each capability, decide who may use it, which data it exposes, and what authentication and monitoring are appropriate. These choices affect what the interface promises and who can act on that promise.
GOV.UK’s standards generally recommend authentication for API users, particularly where personal or sensitive data is involved. They say anonymous endpoints, when needed, should be limited to open data. This is guidance for the government context, not a universal access policy for every private-sector API; organizations should set controls appropriate to their data, users, and obligations.
6. Keep meaning and business rules coherent
The website and API can have different interaction patterns, but they should not quietly describe different versions of what the business offers or permits. Align important terms, eligibility or policy rules, and promised outcomes across surfaces. This is a design recommendation drawn from the W3C’s emphasis on understanding intent across APIs and web interfaces; it does not require identical wording or flows everywhere.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Where AI-oriented machine access fits
Machine-facing interfaces are not limited to AI agents. A UK government guide on making datasets ready for AI discusses machine-readable API practices including OpenAPI standards, clear HTTP methods, pagination, filtering, and rate limiting. It also describes MCP as an open standard through which providers can share resources using servers that describe their capabilities in machine-readable form.
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These are adjacent developments for teams with a relevant machine-client use case, not evidence that every business needs an AI-agent interface. Start with a real consumer need and a capability worth exposing; do not add a new interface merely because a new standard exists.
What the evidence can—and cannot—establish
The official sources behind this design approach are standards and guidance on API design, documentation, user needs, and data access. They do not establish a universal operating model or quantify a commercial benefit from serving both people and software. No directly relevant statistic about performance, adoption, or commercial effect is available in those sources, so the case for a two-interface design should rest on the needs of actual users and developers rather than an assumed business outcome.
The cited guidance includes the GOV.UK API technical and data standards (first published 7 February 2018; reported updated 30 September 2026), Documenting APIs (first published 15 February 2019; last updated 17 January 2022), and the W3C Web Platform Design Principles (surfaced as published in September 2026). Government context and publication dates matter: these materials provide authoritative design guidance, not a claim that every organization is subject to the same rules.
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