Plugin architecture

Plugins are the packages people discover, install, share, and publish in ChatGPT and Codex. A plugin can contain:

• Skills that give the model instructions and resources for repeatable workflows.
• An MCP server that exposes tools and connects to external systems.
• Both skills and an MCP server when the model needs workflow guidance and server-backed capabilities.

ChatGPT and Codex share one universal plugin directory. When you publish a public plugin, people can discover the same listing from supported surfaces in either product. Individual capabilities can still be surface-specific; for example, a plugin can include hooks that run only in Codex.

An MCP server can return structured data and model-readable text without custom UI. When a task benefits from visual interaction, the server can also return a UI resource.

Plugin
├── Skills
└── MCP server (optional)
    ├── Tools and structured results
    └── UI resources (optional)

Skills

A skill is a folder containing a SKILL.md file and, when needed, supporting scripts, references, templates, or assets. Skills describe when to use a workflow, which steps to follow, and what a successful result looks like.

Use skills when instructions and the tools already available to the model are enough to complete the task. A plugin can package one skill or group related skills into one installable experience.

For example, a meeting follow-up plugin might include separate skills for drafting a recap, identifying action items, and preparing a customer email.

MCP servers

Build an MCP server when your plugin must connect to a service, expose a controlled set of tools, authenticate users, or run behavior on infrastructure you operate. The server defines:

• The tools the model can call.
• Input and output schemas for those tools.
• Authentication and authorization requirements.
• Structured results and model-readable content.
• Optional UI resources.

An MCP server gives you control over which capabilities you expose. It also lets you update server behavior independently and observe requests made to your infrastructure.

Optional UI

Custom UI is not required for an MCP server. Use model responses or structured results when they communicate the outcome.

Add UI when people need to inspect, compare, edit, confirm, or navigate structured information. For example, a product comparison, editable schedule, or map can benefit from a component, while a background status lookup often does not.

ChatGPT supports the open MCP Apps UI standard. Start with the shared standard, then add optional ChatGPT extensions only when the UI needs capabilities the standard does not cover. Keep tools useful without the component so the model can complete headless workflows and decide when UI adds value.

Choose a plugin shape

After choosing a shape, build the skills or build the MCP server. Add UI to the MCP server only when a use case requires it, then package the plugin.