ChatGPT plugin extensions add sidebar apps and file viewers; Free and Go web support is still coming

OpenAI's [September 29 DevDay recap](https://openai.com/index/devday‑2026‑recap/) introduced plugin extensions as a way for developers to put experiences into ChatGPT's own interface. A plugin can gain a persistent sidebar home, show an interactive panel beside a conversation or open a viewer for files its product supports. That changes the build decision for a team whose users need to inspect, edit or navigate something while they talk to ChatGPT: a tool result alone may no longer be the best surface for the job. This is an extension of the plugin architecture, not a requirement to rebuild every integration. OpenAI's [architecture guide](https://developers.openai.com/plugins/concepts/plugins) says a plugin can contain skills, an MCP server with tools and optional UI resources. An MCP tool can still return structured data without custom UI. The new extension reference gives developers entrypoints when a fuller interface is useful. For example, a design or document product could let a user open a relevant item and keep the conversation visible alongside an editor. That is a practical use case inferred from the documented surfaces, not a claim that every plugin already offers it.

A [sidebar app](https://developers.openai.com/plugins/build/extensions) gives a plugin a global entrypoint that opens its MCP App fullscreen. A conversation panel is an entrypoint in a thread's side panel, useful when the user needs the app and the chat together. A file viewer or editor instead opens when the user selects a supported file extension; the documentation shows the plugin declaring a file entrypoint and receiving a resource URI for that file. These are different triggers and layouts, so a team should choose by the user's starting point rather than add every surface by default. OpenAI's reference also lists plugin settings, deep links, display modes, model‑app context, composer mentions and rich forms. The useful architectural distinction is that the MCP server and its tools remain the controlled capability layer, while an extension tells ChatGPT where and how to present the app. The [plugin architecture guide](https://developers.openai.com/plugins/concepts/plugins) recommends adding UI when people need to inspect, compare, edit, confirm or navigate structured information. A simple background lookup can stay a tool response.

The DevDay recap labels plugin extensions as available to all plans. The more specific [developer reference](https://developers.openai.com/plugins/build/extensions), checked September 30, says extensions on the web are still coming to ChatGPT Free and Go users, while existing plugin functionality is unaffected. It also says composer mentions are available only in the ChatGPT desktop app. Those notes matter if a developer is choosing a launch audience or writing support instructions: a plugin may be installable for a user without every extension surface being live in that user's client. OpenAI points builders to TypeScript and Python SDK examples, a protocol specification and a sample Bits & Bolts plugin in the [extension guide](https://developers.openai.com/plugins/build/extensions). A sensible first implementation is one surface that directly serves the product's main task, followed by a check of that surface in the intended ChatGPT client and plan.

Plugin extensions concern where an existing integration appears and how a person interacts with it. OpenAI announced separate changes to [plugin creation, submission and discovery](https://openai.com/index/devday‑2026‑recap/) and separate support for proposed MCP Events. Those may change how a plugin is built, found or triggered, but they are not needed to understand a sidebar app or file viewer. The right durable product link for this launch is [ChatGPT](https://opentools.ai/tools/chatgpt), which is made by [OpenAI](https://opentools.ai/organizations/openai); an extension is a feature of that platform rather than a newly named foundation model or standalone OpenAI tool.