Codex Build iOS Apps Plugin: Mirror the Simulator in the Browser and Hot-Reload SwiftUI Previews

OpenAI's Build iOS Apps plugin for Codex gained a browser mirror on June 3, 2026: an ios-simulator-browser skill that streams the iOS Simulator into the Codex in-app browser and hot-reloads Swift Package-backed SwiftUI previews as you edit. The plugin itself is older. Its directory in OpenAI's plugins repository has carried the build-ios-apps name since March 24, 2026, XcodeBuildMCP was wired in on April 7, and the manifest now reads version 0.1.2. The plugin manifest describes it as "workflows for App Intents, SwiftUI UI work, Simulator mirroring in the Codex in-app browser, performance profiling, leak investigation, and simulator debugging." OpenAI's developer docs point at it from the Build for iOS use-case guide.

For context on how Codex keeps adding surfaces, see our writeup of the Codex Chrome extension and Codex computer use and thread automations. For where Codex sits against other agentic coding tools, see Codex CLI vs Claude Code vs Cursor architecture. Anthropic's answer to the same problem is a simulator pane in Claude Code Desktop.

Key Takeaways

• The June 2026 feature: mirror the iOS Simulator in Codex's in-app browser and hot-reload package-backed SwiftUI previews, with no trip back to Xcode to see an edit.
• Nine skills, not one. The README lists ios-debugger-agent, ios-simulator-browser, ios-ettrace-performance, ios-memgraph-leaks, ios-app-intents, swiftui-liquid-glass, swiftui-performance-audit, swiftui-ui-patterns, and swiftui-view-refactor.
• Open source. github.com/openai/plugins under plugins/build-ios-apps, MIT license.
• XcodeBuildMCP-backed. The plugin's .mcp.json launches xcodebuildmcp@latest with the simulator, UI automation, debugging, and logging workflows enabled.
• Package-backed previews only. The skill supports PreviewProvider and #Preview declarations in an importable Swift package and will not modify your Xcode project to force previews.
• Install from /plugins in Codex CLI or the Plugins tab in the ChatGPT desktop app; the IDE extension does not support plugins.

What the Plugin Does

Per the plugin README, fetched September 8, 2026, the plugin covers:

• designing App Intents, app entities, and App Shortcuts for system surfaces
• building and refactoring SwiftUI UI using current platform patterns
• reviewing or adopting iOS 26+ Liquid Glass APIs
• auditing SwiftUI performance and guiding profiling workflows
• capturing symbolicated ETTrace simulator profiles for focused app flows
• capturing and comparing iOS memgraphs to root-cause leaks
• debugging iOS apps on simulators with XcodeBuildMCP-backed flows
• mirroring Simulator in the browser and hot-reloading package-backed SwiftUI previews
• restructuring large SwiftUI views toward smaller, more stable compositions

The manifest's long description adds the reader-facing version: "design App Intents and App Shortcuts, build or refactor SwiftUI UI, render SwiftUI previews in the Codex in-app browser, adopt modern iOS patterns such as Liquid Glass, audit runtime performance, capture ETTrace profiles, investigate memory leaks, and debug apps on simulators with XcodeBuildMCP-backed workflows."

How the Plugin Got Here

The repository history, read through the GitHub API on September 8, 2026:

So the June 4 attention was for one skill landing in a plugin that already existed. That matters if you tried the plugin before June and found no browser mirror: it was not there.

The In-App Browser Preview Loop

The skill file describes two workflows.

Browser mirror. The skill takes an explicit Simulator UDID (from the build workflow or xcrun simctl list devices available), starts serve-sim pinned to that simulator in a long-running terminal, and opens the local URL serve-sim prints in the Codex in-app browser. The skill tells the agent to confirm "a real frame is rendering before reporting success. A loaded page alone is not proof that the simulator stream is healthy." It also warns the agent never to run an unscoped serve-sim --kill, because another thread may own a different simulator mirror.

SIM="<simulator-udid>"
npx --yes serve-sim@latest "$SIM"

serve-sim is a third-party npm package, not an OpenAI one: the registry describes it as "The npx serve of Apple Simulators", first published April 17, 2026, latest 0.1.46 on August 22, 2026, repository EvanBacon/serve-sim.

Preview hot reload. A bundled launcher, swiftui-preview-browser.mjs, takes a Package.swift path, a package target, and a device UDID. Per the skill file, it "generates a disposable host project outside the user's source tree, installs and launches that host in Simulator, and watches the package for edits." On a source edit "the launcher rebuilds a generated dylib and hot-swaps it into the running host without relaunching the app." The host shows every preview variant in the selected target with in-simulator page controls; --preview-filter narrows it to a regex on display names or identifiers. Once the launcher prints the Simulator UDID, the agent starts serve-sim on the same UDID and opens the mirror.

The support boundary, verbatim from the skill file: "Support Swift Package-backed PreviewProvider and #Preview declarations through the generated host. Do not edit the user's .xcodeproj, .xcworkspace, Package.swift, schemes, or build settings to force preview support." If your previews live only in an app target rather than an importable package, this loop does not reach them.

How It Fits With XcodeBuildMCP

The plugin's .mcp.json declares one server:

{
  "mcpServers": {
    "xcodebuildmcp": {
      "command": "npx",
      "args": ["-y", "xcodebuildmcp@latest", "mcp"],
      "env": {
        "XCODEBUILDMCP_ENABLED_WORKFLOWS": "simulator,ui-automation,debugging,logging"
      }
    }
  }
}

OpenAI's Build for iOS guide frames when that matters: "Use XcodeBuildMCP once you're inside a full Xcode project and need deeper automation. That's when schemes, targets, simulator control, screenshots, logs, and UI interaction matter enough that plain shell commands stop being the whole story." The same guide says to keep the loop CLI-first with xcodebuild or Tuist, and that "for that first pass, you often don't need any skill or MCP server."

So the division of labor is: XcodeBuildMCP handles build, run, and inspect against the simulator; the plugin's skills layer iOS-specific workflows (SwiftUI patterns, App Intents, profiling) and the browser mirror on top. The npm registry lists xcodebuildmcp 2.7.0 as latest, published July 23, 2026.

How to Install It

OpenAI's plugins documentation states the surfaces: "Plugins work in Chat and Work across ChatGPT on the web, desktop, and mobile, and in Codex in the ChatGPT desktop app. Codex CLI also has a plugin browser for Codex environments. The IDE extension doesn't support plugins."

• Codex CLI: "enter /plugins to open the plugin browser. Install a plugin from a configured marketplace, then start a new session before using its bundled skills or tools."
• ChatGPT desktop app: open the Plugins tab, browse or search, select the plus button to install, then start a new chat.

The same doc shows public plugins listed in one universal directory shared by ChatGPT and Codex. Nothing in the plugins doc, the use-case guide, or the repository states a plan requirement or a beta label for this plugin.

How a Reader Uses This

Pattern 1: Tight SwiftUI UI Iteration

Your views live in a Swift package the app imports. Point the launcher at Package.swift and the target, start serve-sim on the UDID it prints, and describe the change. Codex edits the package, the dylib rebuilds and hot-swaps, and the mirrored frame in the in-app browser updates without an Xcode rebuild.

Pattern 2: Adding System Integration

You want your app to expose actions to Shortcuts and Spotlight. The ios-app-intents skill covers designing app intents, entities, and App Shortcuts for those system surfaces, then wiring them in.

Pattern 3: Performance and Leak Triage

An app flow feels slow or is leaking memory. The ios-ettrace-performance skill captures symbolicated ETTrace simulator profiles for a focused flow and ios-memgraph-leaks captures and compares iOS memgraphs, giving Codex concrete signals rather than guesses.

Caveats and Open Questions

• The announcement wording is unverifiable. The June 4, 2026 OpenAIDevs post on X was the discovery source for this page. As of September 8, 2026, fetching it returns a JavaScript shell with none of the post text, so the claim that Codex lets you "view and test your iOS app in the in-app browser, open SwiftUI previews, and hot reload edits without leaving Codex" is attributed to that post and cannot be re-checked. Every capability on this page now rests on the plugin manifest, README, skill file, and OpenAI's docs instead.
• No plan, price, or status is stated. The official sources reviewed do not state availability tiers, pricing, GA or beta status, or a plugin-catalog count, so this page does not either.
• Requires a local Xcode and simulator setup. The simulator-backed flows depend on XcodeBuildMCP, xcrun simctl, and serve-sim; the plugin orchestrates that toolchain rather than replacing it.
• Open source means it can change. The README is the current capability list; the history table above is where to look when a capability you expected is missing.

Corrections and Updates

September 8, 2026: This page said OpenAI "shipped a Build iOS Apps plugin" on June 4, 2026. The plugin had lived in OpenAI's plugins repository under that name since March 24, 2026, with XcodeBuildMCP wired in on April 7; what landed on June 3 was the ios-simulator-browser skill that added the browser mirror and preview hot reload. The page also sourced its headline claim and one FAQ answer to an X post alone; the capability claims now cite the plugin manifest, README, skill file, and OpenAI's docs, and the README's Liquid Glass and view-restructuring lines, present on publish day, are included.

Sources

• Build iOS Apps plugin README: https://github.com/openai/plugins/tree/main/plugins/build-ios-apps (fetched September 8, 2026)
• Plugin manifest, version 0.1.2: https://github.com/openai/plugins/blob/main/plugins/build-ios-apps/.codex-plugin/plugin.json
• Plugin MCP config: https://github.com/openai/plugins/blob/main/plugins/build-ios-apps/.mcp.json
• iOS Simulator Browser skill: https://github.com/openai/plugins/blob/main/plugins/build-ios-apps/skills/ios-simulator-browser/SKILL.md (added June 3, 2026, PR #308)
• Repository history for plugins/build-ios-apps via the GitHub commits API (rename March 24, 2026, PR #95; XcodeBuildMCP April 7, 2026, PR #128)
• OpenAI Codex, Build for iOS: https://developers.openai.com/codex/use-cases/native-ios-apps
• OpenAI, Plugins: https://learn.chatgpt.com/docs/plugins
• npm registry: serve-sim (0.1.46, August 22, 2026) and xcodebuildmcp (2.7.0, July 23, 2026)
• OpenAIDevs announcement (X), June 4, 2026: https://x.com/OpenAIDevs/status/2062599291479478275 (not readable by any tool as of September 8, 2026)