Porting between agent-UI protocols

Most ports are mechanical if the app was built with an adapter, and a rewrite if it was not. Establish the adapter first, then port.

The adapter layer

Every host interaction goes through one module. Components never touch window.openai, app.*, or postMessage directly.

// bridge.ts — the only file that knows which protocol you are on
export interface HostBridge {
  onToolInput(cb: (input: unknown) => void): void;
  onToolResult(cb: (result: ToolResult) => void): void;
  callTool(name: string, args: unknown): Promise<ToolResult>;
  sendMessage(text: string): Promise<void>;
  updateModelContext(ctx: unknown): Promise<void>;
  openLink(url: string): Promise<void>;
  setSize(w: number, h: number): void;
  requestDisplayMode(mode: DisplayMode): Promise<DisplayMode>;
  getContext(): HostContext | undefined;
  capabilities(): Capabilities;
}

With this in place a port is: write a new HostBridge implementation, swap the import, adjust the server registration. Without it, every component is a migration site.

Building an app that must reach two protocols at once? Implement both and select at init by probing (app.getHostCapabilities(), then window.openai, then mcp-ui postMessage liveness).

1. OpenAI Apps SDK → portable MCP Apps

The most common port, and the cheapest — ChatGPT already implements MCP Apps, so this is de-risking, not rewriting.

Server

Keep the openai/* aliases alongside the standard fields during rollout — ChatGPT honors both, and dual-writing means no flag day.

View

No standard equivalent — keep these feature-detected: requestCheckout, uploadFile, selectFiles, getFileDownloadUrl, requestModal, widgetState / setWidgetState.

For setWidgetState, the portable substitute is app.updateModelContext() for the model-visible slice. There is no portable durable-state mechanism — that slice moves to your server.

Order of operations

1. Introduce the adapter; components stop touching window.openai.
2. Dual-write server _meta (standard + aliases).
3. Swap the View to the App class, keeping window.openai fallbacks for the six extension APIs.
4. Verify in ChatGPT (nothing should change) and in basic-host (it should now work at all).
5. Drop the aliases once no host you support needs them.

2. mcp-ui legacy actions → the ui/* bridge

intent is the one real loss. Its value was letting the host route a user-expressed intent without the View knowing the tool name. Replacements, in order of preference: a generic server-side handle_intent tool; or keep the intent post as a fallback branch for mcp-ui hosts.

externalUrl and remoteDom have no MCP Apps equivalent. An externalUrl resource ports by inlining the app or serving it as the resource HTML (and declaring its origins in CSP). A remoteDom resource is a genuine rewrite — consider A2UI instead, which is what remoteDom was reaching toward.

3. MCP Apps → A2UI

Not a port. A reimplementation, because the model inverts: you stop shipping markup and start describing components the client owns.

What survives:

• The tool/data layer, essentially unchanged.
• structuredContent becomes the A2UI data model (JSON Pointer paths).
• Your server logic and authorization.

What is rewritten:

• HTML/CSS/JS → a component catalog plus updateComponents messages.
• Event handlers → action.event (agent round trip) or action.functionCall (local).
• Client-side validation → CheckRule objects.
• Styling → deleted. A2UI v1.0 defers all visuals to the renderer's theme.

Do this when the client must render natively (Flutter, SwiftUI), when a design system must own every pixel, or when executing model-influenced markup is unacceptable.

A2UI v0.9 → v1.0

• Function calls are now bidirectional and explicit (callRendererFunction / callAgentFunction), verified against catalog definitions.
• createSurface may carry components and dataModel in one message.
• Theme properties removed — strip every hardcoded color; move design intent into the catalog's instructions field.
• Catalog functions become object maps with $schema / $id.
• Identifiers must satisfy UAX #31; @ is reserved.

4. Adding AG-UI to an existing app

Additive, not a replacement. Keep MCP Apps for the final artifact and add AG-UI for the run.

1. Wrap the agent so it emits RunStarted → content events → RunFinished/RunError.
2. Map existing progress logs to StepStarted/StepFinished and Activity*.
3. Emit StateSnapshot on connect, StateDelta (RFC 6902) thereafter.
4. Point CopilotKit or your own subscriber at the endpoint.
5. Keep the MCP tools intact — the widget still renders the result.

Preserve rawEvent while wrapping. It is the only way to debug a translated stream.

Port checklist

• Adapter layer in place; no component touches a host API directly
• Field mapping applied server-side; aliases dual-written during rollout
• Every non-portable API feature-detected with a working fallback
• Resource URI bumped if the UI changed in a breaking way
• Text-only path (content) still completes the workflow
• Verified on the origin host (no regression) and the new host (works)
• CSP re-derived — origins often change with delivery mode
• Old aliases removed only after every supported host is confirmed

Run /ui:port --from <protocol> --to <protocol> to apply the mechanical parts and produce a report of what needs human judgment.

Related

• protocol-selection — whether the port is the right move at all.
• host-capability-matrix — what the destination host supports.
• openai-apps-sdk, mcp-apps-protocol, a2ui-protocol, ag-ui-protocol.