ui/update-model-context: How should hosts handle multiple instances of the same app resource?

Problem

When a host renders the same MCP App resource URI multiple times in a single session (e.g., the same tool is invoked twice, producing two iframes for the same ui:// resource), each instance independently calls ui/update-model-context. The spec doesn't provide guidance on how the host should scope these updates.

A natural host-side key for storing context is {extension}_{resourceUri}, but this means all instances of the same resource share a single slot. The last instance to send an update overwrites the others, and the model sees only that instance's state on the next turn — potentially the wrong one.

Example

1. User invokes a tool that renders ui://my-extension/editor — Instance A appears
2. User invokes the same tool again — Instance B appears
3. User interacts with Instance A, which sends ui/update-model-context with { counter: 5 }
4. User interacts with Instance B, which sends ui/update-model-context with { counter: 0 }
5. On the next turn, the model sees counter: 0 — Instance B's state silently overwrote Instance A's

Questions

1. Should the spec recommend instance-scoped keys? For example, the host could append a render-specific identifier (message ID, sequence number) to the context key so each instance gets its own slot.

2. Or should last-write-wins be the expected behavior? If the spec considers each resource URI to represent a single logical piece of state, then the latest update is correct by definition — the "active" instance is whichever the user touched last.

3. Does this interact with the slots proposal (#500)? Slots address multiple context dimensions within a single app instance. Multi-instance scoping is orthogonal — you could have per-instance slots, or per-instance single context. But the two proposals share the question of key granularity.

Context

We ran into this while implementing ui/update-model-context in goose. Our current implementation uses last-write-wins keyed by {extension}__{resourceUri}, which is simple and works for the common case (one instance per resource). We'd like spec guidance before adding instance-scoped keys, since it affects how apps reason about their context lifecycle.

Start by reading the ui/update-model-context behavior described here and compare it with the slots proposal in issue #500. Review the instance-scoping and last-write-wins alternatives, then define what spec guidance is needed for repeated resource instances and how completion would be verified.