Is there a reason the _meta fields are not prefixed?

[mkouba]

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Your Question

If I understand it correctly, a tool _meta may contain a key ui (with no prefix, see https://modelcontextprotocol.io/specification/2025-11-25/basic/index#_meta) and the value is an object that contains for example property resourceUri.

Is there a reason a reserved prefix (e.g. mcp) is not used? That would help to resolve ambiguities and collisions. So instead of _meta["ui"].resourceUri the apps would use _meta["mcp/ui"].resourceUri.

[mkouba]

I do understand that io.modelcontextprotocol/ui is the id of the MCP Apps extension. However, from what I know, the spec does not allow an extension to reserve a bare name in the _meta. Even the built-in keys, like io.modelcontextprotocol/protocolVersion , are always prefixed. The only unprefixed keys in the spec are ones the base spec grants itself as explicit exceptions - progressToken, and the OpenTelemetry keys.

Moreover, the spec states: "Official extensions define additional _meta keys under the io.modelcontextprotocol/ prefix, and third-party extensions use their own vendor prefix. In both cases the keys are specified in the extension’s documentation." See also https://modelcontextprotocol.io/specification/2026-07-28/basic#_meta.

So IIUIC, the MCP Apps extension should use the io.modelcontextprotocol/ui prefix instead, i.e. _meta.ui.resourceUri should become _meta["io.modelcontextprotocol/ui"].resourceUri.

Now, _meta["io.modelcontextprotocol/ui"].resourceUri is obviously not very convenient, which is why I originally proposed something shorter like mcp/ui. But that particular form doesn't help either - the reservation rule is worded as "any prefix where the second label is modelcontextprotocol or mcp", and a single-label prefix like mcp/ has no second label, so it isn't reserved for MCP at all.