MCP Apps: `ui/notifications/size-changed` doesn't change the width

[khandrew1]

Describe the bug
When ui/notifications/size-changed gets sent, Goose is deliberately ignoring the width. Is there a reason for this?

Reference: https://github.com/block/goose/blob/HEAD/ui/desktop/src/components/McpApps/McpAppRenderer.tsx#L158-L161

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To Reproduce
Sent a size changed notification with new height and width on an MCP App (with auto resize off) and only the height changed.

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Expected behavior
The width also gets updated

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Please provide the following information

• OS & Arch: MacOS Tahoe 26.1
• Interface: UI
• Version: v1.19.0

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[DOsinga]

it's a good question. when a widget/app renders inside of a conversation, should it not just take the width of the parent? we could make it less wide, but wider doesn't work I think. I imagined that the width was more for a pop-out or standalone mode. /cc @aharvard

[khandrew1]

Seems there's been an update to the spec regarding viewports. My thought process is that height/width should be controlled by the widget because the Host can already declare both a maxHeight and maxWidth in the host context.

Ref: https://github.com/modelcontextprotocol/ext-apps/blob/main/specification/draft/apps.mdx#host-context-in-mcpuiinitializeresult (See containerDimensions).

[aharvard]

@khandrew1, great reference. I was just about to provide more details about that.hostContext.containerDimensions is a new, breaking change and goose is still sending hostContext.viewport. (I've got a forthcoming PR in the works to pivot goose to containerDimensions)

Additionally, goose sends hostContext.displayMode of inline and hostContext.availableDisplayModes of ['inline']. The intent here is that:

1. the host can have dynamic sizing logic based on mode (when we provide more modes, we can ignore certain sizes sent by the guest UI)
2. the guest UI can check what mode they are in and alter the design, if needed

To @DOsinga's point, I too would be worried about an app setting the width larger than the chat window. This makes me wonder if my comment will address the edge case: #6376 (comment)

[aharvard]

FYI, I just opened a thread on the MCP Contributors Discord for the mcp-apps-wg to get some more guidance around this topic https://discord.com/channels/1358869848138059966/1458566824231833731

[owtaylor]

My take on this: #6818 (comment) - the original behavior was right and the width should be ignored - in inline mode, apps simply need to adapt to the width of the container.