[MCP Apps] Add appCapabilities.availableDisplayModes documentation to spec #331

martinalong · 2026-01-23T08:37:40Z

Document the availableDisplayModes field in McpUiAppCapabilities that allows apps to declare which display modes they support. This enables hosts to compute the intersection of host and app capabilities and only offer appropriate display mode controls.

pkg-pr-new · 2026-01-23T08:40:47Z

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commit: 3d182cc

ochafik

Thanks Martina!

ochafik · 2026-01-23T15:52:28Z

specification/draft/apps.mdx


+### App Capabilities in `ui/initialize`
+
+When the Guest UI sends an `ui/initialize` request to the Host, it MUST include its capabilities in the `appCapabilities` field:


now just "view" (cf. #325)

ochafik · 2026-01-23T15:52:45Z

specification/draft/apps.mdx

+   *
+   * @example ["inline", "fullscreen"]
+   */
+  availableDisplayModes?: Array<"inline" | "fullscreen" | "pip">;


I would clarify the purpose of this (which is to allow the host to change the view's display mode).

In the carousel app example you mentioned, the app might support fullscreen but only on its own terms (e.g. when user clicks somewhere specific in their UI), although not sure if that's a use-case worth the complexity.

@idosal wdyt?

If that's the intent, we should remove the requirement for the host to validate intersections, and instead suggest that it honors requests for its declared supported display modes (Host SHOULD honor requests for its declared supported display modes). We may enforce validation (e.g., Host MUST reject the request if the app's display mode isn't supported).

EDIT: I saw you've addressed this in your other comments. Completely agree :)

which is to allow the host to change the view's display mode

Yes, this part is not clear from this PR. Also the dynamic use case (availableDisplayModes changing as a result of user interaction like you mentioned) is not covered here

Oh, didn't consider a dynamic value for this (this might be even more niche), more like the app doesn't want the host to initiate the fullscreen for it, it wants to be in control of when it wants to go fullscreen. But then again, maybe a bit niche, let's forget it for now ;-)

ochafik · 2026-01-23T15:58:20Z

specification/draft/apps.mdx

+**Host Behavior:**
+
+- Host SHOULD intersect its own `availableDisplayModes` with the app's declared modes
+- When responding to `ui/request-display-mode`, host SHOULD only allow modes both support


for separation of concerns, host should only bother checking against its own supported modes. Would be good to spec out what happens when receiving an invalid mode, not sure it that's already covererd in the other availableDisplayModes section

ochafik · 2026-01-23T16:00:01Z

specification/draft/apps.mdx

+}
+```
+
+#### `availableDisplayModes`


Suggested change

#### `availableDisplayModes`

#### View `availableDisplayModes`

(have clear local disambiguation w/ the other

#### Host `availableDisplayModes`

ochafik · 2026-01-23T16:01:23Z

specification/draft/apps.mdx

+- Host SHOULD intersect its own `availableDisplayModes` with the app's declared modes
+- When responding to `ui/request-display-mode`, host SHOULD only allow modes both support
+- Host MAY include the computed available modes in `HostContext.availableDisplayModes`
+- Host MAY assume that all apps support `inline` display mode


host should only assume this if the spec says they MUST support inline above (which I'm not think if fair, e.g. some apps might wanna start in pip from the get go)

I agree. We shouldn't force inline

liady · 2026-01-23T16:25:20Z

specification/draft/apps.mdx


+### App Capabilities in `ui/initialize`
+
+When the Guest UI sends an `ui/initialize` request to the Host, it MUST include its capabilities in the `appCapabilities` field:


Suggested change

When the Guest UI sends an `ui/initialize` request to the Host, it MUST include its capabilities in the `appCapabilities` field:

When the View sends an `ui/initialize` request to the Host, it MUST include its capabilities in the `appCapabilities` field:

martinalong · 2026-01-23T17:32:29Z

Made those changes!

martinalong · 2026-01-23T18:27:51Z

Oop I realized that the reason this whole thing was missing in the first place was b/c my PR to add it to the app capabilities was merged into the wrong branch earlier. Added a commit that actually adds this field to app capabilities!

martinalong · 2026-01-23T18:49:33Z

cc @idosal @liady @ochafik for re-review!

ochafik

Thanks @martinalong !

ochafik · 2026-01-23T19:02:08Z

specification/draft/apps.mdx

+
+**Host Behavior:**
+
+- Host SHOULD honor requests for its declared supported display modes


Sounds like this belongs to the Host availableDisplayModes / ui/request-display-mode sections no?

ochafik · 2026-01-23T19:05:35Z

specification/draft/apps.mdx

+- If omitted, hosts MAY assume the app supports all modes, or only `inline`. Apps that do not specify this field may not be able to take over fullscreen automatically.
+- Apps MUST handle graceful degradation if placed in an unsupported mode
+
+**Host Behavior:**


The host MUST NOT switch the view to a display mode that does not appear in its AppCapabilities.availableDisplayModes, if it is set.

ochafik · 2026-01-23T19:10:15Z

specification/draft/apps.mdx

+}
+```
+
+#### View `availableDisplayModes`


So there's already an untitled section:

Host behavior: * App MUST check if the requested mode is in `availableDisplayModes` from host context. * It is up to the host whether it switches to the requested mode, but the host MUST return the resulting mode (whether updated or not) in the response. * If the requested mode is not available, Host SHOULD return the current display mode in the response. * Host MAY coerce modes on certain platforms (e.g., "pip" to "fullscreen" on mobile). View behavior: * View SHOULD check `availableDisplayModes` in host context before requesting a mode change. * View MUST handle the response mode differing from the requested mode.

Could we merge it and the new material to a #### Display Modes section?

Starts w/ the types

Declaring support from View (AppCapabilities.availableDisplayModes)

Declaring support from Host (HostCapabilities.availableDisplayModes)
Then the methods:

Requesting change from View (ui/request-display-mode)

Notifiying change from Host (ui/notifications/host-context-changed)
Then the MUSTs and SHOULDs.

And this can be referenced from this and other sections.

ochafik · 2026-01-23T19:14:01Z

specification/draft/apps.mdx

+   *
+   * @example ["inline", "fullscreen"]
+   */
+  availableDisplayModes?: Array<"inline" | "fullscreen" | "pip">;


Oh, didn't consider a dynamic value for this (this might be even more niche), more like the app doesn't want the host to initiate the fullscreen for it, it wants to be in control of when it wants to go fullscreen. But then again, maybe a bit niche, let's forget it for now ;-)

ochafik · 2026-01-23T19:16:42Z

specification/draft/apps.mdx

+
+- Apps SHOULD declare all display modes they are designed to support
+- If omitted, hosts MAY assume the app supports all modes, or only `inline`. Apps that do not specify this field may not be able to take over fullscreen automatically.
+- Apps MUST handle graceful degradation if placed in an unsupported mode


Note that from inline to fullscreen, they just need to be reactive and respect insets (which they need to do anyway).

But anyway, why wouldn't we guarantee to an app we won't go against its express wishes?

ochafik · 2026-01-23T19:19:26Z

specification/draft/apps.mdx

+
+**App Behavior:**
+
+- Apps SHOULD declare all display modes they are designed to support


I'd even say MUST, and say that Hosts MAY decline display mode requests from apps that didn't declare said modes in the capabilities.

ochafik

One last nit otherwise looks good, thanks @martinalong !

ochafik · 2026-01-23T20:26:00Z

specification/draft/apps.mdx

+- Host MUST NOT switch the View to a display mode that does not appear in its `appCapabilities.availableDisplayModes`, if set.
+- Host MUST return the resulting mode (whether updated or not) in the response to `ui/request-display-mode`.
+- If the requested mode is not available, Host SHOULD return the current display mode in the response.
+- Host MAY coerce modes on certain platforms (e.g., "pip" to "fullscreen" on mobile).


this conflicts w/ host MUST NOT switch to unsupported display mode above, just drop?

Document the availableDisplayModes field in McpUiAppCapabilities that allows apps to declare which display modes they support. This enables hosts to compute the intersection of host and app capabilities and only offer appropriate display mode controls. 🤖 Generated with [Claude Code](https://claude.com/claude-code) Co-Authored-By: Claude <noreply@anthropic.com>

- Change "Guest UI" to "View" terminology - Add "View" prefix to availableDisplayModes heading for disambiguation - Remove intersection logic - host only checks its own supported modes - Remove assumption that apps must support inline mode - Simplify host behavior to honor/reject based on its own capabilities 🤖 Generated with [Claude Code](https://claude.com/claude-code) Co-Authored-By: Claude <noreply@anthropic.com>

- Add `availableDisplayModes` field to `McpUiAppCapabilities` so apps can declare which display modes they support - Change `HostContext.availableDisplayModes` type from `string[]` to `McpUiDisplayMode[]` for type safety - Regenerate schemas 🤖 Generated with [Claude Code](https://claude.com/claude-code) Co-Authored-By: Claude <noreply@anthropic.com>

- Remove separate "App Capabilities" section for availableDisplayModes - Add display mode requirements to existing ui/request-display-mode section: - Host MUST NOT switch View to mode not in appCapabilities.availableDisplayModes - Host MAY decline requests from Views that didn't declare modes - View MUST declare supported modes during initialization 🤖 Generated with [Claude Code](https://claude.com/claude-code) Co-Authored-By: Claude <noreply@anthropic.com>

- Restore McpUiAppCapabilities interface documentation with availableDisplayModes - Restore "View MUST check if the requested mode is in availableDisplayModes from host context" clause that was accidentally removed 🤖 Generated with [Claude Code](https://claude.com/claude-code) Co-Authored-By: Claude <noreply@anthropic.com>

Consolidate all display mode documentation into a single section: - Display mode types (inline, fullscreen, pip) - Declaring support from View (appCapabilities.availableDisplayModes) - Declaring support from Host (HostContext.availableDisplayModes) - Requesting changes (ui/request-display-mode) - Notifying changes (ui/notifications/host-context-changed) - All MUST/SHOULD requirements for View and Host behavior The ui/request-display-mode in Requests section now references the Display Modes section for behavior details. 🤖 Generated with [Claude Code](https://claude.com/claude-code) Co-Authored-By: Claude <noreply@anthropic.com>

🤖 Generated with [Claude Code](https://claude.com/claude-code) Co-Authored-By: Claude <noreply@anthropic.com>

ochafik requested changes Jan 23, 2026

View reviewed changes

ochafik requested review from idosal and liady January 23, 2026 16:02

liady reviewed Jan 23, 2026

View reviewed changes

martinalong requested review from liady and ochafik January 23, 2026 17:32

ochafik requested changes Jan 23, 2026

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ochafik previously approved these changes Jan 23, 2026

View reviewed changes

martinalong dismissed ochafik’s stale review via 30c2c7c January 23, 2026 20:33

martinalong and others added 7 commits January 23, 2026 12:54

Remove mode coercion clause from display modes

b4a9a5e

🤖 Generated with [Claude Code](https://claude.com/claude-code) Co-Authored-By: Claude <noreply@anthropic.com>

martinalong force-pushed the martinalong/app-capabilities-docs branch from 30c2c7c to b4a9a5e Compare January 23, 2026 20:54

Regenerate schemas with correct version

3d182cc

🤖 Generated with [Claude Code](https://claude.com/claude-code) Co-Authored-By: Claude <noreply@anthropic.com>

ochafik approved these changes Jan 23, 2026

View reviewed changes

ochafik merged commit 89f58bf into main Jan 23, 2026
18 of 19 checks passed


		### App Capabilities in `ui/initialize`

		When the Guest UI sends an `ui/initialize` request to the Host, it MUST include its capabilities in the `appCapabilities` field:

	#### `availableDisplayModes`
	#### View `availableDisplayModes`

	When the Guest UI sends an `ui/initialize` request to the Host, it MUST include its capabilities in the `appCapabilities` field:
	When the View sends an `ui/initialize` request to the Host, it MUST include its capabilities in the `appCapabilities` field:


		Host Behavior:

		- Host SHOULD honor requests for its declared supported display modes


		App Behavior:

		- Apps SHOULD declare all display modes they are designed to support

[MCP Apps] Add appCapabilities.availableDisplayModes documentation to spec #331

[MCP Apps] Add appCapabilities.availableDisplayModes documentation to spec #331

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