Rich UI in Gemini Enterprise: A Field Report on A2A Agents and A2UI Primitives

Card

container Chat Cards v2 ✓

A single-child container with card styling.

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child: <id>i. Bare component-id, not a wrapped list — worth one child only (a Column, if the card needs more than one thing).

Text (title) + Text (body)ii. Two Text blocks doing different jobs by usageHint — the catalog's least glamorous primitive, load-bearing everywhere.

Card ⊂ Cardiii. A Card nested inside a Card — the depth GE actually renders, confirmed rather than assumed.

iv. Thumbs / copy / more — Gemini Enterprise's own turn chrome, not part of the A2UI payload.

Row

container Chat Cards v2 ✓

Horizontal layout container.

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children: {explicitList: […]}i. Every button row in this agent's live decks — weather-city suggestions, the workspace refresh control — is a Row underneath.

First · Second · Thirdii. Even spacing across the full width is GE's own layout choice for Row's children, not a field the payload set explicitly.

children-list mutation — inferrediii. Column's own children list is confirmed live-mutable both ways (see Column below). Row shares the identical field shape but hasn't been separately re-tested — treat as likely, not confirmed, until checked.

Column

container Chat Cards v2 ✓

Vertical layout container.

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children: {explicitList: […]}i. The single most-used primitive in this agent's codebase — every Card on this page wraps exactly one Column.

Top · Middle · Bottomii. Stacked top-to-bottom in declaration order — no separate 'align' field was needed for this simple case.

children-list mutation — confirmed both waysiii. The single biggest mechanical finding in this whole catalog: resending a Column's full children array on an OPEN surface can both ADD a new id (with a definition for that leaf) and REMOVE one — confirmed live, no stuck node, no error either direction. The safe default (declare the full tree up front, only patch leaf content afterward) is still right to reach for first — this just means it's no longer the only option.

List

container Chat Cards v2 ✓

A scrollable list container.

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direction: verticali. Scrollable rather than paginated — for long children sets this is the one to reach for over Column.

children: {explicitList: […]}ii. Same convention as Column's children — no surprises versus what Card/Row/Column already taught.

Button

action · round-trip Chat Cards v2 ✓

A tappable button that dispatches an action to the agent.

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userAction → agenti. Not just "renders" — a real click dispatches a userAction DataPart ({name, sourceComponentId, surfaceId, context}) back to the agent, confirmed round-trip.

Text ×2 (title, caption)ii. Same Text primitive from the Containers group, reused as heading and sub-caption — no separate "heading" primitive exists in the 18.

CheckBox

action · client-bound Chat Cards v2 ✓

A labelled checkbox bound to a boolean value.

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checked: {path: …}i. Native checkbox with its label as PART of the same primitive — not a separate Text block sitting next to it.

two-way boundii. Toggling flips client-side state immediately; nothing reaches the agent until the next Button tap carries it along in context.

TextField

action · round-trip Chat Cards v2 ✓

A text input, two-way bound to the data model.

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value: {path: …}i. Type a city, hit submit: the resolved value comes back on the following Button's action.context — real two-way binding, not a static field.

→ action.context on next Buttonii. The genuinely useful primitive of the whole catalog for an agent that takes typed input instead of only picking from suggested replies.

Slider

input · client-bound Chat Cards v2 ✗

Selects a numeric value within a min/max range.

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min / max / value: {path: …}i. Draggable thumb position; the numeric label underneath is a separate Text block GE doesn't merge onto the thumb itself.

value echoed as textii. The seeded value (95) shows both in the handle position and echoed underneath — a payload convenience, not automatic.

MultipleChoice

input · client-bound Chat Cards v2 ✓

One or more selections from a list.

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options: [{label,value}] (bare array)i. The schema asks for a selectable list; GE's client chose the chip affordance here — a different host could legitimately render checkboxes instead.

selections: {path: …}ii. Note the field is selections, not value — resolves to an array of the selected option values, confirmed live with one chip selected.

variant: "chips" — required alongside typeiii. Undocumented anywhere else in this catalog: type:"chips" alone renders a collapsed dropdown ("N Selected" + chevron) that expands into a checkbox list, not flat pills. Both fields are needed for the chip look shown here — found by diffing against a working reference, not from any spec.

DateTimeInput

input · client-bound Chat Cards v2 ✓

A native date and/or time picker.

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enableTime: false — not honoredi. Sent enableDate: true, enableTime: false for a date-only field; GE showed a full date-AND-time picker anyway. Functionally usable, just not what was asked for — a confirmed client quirk, not a guess.

Tabs

navigation · client-only Chat Cards v2 ✗

Tabbed panels, one visible at a time.

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tabItems: [{title,child}] (bare array)i. The fiddliest primitive to land: wrapping tabItems in {literalArray:…} — the same wrapper children correctly uses — crashes with an opaque teTabs` exception. Bare array is the real v0.8 shape.

tabItems[1] · inactiveii. A tap here swaps the visible panel entirely client-side — confirmed no round trip is needed for the switch itself.

active panel · Textiii. Whatever the active tab's own child is — here, one more Text block.

Modal

overlay · full tree host Chat Cards v2 ~

A trigger that opens a dialog over the conversation.

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Modal's own dismiss controli. The × is Modal's built-in affordance, not a Button block the payload had to add.

backdrop · tap-to-closeii. GE's own client chrome — dismissing by tapping outside the dialog was never declared in the payload either.

contentChild → full treeiii. Whatever contentChild points to renders in full — here a real chart Image, not just text. entryPointChild/contentChild, not trigger/content — and there's no open field; Modal owns its own open state.

entryPointChild · Buttoniv. A2UI Buttons always carry an action — GE dispatches it back to the agent on tap even though opening the modal itself is purely client-side, so the trigger fires a round-trip worth handling (or harmlessly swallowing).

Text

display Chat Cards v2 ✓

Displays text with simple Markdown formatting.

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markdown-lite: bold, italic, codei. No full Markdown — no links or lists inline here, just the three inline styles confirmed rendering.

usageHint: h1–h5 · body · captionii. The only primitive with a type-scale field — why it's the load-bearing one behind every other primitive's title/caption pairs.

Image

display Chat Cards v2 ✓

Displays an image from a URL.

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url (literal or data-bound)i. sankey_flow here isn't a live chart primitive — it's a server-rendered PNG pushed through Image, since GE's client only recognises the 18 standard primitives.

static raster, no interactivityii. Every pixel is baked in ahead of time — no live values, no data-binding once it's an Image. The workaround that makes any custom atom displayable in GE today.

usageHint: real enum, no "hero"iii. The valid values are icon, avatar, smallFeature, mediumFeature, largeFeature, header — "hero" isn't one of them. GE tolerates the invalid value on a fetched-URL Image and just fails silently; on an inline data: URI Image it also fails silently, no error either way — the dangerous part is neither path tells you why the image vanished.

Icon

display Chat Cards v2 ✓

A system icon (Material Symbols).

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name: <material-symbol>i. Confirmed a real Material Symbols glyph, vector-rendered — NOT emoji substituted into a Text block, the distinction an earlier dedicated probe existed to settle.

Video

display Chat Cards v2 ✗

A native video player from a URL.

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controls: true (required)i. Listed as a rendering guideline, not a default — omit it and GE renders a silent player with no visible play button at all.

native <video>, scrubbing includedii. A real element, not a thumbnail-then-launch pattern — full timeline scrub confirmed.

AudioPlayer

display Chat Cards v2 ✗

A native audio player from a URL.

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same url shape as Video, no extra fieldsi. A real native audio element — duration (5:48) read straight from the file, GE isn't faking the control chrome.

A horizontal or vertical dividing line.

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axis: horizontal | verticali. The simplest primitive in the catalog and it shows — this one rule IS the entire primitive; everything else on screen is surrounding Text blocks.