prefab: a TypeScript agent-side emitter for A2UI v1.0

[quotentiroler]

The ecosystem page lists a lot of excellent renderers. This is the other side of
the wire: agent_sdks/ currently holds Python only, so here is a TypeScript way
to produce A2UI.

@maxhealth.tech/prefab is a
server-side component library for agent UIs. You describe the interface once
with a typed API and it emits A2UI v1.0:

const app = new PrefabApp({
  view: Column({ children: [H1('Users'), autoTable(rows)] }),
})

const { messages, diagnostics } = app.toA2UI()

Try it without installing anything: the
playground has an A2UI tab. Pick an
example, switch to it, and you get the translated payload plus a list of
everything the translation cost.

What maps

• The component tree flattens into the adjacency list, with ids allocated
  deterministically so the same tree always emits identical output.
• {{ user.name }} becomes { path: '/user/name' }, and mixed text such as
  Score: {{ score }} goes through formatString as Score: ${/score}.
• Formatting pipes become catalog functions: currency to formatCurrency,
  number and round to formatNumber, the date pipes to formatDate,
  pluralize to pluralize.
• Loops become the child template, following
  catalogs/basic/examples/18_track-list.json. Data tables use the same shape,
  so a table stays one template rather than N inlined rows.
• Named definitions are expanded at emit time, since the agent can do that
  before sending and A2UI then needs no concept for it.
• Form validation becomes checks, so a required field stays required.
• Served over MCP under application/a2ui+json, as an a2ui:// resource or an
  embedded resource in a tool result, per the
  A2UI over MCP guide.

What does not

prefab has 115+ components and the Basic catalog has 18, so parts of a tree
change shape. Every change is reported rather than happening quietly:

degraded     Metric      rendered as a Column of Text; trend and delta styling dropped
unsupported  LineChart   charts are outside the A2UI Basic catalog
expression   Text        "count + 1" is richer than a JSON Pointer

Most of those are gaps in the catalog. Conditionals are a genuine difference
between the two protocols. A2UI has no declarative if: the renderer
draws what the adjacency list says and the agent sends a fresh
updateComponents when the shape should change. prefab runs a reactive client
that re-shapes itself without a round trip. A one-item list template would look
like a conditional and behave like one only by accident, so the emitter reports
the loss instead of faking it.

Conformance

Payloads are validated in CI against the official v1.0 JSON Schemas, with the
Basic catalog bound into the catalog.json slot, plus the two structural rules
the schemas cannot express: every child reference resolves, and every component
is reachable from root. Every shipped example runs through that gate, which is
what caught a case where a component that failed to map left its children
stranded.

I read conformance/*.yaml first and concluded it targets SDK internals
(streaming parser, catalog pruning, validator behaviour) that a pure producer
does not implement. If that is wrong and there is a producer-facing subset I
should be running, I would like to know.

• Repo: https://github.com/Max-Health-Inc/prefab
• Mapping table and diagnostics: https://maxhealth.tech/prefab/guide/a2ui
• npm: https://www.npmjs.com/package/@maxhealth.tech/prefab

Corrections welcome on anything I have mapped wrong. Two I am least sure about:
the date pipes pick a fixed TR35 pattern because prefab renders in the reader's
locale and there is no locale-following option, and {{ $item }} on a list of
plain strings has no relative path to bind to.