Validating and adapting an A2UI stream

Take a streaming agent response, turn it into typed A2UI messages, narrow the dynamic values, and feed a renderer — with the right amount of validation for your trust level.

@threadplane/a2ui is the protocol layer beneath any A2UI integration. This guide shows how to consume a stream, guard the values, build test payloads, and back a custom renderer.

#Consume a streaming response

The parser is fed-by-chunk and line-oriented. Hand each chunk of the response to push; it returns the A2uiMessage[] it could complete from everything buffered so far.

import { createA2uiMessageParser } from '@threadplane/a2ui';
 
const parser = createA2uiMessageParser();
 
for await (const chunk of streamChunks) {
  for (const message of parser.push(chunk)) {
    applyToSurfaceStore(message);
  }
}

The posture (straight from parser.ts) is conservative fallback:

• Malformed lines are skipped silently — partial JSONL is normal mid-stream.
• Lines whose object has no known envelope key are ignored.
• Incomplete JSON buffers until a newline arrives.

That last point in practice:

parser.push('{"beginRendering":');                    // -> []  (buffers)
parser.push('{"surfaceId":"s1","root":"root"}}\n');   // -> 1 message

A line is only attempted once its trailing newline lands, so a split-mid-value chunk never throws.

#Validate and narrow values

When you walk a component's props, you need to tell a literal from a path reference. The package exports four guards:

import {
  isPathRef,
  isLiteralString,
  isLiteralNumber,
  isLiteralBoolean,
} from '@threadplane/a2ui';
 
isPathRef({ path: '/x' });               // true
isLiteralString({ literalString: 'x' }); // true

For most rendering you don't branch on guards at all — resolveDynamic already handles literals, paths, arrays, and passthrough in one call. Reach for the guards when you need to narrow a type or make a decision before resolving.

#Build payloads for tests

The cleanest way to test an adapter is to drive the real parser with assembled lines, then assert the envelope kinds. This mirrors the parser's own multi-message test.

import { createA2uiMessageParser } from '@threadplane/a2ui';
 
const parser = createA2uiMessageParser();
 
const chunk = [
  JSON.stringify({ surfaceUpdate: { surfaceId: 's1', components: [] } }),
  JSON.stringify({ dataModelUpdate: { surfaceId: 's1', contents: [] } }),
  JSON.stringify({ beginRendering: { surfaceId: 's1', root: 'root' } }),
].join('\n') + '\n';
 
const messages = parser.push(chunk);
 
messages.map(m => Object.keys(m)[0]);
// ['surfaceUpdate', 'dataModelUpdate', 'beginRendering']

Each A2uiMessage is a single-key envelope object, so Object.keys(m)[0] is the envelope kind — handy for assertions.

#Build a custom renderer

To render a surface, resolve each component's props against the surface's data model and emit your own UI. The resolver does the literal/path collapsing:

import { resolveDynamic } from '@threadplane/a2ui';
 
function renderText(props: { text: unknown }, model: Record<string, unknown>) {
  const text = resolveDynamic(props.text, model); // literal or path -> string
  return makeTextNode(text);
}

The full mechanics — component resolution, event dispatch, action emission, surface store — are exactly what Threadplane's own Angular renderer, @threadplane/chat's <a2ui-surface>, already implements. If you're on Angular, use it rather than re-deriving it. A custom renderer makes sense when you're on another platform or have rendering needs the component doesn't cover.

#A tradeoff: the parser swallows parse errors

For me, the parser's silent-skip behavior is the right default — it's what lets a half-streamed line not blow up a live render, and it's why feeding raw agent output Just Works. The cost is honest: the parser is not a validator. It will quietly drop a malformed line and ignore an unknown envelope, so a structurally-wrong payload simply produces fewer messages, not an error you can catch.

So the rule of thumb: if you need strictness, validate the parsed A2uiMessage[] after push returns — assert the envelope kinds and shapes you expect, rather than counting on the parser to reject bad input. The parser optimizes for streaming resilience; strict validation is your boundary's job.

#Next

• Quick Start — the parse / build / resolve loop end to end.
• The A2UI message protocol — surfaces, components, envelopes, and actions.