@json-render/yaml

YAML wire format for

@json-render/core

Key Concepts

• YAML wire format: Alternative to JSONL that uses code fences (

  yaml-spec

  ,

  yaml-edit

  ,

  yaml-patch

  ,

  diff

  )
• Streaming parser: Incrementally parses YAML, emits JSON Patch operations via diffing
• Edit modes: Patch (RFC 6902), merge (RFC 7396), and unified diff
• AI SDK transform:

  TransformStream

  that converts YAML fences into json-render patches

Generating YAML Prompts

import { yamlPrompt } from "@json-render/yaml";
import { catalog } from "./catalog";

// Standalone mode (LLM outputs only YAML)
const systemPrompt = yamlPrompt(catalog, {
  mode: "standalone",
  editModes: ["merge"],
  customRules: ["Always use dark theme"],
});

// Inline mode (LLM responds conversationally, wraps YAML in fences)
const chatPrompt = yamlPrompt(catalog, { mode: "inline" });

Options:

• system

  (string) — Custom system message intro

• mode

  ("standalone" | "inline") — Output mode, default "standalone"

• customRules

  (string[]) — Additional rules appended to prompt

• editModes

  (EditMode[]) — Edit modes to document, default ["merge"]

AI SDK Transform

Use

pipeYamlRender

pipeJsonRender

import { pipeYamlRender } from "@json-render/yaml";
import { createUIMessageStream, createUIMessageStreamResponse } from "ai";

const stream = createUIMessageStream({
  execute: async ({ writer }) => {
    writer.merge(pipeYamlRender(result.toUIMessageStream()));
  },
});
return createUIMessageStreamResponse({ stream });

For multi-turn edits, pass the previous spec:

pipeYamlRender(result.toUIMessageStream(), {
  previousSpec: currentSpec,
});

The transform recognizes four fence types:

• yaml-spec

  — Full spec, parsed progressively line-by-line

• yaml-edit

  — Partial YAML deep-merged with current spec (RFC 7396)

• yaml-patch

  — RFC 6902 JSON Patch lines

• diff

  — Unified diff applied to serialized spec

Streaming Parser (Low-Level)

import { createYamlStreamCompiler } from "@json-render/yaml";

const compiler = createYamlStreamCompiler<Spec>();

// Feed chunks as they arrive from any source
const { result, newPatches } = compiler.push("root: main\n");
compiler.push("elements:\n  main:\n    type: Card\n");

// Flush remaining data at end of stream
const { result: final } = compiler.flush();

// Reset for next stream (optionally with initial state)
compiler.reset({ root: "main", elements: {} });

Methods:

push(chunk)

flush()

getResult()

getPatches()

reset(initial?)

Edit Modes (from @json-render/core)

The YAML package uses the universal edit mode system from core:

import { buildEditInstructions, buildEditUserPrompt } from "@json-render/core";
import type { EditMode } from "@json-render/core";

// Generate edit instructions for YAML format
const instructions = buildEditInstructions({ modes: ["merge", "patch"] }, "yaml");

// Build user prompt with current spec context
const userPrompt = buildEditUserPrompt({
  prompt: "Change the title to Dashboard",
  currentSpec: spec,
  config: { modes: ["merge"] },
  format: "yaml",
  serializer: (s) => yamlStringify(s, { indent: 2 }).trimEnd(),
});

Fence Constants

For custom parsing, use the exported constants:

import {
  YAML_SPEC_FENCE,   // "```yaml-spec"
  YAML_EDIT_FENCE,   // "```yaml-edit"
  YAML_PATCH_FENCE,  // "```yaml-patch"
  DIFF_FENCE,        // "```diff"
  FENCE_CLOSE,       // "```"
} from "@json-render/yaml";

Key Exports

yamlPrompt

createYamlTransform

pipeYamlRender

pipeJsonRender

createYamlStreamCompiler

YAML_SPEC_FENCE

YAML_EDIT_FENCE

YAML_PATCH_FENCE

DIFF_FENCE

FENCE_CLOSE

diffToPatches

deepMergeSpec