---name: yamldescription: YAML wire format for json-render with streaming parser, prompt generation, and AI SDK transform. Use when working with @json-render/yaml, YAML-based spec streaming, yaml-spec/yaml-edit fences, or YAML prompt generation.---

@json-render/yaml

YAML wire format for @json-render/core. Progressive rendering and surgical edits via streaming YAML.

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 as a drop-in replacement for 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

Export	Description
yamlPrompt	Generate YAML system prompt from catalog
createYamlTransform	AI SDK TransformStream for YAML fences
pipeYamlRender	Convenience pipe wrapper (replaces pipeJsonRender)
createYamlStreamCompiler	Streaming YAML parser with patch emission
YAML_SPEC_FENCE	Fence constant for yaml-spec
YAML_EDIT_FENCE	Fence constant for yaml-edit
YAML_PATCH_FENCE	Fence constant for yaml-patch
DIFF_FENCE	Fence constant for diff
FENCE_CLOSE	Fence close constant
diffToPatches	Re-export: object diff to JSON Patch
deepMergeSpec	Re-export: RFC 7396 deep merge