@json-render/core

Core package for schema definition, catalog creation, and spec streaming.

Key Concepts

• Schema: Defines the structure of specs and catalogs (use defineSchema)
• Catalog: Maps component/action names to their definitions (use defineCatalog)
• Spec: JSON output from AI that conforms to the schema
• SpecStream: JSONL streaming format for progressive spec building

Experimental Decision-Model Composition

For decision-model composition, import experimental_composeSpec and experimental_createEvaluator from @json-render/core. These APIs are available since v0.21.0; pin exact versions and keep json-render packages on the same version. Experimental exports and Experimental_ types can change in any release.

• Run the Gateway evaluator server-side with { model: "typesafe-ai/jev", apiKey: process.env.AI_GATEWAY_API_KEY! }. A plain model identifier is required; Jev is the current example; do not import a provider constructor.
• Call experimental_composeSpec({ catalog, candidates, prompt, evaluate, initialState, signal }). It is an async generator; stream step.spec snapshots to your existing renderer and inspect complete.stopReason (finish, limit, unavailable). Errors and cancellation throw; retain the last snapshot as partial UI.
• New trees default to strategy: "batch": one evaluation selects root/membership, then a second arranges the selected elements when needed. The first snapshot contains selected content in catalog order under the root's default/first slot. Resource variants share one exclusive question; repeated counts include the root. Root selection takes precedence over conflicting speculative membership for that recipe/resource. Equal sibling positions retain catalog order. Combined layouts are validated before publication; cycles or excessive depth throw. maxElements caps batched creation (default 32). Limit-truncated selections or a missing required layout call return limit. Use strategy: "sequential" for legacy next/parent adapters or sequential creation. Edits stay sequential.
• Batched trace steps use choice: "select" | "layout" and an answers record. Count each trace as one evaluation, including its tokens and latency once. Custom evaluators must answer every offered question; names/choices are opaque and include root/select_*, then parent_*/order_* for batches.
• For follow-up edits, pass the selected version as initialSpec. It is cloned and validated; the evaluator may add, replace, remove non-root subtrees, or move/reorder them. Unchanged IDs, bindings, and state are preserved. Optional elementDescriptions shares identifying descriptions without exposing raw props/state. initialState overrides the seed state. Seeds must be valid trees within the catalog, expression subset, and depth limit. Matching recipes consume usage/resource limits; removals/replacements release them. Replacements/moves use two evaluations (select target, then recipe/destination), each counted against the budget. Treat operation and position keys as opaque.
• Supply atomic candidates with { id, description, element: { type, props, on?, visible? }, root?, maxUses?, resource? }. Catalog alone is insufficient: the app must supply values and binding recipes. Jev chooses elements and parent slots, never free-form text or code. It never executes actions.
• Candidates are configured component instances, not page templates. Build them from current app records/operations or bind props to initialState; offer explicit alternatives for chart types, field configurations, and layout variants. The model chooses grouping and order within those options. Name required sections in prompts; structural validity does not imply semantic completeness.
• V1 supports flat Spec catalogs, named slots, literals, $state, $bindState, and state visibility. No prebuilt children, repeat/watch, computed/template/conditional props, or custom directives. Success/error callbacks must reference allowed actions. Events must be declared in the component catalog.
• Props and action params are validated against initial state without applying schema transforms/defaults. Supply valid initial values and validate/authorize action calls at runtime. Built-ins without parameter schemas get name validation only.
• root defaults true, maxUses defaults one, shared resource values make alternatives mutually exclusive. Defaults: 32 evaluations (terminal calls included; no extra finish call for batches), depth eight, 10-second Gateway timeout per call. Supply an overall abort signal.
• Candidate descriptions, prompt, instructions, topology, and explicit context are sent to the evaluator. Initial state and raw props/binding values are not sent automatically.
• For custom providers implement Experimental_CompositionEvaluator: accept { state, questions, signal }, return { answers: { [question]: { choice, confidence? } }, usage?: { inputTokens? } }. Only return offered criteria keys.

See packages/core/README.md and /docs/jev for app integration and installation instructions. The web playground is an example consumer, not a dependency of the API.

Defining a Schema

import { defineSchema } from "@json-render/core";

export const schema = defineSchema((s) => ({
  spec: s.object({
    // Define spec structure
  }),
  catalog: s.object({
    components: s.map({
      props: s.zod(),
      description: s.string(),
    }),
  }),
}), {
  promptTemplate: myPromptTemplate, // Optional custom AI prompt
});

Creating a Catalog

import { defineCatalog } from "@json-render/core";
import { schema } from "./schema";
import { z } from "zod";

export const catalog = defineCatalog(schema, {
  components: {
    Button: {
      props: z.object({
        label: z.string(),
        variant: z.enum(["primary", "secondary"]).nullable(),
      }),
      description: "Clickable button component",
    },
  },
});

Generating AI Prompts

const systemPrompt = catalog.prompt(); // Uses schema's promptTemplate
const systemPrompt = catalog.prompt({ customRules: ["Rule 1", "Rule 2"] });

SpecStream Utilities

For streaming AI responses (JSONL patches):

import { createSpecStreamCompiler } from "@json-render/core";

const compiler = createSpecStreamCompiler<MySpec>();

// Process streaming chunks
const { result, newPatches } = compiler.push(chunk);

// Get final result
const finalSpec = compiler.getResult();

Form Values in Action Handlers

Use findFormValue("email", params, state) to read a direct parameter, a dotted parameter key (such as "form.email"), a matching flat state key, or a slash-delimited path (such as "/form/email") in nested state. Parameter values are literal, so emails and URLs containing dots are preserved. For action bindings that read nested state, use { $state: "/form/email" }; the resolver passes that value to the handler. A bare "email" field name does not search nested state.form.email.

Dynamic Prop Expressions

Any prop value can be a dynamic expression resolved at render time:

• { "$state": "/state/key" } - reads a value from the state model (one-way read)
• { "$bindState": "/path" } - two-way binding: reads from state and enables write-back. Use on the natural value prop (value, checked, pressed, etc.) of form components.
• { "$bindItem": "field" } - two-way binding to a repeat item field. Use inside repeat scopes.
• { "$cond": <condition>, "$then": <value>, "$else": <value> } - evaluates a visibility condition and picks a branch
• { "$template": "Hello, ${/user/name}!" } - interpolates ${/path} references with state values
• { "$computed": "fnName", "args": { "key": <expression> } } - calls a registered function with resolved args

$cond uses the same syntax as visibility conditions ($state, eq, neq, not, arrays for AND). $then and $else can themselves be expressions (recursive).

Components do not use a statePath prop for two-way binding. Instead, use { "$bindState": "/path" } on the natural value prop (e.g. value, checked, pressed).

{
  "color": {
    "$cond": { "$state": "/activeTab", "eq": "home" },
    "$then": "#007AFF",
    "$else": "#8E8E93"
  },
  "label": { "$template": "Welcome, ${/user/name}!" },
  "fullName": {
    "$computed": "fullName",
    "args": {
      "first": { "$state": "/form/firstName" },
      "last": { "$state": "/form/lastName" }
    }
  }
}

import { resolvePropValue, resolveElementProps } from "@json-render/core";

const resolved = resolveElementProps(element.props, { stateModel: myState });

State Watchers

Elements can declare a watch field (top-level, sibling of type/props/children) to trigger actions when state values change:

{
  "type": "Select",
  "props": { "value": { "$bindState": "/form/country" }, "options": ["US", "Canada"] },
  "watch": {
    "/form/country": { "action": "loadCities", "params": { "country": { "$state": "/form/country" } } }
  },
  "children": []
}

Watchers only fire on value changes, not on initial render.

Validation

Built-in validation functions: required, email, url, numeric, minLength, maxLength, min, max, pattern, matches, equalTo, lessThan, greaterThan, requiredIf.

Cross-field validation uses $state expressions in args:

import { check } from "@json-render/core";

check.required("Field is required");
check.matches("/form/password", "Passwords must match");
check.lessThan("/form/endDate", "Must be before end date");
check.greaterThan("/form/startDate", "Must be after start date");
check.requiredIf("/form/enableNotifications", "Required when enabled");

User Prompt Builder

Build structured user prompts with optional spec refinement and state context:

import { buildUserPrompt } from "@json-render/core";

// Fresh generation
buildUserPrompt({ prompt: "create a todo app" });

// Refinement with edit modes (default: patch-only)
buildUserPrompt({ prompt: "add a toggle", currentSpec: spec, editModes: ["patch", "merge"] });

// With runtime state
buildUserPrompt({ prompt: "show data", state: { todos: [] } });

Available edit modes: "patch" (RFC 6902 JSON Patch), "merge" (RFC 7396 Merge Patch), "diff" (unified diff).

Spec Validation

Validate spec structure and auto-fix common issues:

import { validateSpec, autoFixSpec } from "@json-render/core";

const { valid, issues } = validateSpec(spec);
// issues include: missing_child, invalid_visible (malformed conditions),
// repeat_without_children, repeat_item_outside_scope, repeat_state_mismatch

const { spec: fixed, fixDetails } = autoFixSpec(spec);
// fixDetails entries are { message, lossy }. Lossless fixes relocate
// misplaced fields; lossy fixes prune dangling children references.
// In a repair loop, withhold lossy fixes until retries are exhausted:
const attempt = autoFixSpec(spec, { lossy: retriesExhausted });

Visibility Conditions

Control element visibility with state-based conditions. VisibilityContext is { stateModel: StateModel }.

import { visibility } from "@json-render/core";

// Syntax
{ "$state": "/path" }                    // truthiness
{ "$state": "/path", "not": true }      // falsy
{ "$state": "/path", "eq": value }      // equality
[ cond1, cond2 ]                         // implicit AND

// Helpers
visibility.when("/path")                 // { $state: "/path" }
visibility.unless("/path")               // { $state: "/path", not: true }
visibility.eq("/path", val)              // { $state: "/path", eq: val }
visibility.and(cond1, cond2)             // { $and: [cond1, cond2] }
visibility.or(cond1, cond2)              // { $or: [cond1, cond2] }
visibility.always                        // true
visibility.never                         // false

Built-in Actions in Schema

Schemas can declare builtInActions -- actions that are always available at runtime and auto-injected into prompts:

const schema = defineSchema(builder, {
  builtInActions: [
    { name: "setState", description: "Update a value in the state model" },
  ],
});

These appear in prompts as [built-in] and don't require handlers in defineRegistry.

StateStore

The StateStore interface allows external state management libraries (Redux, Zustand, XState, etc.) to be plugged into json-render renderers. The createStateStore factory creates a simple in-memory implementation:

import { createStateStore, type StateStore } from "@json-render/core";

const store = createStateStore({ count: 0 });

store.get("/count");         // 0
store.set("/count", 1);      // updates and notifies subscribers
store.update({ "/a": 1, "/b": 2 }); // batch update

store.subscribe(() => {
  console.log(store.getSnapshot()); // { count: 1 }
});

The StateStore interface: get(path), set(path, value), update(updates), getSnapshot(), subscribe(listener).

Key Exports