feat: add custom directives API and @json-render/directives package (#279)

* feat: add custom directives API and @json-render/directives package

Add a `defineDirective` API that lets users register custom `$`-prefixed
dynamic values with schemas, resolvers, and prompt instructions — extending
the spec language without forking core.

* fixes

* perf(core): optimize findDirective to iterate registry instead of object keys

Flip the loop from O(object-keys) to O(registry-size) by iterating
the directive registry and checking `key in value` rather than scanning
all object keys with Object.keys() and startsWith("$").

* fix(core): reject directive names that conflict with built-in keys

defineDirective now throws at registration time if the name collides
with a built-in prop expression key ($state, $cond, etc.), making the
precedence contract explicit rather than relying on check ordering in
resolvePropValue.

* fix(directives): handle future dates in $format and warn on $math NaN coercion

$format relative dates now support future timestamps ("2h from now")
and return "just now" for zero diff. $math emits a console.warn in
dev mode when a non-numeric value is silently coerced to 0.

* fix(directives): remove process.env check that breaks DTS build

The directives package doesn't include @types/node, so referencing
process.env fails during tsup's DTS generation. The console.warn is
unconditional now — it only fires on actual misuse (non-numeric input)
so the cost is negligible.

* feat(directives): rename prompt to description, auto-describe schemas in prompt, add docs

- Rename `prompt` to `description` on DirectiveDefinition — a short
  behavioral label rather than the full AI prompt blob
- Auto-generate directive schema signatures in the system prompt using
  formatZodType, so the AI always sees every field, type, and optionality
- Add docs: guide page, API reference page, nav/title entries, docs-chat

* feat(directives): add standardDirectives export and composition hint

Export a pre-assembled standardDirectives array (all 7 non-factory
directives) for convenience. Add a composition hint to the generated
AI prompt so agents know directives can nest inside each other.

* docs: add directives skill, README entry, and docs-chat listing

- Add skills/directives/SKILL.md with full directive API reference
- Add @json-render/directives row to root README packages table
- Add "directives" to the Available skills list in docs-chat prompt

* perf(core): skip Zod parse in directive hot path

Resolvers are already defensive (coercion, fallbacks, switch defaults),
so runtime validation on every render adds overhead without safety.
The schema remains used for prompt generation and TypeScript inference.

README.md

@@ -136,6 +136,7 @@ function Dashboard({ spec }) {
 | `@json-render/react-email`  | React Email renderer for HTML/plain-text emails from specs             |
 | `@json-render/ink`          | Ink terminal renderer with built-in components for interactive TUIs.   |
 | `@json-render/image`        | Image renderer for SVG/PNG output (OG images, social cards) via Satori |
+| `@json-render/directives`   | Pre-built custom directives — $format, $math, $concat, $count, $truncate, $pluralize, $join, $t (i18n) |
 | `@json-render/codegen`      | Utilities for generating code from json-render UI trees                |
 | `@json-render/devtools`     | Framework-agnostic devtools core — panel UI, event store, picker, stream taps |
 | `@json-render/devtools-react`   | React adapter for `@json-render/devtools` (drop-in `<JsonRenderDevtools />`)     |

apps/web/app/(main)/docs/api/directives/page.mdx

@@ -0,0 +1,407 @@
+import { pageMetadata } from "@/lib/page-metadata"
+export const metadata = pageMetadata("docs/api/directives")
+
+# @json-render/directives
+
+Pre-built custom directives for `@json-render/core`. Drop them into your catalog and renderer to add formatting, math, string manipulation, and i18n.
+
+## Install
+
+```bash
+npm install @json-render/directives
+```
+
+## Quick Start
+
+```typescript
+import { standardDirectives } from '@json-render/directives';
+
+// Wire into prompt generation
+const prompt = catalog.prompt({ directives: standardDirectives });
+
+// Wire into the renderer
+<JSONUIProvider spec={spec} directives={standardDirectives}>
+  ...
+</JSONUIProvider>
+```
+
+To add factory directives like `createI18nDirective`, spread the array:
+
+```typescript
+import { standardDirectives, createI18nDirective } from '@json-render/directives';
+
+const directives = [...standardDirectives, createI18nDirective(config)];
+```
+
+## Directives
+
+### `$format` — Locale-aware value formatting
+
+Formats values using `Intl` formatters. Supports `date`, `currency`, `number`, and `percent`.
+
+```json
+{ "$format": "currency", "value": { "$state": "/cart/total" }, "currency": "USD" }
+```
+
+```json
+{ "$format": "date", "value": { "$state": "/user/createdAt" } }
+```
+
+```json
+{ "$format": "number", "value": 1234567, "notation": "compact" }
+```
+
+```json
+{ "$format": "percent", "value": 0.75 }
+```
+
+Relative dates are also supported:
+
+```json
+{ "$format": "date", "value": { "$state": "/post/createdAt" }, "style": "relative" }
+```
+
+This returns strings like `"3h ago"`, `"2d from now"`, or `"just now"`.
+
+<table>
+  <thead>
+    <tr>
+      <th>Field</th>
+      <th>Type</th>
+      <th>Description</th>
+    </tr>
+  </thead>
+  <tbody>
+    <tr>
+      <td><code>$format</code></td>
+      <td><code>{'\"date\" | \"currency\" | \"number\" | \"percent\"'}</code></td>
+      <td>Format type.</td>
+    </tr>
+    <tr>
+      <td><code>value</code></td>
+      <td><code>unknown</code></td>
+      <td>Value to format. Accepts any dynamic expression.</td>
+    </tr>
+    <tr>
+      <td><code>locale</code></td>
+      <td><code>string</code></td>
+      <td>Optional. Locale for formatting (e.g. <code>"en-US"</code>).</td>
+    </tr>
+    <tr>
+      <td><code>currency</code></td>
+      <td><code>string</code></td>
+      <td>Optional. Currency code for <code>"currency"</code> format. Default: <code>"USD"</code>.</td>
+    </tr>
+    <tr>
+      <td><code>notation</code></td>
+      <td><code>string</code></td>
+      <td>Optional. Notation for <code>"number"</code> format (e.g. <code>"compact"</code>).</td>
+    </tr>
+    <tr>
+      <td><code>style</code></td>
+      <td><code>string</code></td>
+      <td>Optional. Set to <code>"relative"</code> for relative date formatting.</td>
+    </tr>
+    <tr>
+      <td><code>options</code></td>
+      <td><code>{'Record<string, unknown>'}</code></td>
+      <td>Optional. Extra <code>Intl</code> formatter options.</td>
+    </tr>
+  </tbody>
+</table>
+
+### `$math` — Arithmetic operations
+
+Performs arithmetic on one or two operands. Operands accept any dynamic expression.
+
+```json
+{ "$math": "add", "a": { "$state": "/subtotal" }, "b": { "$state": "/tax" } }
+```
+
+```json
+{ "$math": "round", "a": 3.7 }
+```
+
+<table>
+  <thead>
+    <tr>
+      <th>Field</th>
+      <th>Type</th>
+      <th>Description</th>
+    </tr>
+  </thead>
+  <tbody>
+    <tr>
+      <td><code>$math</code></td>
+      <td><code>{'\"add\" | \"subtract\" | \"multiply\" | \"divide\" | \"mod\" | \"min\" | \"max\" | \"round\" | \"floor\" | \"ceil\" | \"abs\"'}</code></td>
+      <td>Operation to perform.</td>
+    </tr>
+    <tr>
+      <td><code>a</code></td>
+      <td><code>unknown</code></td>
+      <td>First operand. Defaults to <code>0</code> if missing.</td>
+    </tr>
+    <tr>
+      <td><code>b</code></td>
+      <td><code>unknown</code></td>
+      <td>Second operand (binary ops only). Defaults to <code>0</code> if missing.</td>
+    </tr>
+  </tbody>
+</table>
+
+Unary operations (`round`, `floor`, `ceil`, `abs`) only use `a`. Division by zero returns `0`.
+
+### `$concat` — String concatenation
+
+Concatenates multiple values into a single string. Each element is resolved then joined.
+
+```json
+{ "$concat": [{ "$state": "/user/firstName" }, " ", { "$state": "/user/lastName" }] }
+```
+
+<table>
+  <thead>
+    <tr>
+      <th>Field</th>
+      <th>Type</th>
+      <th>Description</th>
+    </tr>
+  </thead>
+  <tbody>
+    <tr>
+      <td><code>$concat</code></td>
+      <td><code>{'unknown[]'}</code></td>
+      <td>Array of values to concatenate. Each is resolved, converted to string, and joined.</td>
+    </tr>
+  </tbody>
+</table>
+
+### `$count` — Array/string length
+
+Returns the length of an array or string. Returns `0` for other types.
+
+```json
+{ "$count": { "$state": "/cart/items" } }
+```
+
+<table>
+  <thead>
+    <tr>
+      <th>Field</th>
+      <th>Type</th>
+      <th>Description</th>
+    </tr>
+  </thead>
+  <tbody>
+    <tr>
+      <td><code>$count</code></td>
+      <td><code>unknown</code></td>
+      <td>Value to count. Accepts arrays and strings.</td>
+    </tr>
+  </tbody>
+</table>
+
+### `$truncate` — Text truncation
+
+Truncates text to a maximum length with a configurable suffix.
+
+```json
+{ "$truncate": { "$state": "/post/body" }, "length": 140, "suffix": "..." }
+```
+
+<table>
+  <thead>
+    <tr>
+      <th>Field</th>
+      <th>Type</th>
+      <th>Description</th>
+    </tr>
+  </thead>
+  <tbody>
+    <tr>
+      <td><code>$truncate</code></td>
+      <td><code>unknown</code></td>
+      <td>Value to truncate.</td>
+    </tr>
+    <tr>
+      <td><code>length</code></td>
+      <td><code>number</code></td>
+      <td>Optional. Max character length. Default: <code>100</code>.</td>
+    </tr>
+    <tr>
+      <td><code>suffix</code></td>
+      <td><code>string</code></td>
+      <td>Optional. Suffix to append when truncated. Default: <code>"..."</code>.</td>
+    </tr>
+  </tbody>
+</table>
+
+### `$pluralize` — Singular/plural forms
+
+Selects a singular, plural, or zero form based on a count.
+
+```json
+{ "$pluralize": { "$state": "/cart/itemCount" }, "one": "item", "other": "items", "zero": "no items" }
+```
+
+Output: `"3 items"`, `"1 item"`, or `"no items"`.
+
+<table>
+  <thead>
+    <tr>
+      <th>Field</th>
+      <th>Type</th>
+      <th>Description</th>
+    </tr>
+  </thead>
+  <tbody>
+    <tr>
+      <td><code>$pluralize</code></td>
+      <td><code>unknown</code></td>
+      <td>Count value. Accepts dynamic expressions.</td>
+    </tr>
+    <tr>
+      <td><code>one</code></td>
+      <td><code>string</code></td>
+      <td>Singular form label.</td>
+    </tr>
+    <tr>
+      <td><code>other</code></td>
+      <td><code>string</code></td>
+      <td>Plural form label.</td>
+    </tr>
+    <tr>
+      <td><code>zero</code></td>
+      <td><code>string</code></td>
+      <td>Optional. Label for count of zero. If omitted, uses <code>"0 {'<other>'}"</code>.</td>
+    </tr>
+  </tbody>
+</table>
+
+### `$join` — Join array elements
+
+Joins array elements with a separator string.
+
+```json
+{ "$join": { "$state": "/tags" }, "separator": ", " }
+```
+
+<table>
+  <thead>
+    <tr>
+      <th>Field</th>
+      <th>Type</th>
+      <th>Description</th>
+    </tr>
+  </thead>
+  <tbody>
+    <tr>
+      <td><code>$join</code></td>
+      <td><code>unknown</code></td>
+      <td>Array to join. Non-array values are converted to string.</td>
+    </tr>
+    <tr>
+      <td><code>separator</code></td>
+      <td><code>string</code></td>
+      <td>Optional. Separator between elements. Default: <code>", "</code>.</td>
+    </tr>
+  </tbody>
+</table>
+
+### `createI18nDirective` — Internationalization
+
+Factory function that creates a `$t` directive for translations with `{'{{param}}'}` interpolation.
+
+```typescript
+import { createI18nDirective } from '@json-render/directives';
+
+const tDirective = createI18nDirective({
+  locale: 'en',
+  messages: {
+    en: { "greeting": "Hello, {'{{name}}'}!", "checkout.submit": "Place Order" },
+    es: { "greeting": "Hola, {'{{name}}'}!", "checkout.submit": "Realizar Pedido" },
+  },
+  fallbackLocale: 'en',
+});
+```
+
+Usage in specs:
+
+```json
+{ "$t": "checkout.submit" }
+```
+
+```json
+{ "$t": "greeting", "params": { "name": { "$state": "/user/name" } } }
+```
+
+<table>
+  <thead>
+    <tr>
+      <th>Field</th>
+      <th>Type</th>
+      <th>Description</th>
+    </tr>
+  </thead>
+  <tbody>
+    <tr>
+      <td><code>$t</code></td>
+      <td><code>string</code></td>
+      <td>Translation key.</td>
+    </tr>
+    <tr>
+      <td><code>params</code></td>
+      <td><code>{'Record<string, unknown>'}</code></td>
+      <td>Optional. Interpolation parameters. Values accept dynamic expressions.</td>
+    </tr>
+  </tbody>
+</table>
+
+#### `I18nConfig`
+
+<table>
+  <thead>
+    <tr>
+      <th>Field</th>
+      <th>Type</th>
+      <th>Description</th>
+    </tr>
+  </thead>
+  <tbody>
+    <tr>
+      <td><code>locale</code></td>
+      <td><code>string</code></td>
+      <td>Current locale (e.g. <code>"en"</code>).</td>
+    </tr>
+    <tr>
+      <td><code>messages</code></td>
+      <td><code>{'Record<string, Record<string, string>>'}</code></td>
+      <td>Map of locale to key-value translation pairs.</td>
+    </tr>
+    <tr>
+      <td><code>fallbackLocale</code></td>
+      <td><code>string</code></td>
+      <td>Optional. Fallback locale when a key is missing in the current locale.</td>
+    </tr>
+  </tbody>
+</table>
+
+## Composition
+
+Directives compose naturally. Each resolver calls `resolvePropValue` on its inputs, so you can nest directives:
+
+```json
+{
+  "$format": "currency",
+  "value": { "$math": "multiply", "a": { "$state": "/price" }, "b": { "$state": "/qty" } },
+  "currency": "USD"
+}
+```
+
+```json
+{
+  "$pluralize": { "$count": { "$state": "/items" } },
+  "one": "item",
+  "other": "items"
+}
+```

apps/web/app/(main)/docs/directives/page.mdx

@@ -0,0 +1,124 @@
+import { pageMetadata } from "@/lib/page-metadata"
+export const metadata = pageMetadata("docs/directives")
+
+# Directives
+
+Extend the spec language with custom `$`-prefixed dynamic values. Directives let you add formatting, math, string manipulation, i18n, and any other transformation without modifying core.
+
+## Overview
+
+A directive is a user-defined dynamic value expression, like `$state` or `$computed`, but defined in userland. Each directive has a `$`-prefixed name, a Zod schema for validation, and a resolver function.
+
+```json
+{
+  "type": "Text",
+  "props": {
+    "text": {
+      "$format": "currency",
+      "value": { "$state": "/cart/total" },
+      "currency": "USD"
+    }
+  },
+  "children": []
+}
+```
+
+## Defining a Directive
+
+Use `defineDirective` from `@json-render/core`:
+
+```typescript
+import { defineDirective, resolvePropValue } from '@json-render/core';
+import { z } from 'zod';
+
+const doubleDirective = defineDirective({
+  name: '$double',
+  description: 'Double a numeric value.',
+  schema: z.object({
+    $double: z.unknown(),
+  }),
+  resolve(value, ctx) {
+    const resolved = resolvePropValue(value.$double, ctx);
+    return (resolved as number) * 2;
+  },
+});
+```
+
+The `description` field is optional. When generating prompts, the directive's schema fields are auto-described from the Zod schema; the `description` adds short behavioral context the schema can't express.
+
+## Wiring Directives
+
+Pass directives to both the renderer (for runtime resolution) and the catalog prompt (for AI generation).
+
+### Runtime
+
+```tsx
+import { JSONUIProvider, Renderer } from '@json-render/react';
+import { standardDirectives } from '@json-render/directives';
+
+<JSONUIProvider registry={registry} directives={standardDirectives}>
+  <Renderer spec={spec} registry={registry} />
+</JSONUIProvider>
+```
+
+Or with `createRenderer`:
+
+```tsx
+const MyRenderer = createRenderer(catalog, components);
+
+<MyRenderer spec={spec} directives={directives} />
+```
+
+All four renderers (React, Vue, Svelte, Solid) accept the `directives` prop on their provider and `createRenderer` output.
+
+### Prompt Generation
+
+```typescript
+const prompt = catalog.prompt({ directives });
+```
+
+Each directive's schema is auto-described in the "CUSTOM DYNAMIC VALUES" section of the system prompt. The optional `description` field adds behavioral context inline.
+
+## Pre-built Directives
+
+The `@json-render/directives` package ships ready-to-use directives:
+
+```typescript
+import { standardDirectives, createI18nDirective } from '@json-render/directives';
+```
+
+`standardDirectives` includes `$format`, `$math`, `$concat`, `$count`, `$truncate`, `$pluralize`, and `$join`. Add factory directives by spreading:
+
+```typescript
+const directives = [...standardDirectives, createI18nDirective(config)];
+```
+
+See the [API reference](/docs/api/directives) for details on each directive.
+
+## Composition
+
+Directives compose naturally. Each resolver calls `resolvePropValue` on its inputs, so directives can wrap other directives or built-in expressions like `$state`:
+
+```json
+{
+  "$format": "currency",
+  "value": {
+    "$math": "multiply",
+    "a": { "$state": "/price" },
+    "b": { "$state": "/qty" }
+  },
+  "currency": "USD"
+}
+```
+
+This resolves inside-out: `$state` reads from state, `$math` multiplies the values, and `$format` formats the result as currency.
+
+## Built-in Precedence
+
+Built-in expressions (`$state`, `$computed`, `$cond`, `$template`, etc.) always take precedence over custom directives. `defineDirective` throws if you try to register a name that conflicts with a built-in key.
+
+## Next
+
+- [API Reference](/docs/api/directives) — full directive reference
+- [Computed Values](/docs/computed-values) — `$computed` and `$template` expressions
+- [Data Binding](/docs/data-binding) — `$state`, `$item`, and binding expressions

apps/web/app/api/docs-chat/route.ts

@@ -16,8 +16,8 @@ const SYSTEM_PROMPT = `You are a helpful documentation assistant for json-render
 
 GitHub repository: https://github.com/vercel-labs/json-render
 Documentation: https://json-render.dev/docs
-npm packages: @json-render/core, @json-render/react, @json-render/next, @json-render/ink, @json-render/vue, @json-render/svelte, @json-render/solid, @json-render/shadcn, @json-render/shadcn-svelte, @json-render/react-three-fiber, @json-render/react-native, @json-render/react-email, @json-render/react-pdf, @json-render/image, @json-render/remotion, @json-render/codegen, @json-render/devtools, @json-render/devtools-react, @json-render/devtools-vue, @json-render/devtools-svelte, @json-render/devtools-solid, @json-render/mcp, @json-render/redux, @json-render/zustand, @json-render/jotai, @json-render/xstate, @json-render/yaml
-Skills: json-render ships AI agent skills that teach coding agents how to use each package. Install with "npx skills add vercel-labs/json-render --skill <name>". Available skills: core, react, next, ink, react-pdf, react-email, react-native, shadcn, shadcn-svelte, react-three-fiber, image, remotion, vue, svelte, solid, codegen, devtools, mcp, redux, zustand, jotai, xstate, yaml. See /docs/skills for details.
+npm packages: @json-render/core, @json-render/react, @json-render/next, @json-render/ink, @json-render/vue, @json-render/svelte, @json-render/solid, @json-render/shadcn, @json-render/shadcn-svelte, @json-render/react-three-fiber, @json-render/react-native, @json-render/react-email, @json-render/react-pdf, @json-render/image, @json-render/remotion, @json-render/directives, @json-render/codegen, @json-render/devtools, @json-render/devtools-react, @json-render/devtools-vue, @json-render/devtools-svelte, @json-render/devtools-solid, @json-render/mcp, @json-render/redux, @json-render/zustand, @json-render/jotai, @json-render/xstate, @json-render/yaml
+Skills: json-render ships AI agent skills that teach coding agents how to use each package. Install with "npx skills add vercel-labs/json-render --skill <name>". Available skills: core, react, next, ink, react-pdf, react-email, react-native, shadcn, shadcn-svelte, react-three-fiber, image, remotion, vue, svelte, solid, directives, codegen, devtools, mcp, redux, zustand, jotai, xstate, yaml. See /docs/skills for details.
 
 You have access to the full json-render documentation via the bash and readFile tools. The docs are available as markdown files in the /workspace/docs/ directory.
 

apps/web/lib/docs-navigation.ts

@@ -32,6 +32,7 @@ export const docsNavigation: NavSection[] = [
       { title: "Visibility", href: "/docs/visibility" },
       { title: "Watchers", href: "/docs/watchers" },
       { title: "Validation", href: "/docs/validation" },
+      { title: "Directives", href: "/docs/directives" },
     ],
   },
   {
@@ -86,6 +87,7 @@ export const docsNavigation: NavSection[] = [
         title: "@json-render/react-three-fiber",
         href: "/docs/api/react-three-fiber",
       },
+      { title: "@json-render/directives", href: "/docs/api/directives" },
       { title: "@json-render/codegen", href: "/docs/api/codegen" },
       { title: "@json-render/devtools", href: "/docs/api/devtools" },
       {

apps/web/lib/page-titles.ts

@@ -40,6 +40,7 @@ export const PAGE_TITLES: Record<string, string> = {
   "docs/migration": "Migration Guide",
   "docs/changelog": "Changelog",
   "docs/skills": "Skills",
+  "docs/directives": "Directives",
 
   // API references
   "docs/api/core": "@json-render/core API",
@@ -51,6 +52,7 @@ export const PAGE_TITLES: Record<string, string> = {
   "docs/api/react-email": "@json-render/react-email API",
   "docs/api/react-native": "@json-render/react-native API",
   "docs/api/svelte": "@json-render/svelte API",
+  "docs/api/directives": "@json-render/directives API",
   "docs/api/codegen": "@json-render/codegen API",
   "docs/api/devtools": "@json-render/devtools API",
   "docs/api/devtools-react": "@json-render/devtools-react API",

packages/core/src/directives.test.ts

@@ -0,0 +1,214 @@
+import { describe, it, expect } from "vitest";
+import { z } from "zod";
+import {
+  defineDirective,
+  createDirectiveRegistry,
+  findDirective,
+} from "./directives";
+import { resolvePropValue } from "./props";
+import type { PropResolutionContext } from "./props";
+
+describe("defineDirective", () => {
+  it("returns the definition unchanged", () => {
+    const def = defineDirective({
+      name: "$double",
+      schema: z.object({ $double: z.number() }),
+      resolve: (v) => (v as { $double: number }).$double * 2,
+    });
+    expect(def.name).toBe("$double");
+    expect(typeof def.resolve).toBe("function");
+  });
+
+  it("throws when name does not start with $", () => {
+    expect(() =>
+      defineDirective({
+        name: "double",
+        schema: z.object({ double: z.number() }),
+        resolve: () => 0,
+      }),
+    ).toThrow('Directive name must start with "$"');
+  });
+
+  it("throws when name conflicts with a built-in key", () => {
+    for (const name of [
+      "$state",
+      "$item",
+      "$index",
+      "$bindState",
+      "$bindItem",
+      "$cond",
+      "$computed",
+      "$template",
+    ]) {
+      expect(() =>
+        defineDirective({
+          name,
+          schema: z.object({}),
+          resolve: () => 0,
+        }),
+      ).toThrow(`conflicts with a built-in`);
+    }
+  });
+});
+
+describe("createDirectiveRegistry", () => {
+  it("creates a Map from an array of definitions", () => {
+    const d1 = defineDirective({
+      name: "$a",
+      schema: z.object({ $a: z.string() }),
+      resolve: () => "a",
+    });
+    const d2 = defineDirective({
+      name: "$b",
+      schema: z.object({ $b: z.string() }),
+      resolve: () => "b",
+    });
+    const reg = createDirectiveRegistry([d1, d2]);
+    expect(reg.size).toBe(2);
+    expect(reg.get("$a")).toBe(d1);
+    expect(reg.get("$b")).toBe(d2);
+  });
+
+  it("returns an empty Map for an empty array", () => {
+    const reg = createDirectiveRegistry([]);
+    expect(reg.size).toBe(0);
+  });
+});
+
+describe("findDirective", () => {
+  const d = defineDirective({
+    name: "$upper",
+    schema: z.object({ $upper: z.string() }),
+    resolve: (v) => String((v as { $upper: string }).$upper).toUpperCase(),
+  });
+  const registry = createDirectiveRegistry([d]);
+
+  it("finds a matching directive", () => {
+    expect(findDirective({ $upper: "hello" }, registry)).toBe(d);
+  });
+
+  it("returns undefined for non-matching objects", () => {
+    expect(findDirective({ foo: "bar" }, registry)).toBeUndefined();
+  });
+
+  it("returns undefined when registry is undefined", () => {
+    expect(findDirective({ $upper: "hello" }, undefined)).toBeUndefined();
+  });
+
+  it("returns undefined for empty registry", () => {
+    expect(findDirective({ $upper: "hello" }, new Map())).toBeUndefined();
+  });
+
+  it("ignores $ keys not in registry", () => {
+    expect(findDirective({ $unknown: 1 }, registry)).toBeUndefined();
+  });
+
+  it("throws when multiple directive keys match", () => {
+    const d2 = defineDirective({
+      name: "$lower",
+      schema: z.object({ $lower: z.string() }),
+      resolve: (v) => String((v as { $lower: string }).$lower).toLowerCase(),
+    });
+    const multiRegistry = createDirectiveRegistry([d, d2]);
+    expect(() =>
+      findDirective({ $upper: "hello", $lower: "WORLD" }, multiRegistry),
+    ).toThrow("Ambiguous directive");
+  });
+});
+
+describe("resolvePropValue with custom directives", () => {
+  const doubleDirective = defineDirective({
+    name: "$double",
+    schema: z.object({ $double: z.unknown() }),
+    resolve(value, ctx) {
+      const resolved = resolvePropValue(
+        (value as { $double: unknown }).$double,
+        ctx,
+      );
+      return (resolved as number) * 2;
+    },
+  });
+
+  const upperDirective = defineDirective({
+    name: "$upper",
+    schema: z.object({ $upper: z.unknown() }),
+    resolve(value, ctx) {
+      const resolved = resolvePropValue(
+        (value as { $upper: unknown }).$upper,
+        ctx,
+      );
+      return String(resolved).toUpperCase();
+    },
+  });
+
+  const registry = createDirectiveRegistry([doubleDirective, upperDirective]);
+
+  it("resolves a custom directive", () => {
+    const ctx: PropResolutionContext = { stateModel: {}, directives: registry };
+    expect(resolvePropValue({ $double: 5 }, ctx)).toBe(10);
+  });
+
+  it("resolves a directive with $state sub-value", () => {
+    const ctx: PropResolutionContext = {
+      stateModel: { count: 7 },
+      directives: registry,
+    };
+    expect(resolvePropValue({ $double: { $state: "/count" } }, ctx)).toBe(14);
+  });
+
+  it("resolves nested directives (composition)", () => {
+    const ctx: PropResolutionContext = {
+      stateModel: { val: 3 },
+      directives: registry,
+    };
+    const value = { $double: { $double: { $state: "/val" } } };
+    expect(resolvePropValue(value, ctx)).toBe(12);
+  });
+
+  it("resolves directives inside object props", () => {
+    const ctx: PropResolutionContext = {
+      stateModel: { name: "alice" },
+      directives: registry,
+    };
+    const value = { label: { $upper: { $state: "/name" } }, count: 1 };
+    expect(resolvePropValue(value, ctx)).toEqual({
+      label: "ALICE",
+      count: 1,
+    });
+  });
+
+  it("resolves directives inside arrays", () => {
+    const ctx: PropResolutionContext = {
+      stateModel: { x: 5 },
+      directives: registry,
+    };
+    const value = [{ $double: { $state: "/x" } }, "literal"];
+    expect(resolvePropValue(value, ctx)).toEqual([10, "literal"]);
+  });
+
+  it("falls through to plain object resolution when no directive matches", () => {
+    const ctx: PropResolutionContext = {
+      stateModel: { a: 1 },
+      directives: registry,
+    };
+    expect(resolvePropValue({ foo: { $state: "/a" } }, ctx)).toEqual({
+      foo: 1,
+    });
+  });
+
+  it("cannot register a directive that shadows a built-in key", () => {
+    expect(() =>
+      defineDirective({
+        name: "$state",
+        schema: z.object({ $state: z.string() }),
+        resolve: () => "should-not-reach",
+      }),
+    ).toThrow("conflicts with a built-in");
+  });
+
+  it("works without directives in context (backward compat)", () => {
+    const ctx: PropResolutionContext = { stateModel: { x: 1 } };
+    expect(resolvePropValue({ $state: "/x" }, ctx)).toBe(1);
+    expect(resolvePropValue({ foo: "bar" }, ctx)).toEqual({ foo: "bar" });
+  });
+});

packages/core/src/directives.ts

@@ -0,0 +1,124 @@
+import type { z } from "zod";
+import type { PropResolutionContext } from "./props";
+
+/**
+ * Definition for a custom directive — a user-defined `$`-prefixed dynamic
+ * value that extends the spec language.
+ *
+ * @example
+ * ```ts
+ * const formatDirective = defineDirective({
+ *   name: '$format',
+ *   description: 'Locale-aware value formatting (date, currency, number, percent).',
+ *   schema: z.object({
+ *     $format: z.enum(['date', 'currency', 'number']),
+ *     value: z.unknown(),
+ *   }),
+ *   resolve(value, ctx) {
+ *     const resolved = resolvePropValue(value.value, ctx);
+ *     return new Intl.NumberFormat().format(resolved);
+ *   },
+ * });
+ * ```
+ */
+export interface DirectiveDefinition<TSchema extends z.ZodType = z.ZodType> {
+  /** The `$`-prefixed key that triggers this directive (e.g. `"$format"`). */
+  name: string;
+  /**
+   * Short description of the directive for the AI system prompt.
+   * The schema fields are auto-generated; this adds behavioral context.
+   */
+  description?: string;
+  /** Zod schema for validating the directive object. */
+  schema: TSchema;
+  /**
+   * Resolver function. Receives the raw directive value and the current
+   * {@link PropResolutionContext}. May call `resolvePropValue` on sub-values
+   * to support composition with other dynamic expressions.
+   */
+  resolve: (value: z.infer<TSchema>, ctx: PropResolutionContext) => unknown;
+}
+
+/**
+ * A Map from directive name (e.g. `"$format"`) to its definition.
+ * Passed through {@link PropResolutionContext} for runtime resolution.
+ */
+export type DirectiveRegistry = Map<string, DirectiveDefinition>;
+
+/** Keys handled by built-in prop resolution — directives must not shadow these. */
+const BUILT_IN_KEYS = new Set([
+  "$state",
+  "$item",
+  "$index",
+  "$bindState",
+  "$bindItem",
+  "$cond",
+  "$computed",
+  "$template",
+]);
+
+/**
+ * Define a custom directive.
+ *
+ * This is an identity function that provides type checking and serves as
+ * a documentation convention. Throws if the name collides with a built-in
+ * prop expression key.
+ */
+export function defineDirective<TSchema extends z.ZodType>(
+  definition: DirectiveDefinition<TSchema>,
+): DirectiveDefinition<TSchema> {
+  if (!definition.name.startsWith("$")) {
+    throw new Error(
+      `Directive name must start with "$": got "${definition.name}"`,
+    );
+  }
+  if (BUILT_IN_KEYS.has(definition.name)) {
+    throw new Error(
+      `Directive name "${definition.name}" conflicts with a built-in prop expression key`,
+    );
+  }
+  return definition;
+}
+
+/**
+ * Convert an array of directive definitions into a {@link DirectiveRegistry}.
+ */
+export function createDirectiveRegistry(
+  directives: DirectiveDefinition[],
+): DirectiveRegistry {
+  const registry: DirectiveRegistry = new Map();
+  for (const d of directives) {
+    registry.set(d.name, d);
+  }
+  return registry;
+}
+
+/**
+ * Look up a custom directive for a plain-object value.
+ *
+ * Iterates the registry and checks whether the object contains a matching key.
+ * Returns `undefined` when no match is found or when no registry is provided.
+ *
+ * This is only called **after** all built-in expressions (`$state`, `$cond`,
+ * etc.) have been checked in `resolvePropValue`, so built-ins always take
+ * precedence. {@link defineDirective} enforces this at registration time by
+ * rejecting names that collide with built-in keys.
+ */
+export function findDirective(
+  value: Record<string, unknown>,
+  directives?: DirectiveRegistry,
+): DirectiveDefinition | undefined {
+  if (!directives || directives.size === 0) return undefined;
+  let match: DirectiveDefinition | undefined;
+  for (const [key, def] of directives) {
+    if (key in value) {
+      if (match) {
+        throw new Error(
+          `Ambiguous directive: object has multiple directive keys ("${match.name}" and "${key}")`,
+        );
+      }
+      match = def;
+    }
+  }
+  return match;
+}

packages/core/src/index.ts

@@ -85,6 +85,15 @@ export {
   resolveActionParam,
 } from "./props";
 
+// Custom Directives
+export type { DirectiveDefinition, DirectiveRegistry } from "./directives";
+
+export {
+  defineDirective,
+  createDirectiveRegistry,
+  findDirective,
+} from "./directives";
+
 // Actions
 export type {
   ActionBinding,

packages/core/src/props.ts

@@ -1,6 +1,7 @@
 import type { VisibilityCondition, StateModel } from "./types";
 import { getByPath } from "./types";
 import { evaluateVisibility, type VisibilityContext } from "./visibility";
+import { findDirective, type DirectiveRegistry } from "./directives";
 
 // =============================================================================
 // Prop Expression Types
@@ -61,6 +62,8 @@ export interface PropResolutionContext extends VisibilityContext {
   repeatBasePath?: string;
   /** Named functions available for `$computed` expressions. */
   functions?: Record<string, ComputedFunction>;
+  /** Custom directive registry for user-defined `$`-prefixed dynamic values. */
+  directives?: DirectiveRegistry;
 }
 
 // =============================================================================
@@ -291,8 +294,17 @@ export function resolvePropValue(
     return value.map((item) => resolvePropValue(item, ctx));
   }
 
-  // Plain objects (not expressions): resolve each value recursively
+  // Custom directives: check registry before generic object recursion
   if (typeof value === "object") {
+    const directive = findDirective(
+      value as Record<string, unknown>,
+      ctx.directives,
+    );
+    if (directive) {
+      return directive.resolve(value, ctx);
+    }
+
+    // Plain objects (not expressions): resolve each value recursively
     const resolved: Record<string, unknown> = {};
     for (const [key, val] of Object.entries(value as Record<string, unknown>)) {
       resolved[key] = resolvePropValue(val, ctx);

packages/core/src/schema.ts

@@ -1,6 +1,7 @@
 import { z } from "zod";
 import type { EditMode } from "./edit-modes";
 import { buildEditInstructions } from "./edit-modes";
+import type { DirectiveDefinition } from "./directives";
 
 /**
  * Schema builder primitives
@@ -146,6 +147,12 @@ export interface PromptOptions {
   mode?: "standalone" | "inline" | "generate" | "chat";
   /** Edit modes to document in the system prompt. Default: `["patch"]`. */
   editModes?: EditMode[];
+  /**
+   * Custom directives to include in the system prompt.
+   * Each directive's schema is auto-described; the optional `description`
+   * field adds behavioral context. Pass the same array used at runtime.
+   */
+  directives?: DirectiveDefinition[];
 }
 
 /**
@@ -968,6 +975,22 @@ Note: state patches appear right after the elements that use them, so the UI fil
     lines.push("");
   }
 
+  // Custom directives section — auto-describe schema + optional description
+  const directives = options.directives;
+  if (directives && directives.length > 0) {
+    lines.push("CUSTOM DYNAMIC VALUES:");
+    lines.push("");
+    for (const d of directives) {
+      const desc = d.description ? ` (${d.description})` : "";
+      lines.push(`- ${d.name}${desc}: ${formatZodType(d.schema)}`);
+    }
+    lines.push("");
+    lines.push(
+      "Directives compose: any value field can contain another directive or a $state expression, resolved inside-out.",
+    );
+    lines.push("");
+  }
+
   // Validation section — only emit when at least one component has a `checks` prop
   const hasChecksComponents = allComponents
     ? Object.entries(allComponents).some(([, def]) => {

packages/directives/README.md

@@ -0,0 +1,126 @@
+# @json-render/directives
+
+Pre-built custom directives for `@json-render/core`. Drop them into your catalog and renderer to add formatting, math, string manipulation, and i18n to your AI-generated UIs.
+
+## Install
+
+```bash
+npm install @json-render/directives
+```
+
+## Quick Start
+
+```typescript
+import { standardDirectives } from '@json-render/directives';
+
+// Wire into prompt generation
+const prompt = catalog.prompt({ directives: standardDirectives });
+
+// Wire into the renderer
+<JSONUIProvider spec={spec} directives={directives}>
+  ...
+</JSONUIProvider>
+```
+
+## Available Directives
+
+### `$format` — Locale-aware value formatting
+
+```json
+{ "$format": "currency", "value": { "$state": "/cart/total" }, "currency": "USD" }
+{ "$format": "date", "value": { "$state": "/user/createdAt" } }
+{ "$format": "number", "value": 1234567, "notation": "compact" }
+{ "$format": "percent", "value": 0.75 }
+```
+
+Formats: `date`, `currency`, `number`, `percent`. The `value` field accepts any dynamic expression.
+
+### `$math` — Arithmetic operations
+
+```json
+{ "$math": "add", "a": { "$state": "/subtotal" }, "b": { "$state": "/tax" } }
+{ "$math": "multiply", "a": { "$state": "/price" }, "b": { "$state": "/qty" } }
+{ "$math": "round", "a": 3.7 }
+```
+
+Operations: `add`, `subtract`, `multiply`, `divide`, `mod`, `min`, `max`, `round`, `floor`, `ceil`, `abs`. Unary ops only use `a`.
+
+### `$concat` — String concatenation
+
+```json
+{ "$concat": [{ "$state": "/user/firstName" }, " ", { "$state": "/user/lastName" }] }
+```
+
+Each element is resolved then joined into a single string.
+
+### `$count` — Array/string length
+
+```json
+{ "$count": { "$state": "/cart/items" } }
+```
+
+Returns the length of an array or string. Returns `0` for other types.
+
+### `$truncate` — Text truncation
+
+```json
+{ "$truncate": { "$state": "/post/body" }, "length": 140, "suffix": "..." }
+```
+
+Default length is 100, default suffix is `"..."`.
+
+### `$pluralize` — Singular/plural forms
+
+```json
+{ "$pluralize": { "$state": "/cart/itemCount" }, "one": "item", "other": "items", "zero": "no items" }
+```
+
+Outputs: `"3 items"`, `"1 item"`, or `"no items"`. The `zero` form is optional.
+
+### `$join` — Join array elements
+
+```json
+{ "$join": { "$state": "/tags" }, "separator": ", " }
+```
+
+Default separator is `", "`.
+
+### `createI18nDirective` — Internationalization
+
+```typescript
+import { createI18nDirective } from '@json-render/directives';
+
+const t = createI18nDirective({
+  locale: 'en',
+  messages: {
+    en: { "greeting": "Hello, {{name}}!", "checkout.submit": "Place Order" },
+    es: { "greeting": "Hola, {{name}}!", "checkout.submit": "Realizar Pedido" },
+  },
+  fallbackLocale: 'en',
+});
+```
+
+```json
+{ "$t": "checkout.submit" }
+{ "$t": "greeting", "params": { "name": { "$state": "/user/name" } } }
+```
+
+This is a factory function because it requires locale configuration at setup time.
+
+## Composition
+
+Directives compose naturally — each resolver calls `resolvePropValue` on its inputs:
+
+```json
+{
+  "$format": "currency",
+  "value": { "$math": "multiply", "a": { "$state": "/price" }, "b": { "$state": "/qty" } },
+  "currency": "USD"
+}
+```
+
+This resolves inside-out: `$math` first, then `$format` on the result.
+
+## License
+
+Apache-2.0

packages/directives/package.json

@@ -0,0 +1,58 @@
+{
+  "name": "@json-render/directives",
+  "version": "0.18.0",
+  "license": "Apache-2.0",
+  "description": "Pre-built directives for @json-render/core — $format, $math, $concat, $count, $truncate, $pluralize, $join, and $t (i18n).",
+  "keywords": [
+    "json-render",
+    "directives",
+    "format",
+    "i18n",
+    "math",
+    "generative-ui"
+  ],
+  "repository": {
+    "type": "git",
+    "url": "git+https://github.com/vercel-labs/json-render.git",
+    "directory": "packages/directives"
+  },
+  "homepage": "https://json-render.dev",
+  "bugs": {
+    "url": "https://github.com/vercel-labs/json-render/issues"
+  },
+  "publishConfig": {
+    "access": "public"
+  },
+  "main": "./dist/index.js",
+  "module": "./dist/index.mjs",
+  "types": "./dist/index.d.ts",
+  "exports": {
+    ".": {
+      "types": "./dist/index.d.ts",
+      "import": "./dist/index.mjs",
+      "require": "./dist/index.js"
+    }
+  },
+  "files": [
+    "dist"
+  ],
+  "scripts": {
+    "build": "tsup",
+    "dev": "tsup --watch",
+    "typecheck": "tsc --noEmit",
+    "test": "vitest run"
+  },
+  "dependencies": {
+    "@json-render/core": "workspace:*"
+  },
+  "devDependencies": {
+    "@internal/typescript-config": "workspace:*",
+    "tsup": "^8.0.2",
+    "typescript": "^5.4.5",
+    "vitest": "^4.0.17",
+    "zod": "^4.3.6"
+  },
+  "peerDependencies": {
+    "zod": "^4.0.0"
+  }
+}

packages/directives/src/concat.ts

@@ -0,0 +1,18 @@
+import { z } from "zod";
+import { defineDirective, resolvePropValue } from "@json-render/core";
+
+export const concatDirective = defineDirective({
+  name: "$concat",
+  description: "Concatenate multiple dynamic values into a string.",
+  schema: z.object({
+    $concat: z.array(z.unknown()),
+  }),
+  resolve(raw, ctx) {
+    return raw.$concat
+      .map((part) => {
+        const resolved = resolvePropValue(part, ctx);
+        return resolved != null ? String(resolved) : "";
+      })
+      .join("");
+  },
+});

packages/directives/src/count.ts

@@ -0,0 +1,16 @@
+import { z } from "zod";
+import { defineDirective, resolvePropValue } from "@json-render/core";
+
+export const countDirective = defineDirective({
+  name: "$count",
+  description: "Get the length of an array or string.",
+  schema: z.object({
+    $count: z.unknown(),
+  }),
+  resolve(raw, ctx) {
+    const resolved = resolvePropValue(raw.$count, ctx);
+    if (Array.isArray(resolved)) return resolved.length;
+    if (typeof resolved === "string") return resolved.length;
+    return 0;
+  },
+});

packages/directives/src/directives.test.ts

@@ -0,0 +1,497 @@
+import { describe, it, expect, vi } from "vitest";
+import {
+  resolvePropValue,
+  createDirectiveRegistry,
+  type PropResolutionContext,
+} from "@json-render/core";
+import { formatDirective } from "./format";
+import { mathDirective } from "./math";
+import { concatDirective } from "./concat";
+import { countDirective } from "./count";
+import { truncateDirective } from "./truncate";
+import { pluralizeDirective } from "./pluralize";
+import { joinDirective } from "./join";
+import { createI18nDirective } from "./i18n";
+
+const allDirectives = [
+  formatDirective,
+  mathDirective,
+  concatDirective,
+  countDirective,
+  truncateDirective,
+  pluralizeDirective,
+  joinDirective,
+];
+
+function makeCtx(
+  state: Record<string, unknown> = {},
+  extra: Partial<PropResolutionContext> = {},
+): PropResolutionContext {
+  return {
+    stateModel: state,
+    directives: createDirectiveRegistry(allDirectives),
+    ...extra,
+  };
+}
+
+// ============================================================================
+// $format
+// ============================================================================
+
+describe("$format", () => {
+  it("formats a number", () => {
+    const ctx = makeCtx();
+    const result = resolvePropValue({ $format: "number", value: 1234.56 }, ctx);
+    expect(typeof result).toBe("string");
+    expect(result).toContain("1");
+  });
+
+  it("formats currency", () => {
+    const ctx = makeCtx({ total: 42.5 });
+    const result = resolvePropValue(
+      { $format: "currency", value: { $state: "/total" }, currency: "USD" },
+      ctx,
+    );
+    expect(typeof result).toBe("string");
+    expect(result).toContain("42");
+  });
+
+  it("formats percent", () => {
+    const ctx = makeCtx();
+    const result = resolvePropValue({ $format: "percent", value: 0.75 }, ctx);
+    expect(typeof result).toBe("string");
+    expect(result).toContain("75");
+  });
+
+  it("formats a date", () => {
+    const ctx = makeCtx();
+    const result = resolvePropValue(
+      { $format: "date", value: "2024-01-15" },
+      ctx,
+    );
+    expect(typeof result).toBe("string");
+    expect(result).toContain("2024");
+  });
+
+  it("formats a relative date with injectable now", () => {
+    const ctx = makeCtx();
+    const baseDate = new Date("2024-06-15T12:00:00Z").getTime();
+    const result = resolvePropValue(
+      {
+        $format: "date",
+        value: "2024-06-15T12:00:00Z",
+        style: "relative",
+        now: baseDate + 3 * 60 * 60 * 1000,
+      },
+      ctx,
+    );
+    expect(result).toBe("3h ago");
+  });
+
+  it("formats a future relative date", () => {
+    const ctx = makeCtx();
+    const baseDate = new Date("2024-06-15T12:00:00Z").getTime();
+    const result = resolvePropValue(
+      {
+        $format: "date",
+        value: "2024-06-15T12:00:00Z",
+        style: "relative",
+        now: baseDate - 2 * 60 * 60 * 1000,
+      },
+      ctx,
+    );
+    expect(result).toBe("2h from now");
+  });
+
+  it("returns 'just now' when date equals now", () => {
+    const ctx = makeCtx();
+    const ts = new Date("2024-06-15T12:00:00Z").getTime();
+    const result = resolvePropValue(
+      {
+        $format: "date",
+        value: "2024-06-15T12:00:00Z",
+        style: "relative",
+        now: ts,
+      },
+      ctx,
+    );
+    expect(result).toBe("just now");
+  });
+});
+
+// ============================================================================
+// $math
+// ============================================================================
+
+describe("$math", () => {
+  it("adds two values", () => {
+    const ctx = makeCtx({ a: 10, b: 5 });
+    expect(
+      resolvePropValue(
+        { $math: "add", a: { $state: "/a" }, b: { $state: "/b" } },
+        ctx,
+      ),
+    ).toBe(15);
+  });
+
+  it("subtracts", () => {
+    const ctx = makeCtx();
+    expect(resolvePropValue({ $math: "subtract", a: 10, b: 3 }, ctx)).toBe(7);
+  });
+
+  it("multiplies", () => {
+    const ctx = makeCtx();
+    expect(resolvePropValue({ $math: "multiply", a: 4, b: 5 }, ctx)).toBe(20);
+  });
+
+  it("divides", () => {
+    const ctx = makeCtx();
+    expect(resolvePropValue({ $math: "divide", a: 10, b: 4 }, ctx)).toBe(2.5);
+  });
+
+  it("handles division by zero", () => {
+    const ctx = makeCtx();
+    expect(resolvePropValue({ $math: "divide", a: 10, b: 0 }, ctx)).toBe(0);
+  });
+
+  it("computes modulo", () => {
+    const ctx = makeCtx();
+    expect(resolvePropValue({ $math: "mod", a: 10, b: 3 }, ctx)).toBe(1);
+  });
+
+  it("computes min", () => {
+    const ctx = makeCtx();
+    expect(resolvePropValue({ $math: "min", a: 10, b: 3 }, ctx)).toBe(3);
+  });
+
+  it("computes max", () => {
+    const ctx = makeCtx();
+    expect(resolvePropValue({ $math: "max", a: 10, b: 3 }, ctx)).toBe(10);
+  });
+
+  it("rounds", () => {
+    const ctx = makeCtx();
+    expect(resolvePropValue({ $math: "round", a: 3.7 }, ctx)).toBe(4);
+  });
+
+  it("floors", () => {
+    const ctx = makeCtx();
+    expect(resolvePropValue({ $math: "floor", a: 3.7 }, ctx)).toBe(3);
+  });
+
+  it("ceils", () => {
+    const ctx = makeCtx();
+    expect(resolvePropValue({ $math: "ceil", a: 3.2 }, ctx)).toBe(4);
+  });
+
+  it("computes abs", () => {
+    const ctx = makeCtx();
+    expect(resolvePropValue({ $math: "abs", a: -5 }, ctx)).toBe(5);
+  });
+
+  it("defaults missing operand b to 0", () => {
+    const ctx = makeCtx();
+    expect(resolvePropValue({ $math: "add", a: 5 }, ctx)).toBe(5);
+  });
+
+  it("defaults missing operand a to 0", () => {
+    const ctx = makeCtx();
+    expect(resolvePropValue({ $math: "add", b: 3 }, ctx)).toBe(3);
+  });
+
+  it("warns when a non-numeric value is coerced to 0", () => {
+    const ctx = makeCtx();
+    const spy = vi.spyOn(console, "warn").mockImplementation(() => {});
+    const result = resolvePropValue({ $math: "add", a: "foo", b: 3 }, ctx);
+    expect(result).toBe(3);
+    expect(spy).toHaveBeenCalledWith(
+      "$math: non-numeric value coerced to 0:",
+      "foo",
+    );
+    spy.mockRestore();
+  });
+});
+
+// ============================================================================
+// $concat
+// ============================================================================
+
+describe("$concat", () => {
+  it("concatenates strings", () => {
+    const ctx = makeCtx({ first: "John", last: "Doe" });
+    expect(
+      resolvePropValue(
+        { $concat: [{ $state: "/first" }, " ", { $state: "/last" }] },
+        ctx,
+      ),
+    ).toBe("John Doe");
+  });
+
+  it("handles null values", () => {
+    const ctx = makeCtx();
+    expect(resolvePropValue({ $concat: ["hello", null, "world"] }, ctx)).toBe(
+      "helloworld",
+    );
+  });
+
+  it("converts non-strings", () => {
+    const ctx = makeCtx();
+    expect(resolvePropValue({ $concat: ["count: ", 42] }, ctx)).toBe(
+      "count: 42",
+    );
+  });
+});
+
+// ============================================================================
+// $count
+// ============================================================================
+
+describe("$count", () => {
+  it("counts array items", () => {
+    const ctx = makeCtx({ items: [1, 2, 3] });
+    expect(resolvePropValue({ $count: { $state: "/items" } }, ctx)).toBe(3);
+  });
+
+  it("counts string length", () => {
+    const ctx = makeCtx();
+    expect(resolvePropValue({ $count: "hello" }, ctx)).toBe(5);
+  });
+
+  it("returns 0 for non-countable", () => {
+    const ctx = makeCtx();
+    expect(resolvePropValue({ $count: 42 }, ctx)).toBe(0);
+  });
+
+  it("returns 0 for empty array", () => {
+    const ctx = makeCtx({ items: [] });
+    expect(resolvePropValue({ $count: { $state: "/items" } }, ctx)).toBe(0);
+  });
+});
+
+// ============================================================================
+// $truncate
+// ============================================================================
+
+describe("$truncate", () => {
+  it("truncates long text", () => {
+    const ctx = makeCtx();
+    const text = "a".repeat(200);
+    const result = resolvePropValue(
+      { $truncate: text, length: 10, suffix: "..." },
+      ctx,
+    ) as string;
+    expect(result.length).toBe(13);
+    expect(result).toBe("a".repeat(10) + "...");
+  });
+
+  it("does not truncate short text", () => {
+    const ctx = makeCtx();
+    expect(resolvePropValue({ $truncate: "hello", length: 10 }, ctx)).toBe(
+      "hello",
+    );
+  });
+
+  it("uses default length and suffix", () => {
+    const ctx = makeCtx();
+    const text = "a".repeat(200);
+    const result = resolvePropValue({ $truncate: text }, ctx) as string;
+    expect(result.length).toBe(103); // 100 + "..."
+  });
+
+  it("resolves dynamic values", () => {
+    const ctx = makeCtx({ body: "hello world this is a test" });
+    expect(
+      resolvePropValue(
+        { $truncate: { $state: "/body" }, length: 11, suffix: "…" },
+        ctx,
+      ),
+    ).toBe("hello world…");
+  });
+});
+
+// ============================================================================
+// $pluralize
+// ============================================================================
+
+describe("$pluralize", () => {
+  it("handles singular", () => {
+    const ctx = makeCtx();
+    expect(
+      resolvePropValue({ $pluralize: 1, one: "item", other: "items" }, ctx),
+    ).toBe("1 item");
+  });
+
+  it("handles plural", () => {
+    const ctx = makeCtx();
+    expect(
+      resolvePropValue({ $pluralize: 5, one: "item", other: "items" }, ctx),
+    ).toBe("5 items");
+  });
+
+  it("handles zero with zero form", () => {
+    const ctx = makeCtx();
+    expect(
+      resolvePropValue(
+        { $pluralize: 0, one: "item", other: "items", zero: "no items" },
+        ctx,
+      ),
+    ).toBe("no items");
+  });
+
+  it("handles zero without zero form", () => {
+    const ctx = makeCtx();
+    expect(
+      resolvePropValue({ $pluralize: 0, one: "item", other: "items" }, ctx),
+    ).toBe("0 items");
+  });
+
+  it("resolves count from state", () => {
+    const ctx = makeCtx({ count: 3 });
+    expect(
+      resolvePropValue(
+        { $pluralize: { $state: "/count" }, one: "file", other: "files" },
+        ctx,
+      ),
+    ).toBe("3 files");
+  });
+
+  it("coerces string count to number", () => {
+    const ctx = makeCtx();
+    expect(
+      resolvePropValue({ $pluralize: "3", one: "item", other: "items" }, ctx),
+    ).toBe("3 items");
+  });
+});
+
+// ============================================================================
+// $join
+// ============================================================================
+
+describe("$join", () => {
+  it("joins array with default separator", () => {
+    const ctx = makeCtx({ tags: ["red", "green", "blue"] });
+    expect(resolvePropValue({ $join: { $state: "/tags" } }, ctx)).toBe(
+      "red, green, blue",
+    );
+  });
+
+  it("joins with custom separator", () => {
+    const ctx = makeCtx({ tags: ["a", "b", "c"] });
+    expect(
+      resolvePropValue({ $join: { $state: "/tags" }, separator: " | " }, ctx),
+    ).toBe("a | b | c");
+  });
+
+  it("handles non-array values", () => {
+    const ctx = makeCtx();
+    expect(resolvePropValue({ $join: "hello" }, ctx)).toBe("hello");
+  });
+
+  it("handles null items", () => {
+    const ctx = makeCtx({ items: ["a", null, "b"] });
+    expect(
+      resolvePropValue({ $join: { $state: "/items" }, separator: "-" }, ctx),
+    ).toBe("a--b");
+  });
+});
+
+// ============================================================================
+// $t (i18n)
+// ============================================================================
+
+describe("createI18nDirective", () => {
+  const tDirective = createI18nDirective({
+    locale: "en",
+    messages: {
+      en: {
+        greeting: "Hello, {{name}}!",
+        "checkout.submit": "Place Order",
+        "items.count": "{{count}} items in cart",
+      },
+      es: {
+        greeting: "Hola, {{name}}!",
+        "checkout.submit": "Realizar Pedido",
+      },
+    },
+    fallbackLocale: "en",
+  });
+
+  function makeI18nCtx(
+    state: Record<string, unknown> = {},
+  ): PropResolutionContext {
+    return {
+      stateModel: state,
+      directives: createDirectiveRegistry([tDirective]),
+    };
+  }
+
+  it("translates a simple key", () => {
+    const ctx = makeI18nCtx();
+    expect(resolvePropValue({ $t: "checkout.submit" }, ctx)).toBe(
+      "Place Order",
+    );
+  });
+
+  it("interpolates parameters", () => {
+    const ctx = makeI18nCtx({ name: "Alice" });
+    expect(
+      resolvePropValue(
+        { $t: "greeting", params: { name: { $state: "/name" } } },
+        ctx,
+      ),
+    ).toBe("Hello, Alice!");
+  });
+
+  it("returns key for missing translations", () => {
+    const ctx = makeI18nCtx();
+    expect(resolvePropValue({ $t: "missing.key" }, ctx)).toBe("missing.key");
+  });
+
+  it("handles multiple params", () => {
+    const ctx = makeI18nCtx({ count: 3 });
+    expect(
+      resolvePropValue(
+        { $t: "items.count", params: { count: { $state: "/count" } } },
+        ctx,
+      ),
+    ).toBe("3 items in cart");
+  });
+});
+
+// ============================================================================
+// Composition tests
+// ============================================================================
+
+describe("directive composition", () => {
+  it("composes $math inside $format", () => {
+    const ctx = makeCtx({ price: 10, qty: 3 });
+    const result = resolvePropValue(
+      {
+        $format: "currency",
+        value: {
+          $math: "multiply",
+          a: { $state: "/price" },
+          b: { $state: "/qty" },
+        },
+        currency: "USD",
+      },
+      ctx,
+    );
+    expect(typeof result).toBe("string");
+    expect(result).toContain("30");
+  });
+
+  it("composes $count inside $pluralize", () => {
+    const ctx = makeCtx({ items: [1, 2, 3] });
+    expect(
+      resolvePropValue(
+        {
+          $pluralize: { $count: { $state: "/items" } },
+          one: "item",
+          other: "items",
+        },
+        ctx,
+      ),
+    ).toBe("3 items");
+  });
+});

packages/directives/src/format.ts

@@ -0,0 +1,71 @@
+import { z } from "zod";
+import { defineDirective, resolvePropValue } from "@json-render/core";
+
+export const formatDirective = defineDirective({
+  name: "$format",
+  description:
+    'Locale-aware value formatting (date, currency, number, percent). Supports style: "relative" for relative dates.',
+  schema: z.object({
+    $format: z.enum(["date", "currency", "number", "percent"]),
+    value: z.unknown(),
+    locale: z.string().optional(),
+    currency: z.string().optional(),
+    notation: z.string().optional(),
+    style: z.string().optional(),
+    options: z.record(z.string(), z.unknown()).optional(),
+    now: z.number().optional(),
+  }),
+  resolve(raw, ctx) {
+    const value = resolvePropValue(raw.value, ctx);
+    const locale = raw.locale ?? undefined;
+    const extra = raw.options ?? {};
+
+    switch (raw.$format) {
+      case "date": {
+        const date =
+          value instanceof Date ? value : new Date(value as string | number);
+        if (raw.style === "relative") {
+          const now = raw.now ?? Date.now();
+          const diff = now - date.getTime();
+          if (diff === 0) return "just now";
+          const absDiff = Math.abs(diff);
+          const suffix = diff > 0 ? "ago" : "from now";
+          const seconds = Math.floor(absDiff / 1000);
+          const minutes = Math.floor(seconds / 60);
+          const hours = Math.floor(minutes / 60);
+          const days = Math.floor(hours / 24);
+          if (days > 0) return `${days}d ${suffix}`;
+          if (hours > 0) return `${hours}h ${suffix}`;
+          if (minutes > 0) return `${minutes}m ${suffix}`;
+          return `${seconds}s ${suffix}`;
+        }
+        return new Intl.DateTimeFormat(
+          locale,
+          extra as Intl.DateTimeFormatOptions,
+        ).format(date);
+      }
+      case "currency":
+        return new Intl.NumberFormat(locale, {
+          style: "currency",
+          currency: raw.currency ?? "USD",
+          ...(extra as Intl.NumberFormatOptions),
+        }).format(value as number);
+      case "number":
+        return new Intl.NumberFormat(locale, {
+          ...(raw.notation
+            ? {
+                notation: raw.notation as Intl.NumberFormatOptions["notation"],
+              }
+            : {}),
+          ...(extra as Intl.NumberFormatOptions),
+        }).format(value as number);
+      case "percent":
+        return new Intl.NumberFormat(locale, {
+          style: "percent",
+          ...(extra as Intl.NumberFormatOptions),
+        }).format(value as number);
+      default:
+        return value;
+    }
+  },
+});

packages/directives/src/i18n.ts

@@ -0,0 +1,58 @@
+import { z } from "zod";
+import { defineDirective, resolvePropValue } from "@json-render/core";
+import type { DirectiveDefinition } from "@json-render/core";
+
+export interface I18nConfig {
+  /** Current locale (e.g. "en", "es") */
+  locale: string;
+  /** Map of locale → key → translated string */
+  messages: Record<string, Record<string, string>>;
+  /** Fallback locale when a key is missing in the current locale */
+  fallbackLocale?: string;
+}
+
+/**
+ * Create a `$t` directive for internationalization.
+ *
+ * @example
+ * ```ts
+ * const t = createI18nDirective({
+ *   locale: 'en',
+ *   messages: {
+ *     en: { "greeting": "Hello, {{name}}!" },
+ *     es: { "greeting": "Hola, {{name}}!" },
+ *   },
+ * });
+ * ```
+ */
+export function createI18nDirective(config: I18nConfig): DirectiveDefinition {
+  return defineDirective({
+    name: "$t",
+    description: "Translated text with {{param}} interpolation.",
+    schema: z.object({
+      $t: z.string(),
+      params: z.record(z.string(), z.unknown()).optional(),
+    }),
+    resolve(raw, ctx) {
+      const key = raw.$t;
+
+      const localeMessages = config.messages[config.locale];
+      const fallbackMessages = config.fallbackLocale
+        ? config.messages[config.fallbackLocale]
+        : undefined;
+      let template = localeMessages?.[key] ?? fallbackMessages?.[key] ?? key;
+
+      if (raw.params) {
+        for (const [paramKey, paramValue] of Object.entries(raw.params)) {
+          const resolved = resolvePropValue(paramValue, ctx);
+          template = template.replace(
+            new RegExp(`\\{\\{${paramKey}\\}\\}`, "g"),
+            resolved != null ? String(resolved) : "",
+          );
+        }
+      }
+
+      return template;
+    },
+  });
+}

packages/directives/src/index.ts

@@ -0,0 +1,31 @@
+export { formatDirective } from "./format";
+export { mathDirective } from "./math";
+export { concatDirective } from "./concat";
+export { countDirective } from "./count";
+export { truncateDirective } from "./truncate";
+export { pluralizeDirective } from "./pluralize";
+export { joinDirective } from "./join";
+export { createI18nDirective, type I18nConfig } from "./i18n";
+
+import { formatDirective } from "./format";
+import { mathDirective } from "./math";
+import { concatDirective } from "./concat";
+import { countDirective } from "./count";
+import { truncateDirective } from "./truncate";
+import { pluralizeDirective } from "./pluralize";
+import { joinDirective } from "./join";
+
+/**
+ * All non-factory directives in a single array.
+ * Spread with factory directives as needed:
+ * `[...standardDirectives, createI18nDirective(config)]`
+ */
+export const standardDirectives = [
+  formatDirective,
+  mathDirective,
+  concatDirective,
+  countDirective,
+  truncateDirective,
+  pluralizeDirective,
+  joinDirective,
+];

packages/directives/src/join.ts

@@ -0,0 +1,22 @@
+import { z } from "zod";
+import { defineDirective, resolvePropValue } from "@json-render/core";
+
+export const joinDirective = defineDirective({
+  name: "$join",
+  description: "Join array elements with a separator.",
+  schema: z.object({
+    $join: z.unknown(),
+    separator: z.string().optional(),
+  }),
+  resolve(raw, ctx) {
+    const resolved = resolvePropValue(raw.$join, ctx);
+    const separator = raw.separator ?? ", ";
+
+    if (Array.isArray(resolved)) {
+      return resolved
+        .map((item) => (item != null ? String(item) : ""))
+        .join(separator);
+    }
+    return resolved != null ? String(resolved) : "";
+  },
+});

packages/directives/src/math.ts

@@ -0,0 +1,66 @@
+import { z } from "zod";
+import { defineDirective, resolvePropValue } from "@json-render/core";
+
+function toNum(v: unknown): number {
+  if (v == null) return 0;
+  const n = Number(v);
+  if (Number.isNaN(n)) {
+    console.warn(`$math: non-numeric value coerced to 0:`, v);
+    return 0;
+  }
+  return n;
+}
+
+export const mathDirective = defineDirective({
+  name: "$math",
+  description:
+    'Arithmetic operations. Unary ops (round, floor, ceil, abs) only use "a". Division by zero returns 0.',
+  schema: z.object({
+    $math: z.enum([
+      "add",
+      "subtract",
+      "multiply",
+      "divide",
+      "mod",
+      "min",
+      "max",
+      "round",
+      "floor",
+      "ceil",
+      "abs",
+    ]),
+    a: z.unknown().optional(),
+    b: z.unknown().optional(),
+  }),
+  resolve(raw, ctx) {
+    const a = toNum(resolvePropValue(raw.a, ctx));
+    const b = toNum(resolvePropValue(raw.b, ctx));
+
+    switch (raw.$math) {
+      case "add":
+        return a + b;
+      case "subtract":
+        return a - b;
+      case "multiply":
+        return a * b;
+      case "divide":
+        return b !== 0 ? a / b : 0;
+      case "mod":
+        return b !== 0 ? a % b : 0;
+      case "min":
+        return Math.min(a, b);
+      case "max":
+        return Math.max(a, b);
+      case "round":
+        return Math.round(a);
+      case "floor":
+        return Math.floor(a);
+      case "ceil":
+        return Math.ceil(a);
+      case "abs":
+        return Math.abs(a);
+      default:
+        return a;
+    }
+  },
+});

packages/directives/src/pluralize.ts

@@ -0,0 +1,23 @@
+import { z } from "zod";
+import { defineDirective, resolvePropValue } from "@json-render/core";
+
+export const pluralizeDirective = defineDirective({
+  name: "$pluralize",
+  description:
+    'Select singular/plural/zero form based on count. Output: "3 items", "1 item", or "no items".',
+  schema: z.object({
+    $pluralize: z.unknown(),
+    zero: z.string().optional(),
+    one: z.string(),
+    other: z.string(),
+  }),
+  resolve(raw, ctx) {
+    const resolved = resolvePropValue(raw.$pluralize, ctx);
+    const n = Number(resolved);
+    const count = Number.isNaN(n) ? 0 : n;
+
+    if (count === 0 && raw.zero != null) return raw.zero;
+    if (count === 1) return `${count} ${raw.one}`;
+    return `${count} ${raw.other}`;
+  },
+});

packages/directives/src/truncate.ts

@@ -0,0 +1,21 @@
+import { z } from "zod";
+import { defineDirective, resolvePropValue } from "@json-render/core";
+
+export const truncateDirective = defineDirective({
+  name: "$truncate",
+  description: "Truncate text to a max length with a suffix.",
+  schema: z.object({
+    $truncate: z.unknown(),
+    length: z.number().optional(),
+    suffix: z.string().optional(),
+  }),
+  resolve(raw, ctx) {
+    const resolved = resolvePropValue(raw.$truncate, ctx);
+    const text = resolved != null ? String(resolved) : "";
+    const maxLength = raw.length ?? 100;
+    const suffix = raw.suffix ?? "...";
+
+    if (text.length <= maxLength) return text;
+    return text.slice(0, maxLength) + suffix;
+  },
+});

packages/directives/tsconfig.json

@@ -0,0 +1,9 @@
+{
+  "extends": "@internal/typescript-config/base.json",
+  "compilerOptions": {
+    "outDir": "dist",
+    "rootDir": "src"
+  },
+  "include": ["src"],
+  "exclude": ["node_modules", "dist"]
+}

packages/directives/tsup.config.ts

@@ -0,0 +1,10 @@
+import { defineConfig } from "tsup";
+
+export default defineConfig({
+  entry: ["src/index.ts"],
+  format: ["cjs", "esm"],
+  dts: true,
+  sourcemap: true,
+  clean: true,
+  external: ["@json-render/core", "zod"],
+});

packages/react/package.json

@@ -57,7 +57,8 @@
     "@internal/typescript-config": "workspace:*",
     "@types/react": "19.2.3",
     "tsup": "^8.0.2",
-    "typescript": "^5.4.5"
+    "typescript": "^5.4.5",
+    "zod": "^4.3.6"
   },
   "peerDependencies": {
     "react": "^19.2.3"

packages/react/src/directives.test.tsx

@@ -0,0 +1,110 @@
+import { describe, it, expect } from "vitest";
+import React from "react";
+import { render, screen } from "@testing-library/react";
+import { z } from "zod";
+import type { Spec } from "@json-render/core";
+import { defineDirective, resolvePropValue } from "@json-render/core";
+import {
+  JSONUIProvider,
+  Renderer,
+  type ComponentRenderProps,
+} from "./renderer";
+
+function Text({ element }: ComponentRenderProps<{ text: unknown }>) {
+  const value = element.props.text;
+  return <span data-testid="text">{value == null ? "" : String(value)}</span>;
+}
+
+const registry = { Text };
+
+const doubleDirective = defineDirective({
+  name: "$double",
+  description: "Double a numeric value.",
+  schema: z.object({ $double: z.unknown() }),
+  resolve(value, ctx) {
+    const resolved = resolvePropValue(value.$double, ctx);
+    return (resolved as number) * 2;
+  },
+});
+
+const upperDirective = defineDirective({
+  name: "$upper",
+  schema: z.object({ $upper: z.unknown() }),
+  resolve(value, ctx) {
+    const resolved = resolvePropValue(value.$upper, ctx);
+    return String(resolved).toUpperCase();
+  },
+});
+
+describe("directives in React renderer", () => {
+  it("resolves a custom directive in rendered props", () => {
+    const spec: Spec = {
+      root: "main",
+      elements: {
+        main: {
+          type: "Text",
+          props: { text: { $upper: "hello" } },
+          children: [],
+        },
+      },
+    };
+
+    render(
+      <JSONUIProvider
+        registry={registry}
+        directives={[doubleDirective, upperDirective]}
+      >
+        <Renderer spec={spec} registry={registry} />
+      </JSONUIProvider>,
+    );
+
+    expect(screen.getByTestId("text").textContent).toBe("HELLO");
+  });
+
+  it("resolves a directive that reads from state", () => {
+    const spec: Spec = {
+      root: "main",
+      elements: {
+        main: {
+          type: "Text",
+          props: { text: { $double: { $state: "/count" } } },
+          children: [],
+        },
+      },
+      state: { count: 7 },
+    };
+
+    render(
+      <JSONUIProvider
+        registry={registry}
+        initialState={{ count: 7 }}
+        directives={[doubleDirective, upperDirective]}
+      >
+        <Renderer spec={spec} registry={registry} />
+      </JSONUIProvider>,
+    );
+
+    expect(screen.getByTestId("text").textContent).toBe("14");
+  });
+
+  it("renders without directives (backward compat)", () => {
+    const spec: Spec = {
+      root: "main",
+      elements: {
+        main: {
+          type: "Text",
+          props: { text: "plain" },
+          children: [],
+        },
+      },
+    };
+
+    render(
+      <JSONUIProvider registry={registry}>
+        <Renderer spec={spec} registry={registry} />
+      </JSONUIProvider>,
+    );
+
+    expect(screen.getByTestId("text").textContent).toBe("plain");
+  });
+});

packages/react/src/renderer.tsx

@@ -17,6 +17,8 @@ import type {
   SchemaDefinition,
   StateStore,
   ComputedFunction,
+  DirectiveDefinition,
+  DirectiveRegistry,
 } from "@json-render/core";
 import {
   resolveElementProps,
@@ -26,6 +28,7 @@ import {
   getByPath,
   isDevtoolsActive,
   subscribeDevtoolsActive,
+  createDirectiveRegistry,
   type PropResolutionContext,
   type VisibilityContext as CoreVisibilityContext,
 } from "@json-render/core";
@@ -154,6 +157,18 @@ function useFunctions(): Record<string, ComputedFunction> {
   return React.useContext(FunctionsContext);
 }
 
+// ---------------------------------------------------------------------------
+// DirectivesContext – provides custom directive registry to the element tree
+// ---------------------------------------------------------------------------
+
+const DirectivesContext = React.createContext<DirectiveRegistry | undefined>(
+  undefined,
+);
+
+function useDirectives(): DirectiveRegistry | undefined {
+  return React.useContext(DirectivesContext);
+}
+
 interface ElementRendererProps {
   element: UIElement;
   /** Spec key for this element. Used by the devtools picker. */
@@ -194,8 +209,9 @@ const ElementRenderer = React.memo(function ElementRenderer({
   const { execute } = useActions();
   const { getSnapshot, state: watchState } = useStateStore();
   const functions = useFunctions();
+  const directives = useDirectives();
 
-  // Build context with repeat scope and $computed functions
+  // Build context with repeat scope, $computed functions, and custom directives
   const fullCtx: PropResolutionContext = useMemo(() => {
     const base: PropResolutionContext = repeatScope
       ? {
@@ -206,8 +222,9 @@ const ElementRenderer = React.memo(function ElementRenderer({
         }
       : { ...ctx };
     base.functions = functions;
+    base.directives = directives;
     return base;
-  }, [ctx, repeatScope, functions]);
+  }, [ctx, repeatScope, functions, directives]);
 
   // Evaluate visibility (now supports $item/$index inside repeat scopes)
   const isVisible =
@@ -554,6 +571,8 @@ export interface JSONUIProviderProps {
   >;
   /** Named functions for `$computed` expressions in props */
   functions?: Record<string, ComputedFunction>;
+  /** Custom directives for user-defined `$`-prefixed dynamic values */
+  directives?: DirectiveDefinition[];
   /** Callback when state changes (uncontrolled mode) */
   onStateChange?: (changes: Array<{ path: string; value: unknown }>) => void;
   children: ReactNode;
@@ -570,9 +589,14 @@ export function JSONUIProvider({
   navigate,
   validationFunctions,
   functions,
+  directives,
   onStateChange,
   children,
 }: JSONUIProviderProps) {
+  const directiveRegistry = useMemo(
+    () => (directives ? createDirectiveRegistry(directives) : undefined),
+    [directives],
+  );
   return (
     <StateProvider
       store={store}
@@ -583,8 +607,10 @@ export function JSONUIProvider({
         <ValidationProvider customFunctions={validationFunctions}>
           <ActionProvider handlers={handlers} navigate={navigate}>
             <FunctionsContext.Provider value={functions ?? EMPTY_FUNCTIONS}>
-              {children}
-              <ConfirmationDialogManager />
+              <DirectivesContext.Provider value={directiveRegistry}>
+                {children}
+                <ConfirmationDialogManager />
+              </DirectivesContext.Provider>
             </FunctionsContext.Provider>
           </ActionProvider>
         </ValidationProvider>
@@ -790,6 +816,8 @@ export interface CreateRendererProps {
   onStateChange?: (changes: Array<{ path: string; value: unknown }>) => void;
   /** Named functions for `$computed` expressions in props */
   functions?: Record<string, ComputedFunction>;
+  /** Custom directives for user-defined `$`-prefixed dynamic values */
+  directives?: DirectiveDefinition[];
   /** Whether the spec is currently loading/streaming */
   loading?: boolean;
   /** Fallback component for unknown types */
@@ -844,9 +872,15 @@ export function createRenderer<
     onAction,
     onStateChange,
     functions,
+    directives,
     loading,
     fallback,
   }: CreateRendererProps) {
+    const directiveRegistry = useMemo(
+      () => (directives ? createDirectiveRegistry(directives) : undefined),
+      [directives],
+    );
+
     // Wrap onAction with a Proxy so any action name routes to the callback
     const actionHandlers = onAction
       ? new Proxy(
@@ -874,13 +908,15 @@ export function createRenderer<
           <ValidationProvider>
             <ActionProvider handlers={actionHandlers}>
               <FunctionsContext.Provider value={functions ?? EMPTY_FUNCTIONS}>
-                <Renderer
-                  spec={spec}
-                  registry={registry}
-                  loading={loading}
-                  fallback={fallback}
-                />
-                <ConfirmationDialogManager />
+                <DirectivesContext.Provider value={directiveRegistry}>
+                  <Renderer
+                    spec={spec}
+                    registry={registry}
+                    loading={loading}
+                    fallback={fallback}
+                  />
+                  <ConfirmationDialogManager />
+                </DirectivesContext.Provider>
               </FunctionsContext.Provider>
             </ActionProvider>
           </ValidationProvider>

packages/solid/src/renderer.tsx

@@ -19,6 +19,8 @@ import type {
   SchemaDefinition,
   StateStore,
   ComputedFunction,
+  DirectiveDefinition,
+  DirectiveRegistry,
 } from "@json-render/core";
 import {
   resolveElementProps,
@@ -28,6 +30,7 @@ import {
   getByPath,
   isDevtoolsActive,
   subscribeDevtoolsActive,
+  createDirectiveRegistry,
   type PropResolutionContext,
   type VisibilityContext as CoreVisibilityContext,
 } from "@json-render/core";
@@ -111,6 +114,18 @@ function useFunctions(): Record<string, ComputedFunction> {
   return useContext(FunctionsContext) ?? EMPTY_FUNCTIONS;
 }
 
+// ---------------------------------------------------------------------------
+// DirectivesContext – provides custom directive registry to the element tree
+// ---------------------------------------------------------------------------
+
+const DirectivesContext = createContext<DirectiveRegistry | undefined>(
+  undefined,
+);
+
+function useDirectives(): DirectiveRegistry | undefined {
+  return useContext(DirectivesContext);
+}
+
 interface ElementRendererProps {
   element: UIElement;
   /** Spec key for this element. Used by the devtools picker. */
@@ -142,8 +157,9 @@ function ElementRenderer(props: ElementRendererProps) {
   const stateStore = useStateStore();
   const getSnapshot = () => stateStore.getSnapshot();
   const functions = useFunctions();
+  const directives = useDirectives();
 
-  // Build context with repeat scope and $computed functions
+  // Build context with repeat scope, $computed functions, and custom directives
   const fullCtx = createMemo<PropResolutionContext>(() => {
     const repeatItem = repeatScope?.item;
     const repeatIndex = repeatScope?.index;
@@ -157,6 +173,7 @@ function ElementRenderer(props: ElementRendererProps) {
       ...(repeatIndex !== undefined ? { repeatIndex } : {}),
       ...(repeatBasePath !== undefined ? { repeatBasePath } : {}),
       functions,
+      directives,
     };
   });
 
@@ -519,6 +536,8 @@ export interface JSONUIProviderProps {
   >;
   /** Named functions for `$computed` expressions in props */
   functions?: Record<string, ComputedFunction>;
+  /** Custom directives for user-defined `$`-prefixed dynamic values */
+  directives?: DirectiveDefinition[];
   /** Callback when state changes (uncontrolled mode) */
   onStateChange?: (changes: Array<{ path: string; value: unknown }>) => void;
   children: JSX.Element;
@@ -528,6 +547,9 @@ export interface JSONUIProviderProps {
  * Combined provider for all JSONUI contexts
  */
 export function JSONUIProvider(props: JSONUIProviderProps) {
+  const directiveRegistry = createMemo(() =>
+    props.directives ? createDirectiveRegistry(props.directives) : undefined,
+  );
   return (
     <StateProvider
       store={props.store}
@@ -540,8 +562,10 @@ export function JSONUIProvider(props: JSONUIProviderProps) {
             <FunctionsContext.Provider
               value={props.functions ?? EMPTY_FUNCTIONS}
             >
-              {props.children}
-              <ConfirmationDialogManager />
+              <DirectivesContext.Provider value={directiveRegistry()}>
+                {props.children}
+                <ConfirmationDialogManager />
+              </DirectivesContext.Provider>
             </FunctionsContext.Provider>
           </ActionProvider>
         </ValidationProvider>
@@ -748,6 +772,8 @@ export interface CreateRendererProps {
   onStateChange?: (changes: Array<{ path: string; value: unknown }>) => void;
   /** Named functions for `$computed` expressions in props */
   functions?: Record<string, ComputedFunction>;
+  /** Custom directives for user-defined `$`-prefixed dynamic values */
+  directives?: DirectiveDefinition[];
   /** Whether the spec is currently loading/streaming */
   loading?: boolean;
   /** Fallback component for unknown types */
@@ -796,6 +822,10 @@ export function createRenderer<
 
   // Return the renderer component
   return function CatalogRenderer(props: CreateRendererProps) {
+    const directiveRegistry = createMemo(() =>
+      props.directives ? createDirectiveRegistry(props.directives) : undefined,
+    );
+
     // Wrap onAction with a Proxy so any action name routes to the callback
     const actionHandlers = () =>
       props.onAction
@@ -826,13 +856,15 @@ export function createRenderer<
               <FunctionsContext.Provider
                 value={props.functions ?? EMPTY_FUNCTIONS}
               >
-                <Renderer
-                  spec={props.spec}
-                  registry={registry}
-                  loading={props.loading}
-                  fallback={props.fallback}
-                />
-                <ConfirmationDialogManager />
+                <DirectivesContext.Provider value={directiveRegistry()}>
+                  <Renderer
+                    spec={props.spec}
+                    registry={registry}
+                    loading={props.loading}
+                    fallback={props.fallback}
+                  />
+                  <ConfirmationDialogManager />
+                </DirectivesContext.Provider>
               </FunctionsContext.Provider>
             </ActionProvider>
           </ValidationProvider>

packages/svelte/src/CatalogRenderer.svelte

@@ -1,5 +1,5 @@
 <script module lang="ts">
-  import type { ComputedFunction, Spec, StateStore } from "@json-render/core";
+  import type { ComputedFunction, DirectiveDefinition, Spec, StateStore } from "@json-render/core";
   import type { ComponentRenderer, ComponentRegistry } from "./renderer.js";
 
   export interface CatalogRendererProps {
@@ -10,6 +10,8 @@
     onAction?: (actionName: string, params?: Record<string, unknown>) => void;
     onStateChange?: (changes: Array<{ path: string; value: unknown }>) => void;
     functions?: Record<string, ComputedFunction>;
+    /** Custom directives for user-defined `$`-prefixed dynamic values */
+    directives?: DirectiveDefinition[];
     loading?: boolean;
     fallback?: ComponentRenderer;
   }
@@ -28,6 +30,7 @@
     onAction,
     onStateChange,
     functions,
+    directives,
     loading = false,
     fallback,
   }: CatalogRendererProps = $props();
@@ -50,6 +53,7 @@
   initialState={state}
   handlers={actionHandlers}
   {functions}
+  {directives}
   {onStateChange}>
   <Renderer {spec} {registry} {loading} {fallback} />
 </JsonUIProvider>

packages/svelte/src/ElementRenderer.svelte

@@ -16,6 +16,7 @@
   import { getActionContext } from "./contexts/ActionProvider.svelte";
   import { getRepeatScope } from "./contexts/RepeatScopeProvider.svelte";
   import { getFunctions } from "./contexts/FunctionsContextProvider.svelte";
+  import { getDirectives } from "./contexts/DirectivesContextProvider.svelte";
   import RepeatChildren from "./RepeatChildren.svelte";
   import Self from "./ElementRenderer.svelte";
 
@@ -49,8 +50,9 @@
   const actionCtx = getActionContext();
   const repeatScope = getRepeatScope();
   const functions = getFunctions();
+  const directives = getDirectives();
 
-  // Build context with repeat scope and $computed functions
+  // Build context with repeat scope, $computed functions, and custom directives
   let fullCtx = $derived<PropResolutionContext>(
     repeatScope
       ? {
@@ -59,8 +61,9 @@
           repeatIndex: repeatScope.index,
           repeatBasePath: repeatScope.basePath,
           functions,
+          directives,
         }
-      : { stateModel: stateCtx.state, functions },
+      : { stateModel: stateCtx.state, functions, directives },
   );
 
   // Evaluate visibility

packages/svelte/src/JsonUIProvider.svelte

@@ -1,5 +1,5 @@
 <script module lang="ts">
-  import type { ComputedFunction } from "@json-render/core";
+  import type { ComputedFunction, DirectiveDefinition } from "@json-render/core";
   /**
    * Props for JSONUIProvider
    */
@@ -24,6 +24,8 @@
     >;
     /** Named functions for `$computed` expressions in props */
     functions?: Record<string, ComputedFunction>;
+    /** Custom directives for user-defined `$`-prefixed dynamic values */
+    directives?: DirectiveDefinition[];
     /** Callback when state changes (uncontrolled mode) */
     onStateChange?: (changes: Array<{ path: string; value: unknown }>) => void;
     /** Children snippet */
@@ -39,6 +41,7 @@
   import ValidationProvider from "./contexts/ValidationProvider.svelte";
   import ActionProvider from "./contexts/ActionProvider.svelte";
   import FunctionsContextProvider from "./contexts/FunctionsContextProvider.svelte";
+  import DirectivesContextProvider from "./contexts/DirectivesContextProvider.svelte";
   import ConfirmDialogManager from "./ConfirmDialogManager.svelte";
   import type { ComponentRegistry } from "./renderer.js";
 
@@ -49,6 +52,7 @@
     navigate,
     validationFunctions = {},
     functions,
+    directives,
     onStateChange,
     children,
   }: JSONUIProviderProps = $props();
@@ -59,8 +63,10 @@
     <ValidationProvider customFunctions={validationFunctions}>
       <ActionProvider {handlers} {navigate}>
         <FunctionsContextProvider {functions}>
-          {@render children()}
-          <ConfirmDialogManager />
+          <DirectivesContextProvider {directives}>
+            {@render children()}
+            <ConfirmDialogManager />
+          </DirectivesContextProvider>
         </FunctionsContextProvider>
       </ActionProvider>
     </ValidationProvider>

packages/svelte/src/contexts/DirectivesContextProvider.svelte

@@ -0,0 +1,46 @@
+<script module lang="ts">
+  import { getContext } from "svelte";
+  import type { DirectiveDefinition, DirectiveRegistry } from "@json-render/core";
+  import { createDirectiveRegistry } from "@json-render/core";
+
+  const DIRECTIVES_KEY = Symbol.for("json-render-directives");
+
+  /**
+   * Directives context value
+   */
+  export interface DirectivesContext {
+    /** Custom directive registry */
+    registry: DirectiveRegistry | undefined;
+  }
+
+  /**
+   * Get the directives registry from component tree
+   */
+  export function getDirectives(): DirectiveRegistry | undefined {
+    const ctx = getContext<DirectivesContext>(DIRECTIVES_KEY);
+    return ctx?.registry;
+  }
+</script>
+
+<script lang="ts">
+  import { setContext, type Snippet } from "svelte";
+
+  interface Props {
+    directives?: DirectiveDefinition[];
+    children?: Snippet;
+  }
+
+  let { directives, children }: Props = $props();
+
+  let registry = $derived(
+    directives ? createDirectiveRegistry(directives) : undefined,
+  );
+
+  setContext(DIRECTIVES_KEY, {
+    get registry() {
+      return registry;
+    },
+  } satisfies DirectivesContext);
+</script>
+
+{@render children?.()}

packages/svelte/src/index.ts

@@ -46,6 +46,12 @@ export {
   type FunctionsContext,
 } from "./contexts/FunctionsContextProvider.svelte";
 
+export {
+  default as DirectivesContextProvider,
+  getDirectives,
+  type DirectivesContext,
+} from "./contexts/DirectivesContextProvider.svelte";
+
 // =============================================================================
 // Schema
 // =============================================================================

packages/svelte/src/renderer.ts

@@ -1,6 +1,7 @@
 import type {
   Catalog,
   ComputedFunction,
+  DirectiveDefinition,
   SchemaDefinition,
   Spec,
   StateStore,
@@ -206,6 +207,8 @@ export interface CreateRendererProps {
   onStateChange?: (changes: Array<{ path: string; value: unknown }>) => void;
   /** Named functions for `$computed` expressions in props */
   functions?: Record<string, ComputedFunction>;
+  /** Custom directives for user-defined `$`-prefixed dynamic values */
+  directives?: DirectiveDefinition[];
   loading?: boolean;
   fallback?: Component;
 }
@@ -264,6 +267,9 @@ export function createRenderer<
       get functions() {
         return props.functions;
       },
+      get directives() {
+        return props.directives;
+      },
       get loading() {
         return props.loading;
       },

packages/vue/src/renderer.ts

@@ -21,6 +21,8 @@ import type {
   ComputedFunction,
   SchemaDefinition,
   StateStore,
+  DirectiveDefinition,
+  DirectiveRegistry,
 } from "@json-render/core";
 import {
   resolveElementProps,
@@ -28,6 +30,7 @@ import {
   resolveActionParam,
   evaluateVisibility,
   getByPath,
+  createDirectiveRegistry,
   isDevtoolsActive,
   subscribeDevtoolsActive,
   type PropResolutionContext,
@@ -117,6 +120,36 @@ function useFunctions(): ComputedRef<Record<string, ComputedFunction>> {
   );
 }
 
+// ---------------------------------------------------------------------------
+// DirectivesContext — provides custom directive registry to the element tree
+// ---------------------------------------------------------------------------
+
+const DIRECTIVES_KEY = Symbol("json-render:directives");
+
+const DirectivesProvider = defineComponent({
+  name: "DirectivesProvider",
+  props: {
+    directives: {
+      type: Array as PropType<DirectiveDefinition[]>,
+      default: undefined,
+    },
+  },
+  setup(props, { slots }) {
+    const registry = computed(() =>
+      props.directives ? createDirectiveRegistry(props.directives) : undefined,
+    );
+    provide(DIRECTIVES_KEY, registry);
+    return () => slots.default?.();
+  },
+});
+
+function useDirectives(): ComputedRef<DirectiveRegistry | undefined> {
+  return inject<ComputedRef<DirectiveRegistry | undefined>>(
+    DIRECTIVES_KEY,
+    computed(() => undefined),
+  );
+}
+
 // ---------------------------------------------------------------------------
 // ElementErrorBoundary — catches rendering errors in individual elements
 // ---------------------------------------------------------------------------
@@ -239,8 +272,9 @@ const ElementRenderer = defineComponent({
     const { execute } = useActions();
     const { getSnapshot, state: watchState } = useStateStore();
     const functions = useFunctions();
+    const directives = useDirectives();
 
-    // Build context with repeat scope and $computed functions
+    // Build context with repeat scope, $computed functions, and custom directives
     const fullCtx = computed<PropResolutionContext>(() => {
       const base: PropResolutionContext = repeatScope
         ? {
@@ -251,6 +285,7 @@ const ElementRenderer = defineComponent({
           }
         : { ...visibilityCtx.value };
       base.functions = functions.value;
+      base.directives = directives.value;
       return base;
     });
 
@@ -591,6 +626,8 @@ export interface JSONUIProviderProps {
   >;
   /** Named functions for `$computed` expressions in props */
   functions?: Record<string, ComputedFunction>;
+  /** Custom directives for user-defined `$`-prefixed dynamic values */
+  directives?: DirectiveDefinition[];
   onStateChange?: (changes: Array<{ path: string; value: unknown }>) => void;
 }
 
@@ -638,6 +675,10 @@ export const JSONUIProvider = defineComponent({
       type: Object as PropType<Record<string, ComputedFunction>>,
       default: undefined,
     },
+    directives: {
+      type: Array as PropType<DirectiveDefinition[]>,
+      default: undefined,
+    },
     onStateChange: {
       type: Function as PropType<
         (changes: Array<{ path: string; value: unknown }>) => void
@@ -672,10 +713,17 @@ export const JSONUIProvider = defineComponent({
                               FunctionsProvider,
                               { functions: props.functions },
                               {
-                                default: () => [
-                                  slots.default?.(),
-                                  h(ConfirmationDialogManager),
-                                ],
+                                default: () =>
+                                  h(
+                                    DirectivesProvider,
+                                    { directives: props.directives },
+                                    {
+                                      default: () => [
+                                        slots.default?.(),
+                                        h(ConfirmationDialogManager),
+                                      ],
+                                    },
+                                  ),
                               },
                             ),
                         },
@@ -854,6 +902,8 @@ export interface CreateRendererProps {
   onStateChange?: (changes: Array<{ path: string; value: unknown }>) => void;
   /** Named functions for `$computed` expressions in props */
   functions?: Record<string, ComputedFunction>;
+  /** Custom directives for user-defined `$`-prefixed dynamic values */
+  directives?: DirectiveDefinition[];
   loading?: boolean;
   fallback?: Component;
 }
@@ -922,6 +972,10 @@ export function createRenderer<
         type: Object as PropType<Record<string, ComputedFunction>>,
         default: undefined,
       },
+      directives: {
+        type: Array as PropType<DirectiveDefinition[]>,
+        default: undefined,
+      },
       loading: {
         type: Boolean,
         default: undefined,
@@ -972,15 +1026,22 @@ export function createRenderer<
                               FunctionsProvider,
                               { functions: rendererProps.functions },
                               {
-                                default: () => [
-                                  h(Renderer, {
-                                    spec: rendererProps.spec,
-                                    registry,
-                                    loading: rendererProps.loading,
-                                    fallback: rendererProps.fallback,
-                                  }),
-                                  h(ConfirmationDialogManager),
-                                ],
+                                default: () =>
+                                  h(
+                                    DirectivesProvider,
+                                    { directives: rendererProps.directives },
+                                    {
+                                      default: () => [
+                                        h(Renderer, {
+                                          spec: rendererProps.spec,
+                                          registry,
+                                          loading: rendererProps.loading,
+                                          fallback: rendererProps.fallback,
+                                        }),
+                                        h(ConfirmationDialogManager),
+                                      ],
+                                    },
+                                  ),
                               },
                             ),
                         },

pnpm-lock.yaml

@@ -1871,6 +1871,28 @@ importers:
         specifier: ^3.5.0
         version: 3.5.29(typescript@5.9.3)
 
+  packages/directives:
+    dependencies:
+      '@json-render/core':
+        specifier: workspace:*
+        version: link:../core
+    devDependencies:
+      '@internal/typescript-config':
+        specifier: workspace:*
+        version: link:../typescript-config
+      tsup:
+        specifier: ^8.0.2
+        version: 8.5.1(jiti@2.6.1)(postcss@8.5.6)(tsx@4.21.0)(typescript@5.9.3)(yaml@2.8.3)
+      typescript:
+        specifier: ^5.4.5
+        version: 5.9.3
+      vitest:
+        specifier: ^4.0.17
+        version: 4.0.17(@opentelemetry/api@1.9.0)(@types/node@22.19.6)(jiti@2.6.1)(jsdom@27.4.0)(lightningcss@1.32.0)(msw@2.12.10(@types/node@22.19.6)(typescript@5.9.3))(terser@5.46.0)(tsx@4.21.0)(yaml@2.8.3)
+      zod:
+        specifier: ^4.3.6
+        version: 4.3.6
+
   packages/eslint-config:
     devDependencies:
       '@eslint/js':
@@ -2074,6 +2096,9 @@ importers:
       typescript:
         specifier: ^5.4.5
         version: 5.9.3
+      zod:
+        specifier: ^4.3.6
+        version: 4.3.6
 
   packages/react-email:
     dependencies:
@@ -19476,7 +19501,7 @@ snapshots:
   '@stripe/ui-extension-tools@0.0.1(@babel/core@7.29.0)(babel-jest@27.5.1(@babel/core@7.29.0))':
     dependencies:
       '@types/jest': 28.1.8
-      '@typescript-eslint/eslint-plugin': 5.62.0(@typescript-eslint/parser@5.62.0(eslint@8.57.1)(typescript@4.9.5))(eslint@8.57.1)(typescript@4.9.5)
+      '@typescript-eslint/eslint-plugin': 5.62.0(@typescript-eslint/parser@5.62.0(eslint@9.39.2(jiti@2.6.1))(typescript@4.9.5))(eslint@8.57.1)(typescript@4.9.5)
       '@typescript-eslint/parser': 5.62.0(eslint@8.57.1)(typescript@4.9.5)
       eslint: 8.57.1
       eslint-plugin-react: 7.37.5(eslint@8.57.1)
@@ -20160,7 +20185,7 @@ snapshots:
       '@types/node': 22.19.6
     optional: true
 
-  '@typescript-eslint/eslint-plugin@5.62.0(@typescript-eslint/parser@5.62.0(eslint@8.57.1)(typescript@4.9.5))(eslint@8.57.1)(typescript@4.9.5)':
+  '@typescript-eslint/eslint-plugin@5.62.0(@typescript-eslint/parser@5.62.0(eslint@9.39.2(jiti@2.6.1))(typescript@4.9.5))(eslint@8.57.1)(typescript@4.9.5)':
     dependencies:
       '@eslint-community/regexpp': 4.12.2
       '@typescript-eslint/parser': 5.62.0(eslint@8.57.1)(typescript@4.9.5)

skills/directives/SKILL.md

@@ -0,0 +1,205 @@
+---
+name: directives
+description: Pre-built custom directives for json-render — formatting, math, string manipulation, and i18n. Use when working with @json-render/directives, defining custom directives with defineDirective, or adding $format, $math, $concat, $count, $truncate, $pluralize, $join, or $t to specs.
+---
+
+# @json-render/directives
+
+Pre-built custom directives for `@json-render/core`. Drop them into your catalog and renderer to add formatting, math, string manipulation, and i18n.
+
+## Quick Start
+
+```typescript
+import { standardDirectives } from '@json-render/directives';
+
+// Wire into prompt generation
+const prompt = catalog.prompt({ directives: standardDirectives });
+
+// Wire into the renderer (React example)
+import { JSONUIProvider, Renderer } from '@json-render/react';
+
+<JSONUIProvider registry={registry} directives={standardDirectives}>
+  <Renderer spec={spec} registry={registry} />
+</JSONUIProvider>
+```
+
+To add factory directives like `createI18nDirective`, spread the array:
+
+```typescript
+import { standardDirectives, createI18nDirective } from '@json-render/directives';
+
+const directives = [...standardDirectives, createI18nDirective(config)];
+```
+
+## Defining Custom Directives
+
+Use `defineDirective` from `@json-render/core`:
+
+```typescript
+import { defineDirective, resolvePropValue } from '@json-render/core';
+import { z } from 'zod';
+
+const doubleDirective = defineDirective({
+  name: '$double',
+  description: 'Double a numeric value.',
+  schema: z.object({
+    $double: z.unknown(),
+  }),
+  resolve(value, ctx) {
+    const resolved = resolvePropValue(value.$double, ctx);
+    return (resolved as number) * 2;
+  },
+});
+```
+
+Rules:
+- Name must start with `$`
+- Name must not conflict with built-in keys (`$state`, `$cond`, `$computed`, `$template`, `$item`, `$index`, `$bindState`, `$bindItem`)
+- Resolvers should call `resolvePropValue` on sub-values to support composition
+
+## Built-in Directives
+
+### `$format` — Locale-aware value formatting
+
+Formats values using `Intl` formatters. Supports `date`, `currency`, `number`, and `percent`.
+
+```json
+{ "$format": "currency", "value": { "$state": "/cart/total" }, "currency": "USD" }
+{ "$format": "date", "value": { "$state": "/user/createdAt" } }
+{ "$format": "number", "value": 1234567, "notation": "compact" }
+{ "$format": "percent", "value": 0.75 }
+{ "$format": "date", "value": { "$state": "/post/createdAt" }, "style": "relative" }
+```
+
+Fields: `$format` (date | currency | number | percent), `value` (any expression), `locale?` (string), `currency?` (string, default "USD"), `notation?` (string), `style?` ("relative" for relative dates), `options?` (extra Intl options).
+
+### `$math` — Arithmetic operations
+
+```json
+{ "$math": "add", "a": { "$state": "/subtotal" }, "b": { "$state": "/tax" } }
+{ "$math": "round", "a": 3.7 }
+```
+
+Operations: `add`, `subtract`, `multiply`, `divide`, `mod`, `min`, `max`, `round`, `floor`, `ceil`, `abs`. Unary ops (`round`, `floor`, `ceil`, `abs`) only use `a`. Division by zero returns `0`.
+
+Fields: `$math` (operation enum), `a?` (first operand, default 0), `b?` (second operand, default 0).
+
+### `$concat` — String concatenation
+
+```json
+{ "$concat": [{ "$state": "/user/firstName" }, " ", { "$state": "/user/lastName" }] }
+```
+
+Fields: `$concat` (array of values to resolve and join into a string).
+
+### `$count` — Array/string length
+
+```json
+{ "$count": { "$state": "/cart/items" } }
+```
+
+Returns `.length` of arrays or strings, `0` for other types.
+
+Fields: `$count` (value to count).
+
+### `$truncate` — Text truncation
+
+```json
+{ "$truncate": { "$state": "/post/body" }, "length": 140, "suffix": "..." }
+```
+
+Fields: `$truncate` (value to truncate), `length?` (max chars, default 100), `suffix?` (string, default "...").
+
+### `$pluralize` — Singular/plural forms
+
+```json
+{ "$pluralize": { "$state": "/cart/itemCount" }, "one": "item", "other": "items", "zero": "no items" }
+```
+
+Output: `"3 items"`, `"1 item"`, or `"no items"`.
+
+Fields: `$pluralize` (count value), `one` (singular label), `other` (plural label), `zero?` (zero label).
+
+### `$join` — Join array elements
+
+```json
+{ "$join": { "$state": "/tags" }, "separator": ", " }
+```
+
+Fields: `$join` (array to join), `separator?` (string, default ", ").
+
+### `createI18nDirective` — Internationalization factory
+
+```typescript
+import { createI18nDirective } from '@json-render/directives';
+
+const tDirective = createI18nDirective({
+  locale: 'en',
+  messages: {
+    en: { "greeting": "Hello, {{name}}!", "checkout.submit": "Place Order" },
+    es: { "greeting": "Hola, {{name}}!", "checkout.submit": "Realizar Pedido" },
+  },
+  fallbackLocale: 'en',
+});
+```
+
+Usage in specs:
+
+```json
+{ "$t": "checkout.submit" }
+{ "$t": "greeting", "params": { "name": { "$state": "/user/name" } } }
+```
+
+Fields: `$t` (translation key), `params?` (interpolation parameters, values accept expressions).
+
+Config: `locale` (current locale), `messages` (Record<locale, Record<key, string>>), `fallbackLocale?` (fallback when key missing).
+
+## Composition
+
+Directives compose naturally — each resolver calls `resolvePropValue` on its inputs, so directives can wrap other directives or built-in expressions:
+
+```json
+{
+  "$format": "currency",
+  "value": { "$math": "multiply", "a": { "$state": "/price" }, "b": { "$state": "/qty" } },
+  "currency": "USD"
+}
+```
+
+Resolves inside-out: `$state` reads from state, `$math` multiplies, `$format` formats as currency.
+
+## Wiring into Renderers
+
+All four renderers (React, Vue, Svelte, Solid) accept `directives` on their provider and `createRenderer` output:
+
+```tsx
+// Provider pattern
+<JSONUIProvider registry={registry} directives={directives}>
+  <Renderer spec={spec} registry={registry} />
+</JSONUIProvider>
+
+// createRenderer pattern
+const MyRenderer = createRenderer(catalog, components);
+<MyRenderer spec={spec} directives={directives} />
+```
+
+For prompt generation, pass the same array:
+
+```typescript
+const prompt = catalog.prompt({ directives });
+```
+
+## Key Exports
+
+| Export | Purpose |
+|--------|---------|
+| `formatDirective` | `$format` directive definition |
+| `mathDirective` | `$math` directive definition |
+| `concatDirective` | `$concat` directive definition |
+| `countDirective` | `$count` directive definition |
+| `truncateDirective` | `$truncate` directive definition |
+| `pluralizeDirective` | `$pluralize` directive definition |
+| `joinDirective` | `$join` directive definition |
+| `createI18nDirective` | Factory for `$t` i18n directive |
+| `standardDirectives` | Array of all 7 non-factory directives |
+| `I18nConfig` | Type for i18n configuration |