Package Exports

typemeld
typemeld/repair
typemeld/schema

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typemeld

Parse, validate, and repair messy LLM outputs into clean, typed data.

_{Like zod, but for AI. Zero dependencies. TypeScript-ready. ~3KB gzipped.}

Every developer building with LLMs hits the same wall:

You: "Return JSON with name, age, and tags"

GPT/Claude: "Sure! Here's the data:

```json
{name: 'John', age: "30", tags: "developer",}  // trailing comma, unquoted keys, wrong types

Let me know if you need anything else!"


**typemeld** fixes this in one line:

```javascript
import { parse, tm } from 'typemeld';

const user = parse(llmOutput, tm.object({
  name: tm.string(),
  age: tm.number(),
  tags: tm.array(tm.string()),
}));
// => { name: "John", age: 30, tags: ["developer"] }

It strips the markdown fences, extracts the JSON from surrounding prose, fixes the trailing comma, quotes the keys, coerces "30" to 30, wraps "developer" into ["developer"], and validates every field. One function call.

Install

npm install typemeld

Zero dependencies. Works in Node.js 18+, Bun, Deno, and browsers.

What it repairs

LLM quirk	Example	typemeld handles it
Markdown fences	```json { ... } ```	✅ Stripped
JSON in prose	`"Sure! Here's the data: { ... } Hope that helps!"`	✅ Extracted
Trailing commas	`{ "a": 1, "b": 2, }`	✅ Fixed
Unquoted keys	`{ name: "John" }`	✅ Quoted
Single quotes	`{ 'name': 'John' }`	✅ Converted
JS comments	`{ "a": 1 // comment }`	✅ Stripped
Truncated JSON	`{ "items": ["one", "tw`	✅ Auto-closed
NaN / undefined	`{ "value": NaN }`	✅ → `null`
Infinity	`{ "n": Infinity }`	✅ → `null`
Wrong types	`{ "age": "30" }` with number schema	✅ Coerced to `30`
String → array	`{ "tags": "dev" }` with array schema	✅ Wrapped to `["dev"]`
String booleans	`{ "ok": "true" }` with boolean schema	✅ Coerced to `true`
Case mismatch	`{ "mood": "Positive" }` with `enum(['positive', ...])`	✅ Coerced to `"positive"`

API

`parse(input, schema?)`

Parse and validate. Throws ParseError on failure.

import { parse, tm } from 'typemeld';

// Without schema — just repair JSON
const data = parse('```json\n{"key": "value",}\n```');
// => { key: "value" }

// With schema — repair + validate + coerce
const result = parse(messy_llm_output, tm.object({
  sentiment: tm.enum(['positive', 'negative', 'neutral']),
  confidence: tm.number(),
  summary: tm.string(),
}));

`safeParse(input, schema?)`

Same as parse but never throws. Returns { success, data, errors }.

import { safeParse, tm } from 'typemeld';

const result = safeParse(llmOutput, schema);
if (result.success) {
  console.log(result.data);
} else {
  console.log(result.errors);
  // [{ path: "confidence", message: "Expected number, got undefined", expected: "number" }]
}

`repairJson(input)`

Low-level JSON repair without schema validation.

import { repairJson } from 'typemeld';

repairJson("{name: 'John', age: 30,}");
// => { name: "John", age: 30 }

repairJson('Sure! Here is the data:\n{"result": true}\nHope this helps!');
// => { result: true }

repairJson('{"items": ["one", "two", "thr');
// => { items: ["one", "two", "thr"] }

`extractAll(input)`

Extract multiple JSON objects from a single LLM response.

import { extractAll } from 'typemeld';

const objects = extractAll('First: {"a": 1} and then {"b": 2} finally {"c": 3}');
// => [{ a: 1 }, { b: 2 }, { c: 3 }]

`promptFor(schema, options?)`

Generate a prompt fragment describing the expected output format.

import { promptFor, tm } from 'typemeld';

const schema = tm.object({
  sentiment: tm.enum(['positive', 'negative', 'neutral']).describe('Overall sentiment'),
  confidence: tm.number().describe('Confidence score between 0 and 1'),
  summary: tm.string().describe('One sentence summary'),
});

const systemPrompt = `Analyze the following text.\n${promptFor(schema, { strict: true })}`;
// Generates JSON Schema instructions the LLM can follow

Schema Builder

typemeld includes a lightweight, chainable schema builder. No zod dependency needed.

import { tm } from 'typemeld';

// Primitives
tm.string()
tm.number()
tm.boolean()
tm.any()

// Complex
tm.array(tm.string())                    // string[]
tm.object({ name: tm.string() })         // { name: string }
tm.enum(['a', 'b', 'c'])                 // 'a' | 'b' | 'c'

// Modifiers (chainable)
tm.string().optional()                   // string | undefined
tm.string().nullable()                   // string | null
tm.string().default('hello')             // defaults to "hello" if missing
tm.number().describe('User age in years') // description for LLM prompts

// Constraints
tm.string().min(1).max(100)              // length between 1 and 100
tm.number().min(0).max(1)                // value between 0 and 1
tm.array(tm.string()).min(1).max(10)     // array length between 1 and 10

// Object modes
tm.object({ ... })                       // strips extra keys (default)
tm.object({ ... }).passthrough()         // keeps extra keys
tm.object({ ... }).strict()              // rejects extra keys

// Nested
tm.object({
  user: tm.object({
    name: tm.string(),
    email: tm.string().optional(),
  }),
  scores: tm.array(tm.number()),
  status: tm.enum(['active', 'inactive']).default('active'),
})

Type coercion

The schema validator intelligently coerces values when possible:

// String → Number
parse('{"age": "30"}', tm.object({ age: tm.number() }))
// => { age: 30 }

// String → Boolean
parse('{"ok": "true"}', tm.object({ ok: tm.boolean() }))
// => { ok: true }

// Single value → Array
parse('{"tags": "dev"}', tm.object({ tags: tm.array(tm.string()) }))
// => { tags: ["dev"] }

// Number → String
parse('{"id": 123}', tm.object({ id: tm.string() }))
// => { id: "123" }

// Case-insensitive enum
parse('{"mood": "Positive"}', tm.object({ mood: tm.enum(['positive', 'negative']) }))
// => { mood: "positive" }

Real-world example

import Anthropic from '@anthropic-ai/sdk';
import { parse, tm, promptFor } from 'typemeld';

const schema = tm.object({
  sentiment: tm.enum(['positive', 'negative', 'neutral']),
  confidence: tm.number(),
  topics: tm.array(tm.string()),
  summary: tm.string(),
});

const client = new Anthropic();
const response = await client.messages.create({
  model: 'claude-sonnet-4-20250514',
  max_tokens: 1024,
  system: `You are a text analyzer.\n${promptFor(schema, { strict: true })}`,
  messages: [{ role: 'user', content: `Analyze: "${articleText}"` }],
});

// This just works — even if Claude wraps it in fences or adds commentary
const analysis = parse(response.content[0].text, schema);
console.log(analysis.sentiment);  // "positive"
console.log(analysis.confidence); // 0.92
console.log(analysis.topics);     // ["technology", "innovation"]

Why not just use zod?

zod is excellent for general validation. typemeld is purpose-built for LLM outputs:

Feature	zod	typemeld
JSON repair (fences, commas, quotes)	❌	✅
Extract JSON from prose	❌	✅
Fix truncated JSON	❌	✅
Smart type coercion	Partial	✅ Full
Single value → array coercion	❌	✅
Case-insensitive enum matching	❌	✅
Multiple JSON extraction	❌	✅
LLM prompt generation	❌	✅
Min/max constraints	✅	✅
passthrough / strict modes	✅	✅
TypeScript types	✅ Built-in	✅ Built-in
Zero dependencies	❌ (standalone)	✅
Bundle size	~14KB	~3KB

Use zod for form validation. Use typemeld for LLM outputs. Or use both together.

TypeScript

typemeld ships with built-in TypeScript declarations. Full autocomplete and type inference out of the box:

import { parse, safeParse, tm } from 'typemeld';
import type { Infer, SafeParseResult } from 'typemeld';

const userSchema = tm.object({
  name: tm.string(),
  age: tm.number().optional(),
  roles: tm.array(tm.enum(['admin', 'user'])),
});

type User = Infer<typeof userSchema>;
// { name: string; age?: number; roles: ('admin' | 'user')[] }

const result: SafeParseResult<User> = safeParse(llmOutput, userSchema);

Contributing

Contributions welcome! High-impact areas:

More LLM output edge cases
Streaming JSON support (parse partial chunks)
zod adapter (tm.fromZod(zodSchema))
Retry wrapper (re-prompt LLM on validation failure)
XML/YAML repair modes

git clone https://github.com/suhashollakc/typemeld.git
cd typemeld && npm install && npm test

JSPM

typemeld

Package Exports

Readme

typemeld

Install

What it repairs

API

`parse(input, schema?)`

`safeParse(input, schema?)`

`repairJson(input)`

`extractAll(input)`

`promptFor(schema, options?)`

Schema Builder

Type coercion

Real-world example

Why not just use zod?

TypeScript

Contributing

License