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

// Transforms & refinements
tm.string().transform(s => s.trim().toLowerCase())
tm.number().refine(n => n > 0, 'Must be positive')
tm.string().preprocess(v => v ?? '')     // runs before validation
tm.string().message('Name is required')  // custom error messages

// 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" }

Streaming

Parse JSON as it streams from an LLM. Works with OpenAI, Anthropic, Google, and any SSE-based API.

JsonStream — Incremental parser

import { JsonStream, tm } from 'typemeld';

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

const stream = new JsonStream(schema);

// Push chunks as they arrive from the LLM
stream.push('{"sentiment":');
stream.push(' "positive",');
stream.push(' "confidence": 0.95}');

console.log(stream.value); // partial result available at any time
const final = stream.complete(); // validates against schema
// => { sentiment: "positive", confidence: 0.95 }

streamParse() — Process async iterables

import { streamParse, tm } from 'typemeld';

// With OpenAI
const completion = await openai.chat.completions.create({
  model: 'gpt-4o-mini',
  stream: true,
  messages: [{ role: 'user', content: 'Analyze this text...' }],
});

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

for await (const { partial, done } of streamParse(
  (async function*() {
    for await (const chunk of completion) {
      yield chunk.choices[0]?.delta?.content ?? '';
    }
  })(),
  schema
)) {
  console.log(partial); // progressively more complete object
  if (done) console.log('Final validated result:', partial);
}

parseStream() — Simple async completion

import { parseStream, tm } from 'typemeld';

// Buffer all chunks and return final validated result
const result = await parseStream(
  (async function*() {
    for await (const chunk of anthropicStream) {
      yield chunk.delta?.text ?? '';
    }
  })(),
  schema
);

Zod Adapter

Already using Zod? Use your existing schemas with typemeld's repair engine:

import { z } from 'zod';
import { fromZod, parse } from 'typemeld';

// Your existing Zod schema
const zodSchema = z.object({
  name: z.string().min(1),
  age: z.number().min(0).max(150),
  role: z.enum(['admin', 'user', 'guest']),
  tags: z.array(z.string()).optional(),
});

// Convert to typemeld and parse messy LLM output
const tmSchema = fromZod(zodSchema);
const result = parse(messyLlmOutput, tmSchema);
// typemeld repairs the JSON, then validates with your Zod-equivalent schema

The adapter supports: string, number, boolean, array, object, enum, literal, union, optional, nullable, default, describe, min/max, passthrough, strict, and more.

Retry Wrapper

Automatically re-prompt the LLM when validation fails:

import { withRetry, tm } from 'typemeld';
import OpenAI from 'openai';

const openai = new OpenAI();
const schema = tm.object({
  entities: tm.array(tm.object({
    name: tm.string(),
    type: tm.enum(['person', 'org', 'location']),
    confidence: tm.number().min(0).max(1),
  })).min(1),
});

const result = await withRetry({
  schema,
  maxRetries: 3,
  call: (messages) => openai.chat.completions.create({
    model: 'gpt-4o-mini',
    messages,
  }),
  extract: (res) => res.choices[0].message.content,
  prompt: 'Extract all entities from: "Tim Cook announced Apple\'s new product in Cupertino"',
  onRetry: (attempt, errors) => console.log(`Retry ${attempt}:`, errors),
});
// If the LLM returns invalid output, typemeld re-prompts with error details
// => { entities: [{ name: "Tim Cook", type: "person", confidence: 0.98 }, ...] }

With Anthropic

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

const anthropic = new Anthropic();

const result = await withRetry({
  schema: tm.object({
    summary: tm.string().min(10).max(200),
    keywords: tm.array(tm.string()).min(3),
    sentiment: tm.enum(['positive', 'negative', 'neutral']),
  }),
  call: (messages) => anthropic.messages.create({
    model: 'claude-sonnet-4-20250514',
    max_tokens: 1024,
    system: messages.find(m => m.role === 'system')?.content,
    messages: messages.filter(m => m.role !== 'system'),
  }),
  extract: (res) => res.content[0].text,
  prompt: `Summarize and analyze this article: "${articleText}"`,
});

Real-world examples

Sentiment analysis

const sentiment = parse(llmOutput, tm.object({
  sentiment: tm.enum(['positive', 'negative', 'neutral', 'mixed']),
  confidence: tm.number().min(0).max(1),
  reasoning: tm.string().optional(),
}));

Entity extraction

const entities = parse(llmOutput, tm.object({
  entities: tm.array(tm.object({
    text: tm.string(),
    type: tm.enum(['person', 'organization', 'location', 'date', 'money']),
    start: tm.number().optional(),
    end: tm.number().optional(),
  })),
}));

Product listing

const product = parse(llmOutput, tm.object({
  name: tm.string().min(1),
  price: tm.number().min(0),
  currency: tm.enum(['USD', 'EUR', 'GBP']).default('USD'),
  inStock: tm.boolean().default(true),
  tags: tm.array(tm.string()),
  description: tm.string().max(500),
}));

Code review

const review = parse(llmOutput, tm.object({
  issues: tm.array(tm.object({
    severity: tm.enum(['critical', 'warning', 'info']),
    line: tm.number().optional(),
    message: tm.string(),
    suggestion: tm.string().optional(),
  })),
  overall: tm.enum(['approve', 'request_changes', 'comment']),
  summary: tm.string(),
}));

Translation

const translation = parse(llmOutput, tm.object({
  original: tm.string(),
  translated: tm.string(),
  language: tm.string(),
  confidence: tm.number().min(0).max(1),
  alternatives: tm.array(tm.string()).optional(),
}));

Classification

const classification = parse(llmOutput, tm.object({
  category: tm.enum(['bug', 'feature', 'question', 'docs']),
  priority: tm.enum(['low', 'medium', 'high', 'critical']),
  labels: tm.array(tm.string()),
  assignee: tm.string().optional(),
}));

Structured data extraction from documents

const invoice = parse(llmOutput, tm.object({
  vendor: tm.string(),
  invoiceNumber: tm.string(),
  date: tm.string().transform(s => new Date(s)),
  items: tm.array(tm.object({
    description: tm.string(),
    quantity: tm.number().min(1),
    unitPrice: tm.number().min(0),
  })),
  total: tm.number().min(0),
  currency: tm.enum(['USD', 'EUR', 'GBP', 'JPY']).default('USD'),
}));

Multi-step agent output

const agentStep = parse(llmOutput, tm.object({
  thought: tm.string(),
  action: tm.enum(['search', 'calculate', 'respond', 'ask_user']),
  actionInput: tm.string().optional(),
  observation: tm.string().optional(),
  finalAnswer: tm.string().optional(),
}).passthrough()); // keep any extra fields the LLM adds

With transforms and refinements

const userProfile = parse(llmOutput, tm.object({
  email: tm.string()
    .transform(s => s.trim().toLowerCase())
    .refine(s => s.includes('@'), 'Must be a valid email'),
  age: tm.number()
    .min(0).max(150)
    .refine(n => Number.isInteger(n), 'Age must be a whole number'),
  bio: tm.string()
    .transform(s => s.trim())
    .refine(s => s.length > 0, 'Bio cannot be empty'),
  website: tm.string().optional()
    .transform(s => s?.startsWith('http') ? s : `https://${s}`),
}));

With preprocessing

const config = parse(llmOutput, tm.object({
  temperature: tm.number()
    .preprocess(v => typeof v === 'string' ? parseFloat(v) : v)
    .min(0).max(2),
  model: tm.string()
    .preprocess(v => typeof v === 'string' ? v.trim() : String(v)),
}));

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 | ❌ | ✅ | | Streaming JSON parser | ❌ | ✅ | | LLM retry wrapper | ❌ | ✅ | | Zod schema adapter | N/A | ✅ | | Smart type coercion | Partial | ✅ Full | | Single value → array coercion | ❌ | ✅ | | Case-insensitive enum matching | ❌ | ✅ | | Multiple JSON extraction | ❌ | ✅ | | LLM prompt generation | ❌ | ✅ | | Transform / refine / preprocess | ✅ | ✅ | | Custom error messages | ✅ | ✅ | | 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 with fromZod().

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 and repair strategies
• Integration guides for popular frameworks (LangChain, Vercel AI SDK, AutoGPT)
• XML/YAML repair modes
• Performance benchmarks

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

License

MIT © Suhas Holla