repair-json-stream

Blazing-fast JSON repair for LLM streaming

---

When streaming responses from LLMs like OpenAI or Anthropic, JSON often arrives incomplete:

{"message": "Hello, I'm currently generating your resp

JSON.parse() crashes. repair-json-stream fixes it instantly.

import { repairJson } from 'repair-json-stream'

repairJson('{"message": "Hello, I\'m currently generating your resp')
// → '{"message": "Hello, I\'m currently generating your resp"}'

⚡ Performance

Built for real-time streaming. 1.9x faster than alternatives.

┌────────────────────┬────────────────────┬────────────┬─────────┐
│ Benchmark          │ repair-json-stream │ jsonrepair │ Speedup │
├────────────────────┼────────────────────┼────────────┼─────────┤
│ Small (15 KB)      │ 0.85 ms            │ 1.53 ms    │ 1.8x    │
│ Large (3.9 MB)     │ 295 ms             │ 392 ms     │ 1.3x    │
│ Streaming (1K ops) │ 426 ms             │ 583 ms     │ 1.4x    │
└────────────────────┴────────────────────┴────────────┴─────────┘

📦 Installation

npm install repair-json-stream

pnpm add repair-json-stream

yarn add repair-json-stream

🔧 What It Fixes

Issue	Example	Result
Truncated strings	{"text": "Hello	{"text": "Hello"}
Missing brackets	{"a": [1, 2	{"a": [1, 2]}
Unquoted keys	{name: "John"}	{"name": "John"}
Single quotes	{'key': 'val'}	{"key": "val"}
Python constants	{"x": None, "y": True}	{"x": null, "y": true}
Trailing commas	[1, 2, 3,]	[1, 2, 3]
Comments	{"a": 1} // comment	{"a": 1}
Fenced code blocks	```json {"a":1} ```	{"a":1}
JSONP wrappers	callback({"a": 1})	{"a": 1}
MongoDB types	NumberLong(123)	123
String concatenation	"hello" + "world"	"helloworld"
NDJSON	{"a":1}\n{"b":2}	[{"a":1},{"b":2}]

🚀 Usage

Basic

import { repairJson } from 'repair-json-stream'

const broken = '{"users": [{"name": "Alice'
const fixed = repairJson(broken)
// → '{"users": [{"name": "Alice"}]}'

JSON.parse(fixed) // Works!

Streaming (Node.js)

import { createReadStream, createWriteStream } from 'fs'
import { pipeline } from 'stream'
import { jsonrepairTransform } from 'repair-json-stream/stream'

pipeline(
  createReadStream('broken.json'),
  jsonrepairTransform(),
  createWriteStream('fixed.json'),
  (err) => console.log(err || 'Done!')
)

CLI

# Install globally
npm install -g repair-json-stream

# Repair a file
repair-json-stream broken.json > fixed.json

# Pipe from stdin
cat broken.json | repair-json-stream

# Overwrite in place
repair-json-stream broken.json --overwrite

# Verbose output (show repair actions)
repair-json-stream broken.json --verbose

Browser

<script src="https://unpkg.com/repair-json-stream/dist/repair-json-stream.browser.global.js"></script>
<script>
  const fixed = RepairJsonStream.repairJson('{"broken": true')
  console.log(fixed) // '{"broken": true}'
</script>

📖 API Reference

repairJson(json: string): string

Repairs broken JSON and returns valid JSON. Never throws.

repairJson('{"a": tru')     // '{"a": true}'
repairJson("{'b': None}")   // '{"b": null}'
repairJson('[1, 2,')        // '[1, 2]'

repairJson(input, onRepair?)

Track what was fixed using the callback:

repairJson('{"a": 1,', (action, idx, context) => {
  console.log(`Action: ${action} at index ${idx} (${context})`)
})
// Output: Action: closed_object at index 7 (Closing missing })

preprocessJson(input: string): string

Strips wrappers (fenced blocks, JSONP, escaped strings) before repair.

preprocessJson('```json\n{"a": 1}\n```')  // '{"a": 1}'
preprocessJson('callback({"a": 1})')      // '{"a": 1}'

jsonrepairTransform(options?): Transform

Node.js Transform stream for piping.

import { jsonrepairTransform } from 'repair-json-stream/stream'

Web Streams API

Universal TransformStream for Deno, Bun, Cloudflare Workers, and modern browsers.

import { jsonRepairStream } from 'repair-json-stream/web-stream'

// Use with fetch
const response = await fetch('/api/llm')
const repairedStream = response.body
  .pipeThrough(new TextDecoderStream())
  .pipeThrough(jsonRepairStream())

// Read repaired JSON
const reader = repairedStream.getReader()
const { value } = await reader.read()
JSON.parse(value) // Always valid!

Incremental Stateful Repair

True streaming repair with immediate partial output. Perfect for live UI updates.

import { IncrementalJsonRepair } from 'repair-json-stream/incremental'

const repairer = new IncrementalJsonRepair()

// As LLM streams chunks...
let output = ''
for await (const chunk of llmStream) {
  output += repairer.push(chunk)
  updateUI(output) // Live update!
}
output += repairer.end()

JSON.parse(output) // Always valid!

Methods:

• .push(chunk) - Process chunk, returns repaired output
• .end() - Finalize and close open structures
• .snapshot() - Get valid JSON at any point (non-destructive)
• .reset() - Reset state for reuse

LLM Garbage Filtering

Extract JSON from messy LLM outputs containing prose, thinking blocks, or markdown.

import { extractJson, extractAllJson, stripLlmWrapper } from 'repair-json-stream/extract'

// Extract JSON from prose
extractJson('Sure! Here is the data: {"name": "John"} Hope this helps!')
// → '{"name": "John"}'

// Handle thinking blocks (DeepSeek, Claude, etc.)
extractJson('<thought>Let me think...</thought>\n{"result": true}')
// → '{"result": true}'

// Extract multiple JSON blocks
extractAllJson('First: {"a": 1} Second: {"b": 2}')
// → ['{"a": 1}', '{"b": 2}']

// Full cleanup (markdown, prose, thinking blocks)
stripLlmWrapper(`
<thinking>reasoning here</thinking>
\`\`\`json
{"data": [1, 2, 3]}
\`\`\`
Let me know if you need anything else!
`)
// → '{"data": [1, 2, 3]}'

🏗️ Architecture

• O(n) single-pass - No multiple iterations
• Stack-based state machine - No regex parsing
• Zero dependencies - Minimal attack surface
• ReDoS-safe - No exponential backtracking
• 5.5 KB minified - Lightweight for browsers

📄 License

MIT © sonka