@atproto/lexicon

Overview

The @atproto/lexicon package provides TypeScript implementation of the Lexicon schema language for AT Protocol. Lexicon is used to define and validate data schemas for records, XRPC endpoints, and other protocol elements.

Installation

npm install @atproto/lexicon

Core Concepts

Lexicon schemas define:

Record types: Structure of user data (posts, profiles, likes, etc.)
XRPC procedures: Request/response schemas for API methods
XRPC queries: Parameter and response schemas for API queries
Data types: Primitives, objects, arrays, unions, and references

Schemas use familiar concepts like types, required/optional fields, string formats, and validation constraints.

Main Classes

Lexicons

The main class for managing and validating against Lexicon schemas.

import { Lexicons } from '@atproto/lexicon'

// Create a lexicon collection
const lexicons = new Lexicons()

// Add schema definitions
lexicons.add({
  lexicon: 1,
  id: 'app.bsky.feed.post',
  defs: {
    main: {
      type: 'record',
      key: 'tid',
      record: {
        type: 'object',
        required: ['text', 'createdAt'],
        properties: {
          text: { type: 'string', maxLength: 300 },
          createdAt: { type: 'string', format: 'datetime' },
          reply: { type: 'ref', ref: '#replyRef' }
        }
      }
    },
    replyRef: {
      type: 'object',
      required: ['root', 'parent'],
      properties: {
        root: { type: 'ref', ref: 'com.atproto.repo.strongRef' },
        parent: { type: 'ref', ref: 'com.atproto.repo.strongRef' }
      }
    }
  }
})

// Validate a record
try {
  lexicons.assertValidRecord('app.bsky.feed.post', {
    $type: 'app.bsky.feed.post',
    text: 'Hello, world!',
    createdAt: new Date().toISOString()
  })
  console.log('Record is valid')
} catch (err) {
  console.error('Validation failed:', err.message)
}

constructor

Lexicons

constructor(docs?: Iterable<LexiconDoc>)

Creates a new Lexicon collection. Optionally initializes with an iterable of schema documents.

docs

Iterable<LexiconDoc>

Initial schema documents to add

Methods:

add

void

add(doc: LexiconDoc): void

Adds a Lexicon schema document to the collection.

doc

LexiconDoc

required

Schema document to add

remove

void

remove(uri: string): void

Removes a schema document by its NSID.

uri

string

required

Schema NSID to remove

get

LexiconDoc | undefined

get(uri: string): LexiconDoc | undefined

Retrieves a schema document by NSID.

getDef

LexUserType | undefined

getDef(uri: string): LexUserType | undefined

Retrieves a specific definition from a schema. Can reference nested definitions using # notation.

const def = lexicons.getDef('app.bsky.feed.post#replyRef')

Record Validation

assertValidRecord

void

assertValidRecord(nsid: string, value: unknown): void

Validates that a value conforms to a record schema. Throws ValidationError on failure.

nsid

string

required

Record type NSID

value

unknown

required

Record data to validate

lexicons.assertValidRecord('app.bsky.feed.post', {
  $type: 'app.bsky.feed.post',
  text: 'Hello',
  createdAt: new Date().toISOString()
})

validateRecord

ValidationResult

validateRecord(nsid: string, value: unknown): ValidationResult

Validates a record and returns a result object instead of throwing.

const result = lexicons.validateRecord('app.bsky.feed.post', record)
if (!result.success) {
  console.error('Validation errors:', result.error)
}

XRPC Validation

assertValidXrpcParams

void

assertValidXrpcParams(nsid: string, params: unknown): void

Validates XRPC query parameters.

lexicons.assertValidXrpcParams('app.bsky.feed.getTimeline', {
  limit: 50,
  cursor: 'abc123'
})

assertValidXrpcInput

void

assertValidXrpcInput(nsid: string, value: unknown): void

Validates XRPC procedure input body.

lexicons.assertValidXrpcInput('com.atproto.repo.createRecord', {
  repo: 'did:plc:...',
  collection: 'app.bsky.feed.post',
  record: { text: 'Hello', createdAt: new Date().toISOString() }
})

assertValidXrpcOutput

void

assertValidXrpcOutput(nsid: string, value: unknown): void

Validates XRPC response body.

assertValidXrpcMessage

void

assertValidXrpcMessage<T = unknown>(nsid: string, value: unknown): void

Validates XRPC subscription message.

Schema Types

Primitive Types

Lexicon supports several primitive types:

// Boolean
{ type: 'boolean', default: false }

// Integer
{ 
  type: 'integer',
  minimum: 1,
  maximum: 100,
  default: 50
}

// String
{
  type: 'string',
  minLength: 1,
  maxLength: 300,
  format: 'datetime' // or 'uri', 'at-uri', 'did', 'handle', etc.
}

// Unknown (any JSON value)
{ type: 'unknown' }

String Formats:

datetime - ISO 8601 datetime
uri - URI
at-uri - AT URI (at://…)
did - DID identifier
handle - Handle (domain name)
at-identifier - DID or handle
nsid - Namespaced identifier
cid - Content identifier
language - BCP-47 language tag
tid - Timestamp identifier
record-key - Record key

Complex Types

Object

{
  type: 'object',
  required: ['name', 'age'],
  properties: {
    name: { type: 'string', maxLength: 100 },
    age: { type: 'integer', minimum: 0 },
    email: { type: 'string', format: 'email' }
  }
}

Array

{
  type: 'array',
  items: { type: 'string' },
  minLength: 1,
  maxLength: 10
}

Reference

// Reference to another definition
{ type: 'ref', ref: 'com.atproto.repo.strongRef' }

// Reference to definition in same document
{ type: 'ref', ref: '#replyRef' }

Union

{
  type: 'union',
  refs: [
    'app.bsky.embed.images',
    'app.bsky.embed.external',
    'app.bsky.embed.record'
  ]
}

IPLD Types

Bytes

{
  type: 'bytes',
  maxLength: 1000000 // 1MB
}

CID Link

{
  type: 'cid-link'
}

Blob

{
  type: 'blob',
  accept: ['image/png', 'image/jpeg'],
  maxSize: 1000000 // bytes
}

Blob Utilities

BlobRef

Represents a blob reference in records.

import { BlobRef } from '@atproto/lexicon'

const blob = BlobRef.make(
  'bafyreiabc123...',
  'image/jpeg',
  50000
)

console.log(blob.ref) // CID object
console.log(blob.mimeType) // 'image/jpeg'
console.log(blob.size) // 50000

// Check if value is a blob ref
if (BlobRef.isBlob(value)) {
  console.log('This is a blob reference')
}

BlobRef.make

BlobRef

static make(
  cid: string | CID,
  mimeType: string,
  size: number
): BlobRef

Creates a blob reference.

cid

string | CID

required

Content identifier

mimeType

string

required

MIME type (e.g., ‘image/jpeg’)

size

number

required

Size in bytes

BlobRef.isBlob

boolean

static isBlob(obj: unknown): obj is BlobRef

Type guard to check if a value is a blob reference.

Type Definitions

LexiconDoc

type LexiconDoc = {
  lexicon: 1
  id: string  // NSID
  revision?: number
  description?: string
  defs: Record<string, LexUserType>
}

LexRecord

Type definition for a record schema.

type LexRecord = {
  type: 'record'
  key?: 'tid' | 'literal' | string
  record: LexObject
}

LexXrpcProcedure

type LexXrpcProcedure = {
  type: 'procedure'
  description?: string
  parameters?: LexXrpcParameters
  input?: LexXrpcBody
  output?: LexXrpcBody
  errors?: LexXrpcError[]
}

LexXrpcQuery

type LexXrpcQuery = {
  type: 'query'
  description?: string
  parameters?: LexXrpcParameters
  output?: LexXrpcBody
  errors?: LexXrpcError[]
}

Serialization Utilities

lexToIpld

Converts a value to IPLD format (for CBOR encoding).

import { lexToIpld } from '@atproto/lexicon'

const record = {
  text: 'Hello',
  createdAt: new Date().toISOString(),
  blob: BlobRef.make('bafyreiabc', 'image/jpeg', 1000)
}

const ipldValue = lexToIpld(record)
// Converts BlobRefs and other special types to IPLD representation

ipldToLex

Converts an IPLD value back to Lexicon format.

import { ipldToLex } from '@atproto/lexicon'

const record = ipldToLex(ipldValue)
// Converts IPLD blob refs back to BlobRef instances

Complete Example

import { Lexicons, BlobRef } from '@atproto/lexicon'
import { CID } from 'multiformats/cid'

// Define schemas
const lexicons = new Lexicons()

lexicons.add({
  lexicon: 1,
  id: 'com.example.post',
  defs: {
    main: {
      type: 'record',
      key: 'tid',
      record: {
        type: 'object',
        required: ['text', 'createdAt'],
        properties: {
          text: {
            type: 'string',
            maxLength: 300,
            minLength: 1
          },
          createdAt: {
            type: 'string',
            format: 'datetime'
          },
          images: {
            type: 'array',
            items: { type: 'blob' },
            maxLength: 4
          },
          tags: {
            type: 'array',
            items: { type: 'string', maxLength: 50 },
            maxLength: 10
          },
          lang: {
            type: 'string',
            format: 'language'
          }
        }
      }
    }
  }
})

lexicons.add({
  lexicon: 1,
  id: 'com.example.getPost',
  defs: {
    main: {
      type: 'query',
      parameters: {
        type: 'params',
        required: ['uri'],
        properties: {
          uri: { type: 'string', format: 'at-uri' },
          depth: { type: 'integer', minimum: 0, maximum: 10, default: 1 }
        }
      },
      output: {
        encoding: 'application/json',
        schema: {
          type: 'object',
          required: ['post'],
          properties: {
            post: { type: 'ref', ref: 'com.example.post' }
          }
        }
      }
    }
  }
})

// Validate a record
const post = {
  $type: 'com.example.post',
  text: 'Check out this photo!',
  createdAt: new Date().toISOString(),
  images: [
    BlobRef.make(
      CID.parse('bafyreia...'),
      'image/jpeg',
      50000
    )
  ],
  tags: ['photo', 'nature'],
  lang: 'en'
}

try {
  lexicons.assertValidRecord('com.example.post', post)
  console.log('✓ Record is valid')
} catch (err) {
  console.error('✗ Validation failed:', err.message)
}

// Validate XRPC parameters
const params = {
  uri: 'at://did:plc:abc123/com.example.post/xyz789',
  depth: 2
}

try {
  lexicons.assertValidXrpcParams('com.example.getPost', params)
  console.log('✓ Parameters are valid')
} catch (err) {
  console.error('✗ Parameter validation failed:', err.message)
}

// Validate output
const output = {
  post: post
}

try {
  lexicons.assertValidXrpcOutput('com.example.getPost', output)
  console.log('✓ Output is valid')
} catch (err) {
  console.error('✗ Output validation failed:', err.message)
}

// Get schema definition
const postDef = lexicons.getDef('com.example.post')
if (postDef) {
  console.log('Post schema:', postDef)
}

// Clone lexicons
const clone = new Lexicons(lexicons)
console.log('Cloned lexicons with', Array.from(clone).length, 'schemas')

@atproto/syntax - Validation for identifiers used in schemas
@atproto/repo - Repository operations that validate against Lexicon schemas
@atproto/api - API client that uses Lexicon for type safety
@atproto/common-web - Common utilities

Client Libraries

Core Packages

OAuth

Server

Overview

Installation

Core Concepts

Main Classes

Lexicons

Record Validation

XRPC Validation

Schema Types

Primitive Types

Complex Types

Object

Array

Reference

Union

IPLD Types

Bytes

CID Link

Blob

Blob Utilities

BlobRef

Type Definitions

LexiconDoc

LexRecord

LexXrpcProcedure

LexXrpcQuery

Serialization Utilities

lexToIpld

ipldToLex

Complete Example

Additional Resources

Build docs developers (and LLMs) love

Client Libraries

Core Packages

OAuth

Server

Documentation Index

​Overview

​Installation

​Core Concepts

​Main Classes

​Lexicons

​Record Validation

​XRPC Validation

​Schema Types

​Primitive Types

​Complex Types

​Object

​Array

​Reference

​Union

​IPLD Types

​Bytes

​CID Link

​Blob

​Blob Utilities

​BlobRef

​Type Definitions

​LexiconDoc

​LexRecord

​LexXrpcProcedure

​LexXrpcQuery

​Serialization Utilities

​lexToIpld

​ipldToLex

​Complete Example

​Related Packages

​Additional Resources

Build docs developers (and LLMs) love

Overview

Installation

Core Concepts

Main Classes

Lexicons

Record Validation

XRPC Validation

Schema Types

Primitive Types

Complex Types

Object

Array

Reference

Union

IPLD Types

Bytes

CID Link

Blob

Blob Utilities

BlobRef

Type Definitions

LexiconDoc

LexRecord

LexXrpcProcedure

LexXrpcQuery

Serialization Utilities

lexToIpld

ipldToLex

Complete Example

Related Packages

Additional Resources