Castle

Castle is a fraud and account-abuse prevention platform that stops bots, credential stuffing, account takeover, and multi-accounting through behavioral analysis and device fingerprinting — without CAPTCHAs or puzzles for legitimate users. Developers integrate a client-side SDK (browser and mobile) that generates a short-lived request token, then call Castle's backend Risk API and Filter API to score authentication and transaction events in real time. Castle returns machine-learning risk scores and signals (bot, proxy/VPN, residential proxy, impossible travel, device reputation) that drive a policy rules engine, Lists, and webhooks. It serves security and trust-and-safety teams across gaming, fintech, marketplaces, and SaaS.

Castle publishes 1 API on the APIs.io network. Tagged areas include Company, Security, Fraud Prevention, Bot Detection, and Device Fingerprinting.

The Castle catalog on APIs.io includes 1 event-driven AsyncAPI specification.

Castle’s developer surface includes documentation, API reference, getting-started guide, support, engineering blog, pricing, signup flow, and 25 more developer resources.

🌐 Visit website 📡 Source on GitHub
43.2/100 thin Agent 39/100 agent aware Full breakdown ↓
scored 2026-07-20 · rubric v0.4
1 APIs 1 MCP Servers 0 Features
CompanySecurityFraud PreventionBot DetectionDevice FingerprintingAccount TakeoverRisk ScoringIdentity

API Rating

API Evangelist API Evangelist Rating How this is scored →
scored 2026-07-20 · rubric v0.4
Composite quality — 43.2/100 · thin
Contract Quality 0.0 / 25
Developer Ergonomics 16.1 / 20
Commercial Clarity 12.1 / 20
Operational Transparency 5.8 / 13
Governance 0.0 / 12
Discoverability 9.3 / 10
Agent readiness — 39/100 · agent aware
Machine-Readable Contract 0 / 18
Agentic Access Contract 0 / 15
MCP Server 12 / 12
Machine-Readable Auth 10 / 10
Idempotency 0 / 9
Stable Error Semantics 8 / 8
Request/Response Examples 0 / 7
Rate-Limit Signaling 0 / 7
Typed Event Surface 6 / 6
Agent Skills 5 / 5
Well-Known Catalog 0 / 4
Consent & Bot Identity 0 / 3
Improve this rating by publishing the missing artifacts — every area above can be raised, and the full rubric is at apis.io/rating/. This rating is computed from github.com/api-evangelist/castle: open an issue to ask a question, or submit a pull request to add artifacts. Want it done for you? Prioritized profiling — $2,500 →

APIs

Castle API

Castle's REST API for real-time fraud and abuse detection. The Risk API scores authenticated user events (login, transaction, profile update), the Filter API scores anonymous/pr...

MCP Servers

castle-mcp.yml

MCP SERVER

Event Specifications

Castle Webhooks

ASYNCAPI

Resources

Get Started 5

Portal, sign-up, and the first successful call

DeveloperPortal
DeveloperPortal
 GettingStarted
GettingStarted
 SignUp
SignUp
 Login
Login
 Sandbox
Sandbox

Documentation 2

Reference material describing how the API behaves

Documentation
Documentation
 APIReference
APIReference

Agent Surfaces 3

MCP servers, agent skills, and machine-readable catalogs

LLMsTxt
LLMsTxt
 MCPServer
MCPServer
 AgentSkill
AgentSkill

Design & Contract 7

Pagination, idempotency, versioning, errors, and events

Conventions
Conventions
 Lifecycle
Lifecycle
 Conformance
Conformance
 ErrorCatalog
ErrorCatalog
 DataModel
DataModel
 Components
Components
 Webhooks
Webhooks

Scroll for all 7

Build 3

SDKs, sample code, and the tooling you integrate with

GitHubOrganization
GitHubOrganization
 Packages
Packages
 SDKs
SDKs

Access & Security 4

Authentication, authorization, and security posture

TrustCenter
TrustCenter
 DomainSecurity
DomainSecurity
 Compliance
Compliance
 Authentication
Authentication

Operate 3

Status, limits, changes, and where to get help

Support
Support
 StatusPage
StatusPage
 ChangeLog
ChangeLog

Commercial 3

Pricing, plans, and the legal terms of use

Pricing
Pricing
 TermsOfService
TermsOfService
 PrivacyPolicy
PrivacyPolicy

Company 2

The organization behind the API

Website
Website
 Blog
Blog

Source (apis.yml)

apis.yml Raw ↑ 
aid: castle
name: Castle
description: Castle is a fraud and account-abuse prevention platform that stops bots, credential stuffing, account takeover,
  and multi-accounting through behavioral analysis and device fingerprinting — without CAPTCHAs or puzzles for legitimate
  users. Developers integrate a client-side SDK (browser and mobile) that generates a short-lived request token, then call
  Castle's backend Risk API and Filter API to score authentication and transaction events in real time. Castle returns machine-learning
  risk scores and signals (bot, proxy/VPN, residential proxy, impossible travel, device reputation) that drive a policy rules
  engine, Lists, and webhooks. It serves security and trust-and-safety teams across gaming, fintech, marketplaces, and SaaS.
url: https://raw.githubusercontent.com/api-evangelist/castle/refs/heads/main/apis.yml
x-type: company
x-source: vc-portfolio
x-backed-by:
- index-ventures
x-tier: stub
x-tier-reason: portfolio-lead
specificationVersion: '0.20'
created: '2026-07-17'
modified: '2026-07-18'
image: https://castle.io/og.png
tags:
- Company
- Security
- Fraud Prevention
- Bot Detection
- Device Fingerprinting
- Account Takeover
- Risk Scoring
- Identity
maintainers:
- FN: Kin Lane
  email: kin@apievangelist.com
- FN: APIs.json
  email: info@apis.io
apis:
- aid: castle:castle-api
  name: Castle API
  description: Castle's REST API for real-time fraud and abuse detection. The Risk API scores authenticated user events (login,
    transaction, profile update), the Filter API scores anonymous/pre-authentication events (registration, password reset),
    the Log API records events without scoring, and the enterprise Events API queries and aggregates historical events. Device
    and Lists endpoints manage device reputation and allow/deny lists.
  humanURL: https://reference.castle.io
  baseURL: https://api.castle.io
  tags:
  - Fraud Prevention
  - Bot Detection
  - Risk Scoring
  properties:
  - type: Documentation
    url: https://docs.castle.io/
  - type: APIReference
    url: https://reference.castle.io
  - type: GettingStarted
    url: https://docs.castle.io/docs/integration-guide
  - type: Authentication
    url: authentication/castle-authentication.yml
  - type: Webhooks
    url: asyncapi/castle-webhooks.yml
common:
- type: TrustCenter
  url: security/castle-trust-center.yml
- type: DomainSecurity
  url: security/castle-domain-security.yml
- type: Website
  url: https://castle.io
- type: DeveloperPortal
  url: https://docs.castle.io/
- type: Documentation
  url: https://docs.castle.io/
- type: APIReference
  url: https://reference.castle.io
- type: GettingStarted
  url: https://docs.castle.io/docs/integration-guide
- type: Support
  url: https://castle.io/contact
- type: Blog
  url: https://blog.castle.io/
- type: GitHubOrganization
  url: https://github.com/castle
- type: Pricing
  url: https://castle.io/pricing
- type: SignUp
  url: https://dashboard.castle.io/signup/new
- type: Login
  url: https://dashboard.castle.io/login
- type: TermsOfService
  url: https://blog.castle.io/terms
- type: PrivacyPolicy
  url: https://blog.castle.io/privacy
- type: StatusPage
  url: https://status.castle.io
- type: Compliance
  url: security/castle-trust-center.yml
- type: Packages
  url: packages/castle-packages.yml
- type: SDKs
  url: packages/castle-packages.yml
- type: LLMsTxt
  url: llms/castle-llms.txt
- type: Authentication
  url: authentication/castle-authentication.yml
- type: Conventions
  url: conventions/castle-conventions.yml
- type: Sandbox
  url: sandbox/castle-sandbox.yml
- type: Lifecycle
  url: lifecycle/castle-lifecycle.yml
- type: ChangeLog
  url: changelog/castle-changelog.yml
- type: Conformance
  url: conformance/castle-conformance.yml
- type: ErrorCatalog
  url: errors/castle-problem-types.yml
- type: DataModel
  url: data-model/castle-data-model.yml
- type: Components
  url: components/castle-components.yml
- type: MCPServer
  url: mcp/castle-mcp.yml
- type: Webhooks
  url: asyncapi/castle-webhooks.yml
- type: AgentSkill
  url: skills/_index.yml
x-enrichment:
  date: '2026-07-19'
  status: backfilled
  pass: local-v1
  note: backfilled from .gitignore signal + verified work evidence