Stax

Stax (formerly Fattmerchant) is an Orlando, Florida payments technology company that sells subscription-priced payment processing to small and large merchants (Stax Pay), embedded payments and merchant onboarding to SaaS platforms and ISVs (Stax Connect), compliant credit-card surcharging (CardX by Stax), automated subscription billing (Stax Bill), and full-stack processing for partners and ISOs (Stax Processing). Its public REST API at apiprod.fattlabs.com covers charges, credits, refunds and voids, stored payment methods and tokenization, customers, invoices and invoice schedules, payment links, catalog items, terminals and card-present devices, merchant enrollment and underwriting, disputes, statements and deposit reporting, teams, users and API keys, and merchant- and partner-level webhooks. Stax.js handles browser tokenization; iOS and Android SDKs cover card-present and Tap to Pay. Stax Payments, Inc. is a registered ISO/MSP of Fifth Third Bank and Synovus Bank and a registered partner/ISO of Elavon, Inc.

Stax publishes 1 API on the APIs.io network. Tagged areas include Payments, Payment Processing, Merchant Services, Embedded Payments, and ACH.

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

Stax’s developer surface includes documentation, API reference, getting-started guide, support, engineering blog, pricing, signup flow, and 37 more developer resources.

🌐 Visit website 📡 Source on GitHub

74.8/100 exemplar ▬ flat Agent 51/100 agent native Full breakdown ↓
scored 2026-09-01 · rubric v0.17.2

1 APIs 1 MCP Servers

Payments Payment Processing Merchant Services Embedded Payments ACH Invoicing Subscription BillingSurchargingPoint-of-Sale Tokenization Financial-Services Fintech Card Present Merchant Onboarding

Kin Score

Kin Score How this is scored →
scored 2026-09-01 · rubric v0.17.2

Composite quality — 74.8/100 · exemplar

Contract Quality 12.3 / 21

Developer Ergonomics 13.4 / 17

Access Clarity 15.7 / 17

Operational Transparency 9.3 / 11

Contract Governance 1.9 / 10

Discoverability 6.5 / 9

Regulatory Posture 10.3 / 15

Agent readiness — 51/100 · agent native

Machine-Readable Contract 18 / 18

Agentic Access Contract 10 / 10

Documented Reversibility 0 / 6

MCP Server 12 / 12

Machine-Readable Auth 10 / 10

Idempotency 9 / 9

Stable Error Semantics 8 / 8

Request/Response Examples 7 / 7

Rate-Limit Signaling 7 / 7

Typed Event Surface 6 / 6

Agent Skills 5 / 5

Well-Known Catalog 0 / 4

Consent & Bot Identity 0 / 3

A2A Agent Card 0 / 8

Dry-Run / Simulate Mode 0 / 4

Delegated User Identity 0 / 6

Protected Resource Metadata 0 / 5

Registration Without a Human 0 / 6

Agentic Commerce Surface 0 / 5

Regulatory Posture applies to this provider. Its tags matched the Payments regime, so Regulatory Posture carries 15 points of the composite. If this regime is wrong for your business, say so on your provider repo — the applicability map is public and we will correct it.

The six quality facets above are damped to 85 points between them, because the conditional facet above carries the other 15. That is why each facet's contribution is shown against a damped maximum: raising a quality facet moves the composite by 85% of its nominal weight, not 100%. The full arithmetic is at apis.io/rating/.

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/stax: 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 1

Individual APIs this provider publishes, each with its own machine-readable definition.

Stax API

The Stax REST API (published as "stax-api") exposes 133 operations across 101 paths at https://apiprod.fattlabs.com — the Merchant API for payments, payment methods, customers, ...

MCP Servers 1

Model Context Protocol servers that expose these APIs to AI agents.

Rate Limits 1

Documented rate limits and quota policies.

AGENTIC

Resources

Get Started 5

Portal, sign-up, and the first successful call

DeveloperPortal

DeveloperPortal

GettingStarted

GettingStarted

SignUp

SignUp

Login

Sandbox

Sandbox

Documentation 3

Reference material describing how the API behaves

Documentation

Documentation

APIReference

APIReference

OpenAPI

OpenAPI

Agent Surfaces 4

MCP servers, agent skills, and machine-readable catalogs

AgenticAccess

MCPServer

LLMsTxt

AgentSkill

Design & Contract 8

Pagination, idempotency, versioning, errors, and events

Lifecycle

Conventions

Idempotency

ErrorCatalog

Webhooks

Components

DataModel

Conformance

Scroll for all 8

Build 6

SDKs, sample code, and the tooling you integrate with

GitHubOrganization

DeclineCodes

ToolCrosswalk

Packages

SDKs

Examples

Access & Security 7

Authentication, authorization, and security posture

Authentication

Compliance

TrustCenter

SecurityTxt

VulnerabilityDisclosure

VulnerabilityDisclosure

Security

Security

DomainSecurity

DomainSecurity

Scroll for all 7

Operate 4

Status, limits, changes, and where to get help

Support

StatusPage

ChangeLog

RateLimits

Commercial 4

Pricing, plans, and the legal terms of use

Pricing

TermsOfService

PrivacyPolicy

Plans

Company 2

The organization behind the API

Website

Website

Blog

Blog

Other 1

Properties that don't map to a standard resource type

Overlay

Overlay

Source (apis.yml)

aid: stax
name: Stax
description: Stax (formerly Fattmerchant) is an Orlando, Florida payments technology company that sells subscription-priced
  payment processing to small and large merchants (Stax Pay), embedded payments and merchant onboarding to SaaS platforms
  and ISVs (Stax Connect), compliant credit-card surcharging (CardX by Stax), automated subscription billing (Stax Bill),
  and full-stack processing for partners and ISOs (Stax Processing). Its public REST API at apiprod.fattlabs.com covers charges,
  credits, refunds and voids, stored payment methods and tokenization, customers, invoices and invoice schedules, payment
  links, catalog items, terminals and card-present devices, merchant enrollment and underwriting, disputes, statements and
  deposit reporting, teams, users and API keys, and merchant- and partner-level webhooks. Stax.js handles browser tokenization;
  iOS and Android SDKs cover card-present and Tap to Pay. Stax Payments, Inc. is a registered ISO/MSP of Fifth Third Bank
  and Synovus Bank and a registered partner/ISO of Elavon, Inc.
url: https://raw.githubusercontent.com/api-evangelist/stax/refs/heads/main/apis.yml
x-type: company
x-source: harvest:secondary-market
x-tier: profiled
x-tier-reason: enrichment
specificationVersion: '0.20'
created: '2026-08-29'
modified: '2026-08-29'
tags:
- Payments
- Payment Processing
- Merchant Services
- Embedded Payments
- ACH
- Invoicing
- Subscription Billing
- Surcharging
- Point-of-Sale
- Tokenization
- Financial-Services
- Fintech
- Card Present
- Merchant Onboarding
tags_raw:
- Payments
- Payment Processing
- Merchant Services
- Embedded Payments
- ACH
- Invoicing
- Subscription Billing
- Surcharging
- Point of Sale
- Tokenization
- Financial Services
- Fintech
- Card Present
- Merchant Onboarding
apis:
- name: Stax API
  description: The Stax REST API (published as "stax-api") exposes 133 operations across 101 paths at https://apiprod.fattlabs.com
    — the Merchant API for payments, payment methods, customers, invoices, payment links, catalog, terminals, transactions
    and webhooks, and the Partner API for merchant creation, enrollment, registration data, API-key issuance, disputes and
    partner-brand webhooks. Authentication is a bearer API key; sandbox and live keys use the same base URL.
  humanURL: https://docs.staxpayments.com/reference/overview
  baseURL: https://apiprod.fattlabs.com
  image: https://staxpayments.com/wp-content/uploads/2024/04/stax-logo-full.png
  tags:
  - Payments
  - Invoicing
  - Merchant Onboarding
  - Webhook
  - Terminals
  tags_raw:
  - Payments
  - Invoicing
  - Merchant Onboarding
  - Webhooks
  - Terminals
  properties:
  - type: OpenAPI
    url: openapi/stax-api-openapi.json
  - type: Overlay
    url: overlays/stax-api-overlay.yaml
  - type: Documentation
    url: https://docs.staxpayments.com/
  - type: APIReference
    url: https://docs.staxpayments.com/reference/overview
  - type: Examples
    url: examples/stax-api-examples.yml
  - type: DataModel
    url: data-model/stax-data-model.yml
  - type: ErrorCatalog
    url: errors/stax-problem-types.yml
  - type: Webhooks
    url: asyncapi/stax-webhooks.yml
  - type: AgenticAccess
    url: agentic-access/stax-agentic-access.yml
maintainers:
- FN: Kin Lane
  email: kin@apievangelist.com
- FN: APIs.json
  email: info@apis.io
common:
- type: AgenticAccess
  url: agentic-access/stax-agentic-access.yml
- type: Website
  url: https://staxpayments.com/
- type: DeveloperPortal
  url: https://docs.staxpayments.com/
- type: Documentation
  url: https://docs.staxpayments.com/
- type: APIReference
  url: https://docs.staxpayments.com/reference/overview
- type: GettingStarted
  url: https://docs.staxpayments.com/docs/getting-started
- type: Support
  url: https://support.fattmerchant.com/
- type: Blog
  url: https://staxpayments.com/blog/
- type: GitHubOrganization
  url: https://github.com/fattmerchantorg
- type: Pricing
  url: https://staxpayments.com/pricing/
- type: SignUp
  url: https://staxpayments.com/request-sandbox/
- type: Login
  url: https://app.staxpayments.com/
- type: TermsOfService
  url: https://staxpayments.com/terms-of-use/
- type: PrivacyPolicy
  url: https://staxpayments.com/privacy/
- type: StatusPage
  url: https://status.staxpayments.com/
- type: ChangeLog
  url: changelog/stax-changelog.yml
- type: Lifecycle
  url: lifecycle/stax-lifecycle.yml
- type: Authentication
  url: authentication/stax-authentication.yml
- type: Conventions
  url: conventions/stax-conventions.yml
- type: Idempotency
  url: conventions/stax-conventions.yml
- type: RateLimits
  url: rate-limits/stax-rate-limits.yml
- type: Plans
  url: plans/stax-plans-pricing.yml
- type: ErrorCatalog
  url: errors/stax-problem-types.yml
- type: DeclineCodes
  url: errors/stax-decline-codes.yml
- type: Webhooks
  url: asyncapi/stax-webhooks.yml
- type: MCPServer
  url: mcp/stax-mcp.yml
- type: ToolCrosswalk
  url: mcp/stax-tool-crosswalk.yml
- type: LLMsTxt
  url: llms/stax-llms.txt
- type: Packages
  url: packages/stax-packages.yml
- type: SDKs
  url: packages/stax-packages.yml
- type: Components
  url: components/stax-components.yml
- type: Sandbox
  url: sandbox/stax-sandbox.yml
- type: DataModel
  url: data-model/stax-data-model.yml
- type: Conformance
  url: conformance/stax-conformance.yml
- type: Compliance
  url: conformance/stax-conformance.yml
- type: TrustCenter
  url: security/stax-trust-center.yml
- type: SecurityTxt
  url: well-known/stax-security.txt
- type: VulnerabilityDisclosure
  url: security/stax-vulnerability-disclosure.yml
- type: Security
  url: security/stax-vulnerability-disclosure.yml
- type: DomainSecurity
  url: security/stax-domain-security.yml
- type: AgentSkill
  url: skills/_index.yml
- type: Examples
  url: examples/stax-api-examples.yml
- type: Overlay
  url: overlays/stax-api-overlay.yaml
- type: OpenAPI
  url: openapi/stax-api-openapi.json
image: https://staxpayments.com/wp-content/uploads/2024/04/stax-logo-full.png
x-enrichment:
  date: '2026-08-29'
  status: enriched
  artifacts_added: 33
  pass: local-v1

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Stax

Kin Score

APIs 1

Stax API

MCP Servers 1

Stax Docs MCP Server

Pricing Plans 1

Stax Plans Pricing

Rate Limits 1

Stax Rate Limits

Event Specifications 1

Stax Webhooks

Security Posture 4

Stax Authentication

Stax Domain Security

Stax Vulnerability Disclosure

Stax Trust Center

Agentic Access 1

Stax Agentic Access