> ## Documentation Index
> Fetch the complete documentation index at: https://docs.stackone.com/llms.txt
> Use this file to discover all available pages before exploring further.

# Legacy Unified APIs Overview

> Navigate the Legacy Unified APIs for HRIS, ATS, CRM, LMS, and Accounting integrations

<Warning>
  The Legacy Unified APIs are no longer in active development. While existing functionality remains supported, no new features or enhancements will be added.
</Warning>

## What is a Unified API?

A **unified API** provides a single, standardized interface to interact with multiple third-party providers. Instead of learning each provider's unique API format, authentication scheme, and data model, you write code once, and StackOne handles the differences.

**What StackOne normalizes:**

* **Data models** – Consistent field names and structures across providers (e.g., `files` in Google Drive, SharePoint, and Dropbox all return the same schema; `users` in Google Workspace, Okta, and 1Password all return the same schema)
* **Authentication** – OAuth, API keys, and tokens are managed per-provider; you just pass an `x-account-id` header
* **Pagination** – Cursor-based pagination works the same regardless of whether the provider uses offsets, pages, or cursors
* **Error formats** – Standardized error responses with provider-specific details when available

<Tip>
  **Building AI agents?** Use the [MCP Server](/mcp/quickstart), an [AI Toolset SDK](/agents/typescript/introduction), or the [Actions API (RPC)](/platform/api-reference/actions/make-an-rpc-call-to-an-action) instead — same data, optimized for LLM tool calling.
</Tip>

## Legacy Unified APIs

The Legacy Unified APIs provide standardized access to various business system categories. Each API follows the same patterns for authentication, pagination, and error handling, but exposes domain-specific data models.

<CardGroup cols={2}>
  <Card title="HRIS" icon="users" href="/hris/introduction">
    Human Resources Information Systems - manage employees, time-off, benefits, and organizational data
  </Card>

  <Card title="ATS" icon="user-plus" href="/ats/introduction">
    Applicant Tracking Systems - manage candidates, jobs, applications, and hiring workflows
  </Card>

  <Card title="CRM" icon="address-book" href="/crm/introduction">
    Customer Relationship Management - manage contacts, accounts, and sales data
  </Card>

  <Card title="LMS" icon="graduation-cap" href="/lms/introduction">
    Learning Management Systems - manage courses, content, assignments, and completions
  </Card>

  <Card title="Accounting" icon="calculator" href="/accounting/introduction">
    Accounting Systems - manage accounts, transactions, and financial data
  </Card>
</CardGroup>

## Common Features Across All APIs

All Legacy Unified APIs share these common features:

| Feature            | Description                                                                                                              |
| ------------------ | ------------------------------------------------------------------------------------------------------------------------ |
| **Authentication** | All APIs use the same [authentication method](/api/authentication) with API keys and `x-account-id` headers              |
| **Pagination**     | Consistent [cursor-based pagination](/guides/pagination) across all endpoints                                            |
| **Synthetic IDs**  | [StackOne Identifiers](/guides/stackone-identifiers) work the same way across all APIs                                   |
| **Rate Limiting**  | Same [rate limiting policies](/guides/rate-limiting) apply to all endpoints                                              |
| **Error Handling** | Standardized [error codes](/legacy-unified-apis/error-codes-and-troubleshooting) across all APIs                         |
| **Policies**       | The IAM Policies API (list, get) was part of the original IAM model and has been superseded by the new IAM entity model. |

## Getting Started

1. **Set up authentication** - Get your API key from the [dashboard](https://app.stackone.com/api_keys) and learn about [authentication](/api/authentication)
2. **Connect an account** - Use [StackOne Hub](/guides/embedding-stackone-hub) to connect your first provider
3. **Make your first request** - Follow the "Making your first request" guide for your chosen API
4. **Explore common features** - Learn about [pagination](/guides/pagination), [field mapping](/guides/field-mapping), and [advanced query parameters](/guides/using-expand-and-include-api-parameters)

## IAM Policies

The IAM Policies API was part of the original IAM model. It has been superseded by the updated [IAM entity model](/iam/introduction), which replaces the Policies/Resources/Permissions structure with a scoped permissions model.

The Policies endpoints remain available here for existing integrations:

<CardGroup cols={2}>
  <Card title="List Policies" icon="shield" href="/iam/api-reference/policies/list-policies">
    Return all policies from the connected identity provider
  </Card>

  <Card title="Get Policy" icon="shield-halved" href="/iam/api-reference/policies/get-policy">
    Return a single policy by ID
  </Card>
</CardGroup>

## Next Steps

<CardGroup cols={2}>
  <Card title="Pagination" icon="list" href="/guides/pagination">
    Learn how to paginate through large datasets
  </Card>

  <Card title="StackOne Identifiers" icon="fingerprint" href="/guides/stackone-identifiers">
    Understand synthetic IDs and how to use them
  </Card>

  <Card title="Field Mapping" icon="table" href="/guides/field-mapping">
    Map custom fields between your system and providers
  </Card>

  <Card title="Webhooks" icon="bell" href="/guides/webhooks">
    Set up real-time notifications for data changes
  </Card>
</CardGroup>