> ## Documentation Index
> Fetch the complete documentation index at: https://hub.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>
  **Legacy documentation.** The Legacy Unified APIs are no longer in active development. Existing functionality remains supported, and no new features or enhancements will be added.

  For the current StackOne platform, connectors, and agent tooling, see [docs.stackone.com](https://docs.stackone.com).
</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](https://docs.stackone.com/embed/call-actions/mcp), an [AI Toolset SDK](https://docs.stackone.com/embed/call-actions/agent-sdk), or the [Actions API (RPC)](https://docs.stackone.com/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](https://docs.stackone.com/platform-api/authentication) with API keys and `x-account-id` headers |
| **Pagination**     | Consistent [cursor-based pagination](/legacy-unified-apis/common-guide/pagination) across all endpoints                                       |
| **Synthetic IDs**  | [StackOne Identifiers](/legacy-unified-apis/common-guide/stackone-identifiers) work the same way across all APIs                              |
| **Rate Limiting**  | Same [rate limiting policies](https://docs.stackone.com/platform-api/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](https://docs.stackone.com/platform-api/authentication)
2. **Connect an account** - Use [StackOne Hub](https://docs.stackone.com/embed/account-linking/overview) 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](/legacy-unified-apis/common-guide/pagination), [field mapping](/legacy-unified-apis/common-guide/field-mapping), and [advanced query parameters](/legacy-unified-apis/common-guide/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](https://docs.stackone.com/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="https://docs.stackone.com/iam/api-reference/policies/list-policies">
    Return all policies from the connected identity provider
  </Card>

  <Card title="Get Policy" icon="shield-halved" href="https://docs.stackone.com/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="/legacy-unified-apis/common-guide/pagination">
    Learn how to paginate through large datasets
  </Card>

  <Card title="StackOne Identifiers" icon="fingerprint" href="/legacy-unified-apis/common-guide/stackone-identifiers">
    Understand synthetic IDs and how to use them
  </Card>

  <Card title="Field Mapping" icon="table" href="/legacy-unified-apis/common-guide/field-mapping">
    Map custom fields between your system and providers
  </Card>

  <Card title="Webhooks" icon="bell" href="/legacy-unified-apis/common-guide/webhooks">
    Set up real-time notifications for data changes
  </Card>
</CardGroup>