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

# Run external API operations

> Use Nango functions to read, write, and automate external APIs on demand.

Some integration work needs to happen immediately: create a CRM contact, send a Slack message, fetch the latest account settings, or run a multi-step workflow after a user clicks a button.

Nango handles these on-demand operations with Tools. You write the API logic once, Nango runs it with the right connection credentials, and your app calls the function from any backend or agent framework.

## When this fits

Use Tools when you need to:

* Run reads or writes against an external API on demand.
* Hide multi-step provider workflows behind one stable interface.
* Normalize the same operation across several APIs.
* Expose selected operations as tools for AI agents.

For continuously replicated data, use [Sync external API data](https://nango.dev/docs/getting-started/use-cases/syncs.md) instead.

## How Nango fits in

1. Your user authorizes an integration with Nango.
2. You call a catalog tool directly or deploy your own tool.
3. Your app calls the function with an input payload.
4. Nango executes the provider API calls with the user's credentials.
5. Your app receives the tool's result.

These functions are the technical foundation behind on-demand API operations, unified APIs, and AI tool calls.

## What to read next

* [Functions guide](https://nango.dev/docs/guides/functions/functions-guide.md) - create, test, deploy, and trigger functions end to end.
* [Tools](https://nango.dev/docs/guides/functions/tools.md) - build and call on-demand operations.
* [Tool calling for AI agents](https://nango.dev/docs/getting-started/use-cases/tool-calling.md) - use functions as AI tools.
* [Build a unified API](https://nango.dev/docs/getting-started/use-cases/unified-apis.md) - use functions behind a normalized product API.
* [Functions SDK reference](https://nango.dev/docs/reference/functions/functions-sdk.md) - runtime methods available inside function code.

> [!NOTE]
>
> You can use tools from the [template catalog](https://www.nango.dev/templates) without writing code. See the [Functions guide](https://nango.dev/docs/guides/functions/functions-guide.md#catalog-tool-stability) to choose between a catalog tool and your own.


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