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

# Fused: one controlled path to every service

> One controlled path from your applications and agents to every service they depend on.

Fused sits between the software making a call and the service being called. For every request it decides whether the call is allowed, resolves the credential, applies the policy, performs the provider interaction, and records what happened.

Your product code stops carrying provider auth, retries, and pagination. Your agents stop holding credentials. Both get one consistent way in.

## Two components

<CardGroup cols={2}>
  <Card title="Registry" icon="cloud">
    The catalogue. It turns an API description into a versioned service contract every team can inherit instead of working out again.
  </Card>

  <Card title="Engine" icon="server">
    The runtime, in your infrastructure. Credentials, execution, policy, and event delivery stay inside it.
  </Card>
</CardGroup>

Registry defines and distributes integration knowledge. Engine applies and runs it inside your workspace boundary. Provider credentials, request payloads, and execution receipts stay with the Engine.

## The four things you will work with

<CardGroup cols={2}>
  <Card title="Workspace" icon="toggle-on" href="/workspace/enable-a-service">
    Which services and versions your company has approved.
  </Card>

  <Card title="App (SDK)" icon="cube" href="/app/build-an-sdk">
    A typed package holding only what one product needs.
  </Card>

  <Card title="MCP" icon="robot" href="/mcp/deploy-a-server">
    An Engine-hosted server giving an agent scoped access across services.
  </Card>

  <Card title="Bucket" icon="key" href="/bucket/store-credentials">
    Where credentials live, and how they reach a call without reaching your code.
  </Card>
</CardGroup>

## How a change reaches the Engine

Everything configurable is a file under `.fused/`, and every change goes through the same two steps.

```bash theme={null}
fused-cli sdk plan     # read-only; reports required permissions, writes a receipt
fused-cli sdk apply    # commits exactly what the receipt describes
```

The receipt binds an apply to that exact file content and that exact Engine. If the file changes, or you point at a different Engine, the apply is refused rather than doing something you did not review. An unchanged apply is a no-op.

Immutable things behave accordingly: re-applying identical content to an existing app version changes nothing, and changing its scope returns `app_version_immutable`. You publish a new version instead of quietly rewriting the contract of software already in production.

## Three credentials

Keeping these apart avoids most early confusion.

| Credential           | Authenticates                                 | Lives in                                |
| -------------------- | --------------------------------------------- | --------------------------------------- |
| CLI API key          | Management calls to the Engine                | Your CLI config, from `fused-cli login` |
| Execution token      | An app or agent calling the Engine at runtime | Your application's secret manager       |
| Provider credentials | The Engine to the provider                    | An Engine bucket                        |

Provider credentials never appear in a config file, a generated package, or an agent's context.

<CardGroup cols={2}>
  <Card title="Get an Engine running" icon="server" href="/deploy-an-engine">
    Hosted or your own infrastructure. Everything else needs one.
  </Card>

  <Card title="Quickstart" icon="rocket" href="/quickstart">
    Install the CLI and ship a working SDK.
  </Card>
</CardGroup>

## Running this for a company

Everything on this site works on a Dev license, which is enough to build the whole thing end to end. What it is not is a production tier.

Scale-up and enterprise teams get production licensing, SSO, and a support agreement. [Tell us what you're building](https://usefused.com/?plan=enterprise#enterprise-interest) and we'll size it with you.