# Overview
URL: /docs/runtimes/custom/overview

Four ways to wire assistant-ui to a backend without a framework adapter.

If no [framework adapter](/docs/runtimes/pick-a-runtime) fits your backend, this section gives you four building blocks. All four are built on one of the two core runtimes; understanding the layering (see [architecture](/docs/runtimes/concepts/architecture)) makes the choice clear.

## The four paths \[#the-four-paths]

| Path                                                              | Layered on                      | Wire shape                                  | Choose when                                                                                               |
| ----------------------------------------------------------------- | ------------------------------- | ------------------------------------------- | --------------------------------------------------------------------------------------------------------- |
| [`LocalRuntime`](/docs/runtimes/custom/local-runtime)             | Core                            | You write a `ChatModelAdapter.run` function | The simplest case: you have a `fetch` call to make, want assistant-ui to handle state                     |
| [`ExternalStoreRuntime`](/docs/runtimes/custom/external-store)    | Core                            | You provide messages + callbacks            | You already have state in redux, zustand, tanstack-query, or anywhere                                     |
| [`DataStream`](/docs/runtimes/custom/data-stream)                 | LocalRuntime + protocol         | Your backend emits the data stream protocol | You want a thin message-stream contract, or you are migrating from AI SDK v4                              |
| [`AssistantTransport`](/docs/runtimes/custom/assistant-transport) | ExternalStoreRuntime + protocol | Your backend streams agent state snapshots  | Your agent has rich internal state and you want it surfaced in the UI; or you need bidirectional commands |

## Decision tree \[#decision-tree]

```text
do you have an AI SDK / LangGraph / Google ADK / A2A backend?
└── yes → use that framework adapter; you do not need this section
└── no → continue
    │
    do you already have message state in redux / zustand / tanstack-query?
    └── yes → ExternalStoreRuntime
    └── no → continue
        │
        do you control the backend wire format?
        └── yes, and i want simple fetch calls → LocalRuntime
        └── yes, and i want to stream a structured agent state → AssistantTransport
        └── no, the backend already speaks data stream protocol → DataStream
```

## What each path gives you \[#what-each-path-gives-you]

**`LocalRuntime`** is the lowest-friction path. You implement one method (`run` or `async *run`). Branching, editing, regeneration, multi-thread, and adapter slots all work without extra code.

**`ExternalStoreRuntime`** is the inverse: you own the message array and provide callbacks for each interaction. UI features turn on based on which callbacks you provide. Ideal when chat state is a first-class entity in your existing store.

**`DataStream`** is `LocalRuntime` plus a wire protocol. You do not write a `ChatModelAdapter`; your backend emits a standardized stream of message parts and the runtime consumes it directly.

**`AssistantTransport`** is `ExternalStoreRuntime` plus a state-streaming protocol. Instead of sending message parts, your backend sends snapshots of its full agent state. The UI is a stateless view on top of that state. Supports custom commands beyond the built-in `add-message` and `add-tool-result`.

## Common building blocks \[#common-building-blocks]

Regardless of which path you pick, the [adapter](/docs/runtimes/concepts/adapters) and [thread](/docs/runtimes/concepts/threads) interfaces work the same way. Wire attachments, history, speech, feedback, suggestions, and multi-thread support through those shared contracts.

## Next \[#next]

<Cards>
  <Card title="LocalRuntime" description="Write a single ChatModelAdapter.run function." href="/docs/runtimes/custom/local-runtime" />

  <Card title="ExternalStoreRuntime" description="Bring redux, zustand, tanstack-query, or your own state." href="/docs/runtimes/custom/external-store" />

  <Card title="Data Stream" description="Standard message-stream protocol on top of LocalRuntime." href="/docs/runtimes/custom/data-stream" />

  <Card title="Assistant Transport" description="Stream full agent state snapshots and bidirectional commands." href="/docs/runtimes/custom/assistant-transport" />
</Cards>