---
title: Patterns for Defining Tools
description: Common patterns for defining tools in durable AI agents using Workflow SDK.
type: guide
summary: Define step-level and workflow-level tools for durable AI agents with stream writing and context access.
prerequisites:
  - /docs/ai
related:
  - /docs/ai/streaming-updates-from-tools
  - /docs/ai/sleep-and-delays
  - /docs/foundations/workflows-and-steps
  - /docs/api-reference/workflow/get-writable
---

This page covers the details for some common patterns when defining tools for AI agents using Workflow SDK.

Using DurableAgent, we model most tools as steps. These can be anything from a simple function call to a entire multi-day long workflow.

## Accessing message context in tools

Just like in regular AI SDK tool definitions, tool in DurableAgent are called with a first argument of the tool's input parameters, and a second argument of the tool call context.

When you tool needs access to the full message history, you can access it via the `messages` property of the tool call context:

```typescript title="tools.ts" lineNumbers
import { Experimental_Agent as Agent } from "ai";
import type { ModelMessage } from "ai";

async function getWeather(
  { city }: { city: string },
  { messages, toolCallId }: { messages: ModelMessage[], toolCallId: string }) { // [!code highlight]
  "use step";
  return `Weather in ${city} is sunny`;
}
```

## Writing to Streams

As discussed in [Streaming Updates from Tools](/docs/ai/streaming-updates-from-tools), it's common to use a step just to call `getWritable()` for writing custom data parts to the stream.

This can be made generic, by creating a helper step function to write arbitrary data to the stream:

```typescript title="tools.ts" lineNumbers
import { getWritable } from "workflow";

async function writeToStream(data: any) {
  "use step";

  const writable = getWritable();
  const writer = writable.getWriter();
  await writer.write(data);
  writer.releaseLock();
}
```

## Step-Level vs Workflow-Level Tools

Tools can be implemented either at the step level or the workflow level, with different capabilities and constraints.

| Capability | Step-Level (`"use step"`) | Workflow-Level (`"use workflow"`) |
|------------|---------------------------|----------------|
| `getWritable()` | ✅ | ❌ |
| Automatic retries | ✅ | ❌ |
| Side-effects (e.g. API calls) allowed | ✅ | ❌ |
| `sleep()` | ❌ | ✅ |
| `createWebhook()` | ❌ | ✅ |

Tools can also combine both by starting out on the workflow level, and calling into steps for I/O operations, like so:

```typescript title="tools.ts" lineNumbers
import { sleep } from "workflow";
import type { LanguageModel, ModelMessage } from "ai";

// Step: handles I/O with retries
async function performFetch(url: string) {
  "use step";
  const response = await fetch(url);
  return response.json();
}

// Workflow-level: orchestrates steps and can use sleep()
async function executeFetchWithDelay({ url }: { url: string }) {
  const result = await performFetch(url);
  await sleep("5s"); // Only available at workflow level
  return result;
}
```
