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

# HarnessMiddleware

> **HarnessMiddleware**\<`Context`, `Result`> = (`context`, `next`) => `Promise`\<`Result`> | `Result`

**Type Parameters**

| Type Parameter |
| - |
|  `Context` |
|  `Result` |

**Parameters**

| Parameter | Type |
| - | - |
| `context` | `Readonly`\<`Context`> |
| `next` | () => `Promise`\<`Result`> |

**Returns**
`Promise`\<`Result`> | `Result`

## runMiddleware()

> **runMiddleware**\<`Context`, `Result`>(`context`, `middleware`, `core`): `Promise`\<`Result`>

Executes one immutable lifecycle snapshot in registered onion order.

**Type Parameters**

| Type Parameter |
| - |
|  `Context` |
|  `Result` |

**Parameters**

| Parameter | Type |
| - | - |
| `context` | `Readonly`\<`Context`> |
| `middleware` | readonly [`HarnessMiddleware`](/docs/agent-sdk/harness-sdk/plug-ins/middleware)\<`Context`, `Result`>\[] |
| `core` | () => `Promise`\<`Result`> |

**Returns**
`Promise`\<`Result`>

## HarnessTurnMiddlewareContext

Immutable turn-local context passed to turn middleware.

**Type Parameters**

| Type Parameter | Default type |
| - | - |
|  `TOOLS` *extends* `ToolSet` | `ToolSet` |
|  `OutputDefinition` *extends* `HarnessOutputDefinition` \| `undefined` | `undefined` |

**Properties**
**config**

> `readonly` **config**: `HarnessRuntimeConfig`\<`LanguageModel`, `TOOLS`, `OutputDefinition`>

**input**

> `readonly` **input**: `string`

**phase**

> `readonly` **phase**: `"turn"`

**signal**

> `readonly` **signal**: `AbortSignal` | `undefined`

## HarnessModelStepMiddlewareContext

Immutable context passed around an actual model invocation.

**Type Parameters**

| Type Parameter | Default type |
| - | - |
|  `TOOLS` *extends* `ToolSet` | `ToolSet` |

**Properties**
**config**

> `readonly` **config**: `HarnessRuntimeConfig`\<`LanguageModel`, `TOOLS`>

**phase**

> `readonly` **phase**: `"modelStep"`

**signal**

> `readonly` **signal**: `AbortSignal` | `undefined`

**stepNumber**

> `readonly` **stepNumber**: `number`

## HarnessToolMiddlewareContext

Immutable context passed around an actual server-tool invocation.

**Type Parameters**

| Type Parameter | Default type |
| - | - |
|  `TOOLS` *extends* `ToolSet` | `ToolSet` |

**Properties**
**config**

> `readonly` **config**: `HarnessRuntimeConfig`\<`LanguageModel`, `TOOLS`>

**phase**

> `readonly` **phase**: `"tool"`

**signal**

> `readonly` **signal**: `AbortSignal` | `undefined`

**toolCallId**

> `readonly` **toolCallId**: `string` | `undefined`

**toolName**

> `readonly` **toolName**: `string`

## HarnessMiddlewareError

Stable error raised when a lifecycle middleware attempts to advance twice.

**Constructors**
**Constructor**

> **new HarnessMiddlewareError**(): `HarnessMiddlewareError`

**Returns**
`HarnessMiddlewareError`

**Properties**
**cause?**

> `optional` **cause?**: `unknown`

**code**

> `readonly` **code**: `"HARNESS_MIDDLEWARE_NEXT"`

**message**

> **message**: `string`

**name**

> **name**: `string`

**stack?**

> `optional` **stack?**: `string`

**stackTraceLimit**

> `static` **stackTraceLimit**: `number`

The `Error.stackTraceLimit` property specifies the number of stack frames
collected by a stack trace (whether generated by `new Error().stack` or
`Error.captureStackTrace(obj)`).

The default value is `10` but may be set to any valid JavaScript number. Changes
will affect any stack trace captured *after* the value has been changed.

If set to a non-number value, or set to a negative number, stack traces will
not capture any frames.

**Methods**
**captureStackTrace()**

> `static` **captureStackTrace**(`targetObject`, `constructorOpt?`): `void`

Creates a `.stack` property on `targetObject`, which when accessed returns
a string representing the location in the code at which
`Error.captureStackTrace()` was called.

```js theme={null}
const myObject = {};
Error.captureStackTrace(myObject);
myObject.stack;  // Similar to `new Error().stack`
```

The first line of the trace will be prefixed with
`${myObject.name}: ${myObject.message}`.

The optional `constructorOpt` argument accepts a function. If given, all frames
above `constructorOpt`, including `constructorOpt`, will be omitted from the
generated stack trace.

The `constructorOpt` argument is useful for hiding implementation
details of error generation from the user. For instance:

```js theme={null}
function a() {
  b();
}

function b() {
  c();
}

function c() {
  // Create an error without stack trace to avoid calculating the stack trace twice.
  const { stackTraceLimit } = Error;
  Error.stackTraceLimit = 0;
  const error = new Error();
  Error.stackTraceLimit = stackTraceLimit;

  // Capture the stack trace above function b
  Error.captureStackTrace(error, b); // Neither function c, nor b is included in the stack trace
  throw error;
}

a();
```

**Parameters**

| Parameter | Type |
| - | - |
| `targetObject` | `object` |
| `constructorOpt?` | `Function` |

**Returns**
`void`

**prepareStackTrace()**

> `static` **prepareStackTrace**(`err`, `stackTraces`): `any`

**Parameters**

| Parameter | Type |
| - | - |
| `err` | `Error` |
| `stackTraces` | `CallSite`\[] |

**Returns**
`any`

**See**
[https://v8.dev/docs/stack-trace-api#customizing-stack-traces](https://v8.dev/docs/stack-trace-api#customizing-stack-traces)