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

# Testing doubles

## Overview

Instructor supports deterministic tests at three different seams.

* use `FakeInferenceDriver` when you want to drive Instructor directly with queued raw responses or streaming deltas
* use `MockHttp` when you want to keep the provider adapter and HTTP response path in play
* use probe helpers when you need to assert streaming timing, call counts, or emission order

Pick the shallowest seam that still exercises the behavior you care about.

## `FakeInferenceDriver`

`FakeInferenceDriver` lives in `packages/instructor/tests/Support` and is the main
contributor-facing fake for deterministic Instructor tests.

Use it when you want to test:

* deserialization and validation behavior
* retry logic
* stream accumulation and final response behavior
* partial and sequence handling without real HTTP or provider adapters

It supports two modes:

* queued `InferenceResponse` objects for sync execution
* queued `PartialInferenceDelta` batches for streaming execution

Minimal example:

```php theme={null}
use Cognesy\Instructor\Tests\Support\FakeInferenceDriver;
use Cognesy\Polyglot\Inference\Data\InferenceResponse;

$driver = new FakeInferenceDriver(
    responses: [new InferenceResponse(content: '{"name":"Jason","age":28}')],
);
// @doctest id="2283"
```

Choose this seam for most unit and regression tests inside `packages/instructor`.

## `MockHttp`

`MockHttp` lives in `packages/instructor/tests` and builds an HTTP client around
`MockHttpDriver`.

Use it when you want to test:

* provider-specific adapter behavior
* request and response wiring that still goes through HTTP
* response payload shapes such as OpenAI or Anthropic fixtures

Minimal example:

```php theme={null}
use Cognesy\Instructor\Tests\MockHttp;

$http = MockHttp::get(['{"name":"Jason","age":28}']);
// @doctest id="2c3d"
```

Choose this seam when the HTTP transport and provider adapter still matter to the
test. If they do not, prefer `FakeInferenceDriver`.

## Streaming Probes

`ProbeStreamDriver` and `ProbeIterator` live in `packages/instructor/tests/Integration/Support`.

Use them when you need to assert:

* that streaming updates are emitted immediately
* how many sync versus stream reads occurred
* exact delta ordering in an integration-style test

These helpers are narrower than `FakeInferenceDriver`. They are for observation,
not for general-purpose fixture setup.

## Which One To Use

Use this rule of thumb:

* `FakeInferenceDriver` for most deterministic Instructor behavior tests
* `MockHttp` for adapter- and payload-level coverage
* probe helpers for streaming immediacy and observation-heavy assertions

If a test only needs structured-output behavior, prefer the fake driver. If the test
is really about provider response shape or HTTP wiring, keep the mock HTTP path.