deepset-ai--haystack/docs-website/docs/pipeline-components/embedders/mockdocumentembedder.mdx

---
title: "MockDocumentEmbedder"
id: mockdocumentembedder
slug: "/mockdocumentembedder"
description: "A Document Embedder that returns deterministic embeddings without calling any API, for tests and quick prototypes."
---

# MockDocumentEmbedder

A Document Embedder that returns deterministic embeddings without calling any API, for tests and quick prototypes.

<div className="key-value-table">

|  |  |
| --- | --- |
| **Most common position in a pipeline** | In place of a real Document Embedder, in tests and prototypes |
| **Mandatory init variables** | None |
| **Mandatory run variables** | `documents`: A list of documents |
| **Output variables** | `documents`: A list of documents enriched with embeddings  <br /> <br />`meta`: A dictionary of metadata |
| **API reference** | [Embedders](/reference/embedders-api) |
| **GitHub link** | https://github.com/deepset-ai/haystack/blob/main/haystack/components/embedders/mock_document_embedder.py |
| **Package name** | `haystack-ai` |

</div>

## Overview

`MockDocumentEmbedder` is a deterministic, zero-cost drop-in replacement for real Document Embedders such as `OpenAIDocumentEmbedder`. It implements `run`, `run_async`, and serialization like any other embedder but never contacts an external service, which makes it ideal for unit tests, smoke tests, and quick prototypes.

The embedding is selected based on how the component is configured:

- **Deterministic (default)**: With no configuration, each document's embedding is derived from a stable hash of its prepared text. The same text always yields the same unit-length embedding, and different texts yield different embeddings, so the mock works in retrieval pipelines and is reproducible across runs and processes.
- **Fixed embedding**: Pass an `embedding` vector. The same vector is assigned to every document.
- **Dynamic embedding**: Pass an `embedding_fn` callable that receives the prepared text of a document and returns the embedding. To support serialization, pass a named function.

`embedding` and `embedding_fn` are mutually exclusive.

Further optional parameters:

- `dimension`: The number of dimensions of the deterministic embedding. Defaults to `768`. Ignored when `embedding` or `embedding_fn` is provided, since their length is determined by the value or callable.
- `model`: The model name reported in the metadata. Defaults to `"mock-model"`.
- `meta`: Additional metadata merged into the output `meta`.
- `prefix` / `suffix`: Strings added to the beginning and end of each text before embedding, mirroring real embedders.
- `meta_fields_to_embed` / `embedding_separator`: Like real Document Embedders, the metadata fields listed in `meta_fields_to_embed` are concatenated with the document content before embedding, so the deterministic embedding reflects the embedded metadata.
- `progress_bar`: Accepted for interface compatibility with real Document Embedders and ignored.

:::info
The deterministic embeddings are derived from a hash: identical texts get identical vectors and the similarity between different texts is stable but arbitrary. For exact-match retrieval in tests this is exactly what you want. Do not expect semantically similar texts to end up close together.
:::

Use `MockDocumentEmbedder` for documents and its counterpart [`MockTextEmbedder`](mocktextembedder.mdx) for queries. With the default deterministic mode, a query whose text matches a document's content produces the same vector, so the document is retrieved as the top hit.

## Usage

### On its own

```python
from haystack import Document
from haystack.components.embedders import MockDocumentEmbedder

embedder = MockDocumentEmbedder(dimension=8)
result = embedder.run([Document(content="I love pizza!")])
print(result["documents"][0].embedding)  # a deterministic list of 8 floats
```

### In a pipeline

Use it in an indexing pipeline exactly like a real Document Embedder — no API key needed:

```python
from haystack import Document, Pipeline
from haystack.components.embedders import MockDocumentEmbedder
from haystack.components.writers import DocumentWriter
from haystack.document_stores.in_memory import InMemoryDocumentStore

document_store = InMemoryDocumentStore()

indexing_pipeline = Pipeline()
indexing_pipeline.add_component("embedder", MockDocumentEmbedder(dimension=8))
indexing_pipeline.add_component("writer", DocumentWriter(document_store=document_store))
indexing_pipeline.connect("embedder.documents", "writer.documents")

indexing_pipeline.run(
    {
        "embedder": {
            "documents": [
                Document(content="My name is Wolfgang and I live in Berlin"),
                Document(content="I saw a black horse running"),
            ],
        },
    },
)
print(document_store.count_documents())  # 2
```