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

# Add Entity to Collection

> Save an entity into a collection

Saves an entity into the specified collection. This operation is idempotent — if the entity is already in the collection, it returns success without creating a duplicate.

## Path Parameters

<ParamField path="collectionId" type="string" required>
  The ID of the collection to save the entity into.
</ParamField>

## Body Parameters

<ParamField body="entityId" type="string" required>
  The ID of the entity to save.
</ParamField>

## Response

<ResponseField name="success" type="boolean">
  Always `true` on success.
</ResponseField>

<ResponseField name="collection" type="object">
  Updated collection metadata.

  <Expandable>
    <ResponseField name="id" type="string">Collection ID.</ResponseField>
    <ResponseField name="entityCount" type="number">New entity count after the save.</ResponseField>
  </Expandable>
</ResponseField>

## Error Responses

<AccordionGroup>
  <Accordion title="Collection Not Found — 404">
    ```json theme={null}
    { "error": "Collection not found", "code": "collection/not-found" }
    ```
  </Accordion>

  <Accordion title="Entity Not Found — 404">
    ```json theme={null}
    { "error": "Entity not found", "code": "entity/not-found" }
    ```
  </Accordion>

  <Accordion title="Server Error — 500">
    ```json theme={null}
    { "error": "Internal server error.", "code": "collection/server-error" }
    ```
  </Accordion>
</AccordionGroup>