> ## 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 Reaction

> Add or update a reaction on an entity

Adds a reaction of the specified type to an entity. If the user already has a reaction on this entity, it is replaced with the new type. The entity's `reactionCounts` map is updated accordingly.

**Authentication required.**

## Path Parameters

<ParamField path="entityId" type="string" required>
  The UUID of the entity to react to.
</ParamField>

## Body Parameters

<ParamField body="reactionType" type="string" required>
  The type of reaction to add. One of: `upvote`, `downvote`, `like`, `love`, `wow`, `sad`, `angry`, `funny`.
</ParamField>

<ParamField body="userId" type="string">
  Override which user the reaction is attributed to. **Service or master key only.** Useful for backfilling reactions from an external system.
</ParamField>

## Response

Returns the updated [Entity](/data-models/entity) object with the new `reactionCounts` map.

<Note>
  The entity owner receives an in-app notification when someone reacts to their entity. Milestone notifications are also sent at reaction count thresholds (e.g., 10, 50, 100 reactions).
</Note>

## Error Responses

<AccordionGroup>
  <Accordion title="Missing User — 400">
    ```json theme={null}
    { "error": "Missing user ID", "code": "entity/missing-user-id" }
    ```

    Returned when no user is authenticated and no `userId` override is provided.
  </Accordion>

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