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# Post Chat Completion

POST https://fai.buildwithfern.com/chat/{domain}
Content-Type: application/json

Reference: https://buildwithfern.com/learn/docs/ai-features/ask-fern/api-reference/chat/post-chat-completion

## Authentication

- `Authorization` header (bearer token, required) — Bearer authentication of the form `Bearer <token>`, where token is your auth token.

## Servers

- `https://fai.buildwithfern.com` (Production, default)
- `https://fai-dev.buildwithfern.com` (Development)
- `http://localhost:8080` (Local)

## Request

### Path parameters

- `domain` (string, required)

### Body (application/json)

This endpoint expects a PostChatCompletionRequest.

- `messages` (list of ChatMessage, required) — The messages to use for the chat completion
- `model` (enum, optional, nullable) — The model to use for the chat completion
  - Allowed values: `claude-4-sonnet`, `claude-4.5-haiku`, `claude-4.5-sonnet`, `claude-4.6-sonnet`
- `max_tokens` (integer, optional, nullable, default: 3000) — The maximum number of tokens to generate. Note: setting a token count lower than 2000 may result in incomplete responses. You can add a custom system prompt to control the verbosity of the response.
- `system_prompt` (string, optional, nullable) — The system prompt to use for the chat completion
- `rewrite_query` (boolean, optional, nullable, default: false) — Whether to rewrite the query using query decomposition
- `user_is_authed` (boolean, optional, default: false) — Whether the requesting user is authenticated. When true, authed chunks are included in results.
- `allowed_roles` (list of string, optional, nullable) — Roles the authenticated user has. Used to filter chunks by role-based access control.
- `source` (string, optional, nullable) — The origin of the request (e.g. 'web', 'slack', 'api').
- `basepaths` (list of string, optional, nullable) — Restrict retrieval to pages under these URL path prefixes (e.g. ['/my-product']). Matches the scoping the Ask AI widget applies when embedded under a product path. Omit to search the whole site.
- `skip_save_query` (boolean, optional, default: false) — Set to true for test or eval traffic. The request is answered (and billed) normally but is excluded from usage analytics.

## Response

### 200

Successful Response

- `turns` (list of ChatMessage, required) — The conversation turns in the chat completion
- `citations` (list of string, required) — List of citation strings
- `sources` (list of ChatSource, optional) — Pages the answer was grounded in, one entry per cited page

## Errors

### 422 Unprocessable Entity Error

Validation Error

- `detail` (list of ValidationError, optional)

## Types

### ChatMessage

- `role` (enum, required)
  - Allowed values: `user`, `assistant`
- `content` (string, required)

### ChatSource

- `url` (string, required) — URL of the cited documentation page
- `title` (string, required) — Title of the cited documentation page

### ValidationError

- `loc` (list of ValidationErrorLocItems, required)
- `msg` (string, required)
- `type` (string, required)

### ValidationErrorLocItems

## Examples

**Request**

```json
{
  "messages": [
    {
      "role": "user",
      "content": "string"
    }
  ]
}
```

**Response**

```json
{
  "turns": [
    {
      "role": "user",
      "content": "string"
    }
  ],
  "citations": [
    "string"
  ],
  "sources": [
    {
      "url": "string",
      "title": "string"
    }
  ]
}
```