Chat Completions

Overview

The Chat Completions endpoint is the most widely used API for interacting with large language models. It accepts a conversation composed of multiple messages and returns the model’s response. CometAPI routes this endpoint to multiple providers — including OpenAI, Anthropic Claude (via compatibility layer), Google Gemini, and others — through a single unified interface. You can switch between models by simply changing the model parameter.

This endpoint follows the OpenAI Chat Completions format. Most OpenAI-compatible SDKs and tools work with CometAPI by changing the base_url to https://api.cometapi.com/v1.

Important Notes

Model-specific behavior — Different models may support different subsets of parameters and return slightly different response fields. For example, reasoning_effort only applies to reasoning models (o-series, GPT-5.1+), and some models may not support logprobs or n > 1.

Response pass-through — CometAPI passes model responses through without modification (except for format normalization when routing between providers), ensuring you receive output consistent with the original API.

OpenAI Pro models — For OpenAI Pro series models (e.g., o1-pro), use the responses endpoint instead.

Message Roles

Role	Description
`system`	Sets the assistant’s behavior and personality. Placed at the start of the conversation.
`developer`	Replaces `system` for newer models (o1+). Provides instructions the model should follow regardless of user input.
`user`	Messages from the end user.
`assistant`	Previous model responses, used to maintain conversation history.
`tool`	Results from tool/function calls. Must include `tool_call_id` matching the original tool call.

For newer models (GPT-4.1, GPT-5 series, o-series), prefer developer over system for instruction messages. Both work, but developer provides stronger instruction-following behavior.

Multimodal Input

Many models support images and audio alongside text. Use the array format for content to send multimodal messages:

{
  "role": "user",
  "content": [
    {"type": "text", "text": "Describe this image"},
    {
      "type": "image_url",
      "image_url": {
        "url": "https://example.com/image.png",
        "detail": "high"
      }
    }
  ]
}

The detail parameter controls image analysis depth:

low — faster, uses fewer tokens (fixed cost)
high — detailed analysis, more tokens consumed
auto — the model decides (default)

Streaming

When stream is set to true, the response is delivered as Server-Sent Events (SSE). Each event contains a chat.completion.chunk object with incremental content:

data: {"id":"chatcmpl-xxx","object":"chat.completion.chunk","choices":[{"index":0,"delta":{"role":"assistant"},"finish_reason":null}]}

data: {"id":"chatcmpl-xxx","object":"chat.completion.chunk","choices":[{"index":0,"delta":{"content":"Hello"},"finish_reason":null}]}

data: {"id":"chatcmpl-xxx","object":"chat.completion.chunk","choices":[{"index":0,"delta":{"content":"!"},"finish_reason":null}]}

data: {"id":"chatcmpl-xxx","object":"chat.completion.chunk","choices":[{"index":0,"delta":{},"finish_reason":"stop"}]}

data: [DONE]

To include token usage statistics in streaming responses, set stream_options.include_usage to true. The usage data appears in the final chunk before [DONE].

Structured Outputs

Force the model to return valid JSON matching a specific schema using response_format:

{
  "response_format": {
    "type": "json_schema",
    "json_schema": {
      "name": "result",
      "strict": true,
      "schema": {
        "type": "object",
        "properties": {
          "answer": {"type": "string"},
          "confidence": {"type": "number"}
        },
        "required": ["answer", "confidence"],
        "additionalProperties": false
      }
    }
  }
}

JSON Schema mode (json_schema) guarantees the output matches your schema exactly. JSON Object mode (json_object) only guarantees valid JSON — the structure is not enforced.

Tool / Function Calling

Enable the model to call external functions by providing tool definitions:

{
  "tools": [
    {
      "type": "function",
      "function": {
        "name": "get_weather",
        "description": "Get current weather for a city",
        "parameters": {
          "type": "object",
          "properties": {
            "location": {"type": "string", "description": "City name"}
          },
          "required": ["location"]
        }
      }
    }
  ],
  "tool_choice": "auto"
}

When the model decides to call a tool, the response will have finish_reason: "tool_calls" and the message.tool_calls array will contain the function name and arguments. You then execute the function and send the result back as a tool message with the matching tool_call_id.

Response Fields

Field	Description
`id`	Unique completion identifier (e.g. `chatcmpl-abc123`).
`object`	Always `chat.completion`.
`model`	The model that generated the response (may include version suffix).
`choices`	Array of completion choices (typically 1 unless `n` > 1).
`choices[].message`	The assistant’s response message with `role`, `content`, and optionally `tool_calls`.
`choices[].finish_reason`	Why the model stopped: `stop`, `length`, `tool_calls`, or `content_filter`.
`usage`	Token consumption breakdown: `prompt_tokens`, `completion_tokens`, `total_tokens`, and detailed sub-counts.
`system_fingerprint`	Backend configuration fingerprint for debugging reproducibility.

Cross-Provider Notes

Parameter support across providers

Parameter	OpenAI GPT	Claude (via compat)	Gemini (via compat)
`temperature`	0–2	0–1	0–2
`top_p`	0–1	0–1	0–1
`n`	1–128	1 only	1–8
`stop`	Up to 4	Up to 4	Up to 5
`tools`	✅	✅	✅
`response_format`	✅	✅ (json_schema)	✅
`logprobs`	✅	❌	❌
`reasoning_effort`	o-series, GPT-5.1+	❌	❌ (use `thinking` for Gemini native)

max_tokens vs max_completion_tokens

max_tokens — The legacy parameter. Works with most models but is deprecated for newer OpenAI models.
max_completion_tokens — The recommended parameter for GPT-4.1, GPT-5 series, and o-series models. Required for reasoning models as it includes both output tokens and reasoning tokens.

CometAPI automatically handles the mapping when routing to different providers.

system vs developer role

system — The traditional instruction role. Works with all models.
developer — Introduced with o1 models. Provides stronger instruction-following for newer models. Falls back to system behavior on older models.

Use developer for new projects targeting GPT-4.1+ or o-series models.

FAQ

How to handle rate limits?

When encountering 429 Too Many Requests, implement exponential backoff:

import time
import random
from openai import OpenAI, RateLimitError

client = OpenAI(
    base_url="https://api.cometapi.com/v1",
    api_key="<COMETAPI_KEY>",
)

def chat_with_retry(messages, max_retries=3):
    for i in range(max_retries):
        try:
            return client.chat.completions.create(
                model="gpt-5.4",
                messages=messages,
            )
        except RateLimitError:
            if i < max_retries - 1:
                wait_time = (2 ** i) + random.random()
                time.sleep(wait_time)
            else:
                raise

How to maintain conversation context?

Include the full conversation history in the messages array:

messages = [
    {"role": "developer", "content": "You are a helpful assistant."},
    {"role": "user", "content": "What is Python?"},
    {"role": "assistant", "content": "Python is a high-level programming language..."},
    {"role": "user", "content": "What are its main advantages?"},
]

What does `finish_reason` mean?

Value	Meaning
`stop`	Natural completion or hit a stop sequence.
`length`	Reached `max_tokens` or `max_completion_tokens` limit.
`tool_calls`	The model invoked one or more tool/function calls.
`content_filter`	Output was filtered due to content policy.

How to control costs?

Use max_completion_tokens to cap output length.
Choose cost-effective models (e.g., gpt-5.4-mini or gpt-5.4-nano for simpler tasks).
Keep prompts concise — avoid redundant context.
Monitor token usage in the usage response field.

Authorizations

Authorization

string

header

required

Bearer token authentication. Use your CometAPI key.

Body

application/json

model

string

default:gpt-5.4

required

Model ID to use for this request. See the Models page for current options.

Example:

"gpt-4.1"

messages

object[]

required

A list of messages forming the conversation. Each message has a role (system, user, assistant, or developer) and content (text string or multimodal content array).

Show child attributes

stream

boolean

If true, partial response tokens are delivered incrementally via server-sent events (SSE). The stream ends with a data: [DONE] message.

temperature

number

default:1

Sampling temperature between 0 and 2. Higher values (e.g., 0.8) produce more random output; lower values (e.g., 0.2) make output more focused and deterministic. Recommended to adjust this or top_p, but not both.

Required range: 0 <= x <= 2

top_p

number

default:1

Nucleus sampling parameter. The model considers only the tokens whose cumulative probability reaches top_p. For example, 0.1 means only the top 10% probability tokens are considered. Recommended to adjust this or temperature, but not both.

Required range: 0 <= x <= 1

integer

default:1

Number of completion choices to generate for each input message. Defaults to 1.

stop

string

Up to 4 sequences where the API will stop generating further tokens. Can be a string or an array of strings.

max_tokens

integer

Maximum number of tokens to generate in the completion. The total of input + output tokens is capped by the model's context length.

presence_penalty

number

default:0

Number between -2.0 and 2.0. Positive values penalize tokens based on whether they have already appeared, encouraging the model to explore new topics.

Required range: -2 <= x <= 2

frequency_penalty

number

default:0

Number between -2.0 and 2.0. Positive values penalize tokens proportionally to how often they have appeared, reducing verbatim repetition.

Required range: -2 <= x <= 2

logit_bias

object

A JSON object mapping token IDs to bias values from -100 to 100. The bias is added to the model's logits before sampling. Values between -1 and 1 subtly adjust likelihood; -100 or 100 effectively ban or force selection of a token.

user

string

A unique identifier for your end-user. Helps with abuse detection and monitoring.

max_completion_tokens

integer

An upper bound for the number of tokens to generate, including visible output tokens and reasoning tokens. Use this instead of max_tokens for GPT-4.1+, GPT-5 series, and o-series models.

response_format

object

Specifies the output format. Use {"type": "json_object"} for JSON mode, or {"type": "json_schema", "json_schema": {...}} for strict structured output.

Show child attributes

tools

object[]

A list of tools the model may call. Currently supports function type tools.

Show child attributes

tool_choice

default:auto

Controls how the model selects tools. auto (default): model decides. none: no tools. required: must call a tool.

logprobs

boolean

default:false

Whether to return log probabilities of the output tokens.

top_logprobs

integer

Number of most likely tokens to return at each position (0-20). Requires logprobs to be true.

Required range: 0 <= x <= 20

reasoning_effort

enum<string>

Controls the reasoning effort for o-series and GPT-5.1+ models.

Available options:

low,

medium,

high

stream_options

object

Options for streaming. Only valid when stream is true.

Show child attributes

service_tier

enum<string>

Specifies the processing tier.

Available options:

auto,

default,

flex,

priority

Response

200 - application/json

Successful chat completion response.

string

Unique completion identifier.

Example:

"chatcmpl-abc123"

object

enum<string>

Available options:

chat.completion

Example:

"chat.completion"

created

integer

Unix timestamp of creation.

Example:

1774412483

model

string

The model used (may include version suffix).

Example:

"gpt-5.4-2025-07-16"

choices

object[]

Array of completion choices.

Show child attributes

usage

object

Show child attributes

service_tier

string

Example:

"default"

system_fingerprint

string | null

Example:

"fp_490a4ad033"

Overview

API Reference

Integration Guides

Errors

Pricing & Billing

Support

Overview

Important Notes

Message Roles

Multimodal Input

Streaming

Structured Outputs

Tool / Function Calling

Response Fields

Cross-Provider Notes

FAQ

How to handle rate limits?

How to maintain conversation context?

What does `finish_reason` mean?

How to control costs?

Authorizations

Body

Response

Overview

API Reference

Integration Guides

Errors

Pricing & Billing

Support

​Overview

​Important Notes

​Message Roles

​Multimodal Input

​Streaming

​Structured Outputs

​Tool / Function Calling

​Response Fields

​Cross-Provider Notes

​FAQ

​How to handle rate limits?

​How to maintain conversation context?

​What does finish_reason mean?

​How to control costs?

Authorizations

Body

Response

Overview

Important Notes

Message Roles

Multimodal Input

Streaming

Structured Outputs

Tool / Function Calling

Response Fields

Cross-Provider Notes

FAQ

How to handle rate limits?

How to maintain conversation context?

What does `finish_reason` mean?

How to control costs?