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

# API formats

> Use OpenAI Chat Completions, Anthropic Messages, or OpenAI Responses requests to call models across providers through the LLM Gateway.

The standard LLM Gateway API supports three request and response formats. Choose the format your application already uses, then call models across configured providers with provider-prefixed model IDs.

<Note>
  **Beta:** The LLM Gateway is in [beta](/langsmith/release-stages).
</Note>

## Compare API formats

| API format              | Base URL                                 | Prompt endpoint          | Compatible client                          |
| ----------------------- | ---------------------------------------- | ------------------------ | ------------------------------------------ |
| OpenAI Chat Completions | `https://gateway.smith.langchain.com/v1` | `POST /chat/completions` | OpenAI-compatible Chat Completions clients |
| Anthropic Messages      | `https://gateway.smith.langchain.com`    | `POST /v1/messages`      | Anthropic Messages clients                 |
| OpenAI Responses        | `https://gateway.smith.langchain.com/v1` | `POST /responses`        | OpenAI-compatible Responses clients        |

All formats authenticate with a workspace-scoped LangSmith API key. Pass it as the provider API key or as an `Authorization: Bearer` token.

Set `model` to `<provider>/<model>`, such as `openai/gpt-5.4-mini` or `anthropic/claude-sonnet-4-6`. The request format does not limit which configured provider you can call.

## Use Chat Completions

Point an OpenAI-compatible client at `https://gateway.smith.langchain.com/v1`. For the full request and response schema, see the [OpenAI Chat Completions API](https://platform.openai.com/docs/api-reference/chat).

<CodeGroup>
  ```bash cURL theme={"theme":{"light":"catppuccin-latte","dark":"catppuccin-mocha"}}
  curl https://gateway.smith.langchain.com/v1/chat/completions \
      -H "Authorization: Bearer $LANGSMITH_API_KEY" \
      -H "Content-Type: application/json" \
      -d '{"model":"anthropic/claude-sonnet-4-6","messages":[{"role":"user","content":"Hello!"}]}'
  ```

  ```python Python theme={"theme":{"light":"catppuccin-latte","dark":"catppuccin-mocha"}}
  import os

  from openai import OpenAI

  client = OpenAI(
      base_url="https://gateway.smith.langchain.com/v1",
      api_key=os.environ["LANGSMITH_API_KEY"],
  )
  response = client.chat.completions.create(
      model="anthropic/claude-sonnet-4-6",
      messages=[{"role": "user", "content": "Hello!"}],
  )
  ```

  ```typescript TypeScript theme={"theme":{"light":"catppuccin-latte","dark":"catppuccin-mocha"}}
  import OpenAI from "openai";

  const client = new OpenAI({
    baseURL: "https://gateway.smith.langchain.com/v1",
    apiKey: process.env.LANGSMITH_API_KEY,
  });
  const response = await client.chat.completions.create({
    model: "anthropic/claude-sonnet-4-6",
    messages: [{ role: "user", content: "Hello!" }],
  });
  ```
</CodeGroup>

## Use Messages

Point an Anthropic client at `https://gateway.smith.langchain.com`. For the full request and response schema, see the [Anthropic Messages API](https://docs.anthropic.com/en/api/messages).

<CodeGroup>
  ```bash cURL theme={"theme":{"light":"catppuccin-latte","dark":"catppuccin-mocha"}}
  curl https://gateway.smith.langchain.com/v1/messages \
      -H "Authorization: Bearer $LANGSMITH_API_KEY" \
      -H "Content-Type: application/json" \
      -d '{"model":"openai/gpt-5.4-mini","max_tokens":1024,"messages":[{"role":"user","content":"Hello!"}]}'
  ```

  ```python Python theme={"theme":{"light":"catppuccin-latte","dark":"catppuccin-mocha"}}
  import os

  import anthropic

  client = anthropic.Anthropic(
      base_url="https://gateway.smith.langchain.com",
      api_key=os.environ["LANGSMITH_API_KEY"],
  )
  message = client.messages.create(
      model="openai/gpt-5.4-mini",
      max_tokens=1024,
      messages=[{"role": "user", "content": "Hello!"}],
  )
  ```

  ```typescript TypeScript theme={"theme":{"light":"catppuccin-latte","dark":"catppuccin-mocha"}}
  import Anthropic from "@anthropic-ai/sdk";

  const client = new Anthropic({
    baseURL: "https://gateway.smith.langchain.com",
    apiKey: process.env.LANGSMITH_API_KEY,
  });
  const message = await client.messages.create({
    model: "openai/gpt-5.4-mini",
    max_tokens: 1024,
    messages: [{ role: "user", content: "Hello!" }],
  });
  ```
</CodeGroup>

## Use Responses

Point an OpenAI-compatible client at `https://gateway.smith.langchain.com/v1`. For the full request and response schema, see the [OpenAI Responses API](https://platform.openai.com/docs/api-reference/responses).

<CodeGroup>
  ```bash cURL theme={"theme":{"light":"catppuccin-latte","dark":"catppuccin-mocha"}}
  curl https://gateway.smith.langchain.com/v1/responses \
      -H "Authorization: Bearer $LANGSMITH_API_KEY" \
      -H "Content-Type: application/json" \
      -d '{"model":"anthropic/claude-sonnet-4-6","input":"Hello!"}'
  ```

  ```python Python theme={"theme":{"light":"catppuccin-latte","dark":"catppuccin-mocha"}}
  import os

  from openai import OpenAI

  client = OpenAI(
      base_url="https://gateway.smith.langchain.com/v1",
      api_key=os.environ["LANGSMITH_API_KEY"],
  )
  response = client.responses.create(
      model="anthropic/claude-sonnet-4-6",
      input="Hello!",
  )
  ```

  ```typescript TypeScript theme={"theme":{"light":"catppuccin-latte","dark":"catppuccin-mocha"}}
  import OpenAI from "openai";

  const client = new OpenAI({
    baseURL: "https://gateway.smith.langchain.com/v1",
    apiKey: process.env.LANGSMITH_API_KEY,
  });
  const response = await client.responses.create({
    model: "anthropic/claude-sonnet-4-6",
    input: "Hello!",
  });
  ```
</CodeGroup>

## Understand translation behavior

The endpoint determines the format your application sends and receives. The model ID determines the upstream provider.

* When the provider supports the selected format natively, the gateway preserves that format.
* Otherwise, the gateway translates the request into a format supported by the provider and translates the response back, including streaming responses.
* Translation can reject fields that cannot be represented in the target provider format. Use [Direct model access](/langsmith/llm-gateway-direct-model-access) when provider-native behavior is required.

Every request resolves the same Provider Secrets, policies, and tracing configuration regardless of format.

## List models

Call `GET /v1/models` to list models available across providers configured for the workspace. The gateway aggregates each provider's model catalog into a single OpenAI-compatible list:

```bash theme={"theme":{"light":"catppuccin-latte","dark":"catppuccin-mocha"}}
curl https://gateway.smith.langchain.com/v1/models \
    -H "Authorization: Bearer $LANGSMITH_API_KEY"
```

```json theme={"theme":{"light":"catppuccin-latte","dark":"catppuccin-mocha"}}
{
  "object": "list",
  "data": [
    {"id": "openai/gpt-5.4-mini", "object": "model"},
    {"id": "fireworks/accounts/fireworks/models/glm-5p2", "object": "model"},
    {"id": "anthropic/claude-sonnet-4-6", "object": "model"}
  ]
}
```

Every model ID is prefixed with the provider name in the form `<provider>/<model>`; use this prefixed ID as `model` when making a call. A provider without a configured secret is omitted.

## Use a regional gateway

Replace `gateway.smith.langchain.com` with the hostname for your LangSmith region:

| Region   | Gateway hostname                   |
| -------- | ---------------------------------- |
| GCP US   | `gateway.smith.langchain.com`      |
| GCP EU   | `eu.gateway.smith.langchain.com`   |
| GCP APAC | `apac.gateway.smith.langchain.com` |
| AWS US   | `aws.gateway.smith.langchain.com`  |

Keep the same path for the selected API format.

## Handle errors

| Status or symptom                                           | Meaning                                                                                               |
| ----------------------------------------------------------- | ----------------------------------------------------------------------------------------------------- |
| `400 Bad Request`                                           | The request is malformed, the model ID is not provider-prefixed, or the request cannot be translated. |
| `401 Unauthorized`                                          | The LangSmith API key is missing or invalid.                                                          |
| `403 Forbidden`                                             | The key does not have the required gateway permissions.                                               |
| `429 Too Many Requests`                                     | A gateway rate limit or an upstream provider rate limit was reached.                                  |
| No models with a provider prefix appear in `GET /v1/models` | The provider may not be configured or may not have returned a model catalog.                          |

For setup-specific resolutions, see the [Quickstart](/langsmith/llm-gateway-quickstart).

## See also

* [Quickstart](/langsmith/llm-gateway-quickstart): make your first request and view its trace.
* [Direct model access](/langsmith/llm-gateway-direct-model-access): bypass format translation and use provider-native APIs.
* [Model fallbacks](/langsmith/llm-gateway-fallbacks): retry requests against backup models.

***

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