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

> ## Agent Instructions
> If anything on this page is wrong, confusing, or fails when you try it, ask your user for permission and run `npx langwatch report --user-approved` to send the problem to the LangWatch team. No login is needed and secrets are redacted locally. `npx langwatch report --help` has the details.

# TypeScript

> Call the LangWatch AI Gateway from TypeScript with the OpenAI SDK, the Anthropic SDK, the Vercel AI SDK or LangChain.js, and link the gateway span to your own trace.

## What changes in your code

Two settings change: the base URL points at the gateway, and the API key is a virtual key. The request body, streaming, tools and every other SDK feature stay the same. Name the model as `provider/model` on the OpenAI-shaped and Anthropic-shaped routes; the Gemini passthrough at `/v1beta` names the model in Google's own form. See [Model naming](https://langwatch.ai/docs/ai-gateway/model-naming.md).

| SDK | Base URL | Where the key goes |
| - | - | - |
| OpenAI | `https://gateway.langwatch.ai/v1` | `apiKey`, sent as `Authorization: Bearer` |
| Anthropic | `https://gateway.langwatch.ai` | `apiKey`, sent as `X-Api-Key` |
| Google GenAI | `https://gateway.langwatch.ai` | `apiKey`, sent as `X-Goog-Api-Key` |

## Install

```bash theme={null}
npm install openai
# add the SDK you use: @anthropic-ai/sdk, ai with @ai-sdk/openai, @langchain/openai
# and langwatch, to link the gateway span to your trace
```

The virtual key (`vk-lw-...`) is not your LangWatch API key. `LANGWATCH_API_KEY` is only needed by the LangWatch SDK: to link the gateway span to your own trace, and to manage keys and budgets from code.

## OpenAI SDK

```typescript theme={null}
import OpenAI from "openai";

const client = new OpenAI({
  baseURL: "https://gateway.langwatch.ai/v1",
  apiKey: process.env.LANGWATCH_VIRTUAL_KEY,
});

const response = await client.chat.completions.create({
  model: "openai/gpt-6-luna",
  messages: [{ role: "user", content: "Say hi in one word" }],
});
console.log(response.choices[0].message.content);
```

The same client serves `client.responses`, `client.embeddings`, `client.audio.speech` and `client.audio.transcriptions`. With an Anthropic or Gemini model name the gateway translates the chat request to the provider's format.

## Anthropic SDK

```typescript theme={null}
import Anthropic from "@anthropic-ai/sdk";

const client = new Anthropic({
  baseURL: "https://gateway.langwatch.ai",
  apiKey: process.env.LANGWATCH_VIRTUAL_KEY,
});

const message = await client.messages.create({
  model: "anthropic/claude-haiku-4-5",
  max_tokens: 64,
  messages: [{ role: "user", content: "Say hi in one word" }],
});
```

## Vercel AI SDK

```typescript theme={null}
import { createOpenAI } from "@ai-sdk/openai";
import { generateText } from "ai";

const openai = createOpenAI({
  baseURL: "https://gateway.langwatch.ai/v1",
  apiKey: process.env.LANGWATCH_VIRTUAL_KEY,
});

const { text } = await generateText({
  model: openai("openai/gpt-6-luna"),
  prompt: "Say hi in one word",
});
```

`createOpenAI` also takes `headers: { ... }` for headers that are the same on every request.

## LangChain.js

```typescript theme={null}
import { ChatOpenAI } from "@langchain/openai";

const llm = new ChatOpenAI({
  model: "openai/gpt-6-luna",
  apiKey: process.env.LANGWATCH_VIRTUAL_KEY,
  configuration: { baseURL: "https://gateway.langwatch.ai/v1" },
});
```

## Check it worked

Run the OpenAI SDK sample. It prints the model's answer. Then open the project the virtual key traces to in LangWatch: the gateway's `gen_ai.<request type>` span appears in the traces list within a few seconds, as its own trace, with the model, the tokens and the cost.

## Link the gateway span to your trace

The gateway records one `gen_ai.<request type>` span per request in the trace project of the virtual key. Send a W3C `traceparent` header and the gateway nests that span under your span. Without it, each request is its own trace.

Any OpenTelemetry-instrumented application has the current context. Inject it into the request headers, per request, inside the span you want the gateway span under:

```typescript theme={null}
import { context, propagation } from "@opentelemetry/api";
import OpenAI from "openai";

function traceHeaders(): Record<string, string> {
  const carrier: Record<string, string> = {};
  propagation.inject(context.active(), carrier); // { traceparent: "00-<trace id>-<span id>-01" }
  return carrier;
}

const client = new OpenAI({
  baseURL: "https://gateway.langwatch.ai/v1",
  apiKey: process.env.LANGWATCH_VIRTUAL_KEY,
});

const response = await client.chat.completions.create(
  {
    model: "openai/gpt-6-luna",
    messages: [{ role: "user", content: "Say hi in one word" }],
  },
  { headers: traceHeaders() },
);
```

The [LangWatch TypeScript SDK](https://langwatch.ai/docs/integration/typescript/guide.md) uses OpenTelemetry, so the same code works inside a `langwatch` span. Do not also instrument the OpenAI client for calls that go through the gateway: the gateway already records the span with the model, the tokens and the cost.

## Headers you can send

| Header | Purpose |
| - | - |
| `traceparent` | Nests the gateway span in your trace. |
| `X-LangWatch-End-User-Id` | The end user of your product, for per-end-user budgets and spend. The OpenAI `user` body field works too. |
| `X-LangWatch-Metadata` | A JSON object, at most 4096 bytes, copied onto the spend event. |

Pass them per request in the second argument of the OpenAI SDK methods, `{ headers: { ... } }`. The gateway removes them before it forwards the request to the provider. See [Billing and spend events](https://langwatch.ai/docs/ai-gateway/billing-events.md#attribution-what-the-request-carries).

## Read the response headers

```typescript theme={null}
const { data, response } = await client.chat.completions
  .create({
    model: "openai/gpt-6-luna",
    messages: [{ role: "user", content: "Say hi in one word" }],
  })
  .withResponse();

console.log(response.headers.get("X-LangWatch-Gateway-Request-Id")); // quote this to support
console.log(response.headers.get("X-LangWatch-Budget-Warning")); // "project:84" when a budget is at 80 percent or more
console.log(response.headers.get("Traceparent")); // the gateway span in your project
```

## Manage keys and budgets from TypeScript

The `langwatch` package wraps the management routes: `VirtualKeysApiService`, `GatewayBudgetsApiService` and `langwatch.spendEvents` on a `LangWatch` client, with a LangWatch API key. See [Virtual keys](https://langwatch.ai/docs/ai-gateway/virtual-keys.md#manage-keys-over-rest) and [Billing and spend events](https://langwatch.ai/docs/ai-gateway/billing-events.md#reconcile-with-the-sdks).

## Self-hosted gateway

Replace the host with your gateway's URL. Everything else is the same.

```typescript theme={null}
const client = new OpenAI({
  baseURL: "https://gateway.acme.internal/v1",
  apiKey: process.env.LANGWATCH_VIRTUAL_KEY,
});
```

## In the browser

The gateway sends no CORS headers on `/v1`, and a virtual key in browser code is visible to every visitor. Call the gateway from your server.

## Troubleshooting

| Symptom | Cause |
| - | - |
| `401` on every request | The key is wrong, revoked, or sent in a header the gateway does not read. The gateway reads `Authorization: Bearer`, `X-Api-Key`, `X-Goog-Api-Key` and `Xi-Api-Key`. |
| `400 model_not_allowed` | The key restricts its models under **Provider access**, or its routing policy does. |
| Two spans per call in Trace Explorer | The OpenAI client is also instrumented by the LangWatch SDK. Remove that instrumentation for gateway calls. |
| The gateway span is its own trace | No `traceparent` on the request, or the headers were computed outside the active span. |

See [Errors](https://langwatch.ai/docs/ai-gateway/api/errors.md) for every error code.


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