Go - LangWatch

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; see Model naming.

SDK	Base URL	Where the key goes
OpenAI	`https://gateway.langwatch.ai/v1/`	`option.WithAPIKey`, sent as `Authorization: Bearer`
Anthropic	`https://gateway.langwatch.ai/`	`option.WithAPIKey`, sent as `X-Api-Key`

Keep the trailing slash on the base URL: the Go SDKs append the route to it.

Install

go get github.com/openai/openai-go/v3
# add the SDK you use: github.com/anthropics/anthropic-sdk-go
# and go.opentelemetry.io/otel, 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 trace your own code.

OpenAI SDK

package main

import (
	"context"
	"fmt"
	"os"

	"github.com/openai/openai-go/v3"
	"github.com/openai/openai-go/v3/option"
)

func main() {
	client := openai.NewClient(
		option.WithBaseURL("https://gateway.langwatch.ai/v1/"),
		option.WithAPIKey(os.Getenv("LANGWATCH_VIRTUAL_KEY")),
	)

	response, err := client.Chat.Completions.New(context.Background(), openai.ChatCompletionNewParams{
		Model: openai.ChatModel("openai/gpt-5-mini"),
		Messages: []openai.ChatCompletionMessageParamUnion{
			openai.UserMessage("Say hi in one word"),
		},
	})
	if err != nil {
		panic(err)
	}
	fmt.Println(response.Choices[0].Message.Content)
}

The same client serves the Responses API and embeddings. With an Anthropic or Gemini model name the gateway translates the chat request to the provider’s format.

Anthropic SDK

package main

import (
	"context"
	"fmt"
	"os"

	"github.com/anthropics/anthropic-sdk-go"
	"github.com/anthropics/anthropic-sdk-go/option"
)

func main() {
	client := anthropic.NewClient(
		option.WithBaseURL("https://gateway.langwatch.ai/"),
		option.WithAPIKey(os.Getenv("LANGWATCH_VIRTUAL_KEY")),
	)

	message, err := client.Messages.New(context.Background(), anthropic.MessageNewParams{
		Model:     anthropic.Model("anthropic/claude-haiku-4-5"),
		MaxTokens: 64,
		Messages: []anthropic.MessageParam{
			anthropic.NewUserMessage(anthropic.NewTextBlock("Say hi in one word")),
		},
	})
	if err != nil {
		panic(err)
	}
	fmt.Println(message.Content[0].Text)
}

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:

import (
	"context"
	"net/http"

	"github.com/openai/openai-go/v3"
	"github.com/openai/openai-go/v3/option"
	"go.opentelemetry.io/otel"
	"go.opentelemetry.io/otel/propagation"
)

func traceOptions(ctx context.Context) []option.RequestOption {
	carrier := propagation.HeaderCarrier(http.Header{})
	otel.GetTextMapPropagator().Inject(ctx, carrier) // traceparent: 00-<trace id>-<span id>-01
	opts := []option.RequestOption{}
	for key, values := range carrier {
		opts = append(opts, option.WithHeader(key, values[0]))
	}
	return opts
}

func ask(ctx context.Context, client openai.Client) (*openai.ChatCompletion, error) {
	return client.Chat.Completions.New(ctx, openai.ChatCompletionNewParams{
		Model: openai.ChatModel("openai/gpt-5-mini"),
		Messages: []openai.ChatCompletionMessageParamUnion{
			openai.UserMessage("Say hi in one word"),
		},
	}, traceOptions(ctx)...)
}

otel.GetTextMapPropagator() returns a no-op until you register a propagator, for example otel.SetTextMapPropagator(propagation.TraceContext{}). The LangWatch Go SDK is built on OpenTelemetry, so the same code works inside a langwatch span. Do not also add the LangWatch OpenAI middleware to clients that call 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 with option.WithHeader("X-LangWatch-End-User-Id", "cust_8841") as an extra argument to the call. The gateway removes them before it forwards the request to the provider. See Billing and spend events.

Read the response headers

var httpResponse *http.Response

response, err := client.Chat.Completions.New(ctx, params, option.WithResponseInto(&httpResponse))
if err != nil {
	return err
}
fmt.Println(httpResponse.Header.Get("X-LangWatch-Gateway-Request-Id")) // quote this to support
fmt.Println(httpResponse.Header.Get("X-LangWatch-Budget-Warning"))     // "project:84" when a budget is at 80 percent or more
fmt.Println(httpResponse.Header.Get("Traceparent"))                    // the gateway span in your project
_ = response

Manage keys and budgets from Go

The LangWatch Go SDK has no helpers for virtual keys, budgets or spend events. Call the management routes over REST with a LangWatch API key. See Virtual keys and Billing and spend events.

Self-hosted gateway

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

client := openai.NewClient(
	option.WithBaseURL("https://gateway.acme.internal/v1/"),
	option.WithAPIKey(os.Getenv("LANGWATCH_VIRTUAL_KEY")),
)

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 client also carries the LangWatch OpenAI middleware. Remove it for gateway calls.
The gateway span is its own trace	No `traceparent` on the request, no propagator registered, or the headers were computed outside the active span.

See Errors for every error code.

Get Started

SDK Integration

Virtual Keys & Budgets

Billing & Metering

Governance

Providers

Features

API Reference

Self-Hosting

Cookbooks

Go

What changes in your code

Install

OpenAI SDK

Anthropic SDK

Check it worked

Link the gateway span to your trace

Headers you can send

Read the response headers

Manage keys and budgets from Go

Self-hosted gateway

Troubleshooting

​What changes in your code

​Install

​OpenAI SDK

​Anthropic SDK

​Check it worked

​Link the gateway span to your trace

​Headers you can send

​Read the response headers

​Manage keys and budgets from Go

​Self-hosted gateway

​Troubleshooting

What changes in your code

Install

OpenAI SDK

Anthropic SDK

Check it worked

Link the gateway span to your trace

Headers you can send

Read the response headers

Manage keys and budgets from Go

Self-hosted gateway

Troubleshooting