API reference

This page covers the key APIs in the Braintrust Go SDK. For setup, see the Quickstart. For the complete reference, see pkg.go.dev.

Tracing

Tracing records what your application does as spans you can inspect in Braintrust. The recommended way to capture AI calls is auto-instrumentation: use the trace/contrib packages to instrument supported provider libraries, either at build time with Orchestrion or with runtime middleware (see Go SDK integrations). Tracing is built on OpenTelemetry, so you trace your own code with the standard OpenTelemetry API. The APIs below create the client, trace your own code, and link to your traces.

`braintrust.New`

Creates a Braintrust client and configures the OpenTelemetry pipeline that exports spans to Braintrust. Call it once on startup, passing your TracerProvider and any options.

#skip-compile

import (
	"github.com/braintrustdata/braintrust-sdk-go"
	"go.opentelemetry.io/otel"
	"go.opentelemetry.io/otel/sdk/trace"
)

tp := trace.NewTracerProvider()
otel.SetTracerProvider(tp)

client, err := braintrust.New(tp, braintrust.WithProject("My project"))
if err != nil {
	log.Fatal(err)
}

Returns: (*braintrust.Client, error). braintrust.New reads BRAINTRUST_API_KEY from the environment. Configure the rest with functional options or environment variables (see Configuration). Because tracing is built on OpenTelemetry, you trace your own application code with the standard OpenTelemetry API, and traced AI calls nest under your spans.

#skip-compile

ctx, span := otel.Tracer("my-app").Start(ctx, "process-request")
defer span.End()

`Client.Permalink`

Builds a Braintrust UI URL for a span, so you can link straight to a trace from your own logs or app.

#skip-compile

url := client.Permalink(span)

Returns: string.

Evaluations

An evaluation runs your task over a set of cases, scores each output, and logs the results to an experiment, which is how you measure quality and catch regressions as you change prompts or models. The recommended pattern is to define an eval once with braintrust.NewEval, then call Run with any dataset. The same definition works for local runs, bt eval <dir> runs from the command line, and remote eval runs triggered from the playground.

`braintrust.NewEval`

Creates a runnable *eval.Eval by combining a client with an eval definition. Call Run on it to execute the evaluation.

#skip-compile

import (
	"context"

	"github.com/braintrustdata/braintrust-sdk-go"
	"github.com/braintrustdata/braintrust-sdk-go/eval"
)

e := braintrust.NewEval(client, &eval.Eval[string, string]{
	Name: "classify",
	Task: eval.T(func(ctx context.Context, input string) (string, error) {
		return classify(input), nil
	}),
	Scorers: []eval.Scorer[string, string]{exactMatch},
	ProjectName: "my-project",
})

_, err := e.Run(ctx, eval.RunOpts[string, string]{
	Dataset: eval.NewDataset([]eval.Case[string, string]{
		{Input: "apple", Expected: "fruit"},
	}),
})

Returns: *eval.Eval[I, R]. Run returns (*eval.Result, error). eval.Eval[I, R] fields:

Name (string, required): eval name, used as the default experiment name and as the registration key for remote evals.
Task (eval.TaskFunc[I, R], required): the function under evaluation. Wrap a plain func(ctx, input) (output, error) with eval.T, or use eval.TaskWithHooks if the task needs to read parameters or other run metadata.
Scorers ([]eval.Scorer[I, R]): scoring functions applied to each case. Provide Scorers, Classifiers, or both.
Classifiers ([]eval.Classifier[I, R]): classifiers applied to each case.
ParameterSchema (eval.ParameterSchema): declares parameters the eval accepts. Each entry becomes a configurable control in the Braintrust playground (see Parameters).
Dataset (eval.Dataset[I, R]): default dataset for the eval. A playground run supplies its own dataset and overrides this. bt eval <dir> and a direct Run call use it when no dataset is supplied at call time.
ProjectName (string): Braintrust project for this eval. Falls back to the client’s configured project.

eval.RunOpts[I, R] fields (passed to Run):

Dataset (eval.Dataset[I, R]): test cases for this run. Defaults to Eval.Dataset; required if the eval defines none.
Experiment (string): experiment name. Defaults to Eval.Name.
ProjectName (string): overrides the project for this run.
ProjectID (string): identifies the project by ID instead of by name. Takes precedence over ProjectName when set.
Tags ([]string): tags to apply to the experiment.
Metadata (eval.Metadata): metadata to attach to the experiment.
Update (bool): append to an existing experiment with the same name. Defaults to false.
Parallelism (int): number of goroutines. Defaults to 1.
Quiet (bool): suppress result output. Defaults to false.

`braintrust.NewEvaluator`

Creates an evaluator for input type I and result type R, bound to a client. Call Run on it with the cases, task, and scorers to execute the evaluation and log an experiment.

#skip-compile

import (
	"context"

	"github.com/braintrustdata/braintrust-sdk-go"
	"github.com/braintrustdata/braintrust-sdk-go/eval"
)

evaluator := braintrust.NewEvaluator[string, string](client)

_, err := evaluator.Run(context.Background(), eval.Opts[string, string]{
	Experiment: "answers-v1",
	Dataset: eval.NewDataset([]eval.Case[string, string]{
		{Input: "How do I reset my password?", Expected: "Use the account recovery flow."},
		{Input: "How do I export my data?", Expected: "Open Settings and choose Export."},
	}),
	Task: eval.T(answerQuestion),
	Scorers: []eval.Scorer[string, string]{
		eval.NewScorer("exact_match", func(_ context.Context, r eval.TaskResult[string, string]) (eval.Scores, error) {
			v := 0.0
			if r.Output == r.Expected {
				v = 1.0
			}
			return eval.S(v), nil
		}),
	},
})

Returns: *eval.Evaluator[I, R]. Run returns (*eval.Result, error). eval.Opts[I, R] fields:

Experiment (string, required): experiment name.
Dataset (eval.Dataset[I, R], required): the cases to run. Build an in-memory one with eval.NewDataset (see Datasets).
Task (eval.TaskFunc[I, R], required): the function under test. Wrap a plain func(ctx, input) (output, error) with eval.T.
Scorers ([]eval.Scorer[I, R]): scorers to apply to each case. Provide Scorers, Classifiers, or both.
Classifiers ([]eval.Classifier[I, R]): classifiers to apply to each case. Provide Scorers, Classifiers, or both.
ProjectName (string): project to log to. Defaults to the client’s configured project.
ProjectID (string): project ID. When set, takes precedence over ProjectName and uses the project as-is rather than creating it.
Tags ([]string): tags to apply to the experiment.
Metadata (eval.Metadata): metadata to attach to the experiment.
Update (bool): append to an existing experiment with the same name. Defaults to false.
Parallelism (int): number of goroutines. Defaults to 1.
TrialCount (int): number of times to run each case. Defaults to 1.
Quiet (bool): suppress result output. Defaults to false.

`eval.NewScorer`

Creates a scorer from a function. A scorer measures how good the task’s output is, returning one or more named scores per case.

#skip-compile

scorer := eval.NewScorer("exact_match", func(_ context.Context, r eval.TaskResult[string, string]) (eval.Scores, error) {
	if r.Output == r.Expected {
		return eval.S(1.0), nil
	}
	return eval.S(0.0), nil
})

Returns: eval.Scorer[I, R]. The score function receives an eval.TaskResult[I, R] (with Input, Output, Expected, and Metadata) and returns eval.Scores. Use eval.S to build a single score.

`eval.NewClassifier`

Creates a classifier from a function. Use a classifier to categorize output instead of scoring it numerically.

#skip-compile

classifier := eval.NewClassifier("topic", func(_ context.Context, r eval.TaskResult[string, string]) (eval.Classifications, error) {
	return eval.Classifications{{ID: "billing", Label: "Billing"}}, nil
})

Returns: eval.Classifier[I, R].

`eval.TaskWithHooks`

Creates a task function that receives *eval.TaskHooks, which gives access to parameters, metadata, tags, and the current spans. Use it when your task needs to read parameter values that were configured in the playground or passed via RunOpts.Parameters.

#skip-compile

task := eval.TaskWithHooks(func(ctx context.Context, input string, hooks *eval.TaskHooks) (string, error) {
	model := hooks.Parameters.String("model")
	return classify(input, model), nil
})

Returns: eval.TaskFunc[I, R]. The hooks give access to Parameters, Metadata, Tags, TrialIndex, TaskSpan, and EvalSpan. Use eval.T instead when the task doesn’t need hooks.

Parameters

Parameters let you declare configurable options on an eval. When the eval runs from the Braintrust playground via a remote eval, each declared parameter becomes a control in the UI. For a local run, the task receives the declared defaults. Declare parameters with eval.ParameterSchema on the eval.Eval definition:

#skip-compile

e := braintrust.NewEval(client, &eval.Eval[string, string]{
	Name: "classify",
	ParameterSchema: eval.ParameterSchema{
		"model": {
			Type:        eval.ParameterTypeModel,
			Default:     "gpt-5-mini",
			Description: "Model to use for classification",
		},
		"threshold": {
			Type:    "number",
			Default: 0.5,
		},
	},
	Task: eval.TaskWithHooks(func(ctx context.Context, input string, hooks *eval.TaskHooks) (string, error) {
		model := hooks.Parameters.String("model")
		threshold := hooks.Parameters.Float64("threshold")
		return classify(input, model, threshold), nil
	}),
	// ...
})

eval.ParameterSchema is map[string]eval.ParameterDef. Each eval.ParameterDef has:

Type (string): "model" renders a model picker. "prompt" renders a prompt editor. "string", "number", "integer", "boolean" render plain inputs. Use the eval.ParameterTypeModel and eval.ParameterTypePrompt constants for the two dedicated controls.
Default (any): used when no value is supplied for this run. This is also what a local run sees.
Description (string): shown alongside the control in the playground.

eval.Parameters (the resolved values, available as hooks.Parameters) has typed accessors that never panic on a type mismatch:

Parameters.String(name) → string
Parameters.Int(name) → int
Parameters.Float64(name) → float64
Parameters.Bool(name) → bool
Parameters.Get(name) → (any, bool)
Parameters.Has(name) → bool

Remote evals

Remote evals let the Braintrust playground trigger your Go eval code on your own infrastructure. The evalrunner package turns a Go binary into a target that the bt CLI can drive.

in public preview and can change before reaching general availability.

`evalrunner.New` and `evalrunner.RegisterEval`

Create a runner and register your evals. The runner reads the bt environment variables, dispatches the right eval, and streams results back.

#skip-compile

package main

import (
	"context"

	"github.com/braintrustdata/braintrust-sdk-go/eval"
	"github.com/braintrustdata/braintrust-sdk-go/evalrunner"
)

func main() {
	r := evalrunner.New()

	evalrunner.RegisterEval(r, &eval.Eval[string, string]{
		Name: "classify",
		Task: eval.TaskWithHooks(func(ctx context.Context, input string, hooks *eval.TaskHooks) (string, error) {
			model := hooks.Parameters.String("model")
			return classify(input, model), nil
		}),
		Scorers: []eval.Scorer[string, string]{exactMatch},
		ParameterSchema: eval.ParameterSchema{
			"model": {Type: eval.ParameterTypeModel, Default: "gpt-5-mini"},
		},
		ProjectName: "my-project",
	})

	evalrunner.Main(r)
}

Then run from the command line:

# Run all evals locally
bt eval ./cmd/evals

# Start the dev server so the playground can trigger runs
bt eval --dev --language go ./cmd/evals

evalrunner.New accepts evalrunner.Option values:

evalrunner.WithLogger(l logger.Logger): custom logger. Logs go to stderr. Stdout is reserved for the bt protocol.
evalrunner.WithTracerProvider(tp *sdktrace.TracerProvider): shared OpenTelemetry provider. Supply one when instrumented code (LLM clients, custom spans) should appear in the same trace as eval spans. When nil, a per-run provider is created and shut down on exit.

evalrunner.RegisterEval[I, R any](r *Runner, ev *eval.Eval[I, R]) registers an eval by its Name. Registering two evals under the same name replaces the first. evalrunner.Main(r *Runner) dispatches and exits. Use evalrunner.Run(ctx, r) instead if you need to handle the error yourself. r.Mode() returns the current evalrunner.Mode: ModeList, ModeEval, ModeBatch, or ModeInspect. Use it to skip expensive setup when bt only wants the list of registered evals:

#skip-compile

if r.Mode() != evalrunner.ModeList {
	warmCaches()
}

Datasets

A dataset is the set of cases an evaluation runs against. Define cases inline in memory, or manage datasets in Braintrust through the API client.

`eval.NewDataset`

Groups cases into an in-memory dataset you pass to Evaluator.Run, as an alternative to loading one from Braintrust.

#skip-compile

dataset := eval.NewDataset([]eval.Case[string, string]{
	{Input: "How do I reset my password?", Expected: "Use the account recovery flow."},
	{Input: "How do I export my data?", Expected: "Open Settings and choose Export."},
})

Returns: eval.Dataset[I, R]. Each eval.Case[I, R] has an Input and optional Expected, Tags, Metadata, and TrialCount.

Attachments

When your traces involve binary content like images or PDFs, log it as an attachment so it appears in Braintrust instead of as an opaque blob. When you trace AI calls, Braintrust automatically converts base64 attachments in provider messages into uploaded attachments, so you rarely need the APIs below for instrumented calls. Reach for them when you’re attaching binary content to a span yourself.

`attachment.From*`

Creates an attachment from bytes, a file, or a URL.

#skip-compile

import "github.com/braintrustdata/braintrust-sdk-go/trace/attachment"

att, err := attachment.FromFile("image/png", "chart.png")

Constructors:

attachment.FromBytes(contentType string, data []byte) → *attachment.Attachment: from raw bytes.
attachment.FromFile(contentType string, path string) → (*attachment.Attachment, error): reads a file.
attachment.FromURL(url string) → (*attachment.Attachment, error): fetches a URL and uses the response content type.
attachment.FromReader(contentType string, r io.Reader) → *attachment.Attachment: from an io.Reader.

API client

For direct access to the Braintrust REST API, use the api package. Reach for it to manage projects, experiments, datasets, and functions programmatically, beyond what the higher-level APIs above cover.

`api.NewClient`

Creates a REST API client from an API key.

#skip-compile

import "github.com/braintrustdata/braintrust-sdk-go/api"

client := api.NewClient(os.Getenv("BRAINTRUST_API_KEY"))

Returns: *api.API. Namespaces:

client.Projects(): project management.
client.Experiments(): experiment management.
client.Datasets(): dataset management. Methods include Create, Insert, InsertEvents, Delete, Fetch, and Query.
client.Functions(): function management, including Invoke(ctx, functionID, input) to call a deployed function.

Configuration

Configure the client with functional options passed to braintrust.New, or with environment variables.

#skip-compile

client, err := braintrust.New(tp,
	braintrust.WithProject("My project"),
	braintrust.WithBlockingLogin(true),
)

Client options:

braintrust.WithAPIKey(apiKey string): API key. Defaults to BRAINTRUST_API_KEY.
braintrust.WithAPIURL(apiURL string): Braintrust API URL.
braintrust.WithAppURL(appURL string): Braintrust app URL, used for permalinks.
braintrust.WithOrgName(orgName string): organization name, useful when credentials can access multiple orgs.
braintrust.WithProject(projectName string): project that receives exported spans.
braintrust.WithProjectID(projectID string): project ID. Takes precedence over the project name.
braintrust.WithBlockingLogin(enabled bool): log in synchronously during New instead of in the background.
braintrust.WithExporter(exporter trace.SpanExporter): supply a custom span exporter. Intended for testing.
braintrust.WithEnableTraceConsoleLog(enabled bool): print spans to the console.
braintrust.WithFilterAISpans(enabled bool): export only AI-related spans.
braintrust.WithEnvironment(environmentType string, name ...string): set span-origin environment provenance. Overrides auto-detection from CI and server environment variables.

Environment variables

BRAINTRUST_API_KEY (required): Braintrust API key.
BRAINTRUST_API_URL: Braintrust API URL. Defaults to https://api.braintrust.dev.
BRAINTRUST_APP_URL: Braintrust app URL, used for permalinks. Defaults to https://www.braintrust.dev.
BRAINTRUST_DEFAULT_PROJECT: project that traced spans route to. Defaults to default-go-project.
BRAINTRUST_DEFAULT_PROJECT_ID: project UUID. Takes precedence over the project name.
BRAINTRUST_ORG_NAME: organization name, useful when credentials can access multiple orgs.
BRAINTRUST_OTEL_FILTER_AI_SPANS: set to true to export only AI-related spans.
BRAINTRUST_ENABLE_TRACE_CONSOLE_LOG: set to true to print spans to the console.
BRAINTRUST_OTEL_ENABLE_BUILTIN_ADK_TRACES: set to true to export spans from Google ADK’s built-in telemetry. Defaults to false.
BRAINTRUST_AUTO_CONVERT_AI_ATTACHMENTS: scan spans for base64 attachments and upload them to object storage. Defaults to true; set to false to disable.
BRAINTRUST_DEBUG: set to true to enable SDK debug logging. Defaults to false.
BRAINTRUST_BLOCKING_LOGIN: set to true to log in synchronously at startup.
BRAINTRUST_ENVIRONMENT_TYPE: environment type for span-origin provenance (for example, ci or server). Detected automatically from common CI and server runtimes when unset.
BRAINTRUST_ENVIRONMENT_NAME: environment name for span-origin provenance (for example, github_actions or aws_lambda). Detected automatically when unset.

Use the SDK

Reference

Tracing

`braintrust.New`

`Client.Permalink`

Evaluations

`braintrust.NewEval`

`braintrust.NewEvaluator`

`eval.NewScorer`

`eval.NewClassifier`

`eval.TaskWithHooks`

Parameters

Remote evals

`evalrunner.New` and `evalrunner.RegisterEval`

Datasets

`eval.NewDataset`

Attachments

`attachment.From*`

API client

`api.NewClient`

Configuration

Environment variables

​Tracing

​braintrust.New

​Client.Permalink

​Evaluations

​braintrust.NewEval

​braintrust.NewEvaluator

​eval.NewScorer

​eval.NewClassifier

​eval.TaskWithHooks

​Parameters

​Remote evals

​evalrunner.New and evalrunner.RegisterEval

​Datasets

​eval.NewDataset

​Attachments

​attachment.From*

​API client

​api.NewClient

​Configuration

​Environment variables

Tracing

`braintrust.New`

`Client.Permalink`

Evaluations

`braintrust.NewEval`

`braintrust.NewEvaluator`

`eval.NewScorer`

`eval.NewClassifier`

`eval.TaskWithHooks`

Parameters

Remote evals

`evalrunner.New` and `evalrunner.RegisterEval`

Datasets

`eval.NewDataset`

Attachments

`attachment.From*`

API client

`api.NewClient`

Configuration

Environment variables