anthropic

Anthropic Plugin

This plugin provides Genkit support for Claude models through Anthropic's native Messages API, using the official Go SDK, github.com/anthropics/anthropic-sdk-go.

Building with a coding agent? Install the Genkit Go skill first.

npx skills add genkit-ai/skills --skill developing-genkit-go

It teaches your agent the current Genkit Go APIs and common gotchas. Source, manual install and skills for other languages: genkit-ai/skills.

Unlike the OpenAI-compatible compat_oai/anthropic plugin, this one speaks the Messages API itself: thinking content comes back as Genkit reasoning parts with their signatures preserved across turns, structured output uses Anthropic's native support, and Anthropic's server-side tools are reachable.

Setup

Set an Anthropic API key:

export ANTHROPIC_API_KEY=<your-api-key>

ANTHROPIC_BASE_URL overrides the endpoint; both settings can also be set as plugin struct fields.

import (
    "context"

    "github.com/firebase/genkit/go/ai"
    "github.com/firebase/genkit/go/genkit"
    "github.com/firebase/genkit/go/plugins/anthropic"
)

ctx := context.Background()
g := genkit.Init(
    ctx,
    genkit.WithPlugins(&anthropic.Anthropic{}),
    genkit.WithDefaultModel("anthropic/claude-sonnet-5"),
)

response, err := genkit.Generate(ctx, g, ai.WithPrompt("Explain constitutional AI."))

Claude's thinking output is returned as Genkit reasoning parts, available through response.Reasoning(), and each block's signature rides along so multi-turn conversations replay their thinking intact.

Models

Init registers nothing: every Claude model resolves on first use, and the plugin lists Anthropic's current catalog through the models API, cached for an hour. Curated entries describe the current families (claude-fable-5, claude-opus-5, claude-sonnet-5, the Claude 4 line, claude-haiku-4-5), dated snapshots such as claude-opus-4-5-20251101 resolve to the same descriptions, and the Models field describes or corrects any model, most often one released after this plugin:

plugin := &anthropic.Anthropic{Models: map[string]ai.ModelOptions{
    "claude-opus-6": {Label: "Claude Opus 6"},
}}

The current model list is at https://platform.claude.com/docs/en/about-claude/models/overview.

Config

Models take the SDK's own request type, anthropic.MessageNewParams, with Anthropic's wire names. This package and the SDK are both named anthropic, so alias one of them:

import (
    sdk "github.com/anthropics/anthropic-sdk-go"

    "github.com/firebase/genkit/go/ai"
    "github.com/firebase/genkit/go/genkit"
    "github.com/firebase/genkit/go/plugins/anthropic"
)

response, err := genkit.Generate(ctx, g,
    ai.WithModel(anthropic.ModelRef("claude-opus-4-5", &sdk.MessageNewParams{
        MaxTokens: 1024,
    })),
    ai.WithPrompt("Answer concisely."),
)

The fields Genkit builds from the request are not config: a config carrying messages, system, model, an output format, or a custom function tool is rejected with an error naming the Genkit option to use instead (ai.WithMessages, ai.WithSystem, ai.WithModel, ai.WithOutputType, ai.WithTools). The config-level tools field stays available for Anthropic's server-side tools, such as web search and code execution.

The advertised schema is reflected from the anthropic-sdk-go version your build links, so a field Anthropic ships tomorrow becomes usable, and validated, by bumping the SDK in your own go.mod.

The API reference is at https://platform.claude.com/docs/en/api/overview.

Live tests

Live tests are skipped unless ANTHROPIC_API_KEY is set:

go test -v ./plugins/anthropic