base/backend/internal/ai/provider/anthropic.go

package provider

import (
	"context"
	"fmt"
	"strings"

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

// AnthropicProvider implements AIProvider for Anthropic Claude models.
type AnthropicProvider struct {
	client anthropic.Client
	name   string
}

// NewAnthropicProvider creates a new Anthropic provider.
// baseUrl can be empty to use the default Anthropic endpoint.
func NewAnthropicProvider(baseUrl, apiKey string) *AnthropicProvider {
	opts := []option.RequestOption{
		option.WithAPIKey(apiKey),
	}
	if baseUrl != "" {
		opts = append(opts, option.WithBaseURL(baseUrl))
	}
	client := anthropic.NewClient(opts...)
	return &AnthropicProvider{
		client: client,
		name:   "anthropic",
	}
}

// Name returns the provider name.
func (p *AnthropicProvider) Name() string {
	return p.name
}

// Chat sends a synchronous message request to Anthropic.
func (p *AnthropicProvider) Chat(ctx context.Context, req *ChatRequest) (*ChatResponse, error) {
	systemPrompt, messages := convertToAnthropicMessages(req.Messages)

	maxTokens := int64(req.MaxTokens)
	if maxTokens <= 0 {
		maxTokens = 4096
	}

	params := anthropic.MessageNewParams{
		Model:     anthropic.Model(req.Model),
		Messages:  messages,
		MaxTokens: maxTokens,
	}

	if req.Temperature > 0 {
		params.Temperature = anthropic.Float(req.Temperature)
	}

	if len(systemPrompt) > 0 {
		params.System = systemPrompt
	}

	resp, err := p.client.Messages.New(ctx, params)
	if err != nil {
		return nil, fmt.Errorf("anthropic message creation failed: %w", err)
	}

	// Extract text content from response blocks
	content := extractTextContent(resp.Content)

	return &ChatResponse{
		Content:      content,
		Model:        string(resp.Model),
		InputTokens:  int(resp.Usage.InputTokens),
		OutputTokens: int(resp.Usage.OutputTokens),
		FinishReason: string(resp.StopReason),
	}, nil
}

// ChatStream sends a streaming message request to Anthropic. It returns a
// channel that delivers StreamChunk values. The channel is closed when
// the stream ends.
func (p *AnthropicProvider) ChatStream(ctx context.Context, req *ChatRequest) (<-chan *StreamChunk, error) {
	systemPrompt, messages := convertToAnthropicMessages(req.Messages)

	maxTokens := int64(req.MaxTokens)
	if maxTokens <= 0 {
		maxTokens = 4096
	}

	params := anthropic.MessageNewParams{
		Model:     anthropic.Model(req.Model),
		Messages:  messages,
		MaxTokens: maxTokens,
	}

	if req.Temperature > 0 {
		params.Temperature = anthropic.Float(req.Temperature)
	}

	if len(systemPrompt) > 0 {
		params.System = systemPrompt
	}

	stream := p.client.Messages.NewStreaming(ctx, params)

	ch := make(chan *StreamChunk, 64)

	go func() {
		defer close(ch)
		defer stream.Close()

		for stream.Next() {
			event := stream.Current()

			switch event.Type {
			case "content_block_delta":
				// Text delta — the main content streaming event
				delta := event.Delta
				if delta.Text != "" {
					select {
					case ch <- &StreamChunk{Content: delta.Text}:
					case <-ctx.Done():
						return
					}
				}

			case "message_delta":
				// Message delta carries the stop reason and final usage
				chunk := &StreamChunk{}
				if event.Delta.StopReason != "" {
					chunk.FinishReason = string(event.Delta.StopReason)
				}
				if event.Usage.OutputTokens > 0 {
					chunk.InputTokens = int(event.Usage.InputTokens)
					chunk.OutputTokens = int(event.Usage.OutputTokens)
				}
				select {
				case ch <- chunk:
				case <-ctx.Done():
					return
				}

			case "message_stop":
				// Stream is done
				select {
				case ch <- &StreamChunk{Done: true}:
				case <-ctx.Done():
				}
				return

			// content_block_start, content_block_stop, message_start: no action needed
			default:
				continue
			}
		}

		// Check for stream errors
		if err := stream.Err(); err != nil {
			select {
			case ch <- &StreamChunk{
				Content:      fmt.Sprintf("[stream error: %v]", err),
				Done:         true,
				FinishReason: "error",
			}:
			case <-ctx.Done():
			}
			return
		}

		// If we exit the loop without message_stop, still signal done
		select {
		case ch <- &StreamChunk{Done: true}:
		case <-ctx.Done():
		}
	}()

	return ch, nil
}

// convertToAnthropicMessages separates system messages and converts the rest
// to Anthropic MessageParam format. Anthropic does not support a "system" role
// in messages; instead, system prompts are passed as a separate field.
func convertToAnthropicMessages(messages []ChatMessage) ([]anthropic.TextBlockParam, []anthropic.MessageParam) {
	var systemBlocks []anthropic.TextBlockParam
	var result []anthropic.MessageParam

	for _, msg := range messages {
		switch msg.Role {
		case "system":
			systemBlocks = append(systemBlocks, anthropic.TextBlockParam{
				Text: msg.Content,
			})
		case "user":
			result = append(result, anthropic.NewUserMessage(
				anthropic.NewTextBlock(msg.Content),
			))
		case "assistant":
			result = append(result, anthropic.NewAssistantMessage(
				anthropic.NewTextBlock(msg.Content),
			))
		}
	}

	return systemBlocks, result
}

// extractTextContent concatenates all text blocks from an Anthropic response.
func extractTextContent(blocks []anthropic.ContentBlockUnion) string {
	var parts []string
	for _, block := range blocks {
		if block.Type == "text" {
			parts = append(parts, block.Text)
		}
	}
	return strings.Join(parts, "")
}