core/server/providers/anthropic.go

package providers

import (
	"bufio"
	"bytes"
	"context"
	"encoding/json"
	"fmt"
	"io"
	"net/http"
	"strings"
)

const anthropicDefaultEndpoint = "https://api.anthropic.com"
const anthropicAPIVersion = "2023-06-01"

// AnthropicProvider handles the Anthropic Messages API.
type AnthropicProvider struct{}

func (p *AnthropicProvider) ID() string { return "anthropic" }

// ── Chat Completion (non-streaming) ─────────

func (p *AnthropicProvider) ChatCompletion(ctx context.Context, cfg ProviderConfig, req CompletionRequest) (*CompletionResponse, error) {
	req.Stream = false

	body, err := p.doRequest(ctx, cfg, req)
	if err != nil {
		return nil, err
	}
	defer body.Close()

	var resp anthropicResponse
	if err := json.NewDecoder(body).Decode(&resp); err != nil {
		return nil, fmt.Errorf("decode response: %w", err)
	}

	// Extract text from content blocks
	var content string
	for _, block := range resp.Content {
		if block.Type == "text" {
			content += block.Text
		}
	}

	return &CompletionResponse{
		Content:      content,
		Model:        resp.Model,
		FinishReason: resp.StopReason,
		InputTokens:  resp.Usage.InputTokens,
		OutputTokens: resp.Usage.OutputTokens,
	}, nil
}

// ── Stream Completion ───────────────────────

func (p *AnthropicProvider) StreamCompletion(ctx context.Context, cfg ProviderConfig, req CompletionRequest) (<-chan StreamEvent, error) {
	req.Stream = true

	body, err := p.doRequest(ctx, cfg, req)
	if err != nil {
		return nil, err
	}

	ch := make(chan StreamEvent, 64)

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

		scanner := bufio.NewScanner(body)
		for scanner.Scan() {
			line := scanner.Text()

			if !strings.HasPrefix(line, "data: ") {
				continue
			}

			data := strings.TrimPrefix(line, "data: ")

			var event anthropicStreamEvent
			if err := json.Unmarshal([]byte(data), &event); err != nil {
				continue
			}

			switch event.Type {
			case "content_block_delta":
				if event.Delta.Type == "text_delta" {
					ch <- StreamEvent{Delta: event.Delta.Text}
				}
			case "message_delta":
				ch <- StreamEvent{
					Done:         true,
					FinishReason: event.Delta.StopReason,
				}
			case "message_stop":
				ch <- StreamEvent{Done: true}
				return
			case "error":
				ch <- StreamEvent{
					Error: fmt.Errorf("anthropic stream error: %s", data),
				}
				return
			}
		}

		if err := scanner.Err(); err != nil {
			ch <- StreamEvent{Error: err}
		}
	}()

	return ch, nil
}

// ── List Models ─────────────────────────────

func (p *AnthropicProvider) ListModels(_ context.Context, _ ProviderConfig) ([]Model, error) {
	// Anthropic doesn't have a list models endpoint.
	// Return the known model families.
	return []Model{
		{ID: "claude-sonnet-4-20250514", Name: "Claude Sonnet 4"},
		{ID: "claude-haiku-4-20250414", Name: "Claude Haiku 4"},
		{ID: "claude-opus-4-20250514", Name: "Claude Opus 4"},
		{ID: "claude-3-5-sonnet-20241022", Name: "Claude 3.5 Sonnet"},
		{ID: "claude-3-5-haiku-20241022", Name: "Claude 3.5 Haiku"},
	}, nil
}

// ── HTTP Layer ──────────────────────────────

func (p *AnthropicProvider) doRequest(ctx context.Context, cfg ProviderConfig, req CompletionRequest) (io.ReadCloser, error) {
	endpoint := cfg.Endpoint
	if endpoint == "" {
		endpoint = anthropicDefaultEndpoint
	}
	endpoint = strings.TrimSuffix(endpoint, "/") + "/v1/messages"

	// Separate system message from conversation
	var system string
	messages := make([]anthropicMessage, 0, len(req.Messages))
	for _, m := range req.Messages {
		if m.Role == "system" {
			system = m.Content
			continue
		}
		messages = append(messages, anthropicMessage{
			Role:    m.Role,
			Content: m.Content,
		})
	}

	// Build Anthropic-format request
	antReq := anthropicRequest{
		Model:     req.Model,
		Messages:  messages,
		MaxTokens: req.MaxTokens,
		Stream:    req.Stream,
	}
	if system != "" {
		antReq.System = system
	}
	if antReq.MaxTokens == 0 {
		antReq.MaxTokens = 4096 // Anthropic requires max_tokens
	}
	if req.Temperature != nil {
		antReq.Temperature = req.Temperature
	}
	if req.TopP != nil {
		antReq.TopP = req.TopP
	}

	body, err := json.Marshal(antReq)
	if err != nil {
		return nil, fmt.Errorf("marshal request: %w", err)
	}

	httpReq, err := http.NewRequestWithContext(ctx, "POST", endpoint, bytes.NewReader(body))
	if err != nil {
		return nil, err
	}
	httpReq.Header.Set("Content-Type", "application/json")
	httpReq.Header.Set("x-api-key", cfg.APIKey)
	httpReq.Header.Set("anthropic-version", anthropicAPIVersion)

	resp, err := http.DefaultClient.Do(httpReq)
	if err != nil {
		return nil, fmt.Errorf("provider request: %w", err)
	}

	if resp.StatusCode != http.StatusOK {
		b, _ := io.ReadAll(resp.Body)
		resp.Body.Close()
		return nil, fmt.Errorf("provider error: HTTP %d: %s", resp.StatusCode, string(b))
	}

	return resp.Body, nil
}

// ── Anthropic Wire Types ────────────────────

type anthropicMessage struct {
	Role    string `json:"role"`
	Content string `json:"content"`
}

type anthropicRequest struct {
	Model       string             `json:"model"`
	Messages    []anthropicMessage `json:"messages"`
	System      string             `json:"system,omitempty"`
	MaxTokens   int                `json:"max_tokens"`
	Temperature *float64           `json:"temperature,omitempty"`
	TopP        *float64           `json:"top_p,omitempty"`
	Stream      bool               `json:"stream"`
}

type anthropicResponse struct {
	Model      string `json:"model"`
	StopReason string `json:"stop_reason"`
	Content    []struct {
		Type string `json:"type"`
		Text string `json:"text"`
	} `json:"content"`
	Usage struct {
		InputTokens  int `json:"input_tokens"`
		OutputTokens int `json:"output_tokens"`
	} `json:"usage"`
}

type anthropicStreamEvent struct {
	Type  string `json:"type"`
	Delta struct {
		Type       string `json:"type"`
		Text       string `json:"text"`
		StopReason string `json:"stop_reason"`
	} `json:"delta"`
}