feat: add Anthropic Messages API support (#18)

Adds --llm-provider flag (openai|anthropic) to switch between API formats.

Anthropic implementation:
- POST /messages endpoint
- x-api-key + anthropic-version headers
- System prompt as top-level field (not a message)
- max_tokens: 8192 for response generation
- Parses content blocks [{type: "text", text: "..."}]

Changes:
- llm/client.go: Provider type, completeAnthropic(), doRequest() shared helper
- cmd/review-bot/main.go: --llm-provider / LLM_PROVIDER flag
- .gitea/actions/review/action.yml: llm-provider input + env
- llm/client_test.go: 4 new tests for Anthropic path

Backwards compatible — default provider is still openai.

Closes #18

llm/client.go

@@ -1,4 +1,6 @@
-// Package llm provides a client for OpenAI-compatible chat completion APIs.
+// Package llm provides clients for LLM chat completion APIs.
+//
+// Supports OpenAI-compatible (default) and Anthropic Messages API providers.
 package llm
 
 import (
@@ -12,24 +14,37 @@ import (
 	"time"
 )
 
-// Client calls an OpenAI-compatible chat completion API.
+// Provider identifies which API format to use.
+type Provider string
+
+const (
+	// ProviderOpenAI uses the OpenAI-compatible chat/completions endpoint.
+	ProviderOpenAI Provider = "openai"
+	// ProviderAnthropic uses the Anthropic Messages API endpoint.
+	ProviderAnthropic Provider = "anthropic"
+)
+
+// Client calls an LLM chat completion API.
 // A Client is safe for concurrent use by multiple goroutines after construction.
-// WithTimeout and WithTemperature must be called during setup, before concurrent use.
+// WithTimeout, WithTemperature, and WithProvider must be called during setup,
+// before concurrent use.
 type Client struct {
 	baseURL     string
 	apiKey      string
 	model       string
 	temperature float64
+	provider    Provider
 	http        *http.Client
 }
 
-// NewClient creates a new LLM client.
+// NewClient creates a new LLM client. Default provider is OpenAI-compatible.
 func NewClient(baseURL, apiKey, model string) *Client {
 	return &Client{
-		baseURL: strings.TrimRight(baseURL, "/"),
-		apiKey:  apiKey,
-		model:   model,
-		http:    &http.Client{Timeout: 5 * time.Minute},
+		baseURL:  strings.TrimRight(baseURL, "/"),
+		apiKey:   apiKey,
+		model:    model,
+		provider: ProviderOpenAI,
+		http:     &http.Client{Timeout: 5 * time.Minute},
 	}
 }
 
@@ -45,20 +60,39 @@ func (c *Client) WithTemperature(t float64) *Client {
 	return c
 }
 
+// WithProvider sets the API provider format (openai or anthropic).
+func (c *Client) WithProvider(p Provider) *Client {
+	c.provider = p
+	return c
+}
+
 // Message represents a chat message.
 type Message struct {
 	Role    string `json:"role"`
 	Content string `json:"content"`
 }
 
-// ChatRequest is the request payload.
+// Complete sends a chat completion request and returns the assistant's response content.
+// The first message with role "system" is treated as the system prompt.
+func (c *Client) Complete(ctx context.Context, messages []Message) (string, error) {
+	switch c.provider {
+	case ProviderAnthropic:
+		return c.completeAnthropic(ctx, messages)
+	default:
+		return c.completeOpenAI(ctx, messages)
+	}
+}
+
+// --- OpenAI-compatible implementation ---
+
+// ChatRequest is the OpenAI request payload.
 type ChatRequest struct {
 	Model       string    `json:"model"`
 	Messages    []Message `json:"messages"`
 	Temperature float64   `json:"temperature,omitempty"`
 }
 
-// ChatResponse is the response from the API.
+// ChatResponse is the OpenAI response.
 type ChatResponse struct {
 	Choices []struct {
 		Message struct {
@@ -67,8 +101,7 @@ type ChatResponse struct {
 	} `json:"choices"`
 }
 
-// Complete sends a chat completion request and returns the assistant's response content.
-func (c *Client) Complete(ctx context.Context, messages []Message) (string, error) {
+func (c *Client) completeOpenAI(ctx context.Context, messages []Message) (string, error) {
 	reqBody := ChatRequest{
 		Model:       c.model,
 		Temperature: c.temperature,
@@ -81,37 +114,126 @@ func (c *Client) Complete(ctx context.Context, messages []Message) (string, erro
 	}
 
 	url := c.baseURL + "/chat/completions"
-	req, err := http.NewRequestWithContext(ctx, "POST", url, bytes.NewReader(data))
+	req, err := http.NewRequestWithContext(ctx, http.MethodPost, url, bytes.NewReader(data))
 	if err != nil {
 		return "", fmt.Errorf("create request: %w", err)
 	}
 	req.Header.Set("Authorization", "Bearer "+c.apiKey)
 	req.Header.Set("Content-Type", "application/json")
 
+	return c.doRequest(req, func(body []byte) (string, error) {
+		var resp ChatResponse
+		if err := json.Unmarshal(body, &resp); err != nil {
+			return "", fmt.Errorf("parse response: %w", err)
+		}
+		if len(resp.Choices) == 0 {
+			return "", fmt.Errorf("no choices in LLM response")
+		}
+		return resp.Choices[0].Message.Content, nil
+	})
+}
+
+// --- Anthropic Messages API implementation ---
+
+type anthropicRequest struct {
+	Model       string            `json:"model"`
+	MaxTokens   int               `json:"max_tokens"`
+	System      string            `json:"system,omitempty"`
+	Messages    []anthropicMsg    `json:"messages"`
+	Temperature float64           `json:"temperature,omitempty"`
+}
+
+type anthropicMsg struct {
+	Role    string `json:"role"`
+	Content string `json:"content"`
+}
+
+type anthropicResponse struct {
+	Content []struct {
+		Type string `json:"type"`
+		Text string `json:"text"`
+	} `json:"content"`
+}
+
+func (c *Client) completeAnthropic(ctx context.Context, messages []Message) (string, error) {
+	// Extract system message (first message with role "system")
+	var system string
+	var userMessages []anthropicMsg
+	for _, m := range messages {
+		if m.Role == "system" {
+			system = m.Content
+		} else {
+			userMessages = append(userMessages, anthropicMsg{
+				Role:    m.Role,
+				Content: m.Content,
+			})
+		}
+	}
+
+	reqBody := anthropicRequest{
+		Model:     c.model,
+		MaxTokens: 8192,
+		System:    system,
+		Messages:  userMessages,
+	}
+	if c.temperature > 0 {
+		reqBody.Temperature = c.temperature
+	}
+
+	data, err := json.Marshal(reqBody)
+	if err != nil {
+		return "", fmt.Errorf("marshal request: %w", err)
+	}
+
+	url := c.baseURL + "/messages"
+	req, err := http.NewRequestWithContext(ctx, http.MethodPost, url, bytes.NewReader(data))
+	if err != nil {
+		return "", fmt.Errorf("create request: %w", err)
+	}
+	req.Header.Set("x-api-key", c.apiKey)
+	req.Header.Set("anthropic-version", "2023-06-01")
+	req.Header.Set("Content-Type", "application/json")
+
+	return c.doRequest(req, func(body []byte) (string, error) {
+		var resp anthropicResponse
+		if err := json.Unmarshal(body, &resp); err != nil {
+			return "", fmt.Errorf("parse response: %w", err)
+		}
+		if len(resp.Content) == 0 {
+			return "", fmt.Errorf("no content in Anthropic response")
+		}
+		// Concatenate all text blocks
+		var sb strings.Builder
+		for _, block := range resp.Content {
+			if block.Type == "text" {
+				sb.WriteString(block.Text)
+			}
+		}
+		result := sb.String()
+		if result == "" {
+			return "", fmt.Errorf("no text content in Anthropic response")
+		}
+		return result, nil
+	})
+}
+
+// --- Shared HTTP execution ---
+
+func (c *Client) doRequest(req *http.Request, parse func([]byte) (string, error)) (string, error) {
 	resp, err := c.http.Do(req)
 	if err != nil {
 		return "", fmt.Errorf("LLM request: %w", err)
 	}
 	defer resp.Body.Close()
 
-	if resp.StatusCode < 200 || resp.StatusCode >= 300 {
-		body, _ := io.ReadAll(resp.Body)
-		return "", fmt.Errorf("LLM API error (status %d): %s", resp.StatusCode, string(body))
-	}
-
 	body, err := io.ReadAll(resp.Body)
 	if err != nil {
 		return "", fmt.Errorf("read response: %w", err)
 	}
 
-	var chatResp ChatResponse
-	if err := json.Unmarshal(body, &chatResp); err != nil {
-		return "", fmt.Errorf("parse response: %w", err)
+	if resp.StatusCode < 200 || resp.StatusCode >= 300 {
+		return "", fmt.Errorf("LLM API error (status %d): %s", resp.StatusCode, string(body))
 	}
 
-	if len(chatResp.Choices) == 0 {
-		return "", fmt.Errorf("no choices in LLM response")
-	}
-
-	return chatResp.Choices[0].Message.Content, nil
+	return parse(body)
 }