llm: add GitHub Copilot provider

- Implement copilot client reading COPILOT_API_KEY
- Wire copilot_base_url and copilot_model config
- Update README and config example; defaults to gpt-4.1
- Keep OpenAI default at gpt-4.1 for consistency

2 files changed, 184 insertions, 23 deletions

diff --git a/internal/llm/copilot.go b/internal/llm/copilot.go
new file mode 100644
index 0000000..a31022f
--- /dev/null
+++ b/internal/llm/copilot.go
@@ -0,0 +1,153 @@
+package llm
+
+import (
+    "bytes"
+    "context"
+    "encoding/json"
+    "errors"
+    "fmt"
+    "net/http"
+    "strings"
+    "time"
+
+    "hexai/internal/logging"
+)
+
+// copilotClient implements Client against GitHub Copilot's Chat Completions API.
+type copilotClient struct {
+    httpClient   *http.Client
+    apiKey       string
+    baseURL      string
+    defaultModel string
+}
+
+func newCopilot(baseURL, model, apiKey string) Client {
+    if strings.TrimSpace(baseURL) == "" {
+        baseURL = "https://api.githubcopilot.com"
+    }
+    if strings.TrimSpace(model) == "" {
+        model = "gpt-4.1"
+    }
+    return &copilotClient{
+        httpClient:   &http.Client{Timeout: 30 * time.Second},
+        apiKey:       apiKey,
+        baseURL:      strings.TrimRight(baseURL, "/"),
+        defaultModel: model,
+    }
+}
+
+type copilotChatRequest struct {
+    Model       string           `json:"model"`
+    Messages    []copilotMessage `json:"messages"`
+    Temperature *float64         `json:"temperature,omitempty"`
+    MaxTokens   *int             `json:"max_tokens,omitempty"`
+    Stop        []string         `json:"stop,omitempty"`
+}
+
+type copilotMessage struct {
+    Role    string `json:"role"`
+    Content string `json:"content"`
+}
+
+type copilotChatResponse struct {
+    Choices []struct {
+        Index   int `json:"index"`
+        Message struct {
+            Role    string `json:"role"`
+            Content string `json:"content"`
+        } `json:"message"`
+        FinishReason string `json:"finish_reason"`
+    } `json:"choices"`
+    Error *struct {
+        Message string `json:"message"`
+        Type    string `json:"type"`
+        Param   any    `json:"param"`
+        Code    any    `json:"code"`
+    } `json:"error,omitempty"`
+}
+
+func (c *copilotClient) Chat(ctx context.Context, messages []Message, opts ...RequestOption) (string, error) {
+    if strings.TrimSpace(c.apiKey) == "" {
+        return nilStringErr("missing Copilot API key")
+    }
+    o := Options{Model: c.defaultModel}
+    for _, opt := range opts {
+        opt(&o)
+    }
+    if o.Model == "" {
+        o.Model = c.defaultModel
+    }
+
+    start := time.Now()
+    logging.Logf("llm/copilot ", "chat start model=%s temp=%.2f max_tokens=%d stop=%d messages=%d", o.Model, o.Temperature, o.MaxTokens, len(o.Stop), len(messages))
+    for i, m := range messages {
+        logging.Logf("llm/copilot ", "msg[%d] role=%s size=%d preview=%s%s%s", i, m.Role, len(m.Content), logging.AnsiCyan, logging.PreviewForLog(m.Content), logging.AnsiBase)
+    }
+
+    req := copilotChatRequest{Model: o.Model}
+    req.Messages = make([]copilotMessage, len(messages))
+    for i, m := range messages {
+        req.Messages[i] = copilotMessage{Role: m.Role, Content: m.Content}
+    }
+    if o.Temperature != 0 {
+        req.Temperature = &o.Temperature
+    }
+    if o.MaxTokens > 0 {
+        req.MaxTokens = &o.MaxTokens
+    }
+    if len(o.Stop) > 0 {
+        req.Stop = o.Stop
+    }
+
+    body, err := json.Marshal(req)
+    if err != nil {
+        logging.Logf("llm/copilot ", "marshal error: %v", err)
+        return "", err
+    }
+
+    endpoint := c.baseURL + "/chat/completions"
+    logging.Logf("llm/copilot ", "POST %s", endpoint)
+    httpReq, err := http.NewRequestWithContext(ctx, http.MethodPost, endpoint, bytes.NewReader(body))
+    if err != nil {
+        logging.Logf("llm/copilot ", "new request error: %v", err)
+        return "", err
+    }
+    httpReq.Header.Set("Content-Type", "application/json")
+    httpReq.Header.Set("Authorization", "Bearer "+c.apiKey)
+    // Some Copilot deployments expect a version header; optional here.
+    // httpReq.Header.Set("X-GitHub-Api-Version", "2023-12-07")
+
+    resp, err := c.httpClient.Do(httpReq)
+    if err != nil {
+        logging.Logf("llm/copilot ", "%shttp error after %s: %v%s", logging.AnsiRed, time.Since(start), err, logging.AnsiBase)
+        return "", err
+    }
+    defer resp.Body.Close()
+    if resp.StatusCode < 200 || resp.StatusCode >= 300 {
+        var apiErr copilotChatResponse
+        _ = json.NewDecoder(resp.Body).Decode(&apiErr)
+        if apiErr.Error != nil && strings.TrimSpace(apiErr.Error.Message) != "" {
+            logging.Logf("llm/copilot ", "%sapi error status=%d type=%s msg=%s duration=%s%s", logging.AnsiRed, resp.StatusCode, apiErr.Error.Type, apiErr.Error.Message, time.Since(start), logging.AnsiBase)
+            return "", fmt.Errorf("copilot error: %s (status %d)", apiErr.Error.Message, resp.StatusCode)
+        }
+        logging.Logf("llm/copilot ", "%shttp non-2xx status=%d duration=%s%s", logging.AnsiRed, resp.StatusCode, time.Since(start), logging.AnsiBase)
+        return "", fmt.Errorf("copilot http error: status %d", resp.StatusCode)
+    }
+
+    var out copilotChatResponse
+    if err := json.NewDecoder(resp.Body).Decode(&out); err != nil {
+        logging.Logf("llm/copilot ", "%sdecode error after %s: %v%s", logging.AnsiRed, time.Since(start), err, logging.AnsiBase)
+        return "", err
+    }
+    if len(out.Choices) == 0 {
+        logging.Logf("llm/copilot ", "%sno choices returned duration=%s%s", logging.AnsiRed, time.Since(start), logging.AnsiBase)
+        return "", errors.New("copilot: no choices returned")
+    }
+    content := out.Choices[0].Message.Content
+    logging.Logf("llm/copilot ", "success choice=0 finish=%s size=%d preview=%s%s%s duration=%s", out.Choices[0].FinishReason, len(content), logging.AnsiGreen, logging.PreviewForLog(content), logging.AnsiBase, time.Since(start))
+    return content, nil
+}
+
+// Provider metadata
+func (c *copilotClient) Name() string         { return "copilot" }
+func (c *copilotClient) DefaultModel() string { return c.defaultModel }
diff --git a/internal/llm/provider.go b/internal/llm/provider.go
index 6c6cf04..dda3d16 100644
--- a/internal/llm/provider.go
+++ b/internal/llm/provider.go
@@ -43,32 +43,40 @@ func WithStop(stop ...string) RequestOption {
 
 // Config defines provider configuration read from the Hexai config file.
 type Config struct {
-	Provider string
-	// OpenAI options
-	OpenAIBaseURL string
-	OpenAIModel   string
-	// Ollama options
-	OllamaBaseURL string
-	OllamaModel   string
+    Provider string
+    // OpenAI options
+    OpenAIBaseURL string
+    OpenAIModel   string
+    // Ollama options
+    OllamaBaseURL string
+    OllamaModel   string
+    // Copilot options
+    CopilotBaseURL string
+    CopilotModel   string
 }
 
 // NewFromConfig creates an LLM client using only the supplied configuration.
 // The OpenAI API key is supplied separately and may be read from the environment
 // by the caller; other environment-based configuration is not used.
-func NewFromConfig(cfg Config, openAIAPIKey string) (Client, error) {
-	p := strings.ToLower(strings.TrimSpace(cfg.Provider))
-	if p == "" {
-		p = "openai"
-	}
-	switch p {
-	case "openai":
-		if strings.TrimSpace(openAIAPIKey) == "" {
-			return nil, errors.New("missing OPENAI_API_KEY for provider openai")
-		}
-		return newOpenAI(cfg.OpenAIBaseURL, cfg.OpenAIModel, openAIAPIKey), nil
-	case "ollama":
-		return newOllama(cfg.OllamaBaseURL, cfg.OllamaModel), nil
-	default:
-		return nil, errors.New("unknown LLM provider: " + p)
-	}
+func NewFromConfig(cfg Config, openAIAPIKey, copilotAPIKey string) (Client, error) {
+    p := strings.ToLower(strings.TrimSpace(cfg.Provider))
+    if p == "" {
+        p = "openai"
+    }
+    switch p {
+    case "openai":
+        if strings.TrimSpace(openAIAPIKey) == "" {
+            return nil, errors.New("missing OPENAI_API_KEY for provider openai")
+        }
+        return newOpenAI(cfg.OpenAIBaseURL, cfg.OpenAIModel, openAIAPIKey), nil
+    case "ollama":
+        return newOllama(cfg.OllamaBaseURL, cfg.OllamaModel), nil
+    case "copilot":
+        if strings.TrimSpace(copilotAPIKey) == "" {
+            return nil, errors.New("missing COPILOT_API_KEY for provider copilot")
+        }
+        return newCopilot(cfg.CopilotBaseURL, cfg.CopilotModel, copilotAPIKey), nil
+    default:
+        return nil, errors.New("unknown LLM provider: " + p)
+    }
 }