fix: voice transcription via native Gemini API instead of OpenAI-compat

The OpenAI-compatible endpoint silently ignores input_audio parts,
causing Gemini to respond with "provide the audio file" instead of
transcribing. Switch to native Gemini generateContent API with
inline_data for reliable audio transcription.

- Add transcribe.go with direct Gemini REST call (inline_data + mime_type)
- TranscribeAudio now uses TranscribeConfig (model + API key)
- EnableTranscription() wired in main.go from gemini-flash config
- Pass audio/ogg MIME type from handler instead of format string

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: Alexey Panfilov

cmd/agent/main.go

@@ -171,6 +171,15 @@ func main() {
 
 	ag := agent.New(router, s, mcpClient, compacter, string(sysPromptBytes), logger)
 
+	// Enable voice transcription if a multimodal Gemini model is configured.
+	if cfg.Models.GeminiFlash.APIKey != "" && cfg.Models.GeminiFlash.Model != "" {
+		ag.EnableTranscription(agent.TranscribeConfig{
+			Model:  cfg.Models.GeminiFlash.Model,
+			APIKey: cfg.Models.GeminiFlash.APIKey,
+		})
+		logger.Info("voice transcription enabled", "model", cfg.Models.GeminiFlash.Model)
+	}
+
 	if cfg.WebSearch.Enabled {
 		baseURL := cfg.WebSearch.BaseURL
 		if baseURL == "" {

internal/agent/agent.go

@@ -33,14 +33,15 @@ const (
 )
 
 type Agent struct {
-	router    *llm.Router
-	store     store.Store
-	mcp       *mcp.Client
-	compacter *Compacter
-	cache     *ResponseCache
-	sysPrompt string
-	logger    *slog.Logger
-	webSearch *WebSearchConfig // nil = disabled
+	router     *llm.Router
+	store      store.Store
+	mcp        *mcp.Client
+	compacter  *Compacter
+	cache      *ResponseCache
+	sysPrompt  string
+	logger     *slog.Logger
+	webSearch  *WebSearchConfig  // nil = disabled
+	transcribe *TranscribeConfig // nil = disabled
 }
 
 func New(router *llm.Router, s store.Store, mcpClient *mcp.Client, compacter *Compacter, sysPrompt string, logger *slog.Logger) *Agent {
@@ -55,21 +56,22 @@ func New(router *llm.Router, s store.Store, mcpClient *mcp.Client, compacter *Co
 	}
 }
 
-// TranscribeAudio sends audio data to the multimodal LLM for transcription.
+// TranscribeAudio transcribes audio data to text via the native Gemini API.
 // Returns the transcribed text. The audio is not stored in conversation history.
-func (a *Agent) TranscribeAudio(ctx context.Context, audioData []byte, format string) (string, error) {
-	msg := llm.Message{
-		Role: "user",
-		Parts: []llm.ContentPart{
-			{Type: "text", Text: "Transcribe this voice message exactly as spoken. Return only the transcription, no commentary."},
-			{Type: "input_audio", InputAudio: &llm.InputAudio{Data: encodeBase64(audioData), Format: format}},
-		},
+func (a *Agent) TranscribeAudio(ctx context.Context, audioData []byte, mimeType string) (string, error) {
+	if a.transcribe == nil {
+		return "", fmt.Errorf("transcription not configured")
 	}
-	resp, err := a.router.Chat(ctx, []llm.Message{msg}, "", nil)
+	text, err := transcribeViaGemini(ctx, *a.transcribe, audioData, mimeType)
 	if err != nil {
-		return "", fmt.Errorf("transcribe: %w", err)
+		return "", err
 	}
-	return strings.TrimSpace(resp.Content), nil
+	return strings.TrimSpace(text), nil
+}
+
+// EnableTranscription configures audio transcription via native Gemini API.
+func (a *Agent) EnableTranscription(cfg TranscribeConfig) {
+	a.transcribe = &cfg
 }
 
 // EnableWebSearch activates the Ollama web search tool.

internal/agent/transcribe.go

@@ -0,0 +1,90 @@
+package agent
+
+import (
+	"bytes"
+	"context"
+	"encoding/base64"
+	"encoding/json"
+	"fmt"
+	"io"
+	"net/http"
+	"time"
+)
+
+const (
+	geminiGenerateURL  = "https://generativelanguage.googleapis.com/v1beta/models/%s:generateContent"
+	transcribePrompt   = "Transcribe this voice message exactly as spoken. Return only the transcription, no commentary."
+	transcribeTimeout_ = 30 * time.Second
+)
+
+var transcribeHTTPClient = &http.Client{Timeout: transcribeTimeout_}
+
+// TranscribeConfig holds the Gemini model and API key used for audio transcription.
+type TranscribeConfig struct {
+	Model  string // e.g. "gemini-2.0-flash"
+	APIKey string
+}
+
+// transcribeViaGemini calls the native Gemini generateContent API with inline audio data.
+// This bypasses the OpenAI-compat layer which does not reliably support input_audio.
+func transcribeViaGemini(ctx context.Context, cfg TranscribeConfig, audioData []byte, mimeType string) (string, error) {
+	reqBody := map[string]any{
+		"contents": []map[string]any{
+			{
+				"parts": []map[string]any{
+					{"text": transcribePrompt},
+					{
+						"inline_data": map[string]any{
+							"mime_type": mimeType,
+							"data":      base64.StdEncoding.EncodeToString(audioData),
+						},
+					},
+				},
+			},
+		},
+	}
+
+	body, err := json.Marshal(reqBody)
+	if err != nil {
+		return "", fmt.Errorf("transcribe: marshal: %w", err)
+	}
+
+	url := fmt.Sprintf(geminiGenerateURL, cfg.Model)
+	req, err := http.NewRequestWithContext(ctx, "POST", url, bytes.NewReader(body))
+	if err != nil {
+		return "", fmt.Errorf("transcribe: request: %w", err)
+	}
+	req.Header.Set("Content-Type", "application/json")
+	req.Header.Set("x-goog-api-key", cfg.APIKey)
+
+	resp, err := transcribeHTTPClient.Do(req)
+	if err != nil {
+		return "", fmt.Errorf("transcribe: %w", err)
+	}
+	defer resp.Body.Close()
+
+	respBody, err := io.ReadAll(io.LimitReader(resp.Body, 1*1024*1024))
+	if err != nil {
+		return "", fmt.Errorf("transcribe: read: %w", err)
+	}
+	if resp.StatusCode != 200 {
+		return "", fmt.Errorf("transcribe: HTTP %d: %s", resp.StatusCode, string(respBody))
+	}
+
+	var result struct {
+		Candidates []struct {
+			Content struct {
+				Parts []struct {
+					Text string `json:"text"`
+				} `json:"parts"`
+			} `json:"content"`
+		} `json:"candidates"`
+	}
+	if err := json.Unmarshal(respBody, &result); err != nil {
+		return "", fmt.Errorf("transcribe: parse: %w", err)
+	}
+	if len(result.Candidates) == 0 || len(result.Candidates[0].Content.Parts) == 0 {
+		return "", fmt.Errorf("transcribe: empty response")
+	}
+	return result.Candidates[0].Content.Parts[0].Text, nil
+}

internal/telegram/handler.go

@@ -452,7 +452,7 @@ func (h *Handler) transcribeVoice(chatID int64, msg *tgbotapi.Message) string {
 	ctx, cancel := context.WithTimeout(context.Background(), transcribeTimeout)
 	defer cancel()
 
-	text, err := h.agent.TranscribeAudio(ctx, data, "ogg")
+	text, err := h.agent.TranscribeAudio(ctx, data, "audio/ogg")
 	if err != nil {
 		h.logger.Error("transcription failed", "chat_id", chatID, "err", err)
 		return ""