fix(stream): extract input tokens from message_start event

message_delta only contains output_tokens. Input tokens are in the
message_start event under message.usage.input_tokens. This was causing
input token counts to be near-zero for all streaming requests.

internal/proxy/handler.go

@@ -278,11 +278,13 @@ func handleStream(c *gin.Context, upstream *UpstreamClient, san *Sanitizer, pool
 		c.Writer.WriteString(line + "\n")
 		flusher.Flush()
 
-		// Extract token usage from message_delta event
 		if len(line) > 5 && line[:5] == "data:" {
 			data := line[5:]
-			if gjson.Get(data, "type").String() == "message_delta" {
-				inputTokens = gjson.Get(data, "usage.input_tokens").Int()
+			eventType := gjson.Get(data, "type").String()
+			switch eventType {
+			case "message_start":
+				inputTokens = gjson.Get(data, "message.usage.input_tokens").Int()
+			case "message_delta":
 				outputTokens = gjson.Get(data, "usage.output_tokens").Int()
 			}
 		}