Fix critical issues: panic risk and body size limits

- Add type assertion check for session ID context value to prevent panic\n- Add 10MB request body size limit to prevent memory exhaustion\n- Return 413 (Request Entity Too Large) for oversized requests\n- Add maxBodySize constant for easy adjustment

handler.go

@@ -22,6 +22,9 @@ var config *Config
 // httpClient is a shared HTTP client for all upstream requests
 var httpClient = &http.Client{Timeout: 300 * time.Second}
 
+// maxBodySize limits request body size to prevent memory exhaustion
+const maxBodySize = 10 << 20 // 10MB
+
 // blockedHeaders are headers that should never be forwarded to upstream
 // for security/privacy reasons. These headers could leak internal URLs,
 // session information, or other sensitive data.
@@ -81,9 +84,15 @@ func handleChatCompletions(w http.ResponseWriter, r *http.Request) {
 	}
 	apiKey := strings.TrimPrefix(authHeader, "Bearer ")
 
-	// Read body
+	// Read body (with size limit to prevent memory exhaustion)
+	r.Body = http.MaxBytesReader(w, r.Body, maxBodySize)
 	body, err := io.ReadAll(r.Body)
 	if err != nil {
+		// Check if this is a max bytes error
+		if err.Error() == "http: request body too large" {
+			writeError(w, http.StatusRequestEntityTooLarge, "Request body exceeds maximum size limit (10MB)", "invalid_request_error", "body_too_large")
+			return
+		}
 		writeError(w, http.StatusBadRequest, "Failed to read request body", "invalid_request_error", "body_read_error")
 		return
 	}
@@ -96,7 +105,11 @@ func handleChatCompletions(w http.ResponseWriter, r *http.Request) {
 	}
 
 	// Get session ID from context (set by main)
-	sessionID := r.Context().Value(sessionIDKey).(string)
+	sessionID, ok := r.Context().Value(sessionIDKey).(string)
+	if !ok {
+		writeError(w, http.StatusInternalServerError, "Internal server error", "internal_error", "session_missing")
+		return
+	}
 
 	// Convert to Anthropic format — always non-streaming to upstream
 	// (ZAI's streaming returns empty for GLM models)