diff --git a/cmd/anvilmcp/execute.go b/cmd/anvilmcp/execute.go
index e5bdbdd..4838a0d 100644
--- a/cmd/anvilmcp/execute.go
+++ b/cmd/anvilmcp/execute.go
@@ -85,6 +85,15 @@ func recordValidationFailure() {
 	}
 }
 
+func isPermissionError(err error) bool {
+	if err == nil {
+		return false
+	}
+	msg := strings.ToLower(err.Error())
+	return strings.Contains(msg, "permission denied") ||
+		strings.Contains(msg, "no such file or directory")
+}
+
 func validateCode(code string) error {
 	if err := checkRateLimit(); err != nil {
 		return err
diff --git a/cmd/anvilmcp/main.go b/cmd/anvilmcp/main.go
index 9234976..ba6e3de 100644
--- a/cmd/anvilmcp/main.go
+++ b/cmd/anvilmcp/main.go
@@ -134,6 +134,7 @@ func handleToolCall(req MCPRequest) {
 			timeout = int(t)
 		}
 		sandbox, _ := params.Arguments["sandbox"].(string)
+		sandboxExplicit := sandbox != ""
 		if sandbox == "" {
 			sandbox = "anvilmcp"
 		}
@@ -145,6 +146,12 @@ func handleToolCall(req MCPRequest) {
 		fmt.Fprintf(os.Stderr, "[anvilmcp] Executing bash code (timeout: %ds, sandbox: %s)\n", timeout, sandbox)
 		result, err := executeCode(code, language, timeout, sandbox)
 
+		// Fallback to default sandbox on permission errors (only if sandbox wasn't explicit)
+		if err != nil && !sandboxExplicit && isPermissionError(err) {
+			fmt.Fprintf(os.Stderr, "[anvilmcp] Permission error in %s sandbox, falling back to default\n", sandbox)
+			result, err = executeCode(code, language, timeout, "default")
+		}
+
 		// Log token comparison
 		codeTokens := estimateTokens(code)
 		outputTokens := estimateTokens(result)