diff --git a/src/lisp/kernel/cleavir/ast-interpreter.lisp b/src/lisp/kernel/cleavir/ast-interpreter.lisp
index 98bd19cbdc..d13840d285 100644
--- a/src/lisp/kernel/cleavir/ast-interpreter.lisp
+++ b/src/lisp/kernel/cleavir/ast-interpreter.lisp
@@ -1,9 +1,8 @@
 (defpackage #:interpret-ast
   (:use #:cl)
   (:export #:interpret)
-  (:shadow #:variable)
-  ;; FIXME: lose this asap
-  (:export #:*system*))
+  (:export #:cannot-interpret #:cannot-interpret-ast)
+  (:shadow #:variable))
 
 ;;;; NOTE: Some methods in this file must be compiled with cleavir,
 ;;;; as they use cleavir special operators - metacircularity, eh?
@@ -50,20 +49,26 @@
     ;; Do actual work
     (interpret-ast ast env)))
 
+(define-condition cannot-interpret (error)
+  ((ast :reader cannot-interpret-ast :initarg :ast))
+  (:report (lambda (condition stream)
+             (format stream "Interpreter not implemented for AST: ~a"
+                     (cannot-interpret-ast condition)))))
+
 ;;; meat
 
 (defgeneric interpret-ast (ast env))
 
 (defmethod interpret-ast (ast env)
   (declare (ignore env))
-  (error "AST interpreter not implemented for ~a" ast))
+  (error 'cannot-interpret :ast ast))
 
 ;;; distinguished only to make sure the input ast is correct
 (defgeneric interpret-boolean (condition env))
 
 (defmethod interpret-boolean (condition env)
   (declare (ignore then else env))
-  (error "AST boolean interpreter not implemented for ~a" condition))
+  (error 'cannot-interpret :ast condition))
 
 (defmethod interpret-ast ((ast cleavir-ast:immediate-ast) env)
   (declare (ignore env))
diff --git a/src/lisp/kernel/cleavir/setup.lisp b/src/lisp/kernel/cleavir/setup.lisp
index 9d8a56e3e6..3f8fc22871 100644
--- a/src/lisp/kernel/cleavir/setup.lisp
+++ b/src/lisp/kernel/cleavir/setup.lisp
@@ -311,7 +311,12 @@ when this is t a lot of graphs will be generated.")
   (cond (core:*use-interpreter-for-eval*
          (core:interpret form (cleavir-env->interpreter env)))
         (*use-ast-interpreter*
-         (ast-interpret-form form env))
+         (handler-case
+             (ast-interpret-form form env)
+           (interpret-ast:cannot-interpret (c)
+             (declare (ignore c))
+             ;; If the AST interpreter doesn't work, fall back.
+             (cclasp-eval form env))))
         (t
          (cclasp-eval form env))))
 
diff --git a/src/lisp/kernel/cmp/compile-file-parallel.lsp b/src/lisp/kernel/cmp/compile-file-parallel.lsp
index d7c4ba3ecc..739a14a7d2 100644
--- a/src/lisp/kernel/cmp/compile-file-parallel.lsp
+++ b/src/lisp/kernel/cmp/compile-file-parallel.lsp
@@ -370,27 +370,7 @@ Compile a lisp source file into an LLVM module."
               (bformat t "conditions: %s%N" c)))
           (compile-file-results output-path conditions))))))
 
-
-(defun contains-defcallback-p (path)
-  (let ((data (with-open-file (stream path)
-                (let ((data (make-string (file-length stream))))
-                  (read-sequence data stream)
-                  data))))
-    (search "defcallback" data :test #'string-equal)))
-
-
 (defvar *compile-file-parallel* nil)
-#+(or)
-(defun cl:compile-file (input-file &rest args)
-  "Ok, this is a horrible hack - if the file DOESN'T contain defcallback then compile-file-parallel it"
-  (if *compile-file-parallel*
-      (if (null (contains-defcallback-p input-file))
-          (apply #'compile-file-parallel input-file args)
-          (progn
-            (format t "!~%!~%!~% Falling back to compile-file-serial for ~s because it contains DEFCALLBACK~%!~%~~%"
-                    input-file)
-            (apply #'compile-file-serial input-file args)))
-      (apply #'compile-file-serial input-file args)))
                               
 (defun cl:compile-file (input-file &rest args)
   (if *compile-file-parallel*
@@ -399,5 +379,4 @@ Compile a lisp source file into an LLVM module."
 
 (eval-when (:load-toplevel)
   (setf *compile-file-parallel* t)
-  #-cst
   (setf clasp-cleavir::*use-ast-interpreter* t))