diff --git a/tools/build/errors.ts b/tools/build/errors.ts
new file mode 100644
index 00000000..3051f8bd
--- /dev/null
+++ b/tools/build/errors.ts
@@ -0,0 +1,3 @@
+
+// User-facing build failure: printed without a stack trace.
+export class BuildError extends Error {}
diff --git a/tools/build/graph.ts b/tools/build/graph.ts
index d7008170..ba25dad7 100644
--- a/tools/build/graph.ts
+++ b/tools/build/graph.ts
@@ -1,7 +1,8 @@
 
 import type {RuleDecl} from './api.ts';
+import {BuildError} from './errors.ts';
 
-export class BuildError extends Error {}
+export {BuildError};
 
 export interface GraphResult {
     order : RuleDecl[];           // topological, stable w.r.t. declaration order