Avoid duplicating nested error causes

nested_error_contract_test.go

@@ -3,11 +3,27 @@ package promptkit_test
 import (
 	"context"
 	"errors"
+	"strings"
 	"testing"
 
 	"go.maximumdirect.net/promptkit"
 )
 
+func TestNestedErrorsRetainEachCauseWithoutDuplicatingDiagnostics(t *testing.T) {
+	cause := errors.New("independent-leaf-diagnostic")
+	for range 10 {
+		parent := newSimpleEngine(t, "parent", promptkit.WithLLMClient(&fakeLLMClient{err: cause}))
+		_, err := parent.Run(context.Background(), promptkit.RunRequest{PromptID: "prompt"})
+		if !errors.Is(err, cause) || !errors.Is(err, promptkit.ErrLLMGenerate) {
+			t.Fatalf("lost owner or cause: %v", err)
+		}
+		if strings.Count(err.Error(), "independent-leaf-diagnostic") != 1 {
+			t.Fatalf("duplicated leaf: %v", err)
+		}
+		cause = err
+	}
+}
+
 type nestedErrorReader struct{ cause error }
 
 func (r nestedErrorReader) Read(context.Context, promptkit.ArtifactRef) (*promptkit.Artifact, error) {