Avoid duplicating nested error causes

docs/roadmap/remediation.md

@@ -121,7 +121,12 @@ the selected self-contained contract is not satisfied.
 
 ### 3. Avoid Duplicating An Already-Retained Error Cause [P3]
 
-**Status:** Open. **Stages:** 3–4.
+**Status:** Complete. **Stages:** 3–4.
+
+Completion: ordinary operation projection retains its mapped cause once, without
+joining the original subtree a second time. Multi-engine regression coverage
+checks exact cause/owner matching and one occurrence of an independent leaf;
+specialized projection and redaction tests remain intact.
 
 **Location:** [public error projection](../../errors.go), `mapPublicError`,
 particularly lines 26–38.

errors.go

@@ -30,12 +30,9 @@ func mapPublicError(err error) error {
 			if errors.Is(operation.Cause, llm.ErrMalformedResponse) {
 				mapped = errors.Join(ErrMalformedResponse, mapped)
 			}
-			var internalCapacity *usecase.CapacityError
-			var internalProvider *llm.ProviderHTTPError
-			if errors.As(operation.Cause, &internalCapacity) || errors.As(operation.Cause, &internalProvider) {
-				return errors.Join(owner, mapped)
-			}
-			return errors.Join(owner, mapped, operation.Cause)
+			// The projection already retains ordinary causes; adding the original
+			// again would duplicate its entire subtree at each enclosing operation.
+			return errors.Join(owner, mapped)
 		}
 	}
 	return mapUnownedPublicError(err)

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) {