Add internal capacity error identity

internal/capacity/manager_test.go

@@ -124,6 +124,11 @@ func TestManagerAdmissionHonorsContextAndUnlimitedBackends(t *testing.T) {
 	if err != nil {
 		t.Fatalf("construct manager: %v", err)
 	}
+	release, err := manager.Admit(context.Background(), "limited")
+	if err != nil {
+		t.Fatalf("fill limited pool: %v", err)
+	}
+	defer release()
 
 	ctx, cancel := context.WithCancel(context.Background())
 	cancel()

internal/usecase/capacity_error.go

@@ -0,0 +1,24 @@
+package usecase
+
+import (
+	"fmt"
+	"strings"
+
+	"gitea.maximumdirect.net/eric/promptkit/internal/capacity"
+)
+
+// CapacityError identifies bounded admission rejected for one selected backend.
+type CapacityError struct {
+	BackendID string
+}
+
+func (e *CapacityError) Error() string {
+	if e == nil || strings.TrimSpace(e.BackendID) == "" {
+		return capacity.ErrCapacityExceeded.Error()
+	}
+	return fmt.Sprintf("backend %q admission: %v", e.BackendID, capacity.ErrCapacityExceeded)
+}
+
+func (e *CapacityError) Unwrap() error {
+	return capacity.ErrCapacityExceeded
+}

internal/usecase/runner.go

@@ -390,8 +390,8 @@ func (r *Runner) admitRun(ctx context.Context, backendID string) (func(), error)
 	}
 	release, err := r.admitter.Admit(ctx, backendID)
 	if err != nil {
-		if errors.Is(err, capacity.ErrCapacityExceeded) {
-			return nil, fmt.Errorf("backend %q admission: %w", backendID, err)
+		if errors.Is(err, capacity.ErrCapacityExceeded) && strings.TrimSpace(backendID) != "" {
+			return nil, &CapacityError{BackendID: backendID}
 		}
 		return nil, err
 	}

internal/usecase/runner_test.go

@@ -1357,14 +1357,14 @@ func TestRunnerAdmissionUsesResolvedBackendIdentity(t *testing.T) {
 
 func TestRunnerAdmissionFailureSkipsCompletionCollaborators(t *testing.T) {
 	tests := []struct {
-		name               string
-		admissionError     error
-		wantBackendContext bool
+		name             string
+		admissionError   error
+		wantCapacityType bool
 	}{
 		{
-			name:               "capacity exhausted",
-			admissionError:     capacity.ErrCapacityExceeded,
-			wantBackendContext: true,
+			name:             "capacity exhausted",
+			admissionError:   capacity.ErrCapacityExceeded,
+			wantCapacityType: true,
 		},
 		{
 			name:           "context canceled",
@@ -1414,8 +1414,16 @@ func TestRunnerAdmissionFailureSkipsCompletionCollaborators(t *testing.T) {
 			if errors.Is(err, ErrInvalidRequest) || errors.Is(err, ErrLLMGenerate) {
 				t.Fatalf("admission error was recategorized: %v", err)
 			}
-			if tc.wantBackendContext && !strings.Contains(err.Error(), "custom") {
-				t.Fatalf("capacity error lacks backend context: %v", err)
+			var capacityErr *CapacityError
+			if tc.wantCapacityType {
+				if !errors.As(err, &capacityErr) {
+					t.Fatalf("capacity error=%v, want internal typed identity", err)
+				}
+				if capacityErr.BackendID != "custom" {
+					t.Fatalf("capacity backend ID=%q, want custom", capacityErr.BackendID)
+				}
+			} else if errors.As(err, &capacityErr) {
+				t.Fatalf("non-capacity admission error exposed typed capacity identity: %v", err)
 			}
 			if !reflect.DeepEqual(admitter.backendIDs, []string{"custom"}) {
 				t.Fatalf("admitted backend IDs=%#v, want custom", admitter.backendIDs)