Resolve profiles through built-in backends

public_contract_test.go

@@ -3,6 +3,7 @@ package promptkit_test
 import (
 	"context"
 	"encoding/json"
+	"errors"
 	"fmt"
 	"strings"
 	"sync"
@@ -26,6 +27,62 @@ func TestPreparedRunJSONOmitsZeroTimingValues(t *testing.T) {
 	}
 }
 
+func TestBackendIdentityJSONNamesAndOmission(t *testing.T) {
+	values := []struct {
+		name  string
+		value any
+		field string
+	}{
+		{name: "execution target", value: promptkit.ExecutionTarget{BackendID: promptkit.BackendOpenRouter}, field: "backend_id"},
+		{name: "prepared run", value: promptkit.PreparedRun{SelectedBackendID: promptkit.BackendOpenRouter}, field: "selected_backend_id"},
+		{name: "run result", value: promptkit.RunResult{SelectedBackendID: promptkit.BackendOpenRouter}, field: "selected_backend_id"},
+	}
+	for _, tt := range values {
+		t.Run(tt.name, func(t *testing.T) {
+			payload, err := json.Marshal(tt.value)
+			if err != nil {
+				t.Fatalf("marshal populated value: %v", err)
+			}
+			var object map[string]any
+			if err := json.Unmarshal(payload, &object); err != nil {
+				t.Fatalf("decode populated value: %v", err)
+			}
+			if object[tt.field] != promptkit.BackendOpenRouter {
+				t.Fatalf("expected %s=%q, got %s", tt.field, promptkit.BackendOpenRouter, payload)
+			}
+		})
+	}
+
+	emptyValues := []any{promptkit.ExecutionTarget{}, promptkit.PreparedRun{}, promptkit.RunResult{}}
+	for _, value := range emptyValues {
+		payload, err := json.Marshal(value)
+		if err != nil {
+			t.Fatalf("marshal empty value: %v", err)
+		}
+		if strings.Contains(string(payload), `"backend_id"`) || strings.Contains(string(payload), `"selected_backend_id"`) {
+			t.Fatalf("empty backend identity was not omitted: %s", payload)
+		}
+	}
+}
+
+func TestUnknownProfileBackendHasProfileLoadIdentity(t *testing.T) {
+	engine, err := promptkit.NewEngine(promptkit.Config{},
+		promptkit.WithPromptFS(contractPromptFS("prompt", "profile", "message"), "."),
+		promptkit.WithProfiles(promptkit.Profile{ID: "profile", BackendID: "unknown", Model: "model"}),
+	)
+	if err != nil {
+		t.Fatalf("construct engine: %v", err)
+	}
+
+	_, err = engine.Prepare(context.Background(), promptkit.RunRequest{PromptID: "prompt"})
+	if !errors.Is(err, promptkit.ErrProfileLoad) {
+		t.Fatalf("expected ErrProfileLoad, got %v", err)
+	}
+	if errors.Is(err, promptkit.ErrInvalidRequest) {
+		t.Fatalf("unknown backend should not have invalid-request identity: %v", err)
+	}
+}
+
 func TestPreparedRunJSONTimingRoundTrips(t *testing.T) {
 	start := time.Date(2026, time.July, 29, 12, 0, 0, 0, time.UTC)
 	prepared := promptkit.PreparedRun{