Identify configuration source files in load errors

internal/cli/configkit_load.go

@@ -30,6 +30,18 @@ func writeConfigLoadError(output io.Writer, err error) {
 	var loadError *configkit.LoadError
 	if errors.As(err, &loadError) {
 		if writeErr := configkit.WriteDiagnostics(output, loadError.Diagnostics()); writeErr == nil {
+			// ConfigKit's locations contain source IDs. Keep its renderer and
+			// supply the acquired-source legend so imported/profile errors can
+			// be traced to a file. Quoting also escapes terminal controls.
+			for _, source := range loadError.Sources() {
+				label := source.Path
+				if label == "" {
+					label = source.Label
+				}
+				if _, err := fmt.Fprintf(output, "source %q: %q\n", source.ID, label); err != nil {
+					return
+				}
+			}
 			return
 		}
 	}

internal/cli/configkit_load_test.go

@@ -4,12 +4,45 @@ import (
 	"bytes"
 	"os"
 	"path/filepath"
+	"strconv"
 	"strings"
 	"testing"
 
 	configkit "gitea.maximumdirect.net/eric/ConfigKit"
 )
 
+func TestConfigLoadDiagnosticsIdentifyComposedSources(t *testing.T) {
+	t.Parallel()
+	for _, profile := range []bool{false, true} {
+		t.Run(strconv.FormatBool(profile), func(t *testing.T) {
+			root := t.TempDir()
+			main := filepath.Join(root, "config.yml")
+			fragment := filepath.Join(root, "pipeline\n\x1b.yml")
+			args := []string{"config", "validate", "--config", main}
+			if profile {
+				fragment = filepath.Join(root, "profiles", "development.yml")
+				writeConfigKitFile(t, main, "version: 5\n")
+				args = append(args, "--config-profile", "development")
+			} else {
+				writeConfigKitFile(t, main, "version: 5\nconfigkit:\n  imports:\n    - path: "+strconv.Quote(filepath.Base(fragment))+"\n")
+			}
+			writeConfigKitFile(t, fragment, "concurrency: {total_llm: invalid-private-value}\n")
+			var stdout, stderr bytes.Buffer
+			if code := RunWithOptions(args, &stdout, &stderr, commandContractOptions(t)); code != 1 {
+				t.Fatalf("status = %d; %s", code, stderr.String())
+			}
+			got := stderr.String()
+			if stdout.Len() != 0 || !strings.Contains(got, "decode.type") ||
+				!strings.Contains(got, "source-2:1:") ||
+				!strings.Contains(got, strconv.Quote(fragment)) ||
+				strings.Contains(got, "invalid-private-value") ||
+				strings.ContainsRune(got, '\x1b') {
+				t.Fatalf("stdout = %q; stderr = %q", stdout.String(), got)
+			}
+		})
+	}
+}
+
 func TestLoadConfigKitUsesDiscoveryProfileAndOverrides(t *testing.T) {
 	t.Parallel()
 	root := t.TempDir()