diff --git a/src/cli.ts b/src/cli.ts
index 9479091..d868914 100644
--- a/src/cli.ts
+++ b/src/cli.ts
@@ -7,7 +7,7 @@ import { lstsvToDataCsv, lstsvToDdiXml } from './pipelines/lstsv2ddi/index.js';
 import { lstsvToXlsform } from './pipelines/lstsv2xlsform/index.js';
 import type { Submission } from './pipelines/xlsform2ddi/index.js';
 import { XLSFormToTSVConverter } from './pipelines/xlsform2lstsv/index.js';
-import type { XLSFormData } from './config/types.js';
+import type { XLSFormData } from './xlsform/types.js';
 import { XLSValidator } from './xlsform/validate.js';
 import type { SubsetTarget } from './xlsform/validate.js';
 import {
diff --git a/src/cliShared.ts b/src/cliShared.ts
index 3e7f31e..dedd316 100644
--- a/src/cliShared.ts
+++ b/src/cliShared.ts
@@ -6,7 +6,7 @@
 import { readFileSync, writeFileSync } from 'node:fs';
 import { parseArgs, ParseArgsConfig } from 'node:util';
 
-import { XLSFormData } from './config/types.js';
+import { XLSFormData } from './xlsform/types.js';
 import type { BuildDdiOptions } from './ddi/codebook.js';
 import {
   buildDataCsv,
diff --git a/src/config/ConfigManager.ts b/src/config/ConfigManager.ts
index ee16341..eeb2589 100644
--- a/src/config/ConfigManager.ts
+++ b/src/config/ConfigManager.ts
@@ -1,64 +1,34 @@
-import { deepMerge } from '../utils/helpers.js';
-
-import { ConversionConfig, defaultConfig } from './types.js';
+import { resolveConfig } from './resolveConfig.js';
+import { LstsvConfig } from './types.js';
 
 export { ConversionConfig } from './types.js';
 
+/**
+ * @deprecated Use {@link resolveConfig}. Kept as a thin wrapper so existing
+ * consumers keep working; it will go with the public-API cleanup.
+ */
 export class ConfigManager {
-  private config: ConversionConfig;
-
-  constructor(config?: Partial<ConversionConfig>) {
-    this.config = this.mergeConfig(config || {});
-  }
+  private config: Readonly<LstsvConfig>;
 
-  private mergeConfig(
-    partialConfig: Partial<ConversionConfig>,
-  ): ConversionConfig {
-    return deepMerge(structuredClone(defaultConfig), partialConfig);
+  constructor(config?: Partial<LstsvConfig>) {
+    this.config = resolveConfig(config);
   }
 
-  getConfig(): ConversionConfig {
+  getConfig(): Readonly<LstsvConfig> {
     return this.config;
   }
 
-  getDefaults(): ConversionConfig['defaults'] {
+  getDefaults(): LstsvConfig['defaults'] {
     return this.config.defaults;
   }
 
-  getAdvancedOptions() {
-    return {
-      autoCreateGroups: true, // Always auto-create groups (hardcoded)
-      handleRepeats: this.config.handleRepeats ?? 'warn',
-      debugLogging: this.config.debugLogging ?? false,
-    };
+  /** Replaces the options: `partialConfig` merged over the defaults (as before). */
+  updateConfig(partialConfig: Partial<LstsvConfig>): void {
+    this.config = resolveConfig(partialConfig);
   }
 
-  /**
-   * Update configuration at runtime
-   */
-  updateConfig(partialConfig: Partial<ConversionConfig>): void {
-    this.config = this.mergeConfig(partialConfig);
-  }
-
-  /**
-   * Validate configuration
-   */
+  /** Validation now happens in {@link resolveConfig}; kept for compatibility. */
   validateConfig(): void {
-    const { defaults } = this.config;
-
-    // Validate handleRepeats if provided
-    if (
-      this.config.handleRepeats &&
-      !['warn', 'error', 'ignore'].includes(this.config.handleRepeats)
-    ) {
-      throw new Error(
-        `Invalid handleRepeats option: ${this.config.handleRepeats}`,
-      );
-    }
-
-    // Validate defaults
-    if (!defaults.language || defaults.language.length !== 2) {
-      throw new Error('defaults.language must be a 2-character language code');
-    }
+    resolveConfig(this.config);
   }
 }
diff --git a/src/config/resolveConfig.ts b/src/config/resolveConfig.ts
new file mode 100644
index 0000000..774116a
--- /dev/null
+++ b/src/config/resolveConfig.ts
@@ -0,0 +1,27 @@
+import { deepMerge } from '../utils/helpers.js';
+
+import { defaultConfig, LstsvConfig } from './types.js';
+
+const REPEAT_MODES = ['warn', 'error', 'ignore'];
+
+/**
+ * Merge a partial config over the defaults and validate it, in one step. The
+ * result is frozen: a conversion reads its options, it never changes them.
+ */
+export function resolveConfig(
+  partial: Partial<LstsvConfig> = {},
+): Readonly<LstsvConfig> {
+  const config = deepMerge(structuredClone(defaultConfig), partial);
+
+  if (config.handleRepeats && !REPEAT_MODES.includes(config.handleRepeats)) {
+    throw new Error(`Invalid handleRepeats option: ${config.handleRepeats}`);
+  }
+  // The whole xlsform2lstsv pipeline handles 2-letter codes only
+  // (isValidLanguageCode), although convention:languageTagging allows BCP 47
+  // tags such as fr-BE; tracked separately.
+  if (!config.defaults.language || config.defaults.language.length !== 2) {
+    throw new Error('defaults.language must be a 2-character language code');
+  }
+
+  return Object.freeze(config);
+}
diff --git a/src/config/types.ts b/src/config/types.ts
index dde6380..dff545f 100644
--- a/src/config/types.ts
+++ b/src/config/types.ts
@@ -1,65 +1,25 @@
 /**
- * Represents a row in the survey section of an XLSForm
+ * Configuration for the XLSForm → LimeSurvey TSV conversion.
+ *
+ * The XLSForm row types used to live here; they are in `src/xlsform/types.ts`
+ * now and re-exported below for compatibility.
  */
-export interface SurveyRow {
-  type?: string;
-  name?: string;
-  label?: string | Record<string, string>;
-  hint?: string | Record<string, string>;
-  required?: string;
-  relevant?: string;
-  constraint?: string;
-  constraint_message?: string;
-  calculation?: string;
-  default?: string;
-  _languages?: string[];
-  [key: string]: unknown;
-}
-
-/**
- * Represents a row in the choices section of an XLSForm
- */
-export interface ChoiceRow {
-  list_name?: string;
-  name?: string;
-  label?: string | Record<string, string>;
-  filter?: string;
-  _languages?: string[];
-  [key: string]: unknown;
-}
-
-/**
- * Represents a row in the settings section of an XLSForm
- */
-export interface SettingsRow {
-  form_title?: string;
-  form_id?: string;
-  default_language?: string;
-  style?: string;
-  [key: string]: unknown;
-}
+export type {
+  SurveyRow,
+  ChoiceRow,
+  SettingsRow,
+  XLSFormData,
+} from '../xlsform/types.js';
 
 /**
- * Result type returned by XLS/XLSX loaders
+ * Options for the XLSForm → LimeSurvey TSV conversion (`xlsform2lstsv`). The
+ * other directions take their own option objects.
  */
-export interface XLSFormData {
-  surveyData: SurveyRow[];
-  choicesData: ChoiceRow[];
-  settingsData: SettingsRow[];
-  hasSurveySheet: boolean;
-  hasChoicesSheet: boolean;
-  hasSettingsSheet: boolean;
-}
-
-export interface ConversionConfig {
-  /**
-   * How to handle repeats: 'warn', 'error', or 'ignore' (default: 'warn')
-   */
+export interface LstsvConfig {
+  /** @deprecated Never read; accepted so existing callers keep compiling. */
   handleRepeats?: 'warn' | 'error' | 'ignore';
 
-  /**
-   * Enable debug logging (default: false)
-   */
+  /** @deprecated Never read; accepted so existing callers keep compiling. */
   debugLogging?: boolean;
 
   /**
@@ -110,7 +70,7 @@ export interface ConversionConfig {
 /**
  * Default configuration with sensible defaults
  */
-export const defaultConfig: ConversionConfig = {
+export const defaultConfig: LstsvConfig = {
   handleRepeats: 'warn',
   debugLogging: false,
   convertWelcomeNote: true,
@@ -127,3 +87,6 @@ export const defaultConfig: ConversionConfig = {
     description: '',
   },
 };
+
+/** @deprecated Use {@link LstsvConfig}. */
+export type ConversionConfig = LstsvConfig;
diff --git a/src/fileChoices.ts b/src/fileChoices.ts
index 77c8d82..1c5c600 100644
--- a/src/fileChoices.ts
+++ b/src/fileChoices.ts
@@ -11,7 +11,7 @@
 import * as fs from 'node:fs';
 import * as path from 'node:path';
 
-import type { ChoiceRow, SurveyRow } from './config/types.js';
+import type { ChoiceRow, SurveyRow } from './xlsform/types.js';
 import { parseVocabCsv, referencedVocabFiles } from './vocab.js';
 
 /**
diff --git a/src/generateFixtures.ts b/src/generateFixtures.ts
index e98be76..1885df0 100644
--- a/src/generateFixtures.ts
+++ b/src/generateFixtures.ts
@@ -18,12 +18,8 @@ import * as fs from 'fs';
 import * as path from 'path';
 import { fileURLToPath } from 'url';
 
-import {
-  SurveyRow,
-  ChoiceRow,
-  SettingsRow,
-  ConversionConfig,
-} from './config/types.js';
+import { LstsvConfig } from './config/types.js';
+import { SurveyRow, ChoiceRow, SettingsRow } from './xlsform/types.js';
 import { XLSLoader } from './xlsform/loader.js';
 import { XLSFormToTSVConverter } from './pipelines/xlsform2lstsv/index.js';
 
@@ -58,7 +54,7 @@ function cleanOutputDirectory(dir: string): void {
 async function generateTSVFromFixture(
   fixturePath: string,
   outputPath: string,
-  config: Partial<ConversionConfig> = {},
+  config: Partial<LstsvConfig> = {},
 ): Promise<void> {
   console.log(`Processing: ${path.basename(fixturePath)}`);
 
diff --git a/src/index.ts b/src/index.ts
index ecdf886..936ff31 100644
--- a/src/index.ts
+++ b/src/index.ts
@@ -83,3 +83,11 @@ export type {
 // ── Config ─────────────────────────────────────────────────────────────
 export { ConfigManager, ConversionConfig } from './config/ConfigManager.js';
 export { defaultConfig } from './config/types.js';
+export type { LstsvConfig } from './config/types.js';
+export { resolveConfig } from './config/resolveConfig.js';
+export type {
+  SurveyRow,
+  ChoiceRow,
+  SettingsRow,
+  XLSFormData,
+} from './xlsform/types.js';
diff --git a/src/pipelines/lstsv2xlsform/toXlsform.ts b/src/pipelines/lstsv2xlsform/toXlsform.ts
index a50ef2b..f84ef86 100644
--- a/src/pipelines/lstsv2xlsform/toXlsform.ts
+++ b/src/pipelines/lstsv2xlsform/toXlsform.ts
@@ -25,7 +25,7 @@
  */
 
 import { defaultConfig } from '../../config/types.js';
-import type { SurveyRow, ChoiceRow, SettingsRow } from '../../config/types.js';
+import type { SurveyRow, ChoiceRow, SettingsRow } from '../../xlsform/types.js';
 import { APPEARANCES } from '../../generated/Appearances.js';
 import { TYPE_MAPPINGS } from '../../generated/TypeMappings.js';
 import { EXCLUSIVE_RULE } from '../../xlsform/exclusive.js';
diff --git a/src/pipelines/xlsform2lstsv/choiceManager.ts b/src/pipelines/xlsform2lstsv/choiceManager.ts
index 22a631a..fb32808 100644
--- a/src/pipelines/xlsform2lstsv/choiceManager.ts
+++ b/src/pipelines/xlsform2lstsv/choiceManager.ts
@@ -1,4 +1,4 @@
-import { ChoiceRow, SurveyRow } from '../../config/types.js';
+import { ChoiceRow, SurveyRow } from '../../xlsform/types.js';
 import { normalizeName } from '../../xlsform/identifiers.js';
 import { FieldSanitizer } from '../../xlsform/sanitize.js';
 import { TypeInfo } from './typeMapper.js';
diff --git a/src/pipelines/xlsform2lstsv/fieldNameHandler.ts b/src/pipelines/xlsform2lstsv/fieldNameHandler.ts
index 297c9f3..44242d9 100644
--- a/src/pipelines/xlsform2lstsv/fieldNameHandler.ts
+++ b/src/pipelines/xlsform2lstsv/fieldNameHandler.ts
@@ -1,4 +1,4 @@
-import { SurveyRow } from '../../config/types.js';
+import { SurveyRow } from '../../xlsform/types.js';
 import { normalizeName } from '../../xlsform/identifiers.js';
 import { FieldSanitizer } from '../../xlsform/sanitize.js';
 
diff --git a/src/pipelines/xlsform2lstsv/groupEmitter.ts b/src/pipelines/xlsform2lstsv/groupEmitter.ts
index ede2098..017a074 100644
--- a/src/pipelines/xlsform2lstsv/groupEmitter.ts
+++ b/src/pipelines/xlsform2lstsv/groupEmitter.ts
@@ -1,4 +1,4 @@
-import { SurveyRow } from '../../config/types.js';
+import { SurveyRow } from '../../xlsform/types.js';
 import { ConfigManager } from '../../config/ConfigManager.js';
 import { RowEmitter } from './rowEmitter.js';
 import { LanguageHandler } from './languageHandler.js';
diff --git a/src/pipelines/xlsform2lstsv/groupProcessor.ts b/src/pipelines/xlsform2lstsv/groupProcessor.ts
index 3595f53..095c5c1 100644
--- a/src/pipelines/xlsform2lstsv/groupProcessor.ts
+++ b/src/pipelines/xlsform2lstsv/groupProcessor.ts
@@ -1,4 +1,4 @@
-import { SurveyRow } from '../../config/types.js';
+import { SurveyRow } from '../../xlsform/types.js';
 import { ConfigManager } from '../../config/ConfigManager.js';
 import { SKIP_TYPES } from './constants.js';
 
diff --git a/src/pipelines/xlsform2lstsv/index.ts b/src/pipelines/xlsform2lstsv/index.ts
index a6ba2da..5091c94 100644
--- a/src/pipelines/xlsform2lstsv/index.ts
+++ b/src/pipelines/xlsform2lstsv/index.ts
@@ -1,5 +1,6 @@
-import { ConfigManager, ConversionConfig } from '../../config/ConfigManager.js';
-import { SurveyRow, ChoiceRow, SettingsRow } from '../../config/types.js';
+import { ConfigManager } from '../../config/ConfigManager.js';
+import type { LstsvConfig } from '../../config/types.js';
+import { SurveyRow, ChoiceRow, SettingsRow } from '../../xlsform/types.js';
 import { FieldSanitizer } from '../../xlsform/sanitize.js';
 import { TSVGenerator } from '../../lstsv/serialize.js';
 import { TypeMapper, TYPE_MAPPINGS } from './typeMapper.js';
@@ -58,9 +59,8 @@ export class XLSFormToTSVConverter {
   private surveySettingsEmitter: SurveySettingsEmitter;
   private surveyDataCache: SurveyRow[] = [];
 
-  constructor(config?: Partial<ConversionConfig>) {
+  constructor(config?: Partial<LstsvConfig>) {
     this.configManager = new ConfigManager(config);
-    this.configManager.validateConfig();
 
     this.fieldSanitizer = new FieldSanitizer();
     this.choiceManager = new ChoiceManager(this.fieldSanitizer);
@@ -135,14 +135,14 @@ export class XLSFormToTSVConverter {
   /**
    * Get the current configuration
    */
-  getConfig(): ConversionConfig {
+  getConfig(): Readonly<LstsvConfig> {
     return this.configManager.getConfig();
   }
 
   /**
    * Update configuration at runtime
    */
-  updateConfig(partialConfig: Partial<ConversionConfig>): void {
+  updateConfig(partialConfig: Partial<LstsvConfig>): void {
     this.configManager.updateConfig(partialConfig);
   }
 
@@ -216,9 +216,8 @@ export class XLSFormToTSVConverter {
       return xfType === 'begin_group';
     });
 
-    // If no groups, add a default group
-    const advancedOptions = this.configManager.getAdvancedOptions();
-    if (!hasGroups && advancedOptions.autoCreateGroups) {
+    // LimeSurvey needs every question in a group; add one if the form has none.
+    if (!hasGroups) {
       this.groupEmitter.addDefaultGroup();
     }
 
diff --git a/src/pipelines/xlsform2lstsv/languageHandler.ts b/src/pipelines/xlsform2lstsv/languageHandler.ts
index e9954f3..06ce7c4 100644
--- a/src/pipelines/xlsform2lstsv/languageHandler.ts
+++ b/src/pipelines/xlsform2lstsv/languageHandler.ts
@@ -1,4 +1,4 @@
-import { SurveyRow, ChoiceRow, SettingsRow } from '../../config/types.js';
+import { SurveyRow, ChoiceRow, SettingsRow } from '../../xlsform/types.js';
 import { getBaseLanguage } from '../../utils/languageUtils.js';
 import { markdownToHtml } from '../../utils/markdownRenderer.js';
 import { ConfigManager } from '../../config/ConfigManager.js';
diff --git a/src/pipelines/xlsform2lstsv/matrixHandler.ts b/src/pipelines/xlsform2lstsv/matrixHandler.ts
index 9c05a97..1330951 100644
--- a/src/pipelines/xlsform2lstsv/matrixHandler.ts
+++ b/src/pipelines/xlsform2lstsv/matrixHandler.ts
@@ -1,4 +1,4 @@
-import { SurveyRow } from '../../config/types.js';
+import { SurveyRow } from '../../xlsform/types.js';
 import { TypeInfo } from './typeMapper.js';
 import { RowEmitter } from './rowEmitter.js';
 import { LanguageHandler } from './languageHandler.js';
diff --git a/src/pipelines/xlsform2lstsv/otherPatternDetector.ts b/src/pipelines/xlsform2lstsv/otherPatternDetector.ts
index 10e0968..bc821a9 100644
--- a/src/pipelines/xlsform2lstsv/otherPatternDetector.ts
+++ b/src/pipelines/xlsform2lstsv/otherPatternDetector.ts
@@ -1,4 +1,4 @@
-import { SurveyRow, ChoiceRow } from '../../config/types.js';
+import { SurveyRow, ChoiceRow } from '../../xlsform/types.js';
 import { TypeInfo } from './typeMapper.js';
 import { ChoiceManager } from './choiceManager.js';
 import { LanguageHandler } from './languageHandler.js';
diff --git a/src/pipelines/xlsform2lstsv/surveySettingsEmitter.ts b/src/pipelines/xlsform2lstsv/surveySettingsEmitter.ts
index e6c88d7..2104a52 100644
--- a/src/pipelines/xlsform2lstsv/surveySettingsEmitter.ts
+++ b/src/pipelines/xlsform2lstsv/surveySettingsEmitter.ts
@@ -1,4 +1,4 @@
-import { SurveyRow, SettingsRow } from '../../config/types.js';
+import { SurveyRow, SettingsRow } from '../../xlsform/types.js';
 import { RowEmitter } from './rowEmitter.js';
 import { LanguageHandler } from './languageHandler.js';
 import { ConfigManager } from '../../config/ConfigManager.js';
diff --git a/src/vocab.ts b/src/vocab.ts
index d0e253e..93781be 100644
--- a/src/vocab.ts
+++ b/src/vocab.ts
@@ -6,7 +6,7 @@
  * the caller's to read and pass through {@link parseVocabCsv}; the node-only
  * `fileChoices.ts` does that from disk for the CLI.
  */
-import type { ChoiceRow, SurveyRow } from './config/types.js';
+import type { ChoiceRow, SurveyRow } from './xlsform/types.js';
 import { VOCABULARY_OPTIONS } from './generated/VocabularyOptions.js';
 import { parseCsvRecords } from './responseFile.js';
 
diff --git a/src/xlsform/exclusive.ts b/src/xlsform/exclusive.ts
index 6af6df9..bcd2b28 100644
--- a/src/xlsform/exclusive.ts
+++ b/src/xlsform/exclusive.ts
@@ -5,7 +5,7 @@
  * `exclude_all_others` question attribute.
  */
 import conventions from '../generated/conventions.js';
-import type { ChoiceRow } from '../config/types.js';
+import type { ChoiceRow } from './types.js';
 
 export const EXCLUSIVE_RULE = conventions.conventions.exclusiveChoice;
 
diff --git a/src/xlsform/loader.ts b/src/xlsform/loader.ts
index 27122d2..96a0fcd 100644
--- a/src/xlsform/loader.ts
+++ b/src/xlsform/loader.ts
@@ -1,11 +1,6 @@
 import * as XLSX from 'xlsx';
 
-import {
-  SurveyRow,
-  ChoiceRow,
-  SettingsRow,
-  XLSFormData,
-} from '../config/types.js';
+import { SurveyRow, ChoiceRow, SettingsRow, XLSFormData } from './types.js';
 import {
   extractBaseColumnName,
   extractLanguageCode,
diff --git a/src/xlsform/parser.ts b/src/xlsform/parser.ts
index 5f04d52..c6dfdff 100644
--- a/src/xlsform/parser.ts
+++ b/src/xlsform/parser.ts
@@ -3,7 +3,7 @@
  */
 
 import { ConversionConfig } from '../config/ConfigManager.js';
-import type { ChoiceRow } from '../config/types.js';
+import type { ChoiceRow } from './types.js';
 
 import { XLSLoader } from './loader.js';
 
diff --git a/src/xlsform/types.ts b/src/xlsform/types.ts
new file mode 100644
index 0000000..0d28ae1
--- /dev/null
+++ b/src/xlsform/types.ts
@@ -0,0 +1,54 @@
+/** XLSForm data as the loaders produce it: one object per sheet row. */
+
+/**
+ * Represents a row in the survey section of an XLSForm
+ */
+export interface SurveyRow {
+  type?: string;
+  name?: string;
+  label?: string | Record<string, string>;
+  hint?: string | Record<string, string>;
+  required?: string;
+  relevant?: string;
+  constraint?: string;
+  constraint_message?: string;
+  calculation?: string;
+  default?: string;
+  _languages?: string[];
+  [key: string]: unknown;
+}
+
+/**
+ * Represents a row in the choices section of an XLSForm
+ */
+export interface ChoiceRow {
+  list_name?: string;
+  name?: string;
+  label?: string | Record<string, string>;
+  filter?: string;
+  _languages?: string[];
+  [key: string]: unknown;
+}
+
+/**
+ * Represents a row in the settings section of an XLSForm
+ */
+export interface SettingsRow {
+  form_title?: string;
+  form_id?: string;
+  default_language?: string;
+  style?: string;
+  [key: string]: unknown;
+}
+
+/**
+ * Result type returned by XLS/XLSX loaders
+ */
+export interface XLSFormData {
+  surveyData: SurveyRow[];
+  choicesData: ChoiceRow[];
+  settingsData: SettingsRow[];
+  hasSurveySheet: boolean;
+  hasChoicesSheet: boolean;
+  hasSettingsSheet: boolean;
+}
diff --git a/src/xlsform/validate.ts b/src/xlsform/validate.ts
index ff1b2e5..3672309 100644
--- a/src/xlsform/validate.ts
+++ b/src/xlsform/validate.ts
@@ -2,7 +2,7 @@ import conventions from '../generated/conventions.js';
 import { APPEARANCES } from '../generated/Appearances.js';
 import { TYPE_MAPPINGS } from '../generated/TypeMappings.js';
 
-import { SurveyRow, ChoiceRow } from '../config/types.js';
+import { SurveyRow, ChoiceRow } from './types.js';
 import { registeredVocabFiles } from '../vocab.js';
 import { EXCLUSIVE_RULE, exclusiveCell, isExclusive } from './exclusive.js';
 
diff --git a/tests/ts/contract/fullRoundtrip.test.ts b/tests/ts/contract/fullRoundtrip.test.ts
index caa1451..74052be 100644
--- a/tests/ts/contract/fullRoundtrip.test.ts
+++ b/tests/ts/contract/fullRoundtrip.test.ts
@@ -11,7 +11,7 @@ import { describe, test, expect } from 'vitest';
 
 import { XLSFormToTSVConverter } from '../../../src/pipelines/xlsform2lstsv/index.js';
 import { lstsvToXlsform } from '../../../src/pipelines/lstsv2xlsform/index.js';
-import type { SurveyRow, ChoiceRow } from '../../../src/config/types.js';
+import type { SurveyRow, ChoiceRow } from '../../../src/xlsform/types.js';
 
 async function roundTrip(
   survey: SurveyRow[],
diff --git a/tests/ts/unit/markdownLabels.test.ts b/tests/ts/unit/markdownLabels.test.ts
index 25baab8..48a3b9f 100644
--- a/tests/ts/unit/markdownLabels.test.ts
+++ b/tests/ts/unit/markdownLabels.test.ts
@@ -1,5 +1,8 @@
 import { describe, test, expect } from 'vitest';
-import { markdownToHtml, htmlToMarkdown } from '../../../src/utils/markdownRenderer';
+import {
+  markdownToHtml,
+  htmlToMarkdown,
+} from '../../../src/utils/markdownRenderer';
 import { convertAndParse, findRowByName, findRowsByClass } from './helpers';
 
 // ==============================================================
diff --git a/tests/ts/unit/resolveConfig.test.ts b/tests/ts/unit/resolveConfig.test.ts
new file mode 100644
index 0000000..c14b870
--- /dev/null
+++ b/tests/ts/unit/resolveConfig.test.ts
@@ -0,0 +1,31 @@
+/** resolveConfig merges over the defaults, validates, and freezes (#64). */
+import { describe, test, expect } from 'vitest';
+
+import { resolveConfig, defaultConfig } from '../../../src/index.js';
+
+describe('resolveConfig', () => {
+  test('merges nested options over the defaults', () => {
+    const c = resolveConfig({
+      hideNoAnswer: false,
+      defaults: { language: 'de' } as never,
+    });
+    expect(c.hideNoAnswer).toBe(false);
+    expect(c.defaults.language).toBe('de');
+    expect(c.defaults.groupName).toBe(defaultConfig.defaults.groupName);
+  });
+
+  test('returns a frozen object and leaves the defaults untouched', () => {
+    const c = resolveConfig();
+    expect(Object.isFrozen(c)).toBe(true);
+    expect(c).not.toBe(defaultConfig);
+  });
+
+  test('rejects invalid options', () => {
+    expect(() => resolveConfig({ handleRepeats: 'skip' as never })).toThrow(
+      /handleRepeats/,
+    );
+    expect(() =>
+      resolveConfig({ defaults: { language: 'deu' } as never }),
+    ).toThrow(/2-character/);
+  });
+});