diff --git a/packages/openapi-generator/test/arrowFunctionFallback.test.ts b/packages/openapi-generator/test/arrowFunctionFallback.test.ts
new file mode 100644
index 00000000..b1471168
--- /dev/null
+++ b/packages/openapi-generator/test/arrowFunctionFallback.test.ts
@@ -0,0 +1,80 @@
+const E = require('fp-ts/lib/Either');
+const assert = require('node:assert/strict');
+const test = require('node:test');
+const { TestProject } = require('./testProject');
+const { parsePlainInitializer } = require('../src');
+
+test('arrow function with parameters should use custom codec definition', async () => {
+  const files = {
+    '/api/v2/admin/migrateOrgShardKey.ts': `
+import * as t from 'io-ts';
+import { NonEmptyString } from './types';
+import { arrayFromArrayOrSingle } from '../../../utils';
+
+export const UpdateShardKeyRequestBody = {
+  shardKey: t.string,
+  collectionType: t.union([
+    t.literal('user'),
+    t.literal('enterprise'),
+    t.literal('enterpriseTemplate')
+  ]),
+  ids: arrayFromArrayOrSingle(NonEmptyString),
+  enterpriseUpdateUsers: t.boolean,
+} as const;
+`,
+    '/api/v2/admin/types.ts': `
+import * as t from 'io-ts';
+
+export const NonEmptyString = t.string;
+`,
+    '/utils/arrayFromArrayOrSingle.ts': `
+import * as t from 'io-ts';
+import { arrayFromSingle } from '@bitgo/public-types';
+
+export const arrayFromArrayOrSingle = <C extends t.Mixed>(codec: C) =>
+  t.union([t.array(codec), arrayFromSingle(codec)]);
+`,
+  };
+
+  const knownImports = {
+    '.': {
+      arrayFromArrayOrSingle: (_: any, innerSchema: any) =>
+        E.right({ type: 'array', items: innerSchema }),
+    },
+  };
+
+  const project = new TestProject(files, knownImports);
+  await project.parseEntryPoint('/api/v2/admin/migrateOrgShardKey.ts');
+  const sourceFile = project.get('/api/v2/admin/migrateOrgShardKey.ts');
+
+  assert.ok(sourceFile !== undefined, 'Source file should exist');
+
+  const declaration = sourceFile.symbols.declarations.find(
+    (s: any) => s.name === 'UpdateShardKeyRequestBody',
+  );
+  assert.ok(
+    declaration !== undefined,
+    'UpdateShardKeyRequestBody declaration should exist',
+  );
+  assert.ok(
+    declaration.init !== undefined,
+    'UpdateShardKeyRequestBody should have init',
+  );
+
+  const result = parsePlainInitializer(project, sourceFile, declaration.init);
+
+  assert.ok(
+    E.isRight(result),
+    `Expected no errors, but got "${E.isLeft(result) ? result.left : ''}"`,
+  );
+
+  assert.ok(
+    result.right.type === 'object',
+    'Should parse UpdateShardKeyRequestBody as object',
+  );
+
+  assert.ok(
+    result.right.properties?.ids?.type === 'array',
+    'Should parse ids field as array (from custom codec definition)',
+  );
+});