diff --git a/packages/openapi-generator/src/codec.ts b/packages/openapi-generator/src/codec.ts
index 175abbd4..e3eb58a4 100644
--- a/packages/openapi-generator/src/codec.ts
+++ b/packages/openapi-generator/src/codec.ts
@@ -450,6 +450,20 @@ export function parsePlainInitializer(
   }
 }
 
+function parseFunctionBody(
+  project: Project,
+  source: SourceFile,
+  func: swc.ArrowFunctionExpression,
+): E.Either<string, Schema> {
+  if (func.body === undefined) {
+    return errorLeft('Function body is undefined');
+  }
+  if (func.body.type === 'BlockStatement') {
+    return errorLeft('BlockStatement arrow functions are not yet supported');
+  }
+  return parseCodecInitializer(project, source, func.body);
+}
+
 export function parseCodecInitializer(
   project: Project,
   source: SourceFile,
@@ -471,8 +485,29 @@ export function parseCodecInitializer(
   } else if (init.type === 'CallExpression') {
     const callee = init.callee;
     if (callee.type !== 'Identifier' && callee.type !== 'MemberExpression') {
-      return errorLeft(`Unimplemented callee type ${init.callee.type}`);
+      return errorLeft(`Unimplemented callee type ${callee.type}`);
+    }
+
+    let calleeName: string | [string, string] | undefined;
+    if (callee.type === 'Identifier') {
+      calleeName = callee.value;
+    } else if (
+      callee.object.type === 'Identifier' &&
+      callee.property.type === 'Identifier'
+    ) {
+      calleeName = [callee.object.value, callee.property.value];
+    }
+
+    if (calleeName !== undefined) {
+      const calleeInitE = findSymbolInitializer(project, source, calleeName);
+      if (E.isRight(calleeInitE)) {
+        const [calleeSourceFile, calleeInit] = calleeInitE.right;
+        if (calleeInit !== null && calleeInit.type === 'ArrowFunctionExpression') {
+          return parseFunctionBody(project, calleeSourceFile, calleeInit);
+        }
+      }
     }
+
     const identifierE = codecIdentifier(project, source, callee);
     if (E.isLeft(identifierE)) {
       return identifierE;
diff --git a/packages/openapi-generator/test/externalModuleApiSpec.test.ts b/packages/openapi-generator/test/externalModuleApiSpec.test.ts
index 08837ced..7522a169 100644
--- a/packages/openapi-generator/test/externalModuleApiSpec.test.ts
+++ b/packages/openapi-generator/test/externalModuleApiSpec.test.ts
@@ -368,3 +368,46 @@ testCase(
   },
   [],
 );
+
+testCase(
+  'simple api spec with util type functions',
+  'test/sample-types/apiSpecWithArrow.ts',
+  {
+    openapi: '3.0.3',
+    info: {
+      title: 'simple api spec with util type functions',
+      version: '1.0.0',
+      description: 'simple api spec with util type functions',
+    },
+    paths: {
+      '/test': {
+        get: {
+          parameters: [],
+          responses: {
+            200: {
+              description: 'OK',
+              content: {
+                'application/json': {
+                  schema: {
+                    type: 'object',
+                    properties: {
+                      hasLargeNumberOfAddresses: {
+                        nullable: true,
+                        type: 'boolean',
+                      },
+                    },
+                    required: ['hasLargeNumberOfAddresses'],
+                  },
+                },
+              },
+            },
+          },
+        },
+      },
+    },
+    components: {
+      schemas: {},
+    },
+  },
+  [],
+);
diff --git a/packages/openapi-generator/test/sample-types/apiSpecWithArrow.ts b/packages/openapi-generator/test/sample-types/apiSpecWithArrow.ts
new file mode 100644
index 00000000..edb6a807
--- /dev/null
+++ b/packages/openapi-generator/test/sample-types/apiSpecWithArrow.ts
@@ -0,0 +1,23 @@
+import * as h from '@api-ts/io-ts-http';
+import * as t from 'io-ts';
+import { BooleanFromString, fromNullable } from 'io-ts-types';
+
+const BooleanFromNullableWithFallback = () =>
+  fromNullable(t.union([BooleanFromString, t.boolean]), false);
+
+export const TEST_ROUTE = h.httpRoute({
+  path: '/test',
+  method: 'GET',
+  request: h.httpRequest({}),
+  response: {
+    200: t.type({
+      hasLargeNumberOfAddresses: BooleanFromNullableWithFallback(),
+    }),
+  },
+});
+
+export const apiSpec = h.apiSpec({
+  'api.test': {
+    get: TEST_ROUTE,
+  },
+});