diff --git a/src/codecTools/toJsonSchema.ts b/src/codecTools/toJsonSchema.ts
index 113849b..7960b3d 100644
--- a/src/codecTools/toJsonSchema.ts
+++ b/src/codecTools/toJsonSchema.ts
@@ -1,7 +1,7 @@
 /* eslint-disable no-underscore-dangle */
 /* eslint-disable @typescript-eslint/no-explicit-any */
-import type { JSONSchema7 } from 'json-schema';
 import * as t from 'io-ts';
+import type { JSONSchema7 } from 'json-schema';
 
 /**
  * io-ts types compatible w/ JSON Schema
@@ -27,10 +27,15 @@ type MappableType =
  * Convert an io-ts codec to a JSON Schema (v7)
  * @param _type - an io-ts codec
  * @param strict - whether to enable strict mode
+ * @param alwaysIncludeRequired - whether to always include required fields (OpenAI requires this)
  * @returns a JSON schema object
  * @see https://json-schema.org/understanding-json-schema/basics.html
  */
-export const toJsonSchema = (_type: any, strict = false): JSONSchema7 => {
+export const toJsonSchema = (
+  _type: any,
+  strict = false,
+  alwaysIncludeRequired = false,
+): JSONSchema7 => {
   const type = _type as MappableType;
 
   if (type._tag === 'StringType') {
@@ -52,11 +57,37 @@ export const toJsonSchema = (_type: any, strict = false): JSONSchema7 => {
     return { type: 'string', enum: Object.keys(type.keys) };
   }
   if (type._tag === 'UnionType') {
-    return { anyOf: type.types.map((t: any) => toJsonSchema(t, strict)) };
+    return {
+      anyOf: type.types.map((t: any) =>
+        toJsonSchema(t, strict, alwaysIncludeRequired),
+      ),
+    };
   }
-  if (type._tag === 'IntersectionType') {
-    return { allOf: type.types.map((t: any) => toJsonSchema(t, strict)) };
+  if (type._tag === 'IntersectionType' && !alwaysIncludeRequired) {
+    return {
+      allOf: type.types.map((t: any) =>
+        toJsonSchema(t, strict, alwaysIncludeRequired),
+      ),
+    };
   }
+  if (type._tag === 'IntersectionType' && alwaysIncludeRequired) {
+    const results = type.types.map((t: any) =>
+      toJsonSchema(t, strict, alwaysIncludeRequired),
+    );
+    if (!results.every((r: any) => r.type === 'object')) {
+      throw new Error('InterfaceType must have all children as type=object');
+    }
+    return {
+      type: 'object',
+      required: results.map((r: any) => r.required).flat(),
+      properties: results.reduce(
+        (acc: any, r: any) => ({ ...acc, ...r.properties }),
+        {} as any,
+      ),
+      ...(strict ? { additionalProperties: false } : {}),
+    };
+  }
+
   if (type._tag === 'InterfaceType') {
     return {
       type: 'object',
@@ -64,7 +95,7 @@ export const toJsonSchema = (_type: any, strict = false): JSONSchema7 => {
       properties: Object.fromEntries(
         Object.entries<t.Type<any>>(type.props).map(([key, subtype]) => [
           key,
-          toJsonSchema(subtype, strict),
+          toJsonSchema(subtype, strict, alwaysIncludeRequired),
         ]),
       ),
       ...(strict ? { additionalProperties: false } : {}),
@@ -73,17 +104,30 @@ export const toJsonSchema = (_type: any, strict = false): JSONSchema7 => {
   if (type._tag === 'DictionaryType') {
     return {
       type: 'object',
-      additionalProperties: toJsonSchema(type.codomain, strict),
+      additionalProperties: toJsonSchema(
+        type.codomain,
+        strict,
+        alwaysIncludeRequired,
+      ),
     };
   }
   if (type._tag === 'PartialType') {
     return {
       type: 'object',
+      ...(alwaysIncludeRequired ? { required: Object.keys(type.props) } : {}),
       properties: Object.fromEntries(
-        Object.entries<t.Type<any>>(type.props).map(([key, subtype]) => [
-          key,
-          toJsonSchema(subtype, strict),
-        ]),
+        Object.entries<t.Type<any>>(type.props).map(([key, subtype]) => {
+          const result = toJsonSchema(subtype, strict, alwaysIncludeRequired);
+          return [
+            key,
+            alwaysIncludeRequired && result.type
+              ? {
+                  ...result,
+                  type: [result.type as any, 'null'],
+                }
+              : result,
+          ];
+        }),
       ),
       ...(strict ? { additionalProperties: false } : {}),
     };
@@ -91,13 +135,15 @@ export const toJsonSchema = (_type: any, strict = false): JSONSchema7 => {
   if (type._tag === 'ArrayType') {
     return {
       type: 'array',
-      items: toJsonSchema(type.type, strict),
+      items: toJsonSchema(type.type, strict, alwaysIncludeRequired),
     };
   }
   if (type._tag === 'TupleType') {
     return {
       type: 'array',
-      items: type.types.map((t: any) => toJsonSchema(t, strict)),
+      items: type.types.map((t: any) =>
+        toJsonSchema(t, strict, alwaysIncludeRequired),
+      ),
     };
   }
   if (type._tag === 'RefinementType') {
@@ -105,12 +151,12 @@ export const toJsonSchema = (_type: any, strict = false): JSONSchema7 => {
       return { type: 'integer' };
     }
     return {
-      ...toJsonSchema(type.type, strict),
+      ...toJsonSchema(type.type, strict, alwaysIncludeRequired),
       description: `Predicate: ${type.predicate.name || type.name}`,
     };
   }
   // could add more here for DateFromISOString, etc. etc.
-  return unhandledType(type);
+  return unhandledType(type as never);
 };
 
 // eslint-disable-next-line @typescript-eslint/explicit-function-return-type, @typescript-eslint/no-unused-vars