diff --git a/.gitignore b/.gitignore
index 323e2c5..d0ccd2c 100644
--- a/.gitignore
+++ b/.gitignore
@@ -130,3 +130,5 @@ dist
 .pnp.*
 
 package-lock.json
+.vscode
+test-storage
\ No newline at end of file
diff --git a/example.js b/example.js
index f61911f..f8b4c18 100644
--- a/example.js
+++ b/example.js
@@ -2,6 +2,8 @@ const Hyperschema = require('.')
 
 const schema = Hyperschema.from('./spec')
 const ns1 = schema.namespace('namespace-1')
+ns1.require('./helpers.js')
+
 const ns2 = schema.namespace('namespace-2')
 
 ns2.register({
@@ -60,6 +62,7 @@ ns1.register({
 ns1.register({
   name: 'basic-struct',
   flagsPosition: 0,
+  validator: 'basicStructValidator',
   fields: [
     {
       name: 'id',
diff --git a/helpers.js b/helpers.js
new file mode 100644
index 0000000..61a51bd
--- /dev/null
+++ b/helpers.js
@@ -0,0 +1,4 @@
+exports.basicStructValidator = (state, m) => {
+  if (!m.id) throw new Error('id is required')
+  if (!m.basicString) throw new Error('basicString is required')
+}
diff --git a/index.js b/index.js
index cb347b2..dd2738a 100644
--- a/index.js
+++ b/index.js
@@ -131,6 +131,7 @@ class Struct extends ResolvedType {
     super(hyperschema, fqn, description, existing)
     this.isStruct = true
     this.default = null
+    this.validator = description.validator
 
     this.fields = []
     this.fieldsByName = new Map()
@@ -188,6 +189,10 @@ class Struct extends ResolvedType {
     }
   }
 
+  getNamespace () {
+    return this.hyperschema.namespaces.get(this.namespace)
+  }
+
   toJSON () {
     return {
       name: this.name,
@@ -203,6 +208,12 @@ class HyperschemaNamespace {
   constructor (hyperschema, name) {
     this.hyperschema = hyperschema
     this.name = name
+    this.helpers = null
+    this.id = hyperschema.namespaces.size
+  }
+
+  require (filename) {
+    this.helpers = p.resolve(filename)
   }
 
   register (description) {
diff --git a/lib/codegen.js b/lib/codegen.js
index 94ebca1..981b4eb 100644
--- a/lib/codegen.js
+++ b/lib/codegen.js
@@ -1,5 +1,6 @@
 const gen = require('generate-object-property')
 const s = require('generate-string')
+const p = require('path')
 
 module.exports = function generateSchema (hyperschema) {
   const structs = []
@@ -33,6 +34,13 @@ module.exports = function generateSchema (hyperschema) {
   str += 'let version = VERSION\n'
   str += '\n'
 
+  for (const ns of hyperschema.namespaces.values()) {
+    if (!ns.helpers) continue
+    const helpers = p.relative(p.resolve(hyperschema.dir), ns.helpers).replaceAll('\\', '/')
+    str += `const helpers${ns.id} = require('${helpers}')\n`
+    str += '\n'
+  }
+
   for (let i = 0; i < structs.length; i++) {
     const struct = structs[i]
     const { encoder } = structsByName.get(struct.fqn)
@@ -102,7 +110,13 @@ module.exports = function generateSchema (hyperschema) {
       str += '\n'
     }
 
-    const preencode = generateEncode(struct, { preencode: true })
+    if (struct.validator) {
+      str += `// ${struct.fqn}.validator\n`
+      str += `const ${id}_validator = ${gen('helpers' + struct.getNamespace().id, struct.validator)}\n`
+      str += '\n'
+    }
+
+    const preencode = generateEncode(struct, { id, preencode: true })
     const encode = generateEncode(struct)
     const decode = generateDecode(struct)
     str += `// ${struct.fqn}\n`
@@ -119,10 +133,12 @@ module.exports = function generateSchema (hyperschema) {
     str += '}\n'
     return str
 
-    function generateEncode (struct, { preencode = false } = {}) {
+    function generateEncode (struct, { id, preencode = false } = {}) {
       const fn = preencode ? 'preencode' : 'encode'
       let str = ''
 
+      str = preencode && struct.validator ? `    ${id}_validator(state, m)\n` : ''
+
       if (struct.optionals.length >= 1) str += '    let flags = 0\n'
       for (const field of struct.optionals) {
         str += `    ${vPrefix(field.version, gen('m', field.name))} flags |= ${field.flag}`
diff --git a/test/basic.js b/test/basic.js
index 8cf76ca..a6c31e4 100644
--- a/test/basic.js
+++ b/test/basic.js
@@ -283,6 +283,40 @@ test('basic required field missing', async t => {
   }
 })
 
+test('basic required field missing with validation', async t => {
+  const schema = await createTestSchema(t)
+
+  await schema.rebuild(schema => {
+    const ns = schema.namespace('test')
+    ns.require('test/helpers/index.js')
+    ns.register({
+      name: 'test-struct',
+      validator: 'testValidator',
+      fields: [
+        {
+          name: 'field1',
+          type: 'string',
+          required: true
+        }
+      ]
+    })
+  })
+
+  t.is(schema.json.version, 1)
+  t.is(schema.module.version, 1)
+
+  {
+    const enc = schema.module.resolveStruct('@test/test-struct')
+    const missingRequired = { field2: 'badField' }
+    try {
+      c.encode(enc, missingRequired)
+      t.fail('expected error')
+    } catch (e) {
+      t.is(e.message, 'field1 is required')
+    }
+  }
+})
+
 test('basic nested struct, version bump', async t => {
   const schema = await createTestSchema(t)
 
diff --git a/test/helpers/index.js b/test/helpers/index.js
index f273f1e..d268c4f 100644
--- a/test/helpers/index.js
+++ b/test/helpers/index.js
@@ -48,6 +48,11 @@ async function createTestSchema (t) {
   return new TestBuilder(dir, t)
 }
 
+function testValidator (state, m) {
+  if (!m.field1) throw new Error('field1 is required')
+}
+
 module.exports = {
-  createTestSchema
+  createTestSchema,
+  testValidator
 }