diff --git a/packages/zod-openapi/README.md b/packages/zod-openapi/README.md
index b0223b12..6bbe75da 100644
--- a/packages/zod-openapi/README.md
+++ b/packages/zod-openapi/README.md
@@ -286,10 +286,25 @@ const client = hc<typeof appRoutes>('http://localhost:8787/')
 
 ## Limitations
 
+### Combining with `Hono`
+
 Be careful when combining `OpenAPIHono` instances with plain `Hono` instances. `OpenAPIHono` will merge the definitions of direct subapps, but plain `Hono` knows nothing about the OpenAPI spec additions. Similarly `OpenAPIHono` will not "dig" for instances deep inside a branch of plain `Hono` instances.
 
 If you're migrating from plain `Hono` to `OpenAPIHono`, we recommend porting your top-level app, then working your way down the router tree.
 
+### Header keys
+
+Header keys that you define in your schema must be in lowercase.
+
+```ts
+const HeadersSchema = z.object({
+  // Header keys must be in lowercase, `Authorization` is not allowed.
+  authorization: z.string().openapi({
+    example: 'Bearer SECRET',
+  }),
+})
+```
+
 ## References
 
 - [Hono](https://hono.dev/)
diff --git a/packages/zod-openapi/package.json b/packages/zod-openapi/package.json
index 723e5e7a..98f81d92 100644
--- a/packages/zod-openapi/package.json
+++ b/packages/zod-openapi/package.json
@@ -42,6 +42,7 @@
     "zod": "3.*"
   },
   "devDependencies": {
+    "@hono/zod-validator": "^0.1.11",
     "hono": "^3.9.1",
     "zod": "^3.22.1"
   },
diff --git a/packages/zod-openapi/test/index.test.ts b/packages/zod-openapi/test/index.test.ts
index de304784..c7fc9404 100644
--- a/packages/zod-openapi/test/index.test.ts
+++ b/packages/zod-openapi/test/index.test.ts
@@ -33,31 +33,20 @@ describe('Basic - params', () => {
   const ParamsSchema = z.object({
     id: z
       .string()
-      .transform((val, ctx) => {
-        const parsed = parseInt(val)
-        if (isNaN(parsed)) {
-          ctx.addIssue({
-            code: z.ZodIssueCode.custom,
-            message: 'Not a number',
-          })
-          return z.NEVER
-        }
-        return parsed
-      })
+      .min(4)
       .openapi({
         param: {
           name: 'id',
           in: 'path',
         },
-        example: 123,
-        type: 'integer',
+        example: '12345',
       }),
   })
 
   const UserSchema = z
     .object({
-      id: z.number().openapi({
-        example: 123,
+      id: z.string().openapi({
+        example: '12345',
       }),
       name: z.string().openapi({
         example: 'John Doe',
@@ -68,6 +57,13 @@ describe('Basic - params', () => {
     })
     .openapi('User')
 
+  const HeadersSchema = z.object({
+    // Header keys must be in lowercase
+    authorization: z.string().openapi({
+      example: 'Bearer SECRET',
+    }),
+  })
+
   const ErrorSchema = z
     .object({
       ok: z.boolean().openapi({
@@ -81,6 +77,7 @@ describe('Basic - params', () => {
     path: '/users/{id}',
     request: {
       params: ParamsSchema,
+      headers: HeadersSchema,
     },
     responses: {
       200: {
@@ -136,17 +133,21 @@ describe('Basic - params', () => {
   })
 
   it('Should return 200 response with correct contents', async () => {
-    const res = await app.request('/users/123')
+    const res = await app.request('/users/12345', {
+      headers: {
+        Authorization: 'Bearer TOKEN',
+      },
+    })
     expect(res.status).toBe(200)
     expect(await res.json()).toEqual({
-      id: 123,
+      id: '12345',
       age: 20,
       name: 'Ultra-man',
     })
   })
 
   it('Should return 400 response with correct contents', async () => {
-    const res = await app.request('/users/abc')
+    const res = await app.request('/users/123')
     expect(res.status).toBe(400)
     expect(await res.json()).toEqual({ ok: false })
   })
@@ -162,7 +163,7 @@ describe('Basic - params', () => {
           User: {
             type: 'object',
             properties: {
-              id: { type: 'number', example: 123 },
+              id: { type: 'string', example: '12345' },
               name: { type: 'string', example: 'John Doe' },
               age: { type: 'number', example: 42 },
             },
@@ -181,11 +182,17 @@ describe('Basic - params', () => {
           get: {
             parameters: [
               {
-                schema: { type: 'integer', example: 123 },
+                schema: { type: 'string', example: '12345', minLength: 4 },
                 required: true,
                 name: 'id',
                 in: 'path',
               },
+              {
+                schema: { type: 'string', example: 'Bearer SECRET' },
+                required: true,
+                name: 'authorization',
+                in: 'header',
+              },
             ],
             responses: {
               '200': {