fix: lowercase type-providers headers types

test/types/type-provider.test-d.ts

@@ -278,6 +278,62 @@ expectAssignable(server.withTypeProvider<TypeBoxProvider>().get(
   }
 ))
 
+// -------------------------------------------------------------------
+// Request headers
+// -------------------------------------------------------------------
+
+// JsonSchemaToTsProvider
+expectAssignable(server.withTypeProvider<JsonSchemaToTsProvider>().get(
+  '/',
+  {
+    schema: {
+      headers: {
+        type: 'object',
+        properties: {
+          lowercase: { type: 'string' },
+          UPPERCASE: { type: 'number' },
+          camelCase: { type: 'boolean' },
+          'KEBAB-case': { type: 'boolean' },
+          PRESERVE_OPTIONAL: { type: 'number' }
+        },
+        required: ['lowercase', 'UPPERCASE', 'camelCase', 'KEBAB-case']
+      } as const
+    }
+  },
+  (req) => {
+    expectType<string>(req.headers.lowercase)
+    expectType<string | string[] | undefined>(req.headers.UPPERCASE)
+    expectType<number>(req.headers.uppercase)
+    expectType<boolean>(req.headers.camelcase)
+    expectType<boolean>(req.headers['kebab-case'])
+    expectType<number | undefined>(req.headers.preserve_optional)
+  }
+))
+
+// TypeBoxProvider
+expectAssignable(server.withTypeProvider<TypeBoxProvider>().get(
+  '/',
+  {
+    schema: {
+      headers: Type.Object({
+        lowercase: Type.String(),
+        UPPERCASE: Type.Number(),
+        camelCase: Type.Boolean(),
+        'KEBAB-case': Type.Boolean(),
+        PRESERVE_OPTIONAL: Type.Optional(Type.Number())
+      })
+    }
+  },
+  (req) => {
+    expectType<string>(req.headers.lowercase)
+    expectType<string | string[] | undefined>(req.headers.UPPERCASE)
+    expectType<number>(req.headers.uppercase)
+    expectType<boolean>(req.headers.camelcase)
+    expectType<boolean>(req.headers['kebab-case'])
+    expectType<number | undefined>(req.headers.preserve_optional)
+  }
+))
+
 // -------------------------------------------------------------------
 // TypeBox Reply Type
 // -------------------------------------------------------------------

types/type-provider.d.ts

@@ -1,5 +1,6 @@
 import { RouteGenericInterface } from './route'
 import { FastifySchema } from './schema'
+import { RecordKeysToLowercase } from './utils'
 
 // -----------------------------------------------------------------------------------------------
 // TypeProvider
@@ -51,7 +52,7 @@ export interface FastifyRequestType<Params = unknown, Querystring = unknown, Hea
 export interface ResolveFastifyRequestType<TypeProvider extends FastifyTypeProvider, SchemaCompiler extends FastifySchema, RouteGeneric extends RouteGenericInterface> extends FastifyRequestType {
   params: ResolveRequestParams<TypeProvider, SchemaCompiler, RouteGeneric>,
   query: ResolveRequestQuerystring<TypeProvider, SchemaCompiler, RouteGeneric>,
-  headers: ResolveRequestHeaders<TypeProvider, SchemaCompiler, RouteGeneric>,
+  headers: RecordKeysToLowercase<ResolveRequestHeaders<TypeProvider, SchemaCompiler, RouteGeneric>>,
   body: ResolveRequestBody<TypeProvider, SchemaCompiler, RouteGeneric>
 }
 

types/utils.d.ts

@@ -69,3 +69,12 @@ export type ReplyKeysToCodes<Key> = [Key] extends [never] ? number :
 export type CodeToReplyKey<Code extends number> = `${Code}` extends `${infer FirstDigit extends CodeClasses}${number}`
   ? `${FirstDigit}xx`
   : never;
+
+export type RecordKeysToLowercase<Input> = Input extends Record<string, unknown>
+  ? {
+    [Key in keyof Input as Key extends string
+      ? Lowercase<Key>
+      : Key
+    ]: Input[Key];
+  }
+  : Input;