Encode response schema

src/core.ts

@@ -10,8 +10,8 @@ import type {
   RawServerBase,
   RawServerDefault,
 } from 'fastify'
-import type { $ZodRegistry, input, output } from 'zod/v4/core'
-import { $ZodType, globalRegistry, safeParse } from 'zod/v4/core'
+import type { $ZodRegistry, output } from 'zod/v4/core'
+import { $ZodType, globalRegistry, safeEncode, safeParse } from 'zod/v4/core'
 import { createValidationError, InvalidSchemaError, ResponseSerializationError } from './errors'
 import { getOASVersion, jsonSchemaToOAS } from './json-to-oas'
 import { type ZodToJsonConfig, zodRegistryToJson, zodSchemaToJson } from './zod-to-json'
@@ -30,7 +30,7 @@ const defaultSkipList = [
 
 export interface ZodTypeProvider extends FastifyTypeProvider {
   validator: this['schema'] extends $ZodType ? output<this['schema']> : unknown
-  serializer: this['schema'] extends $ZodType ? input<this['schema']> : unknown
+  serializer: this['schema'] extends $ZodType ? output<this['schema']> : unknown
 }
 
 interface Schema extends FastifySchema {
@@ -215,7 +215,7 @@ export const createSerializerCompiler =
   (data) => {
     const schema = resolveSchema(maybeSchema)
 
-    const result = safeParse(schema, data)
+    const result = safeEncode(schema, data)
     if (result.error) {
       throw new ResponseSerializationError(method, url, {
         cause: result.error,

test/response-schema.spec.ts

@@ -224,6 +224,76 @@ describe('response schema', () => {
     })
   })
 
+  describe('correctly processes response schema (codec)', () => {
+    let app: FastifyInstance
+    beforeEach(async () => {
+      const REPLY_SCHEMA = z.object({
+        value: z.codec(z.string().regex(z.regexes.integer), z.int(), {
+          decode: (str) => Number.parseInt(str, 10),
+          encode: (num) => num.toString(),
+        }),
+      })
+
+      app = Fastify()
+      app.setValidatorCompiler(validatorCompiler)
+      app.setSerializerCompiler(serializerCompiler)
+
+      app.after(() => {
+        app.withTypeProvider<ZodTypeProvider>().route({
+          method: 'GET',
+          url: '/',
+          schema: {
+            response: {
+              200: REPLY_SCHEMA,
+            },
+          },
+          handler: (_req, res) => {
+            res.send({ value: 1234 })
+          },
+        })
+
+        app.withTypeProvider<ZodTypeProvider>().route({
+          method: 'GET',
+          url: '/incorrect',
+          schema: {
+            response: {
+              200: REPLY_SCHEMA,
+            },
+          },
+          handler: (_req, res) => {
+            res.send({ value: '1234' as any })
+          },
+        })
+      })
+
+      await app.ready()
+    })
+    afterAll(async () => {
+      await app.close()
+    })
+
+    it('returns 200 for correct response', async () => {
+      const response = await app.inject().get('/')
+
+      expect(response.statusCode).toBe(200)
+      expect(response.json()).toEqual({ value: '1234' })
+    })
+
+    it('returns 500 for incorrect response', async () => {
+      const response = await app.inject().get('/incorrect')
+
+      expect(response.statusCode).toBe(500)
+      expect(response.json()).toMatchInlineSnapshot(`
+        {
+          "code": "FST_ERR_RESPONSE_SERIALIZATION",
+          "error": "Internal Server Error",
+          "message": "Response doesn't match the schema",
+          "statusCode": 500,
+        }
+      `)
+    })
+  })
+
   describe('correctly replaces date in stringified response', () => {
     let app: FastifyInstance
     beforeAll(async () => {