diff --git a/benchmark/bench-cmp-lib.js b/benchmark/bench-cmp-lib.js
index 57a377d1..794e2dd4 100644
--- a/benchmark/bench-cmp-lib.js
+++ b/benchmark/bench-cmp-lib.js
@@ -1,6 +1,8 @@
 'use strict'
 
+const assert = require('node:assert/strict')
 const { Bench } = require('tinybench')
+const apiResponses = require('./fixtures/api-responses')
 const suite = new Bench({
   name: 'Library Comparison Benchmarks',
   setup: (_task, mode) => {
@@ -173,139 +175,162 @@ for (let i = 0; i < MULTI_ARRAY_LENGTH; i++) {
   multiArray[i] = obj
 }
 
-suite.add('fast-json-stringify: creation', function () {
-  FJS(schema)
+// Some serializers return unflattened strings. Fastify calls Buffer.byteLength()
+// for Content-Length, which triggers flattening; include that cost in every case.
+suite.add('fast-json-stringify: creation and first stringify', function () {
+  Buffer.byteLength(FJS(schema)(obj))
 })
-suite.add('compile-json-stringify: creation', function () {
-  CJS(schemaCJS)
+suite.add('compile-json-stringify: creation and first stringify', function () {
+  Buffer.byteLength(CJS(schemaCJS)(obj))
 })
-suite.add('AJV: creation', function () {
-  ajv.compileSerializer(schemaAJVJTD)
+suite.add('AJV: creation and first stringify', function () {
+  Buffer.byteLength(ajv.compileSerializer(schemaAJVJTD)(obj))
 })
-suite.add('json-accelerator: creation', function () {
-  createAccelerator(schema)
+suite.add('json-accelerator: creation and first stringify', function () {
+  Buffer.byteLength(createAccelerator(schema)(obj))
 })
 
 suite.add('JSON.stringify: array', function () {
-  JSON.stringify(multiArray)
+  Buffer.byteLength(JSON.stringify(multiArray))
 })
 
 suite.add('fast-json-stringify [default]: array', function () {
-  stringifyArrayDefault(multiArray)
+  Buffer.byteLength(stringifyArrayDefault(multiArray))
 })
 
 suite.add('json-accelerator: array', function () {
-  accelArray(multiArray)
+  Buffer.byteLength(accelArray(multiArray))
 })
 
 suite.add('fast-json-stringify [json-stringify]: array', function () {
-  stringifyArrayJSONStringify(multiArray)
+  Buffer.byteLength(stringifyArrayJSONStringify(multiArray))
 })
 
 suite.add('compile-json-stringify: array', function () {
-  CJSStringifyArray(multiArray)
+  Buffer.byteLength(CJSStringifyArray(multiArray))
 })
 
 suite.add('AJV: array', function () {
-  ajvSerializeArray(multiArray)
+  Buffer.byteLength(ajvSerializeArray(multiArray))
 })
 
 suite.add('JSON.stringify: large array', function () {
-  JSON.stringify(largeArray)
+  Buffer.byteLength(JSON.stringify(largeArray))
 })
 
 suite.add('fast-json-stringify [default]: large array', function () {
-  stringifyArrayDefault(largeArray)
+  Buffer.byteLength(stringifyArrayDefault(largeArray))
 })
 
 suite.add('fast-json-stringify [json-stringify]: large array', function () {
-  stringifyArrayJSONStringify(largeArray)
+  Buffer.byteLength(stringifyArrayJSONStringify(largeArray))
 })
 
 suite.add('compile-json-stringify: large array', function () {
-  CJSStringifyArray(largeArray)
+  Buffer.byteLength(CJSStringifyArray(largeArray))
 })
 
 suite.add('AJV: large array', function () {
-  ajvSerializeArray(largeArray)
+  Buffer.byteLength(ajvSerializeArray(largeArray))
 })
 
 suite.add('JSON.stringify: long string', function () {
-  JSON.stringify(str)
+  Buffer.byteLength(JSON.stringify(str))
 })
 
 suite.add('fast-json-stringify: long string', function () {
-  stringifyString(str)
+  Buffer.byteLength(stringifyString(str))
 })
 
 suite.add('json-accelerator: long string', function () {
-  stringifyString(str)
+  Buffer.byteLength(stringifyString(str))
 })
 
 suite.add('compile-json-stringify: long string', function () {
-  CJSStringifyString(str)
+  Buffer.byteLength(CJSStringifyString(str))
 })
 
 suite.add('AJV: long string', function () {
-  ajvSerializeString(str)
+  Buffer.byteLength(ajvSerializeString(str))
 })
 
 suite.add('JSON.stringify: short string', function () {
-  JSON.stringify('hello world')
+  Buffer.byteLength(JSON.stringify('hello world'))
 })
 
 suite.add('fast-json-stringify: short string', function () {
-  stringifyString('hello world')
+  Buffer.byteLength(stringifyString('hello world'))
 })
 
 suite.add('json-accelerator: short string', function () {
-  accelString('hello world')
+  Buffer.byteLength(accelString('hello world'))
 })
 
 suite.add('compile-json-stringify: short string', function () {
-  CJSStringifyString('hello world')
+  Buffer.byteLength(CJSStringifyString('hello world'))
 })
 
 suite.add('AJV: short string', function () {
-  ajvSerializeString('hello world')
+  Buffer.byteLength(ajvSerializeString('hello world'))
 })
 
 suite.add('JSON.stringify: obj', function () {
-  JSON.stringify(obj)
+  Buffer.byteLength(JSON.stringify(obj))
 })
 
 suite.add('fast-json-stringify: obj', function () {
-  stringify(obj)
+  Buffer.byteLength(stringify(obj))
 })
 
 suite.add('json-accelerator: obj', function () {
-  accelStringify(obj)
+  Buffer.byteLength(accelStringify(obj))
 })
 
 suite.add('compile-json-stringify: obj', function () {
-  CJSStringify(obj)
+  Buffer.byteLength(CJSStringify(obj))
 })
 
 suite.add('AJV: obj', function () {
-  ajvSerialize(obj)
+  Buffer.byteLength(ajvSerialize(obj))
 })
 
 suite.add('JSON.stringify: date', function () {
-  JSON.stringify(date)
+  Buffer.byteLength(JSON.stringify(date))
 })
 
 suite.add('fast-json-stringify: date', function () {
-  stringifyDate(date)
+  Buffer.byteLength(stringifyDate(date))
 })
 
 suite.add('json-accelerate: date', function () {
-  accelDate(date)
+  Buffer.byteLength(accelDate(date))
 })
 
 suite.add('compile-json-stringify: date', function () {
-  CJSStringifyDate(date)
+  Buffer.byteLength(CJSStringifyDate(date))
 })
 
+for (const { name, schema: responseSchema, input } of apiResponses) {
+  const stringifyResponse = FJS(responseSchema)
+  // Check outside the timed loop that the schema preserves the complete response.
+  assert.deepStrictEqual(JSON.parse(stringifyResponse(input)), JSON.parse(JSON.stringify(input)))
+
+  suite.add(`JSON.stringify: ${name}`, function () {
+    Buffer.byteLength(JSON.stringify(input))
+  })
+  suite.add(`fast-json-stringify: ${name}`, function () {
+    Buffer.byteLength(stringifyResponse(input))
+  })
+}
+
+if (process.argv.includes('--fjs-vs-json')) {
+  for (const task of suite.tasks) {
+    if (!task.name.startsWith('fast-json-stringify') && !task.name.startsWith('JSON.stringify:')) {
+      suite.remove(task.name)
+    }
+  }
+}
+
 suite.run().then(() => {
   const results = suite.tasks.map(task => ({
     name: task.name,
diff --git a/benchmark/bench-thread.js b/benchmark/bench-thread.js
index 91f3cab8..89c247b2 100644
--- a/benchmark/bench-thread.js
+++ b/benchmark/bench-thread.js
@@ -18,10 +18,12 @@ const FJS = require('..')
 const stringify = benchmark.compile ? null : FJS(benchmark.schema, benchmark.options)
 
 bench.add(benchmark.name, () => {
+  // Some serializers return unflattened strings. Fastify calls Buffer.byteLength()
+  // for Content-Length, which triggers flattening; include that cost here too.
   if (benchmark.compile) {
-    FJS(benchmark.schema, benchmark.options)(benchmark.input)
+    Buffer.byteLength(FJS(benchmark.schema, benchmark.options)(benchmark.input))
   } else {
-    stringify(benchmark.input)
+    Buffer.byteLength(stringify(benchmark.input))
   }
 }).run().then(() => {
   const task = bench.tasks[0]
diff --git a/benchmark/fixtures/api-responses.js b/benchmark/fixtures/api-responses.js
new file mode 100644
index 00000000..d03ac8a3
--- /dev/null
+++ b/benchmark/fixtures/api-responses.js
@@ -0,0 +1,266 @@
+'use strict'
+
+const addressSchema = {
+  type: 'object',
+  properties: {
+    street: { type: 'string' },
+    city: { type: 'string' },
+    postalCode: { type: 'string' },
+    country: { type: 'string' },
+    apartment: { type: ['string', 'null'] }
+  }
+}
+
+const productSchema = {
+  type: 'object',
+  properties: {
+    id: { type: 'string' },
+    sku: { type: 'string' },
+    name: { type: 'string' },
+    description: { type: 'string' },
+    price: {
+      type: 'object',
+      properties: {
+        amount: { type: 'integer' },
+        currency: { type: 'string' },
+        discount: { type: ['number', 'null'] }
+      }
+    },
+    inventory: {
+      type: 'object',
+      properties: {
+        available: { type: 'boolean' },
+        quantity: { type: 'integer' },
+        warehouse: { type: 'string' }
+      }
+    },
+    tags: { type: 'array', items: { type: 'string' } },
+    images: {
+      type: 'array',
+      items: {
+        type: 'object',
+        properties: {
+          url: { type: 'string' },
+          alt: { type: 'string' },
+          primary: { type: 'boolean' }
+        }
+      }
+    },
+    rating: {
+      type: 'object',
+      properties: {
+        average: { type: 'number' },
+        count: { type: 'integer' }
+      }
+    },
+    updatedAt: { type: 'string' }
+  }
+}
+
+// Deterministic synthetic responses include Unicode and escaping, as API payloads do.
+const address = {
+  street: '12 Example Street',
+  city: 'Łódź',
+  postalCode: '90-001',
+  country: 'PL',
+  apartment: null
+}
+
+const products = Array.from({ length: 50 }, (_, i) => ({
+  id: `product_${i + 1}`,
+  sku: `MUG-${1000 + i}`,
+  name: `Café mug "Everyday" — ${i + 1}`,
+  description: 'Handmade ceramic mug ☕\nDishwasher safe. Available in blue, green, and cream.',
+  price: {
+    amount: 1800 + i * 100,
+    currency: 'EUR',
+    discount: i % 2 === 0 ? null : 0.1
+  },
+  inventory: { available: i % 7 !== 0, quantity: i * 3, warehouse: 'eu-central' },
+  tags: ['home', 'kitchen', i % 2 === 0 ? 'ceramic' : 'stoneware'],
+  images: [
+    { url: `https://example.com/products/${i + 1}/front.jpg`, alt: 'Front view', primary: true },
+    { url: `https://example.com/products/${i + 1}/side.jpg`, alt: 'Side view', primary: false }
+  ],
+  rating: { average: 4.2 + (i % 5) / 10, count: 10 + i * 7 },
+  updatedAt: '2026-01-15T10:30:00.000Z'
+}))
+
+const orderItems = products.slice(0, 5).map((product, i) => ({
+  product,
+  quantity: i + 1,
+  unitPrice: product.price.amount,
+  total: product.price.amount * (i + 1)
+}))
+const subtotal = orderItems.reduce((sum, item) => sum + item.total, 0)
+
+module.exports = [
+  {
+    name: 'user profile response',
+    schema: {
+      type: 'object',
+      properties: {
+        data: {
+          type: 'object',
+          properties: {
+            id: { type: 'string' },
+            email: { type: 'string' },
+            displayName: { type: 'string' },
+            bio: { type: 'string' },
+            avatarUrl: { type: ['string', 'null'] },
+            active: { type: 'boolean' },
+            roles: { type: 'array', items: { type: 'string' } },
+            address: addressSchema,
+            organization: {
+              type: 'object',
+              properties: {
+                id: { type: 'string' },
+                name: { type: 'string' },
+                plan: { type: 'string' }
+              }
+            },
+            preferences: {
+              type: 'object',
+              properties: {
+                locale: { type: 'string' },
+                timezone: { type: 'string' },
+                notifications: {
+                  type: 'object',
+                  properties: {
+                    email: { type: 'boolean' },
+                    push: { type: 'boolean' },
+                    digest: { type: 'string' }
+                  }
+                }
+              }
+            },
+            createdAt: { type: 'string' },
+            lastLoginAt: { type: ['string', 'null'] }
+          }
+        },
+        meta: {
+          type: 'object',
+          properties: { requestId: { type: 'string' }, version: { type: 'integer' } }
+        }
+      }
+    },
+    input: {
+      data: {
+        id: 'user_123',
+        email: 'alex@example.com',
+        displayName: 'Alex Żółć',
+        bio: 'Engineer & coffee enthusiast ☕\nWorking on "Project Atlas".',
+        avatarUrl: null,
+        active: true,
+        roles: ['member', 'billing-admin'],
+        address,
+        organization: { id: 'org_456', name: 'Example & Co.', plan: 'business' },
+        preferences: {
+          locale: 'pl-PL',
+          timezone: 'Europe/Warsaw',
+          notifications: { email: true, push: false, digest: 'weekly' }
+        },
+        createdAt: '2025-06-01T08:00:00.000Z',
+        lastLoginAt: '2026-01-15T10:30:00.000Z'
+      },
+      meta: { requestId: 'req_profile_123', version: 1 }
+    }
+  },
+  {
+    name: 'order response',
+    schema: {
+      type: 'object',
+      properties: {
+        id: { type: 'string' },
+        status: { type: 'string' },
+        customer: {
+          type: 'object',
+          properties: {
+            id: { type: 'string' },
+            name: { type: 'string' },
+            email: { type: 'string' }
+          }
+        },
+        shippingAddress: addressSchema,
+        items: {
+          type: 'array',
+          items: {
+            type: 'object',
+            properties: {
+              product: productSchema,
+              quantity: { type: 'integer' },
+              unitPrice: { type: 'integer' },
+              total: { type: 'integer' }
+            }
+          }
+        },
+        totals: {
+          type: 'object',
+          properties: {
+            subtotal: { type: 'integer' },
+            shipping: { type: 'integer' },
+            tax: { type: 'integer' },
+            total: { type: 'integer' },
+            currency: { type: 'string' }
+          }
+        },
+        payment: {
+          type: 'object',
+          properties: {
+            method: { type: 'string' },
+            paid: { type: 'boolean' },
+            transactionId: { type: ['string', 'null'] }
+          }
+        },
+        notes: { type: ['string', 'null'] },
+        createdAt: { type: 'string' }
+      }
+    },
+    input: {
+      id: 'order_789',
+      status: 'processing',
+      customer: { id: 'user_123', name: 'Alex Żółć', email: 'alex@example.com' },
+      shippingAddress: address,
+      items: orderItems,
+      totals: { subtotal, shipping: 500, tax: 4200, total: subtotal + 4700, currency: 'EUR' },
+      payment: { method: 'card', paid: true, transactionId: 'txn_987' },
+      notes: 'Please ring the bell marked "Office".\nLeave with reception if unavailable.',
+      createdAt: '2026-01-15T10:30:00.000Z'
+    }
+  },
+  {
+    name: 'paginated products response',
+    schema: {
+      type: 'object',
+      properties: {
+        data: { type: 'array', items: productSchema },
+        pagination: {
+          type: 'object',
+          properties: {
+            page: { type: 'integer' },
+            pageSize: { type: 'integer' },
+            total: { type: 'integer' },
+            hasNextPage: { type: 'boolean' }
+          }
+        },
+        links: {
+          type: 'object',
+          properties: {
+            self: { type: 'string' },
+            next: { type: 'string' },
+            previous: { type: ['string', 'null'] }
+          }
+        }
+      }
+    },
+    input: {
+      data: products,
+      pagination: { page: 1, pageSize: 50, total: 1248, hasNextPage: true },
+      links: {
+        self: '/products?page=1&pageSize=50',
+        next: '/products?page=2&pageSize=50',
+        previous: null
+      }
+    }
+  }
+]