diff --git a/src/lib/headers.spec.ts b/src/lib/headers.spec.ts
index a359b107..4b03fa5d 100644
--- a/src/lib/headers.spec.ts
+++ b/src/lib/headers.spec.ts
@@ -7,6 +7,7 @@ import {
   excludeHopByHopHeaders,
   excludeOriginSecurityContextHeaders,
   excludeUnusedHeaders,
+  sortHeaders,
 } from './headers'
 
 describe('appendVaryHeader', () => {
@@ -159,3 +160,39 @@ describe('excludeUnusedHeaders', () => {
     })
   })
 })
+
+describe('sortHeaders', () => {
+  it('returns empty object unchanged', () => {
+    expect(sortHeaders({})).toStrictEqual({})
+  })
+
+  it('sorts keys lexicographically', () => {
+    const result = sortHeaders({
+      'x-rendering-proxy': 'value',
+      'content-type': 'text/html',
+      'accept-ranges': 'bytes',
+    })
+    expect(Object.keys(result)).toStrictEqual([
+      'accept-ranges',
+      'content-type',
+      'x-rendering-proxy',
+    ])
+    expect(result).toStrictEqual({
+      'accept-ranges': 'bytes',
+      'content-type': 'text/html',
+      'x-rendering-proxy': 'value',
+    })
+  })
+
+  it('preserves all header values', () => {
+    const input = { b: '2', a: '1', c: '3' }
+    const result = sortHeaders(input)
+    expect(result).toStrictEqual({ a: '1', b: '2', c: '3' })
+  })
+
+  it('does not mutate the input object', () => {
+    const input = { z: 'last', a: 'first' }
+    sortHeaders(input)
+    expect(Object.keys(input)).toStrictEqual(['z', 'a'])
+  })
+})
diff --git a/src/lib/headers.ts b/src/lib/headers.ts
index 2d8d4fe9..696ec8ed 100644
--- a/src/lib/headers.ts
+++ b/src/lib/headers.ts
@@ -180,3 +180,18 @@ export function excludeUnusedHeaders(headers: Headers): Headers {
     excludeContentDependentHeaders(excludeHopByHopHeaders(headers)),
   )
 }
+
+/**
+ * Return a new headers object with keys sorted lexicographically.
+ * Eliminates non-determinism introduced by browser enumeration order or
+ * upstream server header order.
+ * @param headers {Headers}
+ * @returns headers {Headers}
+ */
+export function sortHeaders(headers: Headers): Headers {
+  const result: Headers = {}
+  for (const key of Object.keys(headers).sort()) {
+    result[key] = headers[key]
+  }
+  return result
+}
diff --git a/src/server/index.ts b/src/server/index.ts
index 2aaf518e..0bae280d 100644
--- a/src/server/index.ts
+++ b/src/server/index.ts
@@ -4,7 +4,11 @@ import type { Browser } from 'playwright'
 import { runWithDefer } from 'with-defer'
 
 import { getBrowser, type SelectableBrowsers } from '../browser'
-import { appendVaryHeader, excludeUnusedHeaders } from '../lib/headers'
+import {
+  appendVaryHeader,
+  excludeUnusedHeaders,
+  sortHeaders,
+} from '../lib/headers'
 import { isAbsoluteURL } from '../lib/url'
 import { waitForProcessExit } from '../lib/wait_for_exit'
 import { getRenderedContent } from '../render'
@@ -64,13 +68,15 @@ async function renderToResponse(
   request: Parameters<typeof getRenderedContent>[1],
 ): Promise<void> {
   const renderedContent = await getRenderedContent(browser, request)
-  const headers = appendVaryHeader(
-    {
-      ...excludeUnusedHeaders(renderedContent.headers),
-      'x-rendering-proxy': JSON.stringify(renderedContent.evaluateResults),
-      'x-rendering-proxy-version': String(PROTOCOL_VERSION),
-    },
-    'x-rendering-proxy',
+  const headers = sortHeaders(
+    appendVaryHeader(
+      {
+        ...excludeUnusedHeaders(renderedContent.headers),
+        'x-rendering-proxy': JSON.stringify(renderedContent.evaluateResults),
+        'x-rendering-proxy-version': String(PROTOCOL_VERSION),
+      },
+      'x-rendering-proxy',
+    ),
   )
   res.writeHead(renderedContent.status, headers)
   res.end(renderedContent.body, 'binary')