diff --git a/packages/start-server-core/src/request-response.ts b/packages/start-server-core/src/request-response.ts
index cf0e426db9c..4c0ab588fbb 100644
--- a/packages/start-server-core/src/request-response.ts
+++ b/packages/start-server-core/src/request-response.ts
@@ -136,8 +136,16 @@ export function setResponseHeaders(
   headers: TypedHeaders<ResponseHeaderMap>,
 ): void {
   const event = getH3Event()
-  for (const [name, value] of Object.entries(headers)) {
-    event.res.headers.set(name, value)
+  const addedHeaderNames: Record<string, true> = {}
+  for (const [name, value] of headers.entries()) {
+    const found = addedHeaderNames[name] ?? false
+    if (!found) {
+      addedHeaderNames[name] = true
+    }
+    // If header already existed in h3 event headers, it will be replaced.
+    // However, headers object in this invocation might have multiple instances of the same header name (.append() was used), let's allow the duplicates.
+    const method = found ? 'append' : 'set'
+    event.res.headers[method](name, value)
   }
 }
 
diff --git a/packages/start-server-core/tests/request-response.test.ts b/packages/start-server-core/tests/request-response.test.ts
new file mode 100644
index 00000000000..8edf78739e8
--- /dev/null
+++ b/packages/start-server-core/tests/request-response.test.ts
@@ -0,0 +1,110 @@
+import { describe, expect, it } from 'vitest'
+import {
+  getResponseHeader,
+  getResponseHeaders,
+  requestHandler,
+  setResponseHeaders,
+} from '../src/request-response'
+
+describe('setResponseHeaders', () => {
+  it('should set a single header via Headers object', async () => {
+    const headers = new Headers()
+    headers.set('X-Custom-Header', 'test-value')
+
+    const handler = requestHandler(() => {
+      setResponseHeaders(headers)
+      const responseHeaders = getResponseHeaders()
+      expect(responseHeaders.get('X-Custom-Header')).toBe('test-value')
+      return new Response('OK')
+    })
+
+    const request = new Request('http://localhost:3000/test')
+    await handler(request, {})
+  })
+
+  it('should set multiple headers via Headers object', async () => {
+    const headers = new Headers()
+    headers.set('X-Custom-Header', 'test-value')
+    headers.set('X-Another-Header', 'another-value')
+    headers.set('Content-Type', 'application/json')
+
+    const handler = requestHandler(() => {
+      setResponseHeaders(headers)
+      const responseHeaders = getResponseHeaders()
+      expect(responseHeaders.get('X-Custom-Header')).toBe('test-value')
+      expect(responseHeaders.get('X-Another-Header')).toBe('another-value')
+      expect(responseHeaders.get('Content-Type')).toBe('application/json')
+      return new Response('OK')
+    })
+
+    const request = new Request('http://localhost:3000/test')
+    await handler(request, {})
+  })
+
+  it('should handle empty Headers object', async () => {
+    const handler = requestHandler(() => {
+      const headers = new Headers()
+      setResponseHeaders(headers)
+      const responseHeaders = getResponseHeaders()
+      expect(responseHeaders).toBeDefined()
+      expect(Array.from(responseHeaders.entries()).length).toEqual(0)
+      return new Response('OK')
+    })
+
+    const request = new Request('http://localhost:3000/test')
+    await handler(request, {})
+  })
+
+  it('should replace existing headers with the same name', async () => {
+    const headers = new Headers()
+    headers.set('X-Custom-Header', 'old-value')
+
+    const handler = requestHandler(() => {
+      setResponseHeaders(
+        new Headers({
+          'X-Custom-Header': 'old-value',
+        }),
+      )
+      expect(getResponseHeader('X-Custom-Header')).toEqual('old-value')
+      setResponseHeaders(
+        new Headers({
+          'X-Custom-Header': 'new-value',
+        }),
+      )
+      expect(getResponseHeader('X-Custom-Header')).toEqual('new-value')
+
+      return new Response('OK')
+    })
+
+    const request = new Request('http://localhost:3000/test')
+    await handler(request, {})
+  })
+
+  it('should handle multiple headers with the same name added via headers.append()', async () => {
+    const headers = new Headers()
+    headers.append('Set-Cookie', 'session=abc123; Path=/; HttpOnly')
+    headers.append('Set-Cookie', 'user=john; Path=/; Secure')
+
+    const handler = requestHandler(() => {
+      setResponseHeaders(headers)
+
+      // When multiple values are appended with the same header name,
+      // headers.entries() returns separate entries for each value.
+      // The implementation uses .set() for the first occurrence and .append() for
+      // subsequent duplicates, preserving all values.
+      // Note: getResponseHeader() uses .get() which returns comma-separated values.
+      const setCookieValue = getResponseHeader('Set-Cookie')
+
+      expect(setCookieValue).toBeDefined()
+
+      // Both cookie values should be present in the result
+      expect(setCookieValue).toContain('session=abc123')
+      expect(setCookieValue).toContain('user=john')
+
+      return new Response('OK')
+    })
+
+    const request = new Request('http://localhost:3000/test')
+    await handler(request, {})
+  })
+})