diff --git a/packages/devtools/src/__tests__/ssr-requests.test.ts b/packages/devtools/src/__tests__/ssr-requests.test.ts
index d2a1f45c..e5d007c6 100644
--- a/packages/devtools/src/__tests__/ssr-requests.test.ts
+++ b/packages/devtools/src/__tests__/ssr-requests.test.ts
@@ -1,6 +1,7 @@
 import '@angular/compiler';
 import { EventEmitter } from 'node:events';
-import type { IncomingMessage, ServerResponse } from 'node:http';
+import { createServer, type IncomingMessage, type ServerResponse } from 'node:http';
+import type { AddressInfo } from 'node:net';
 import { Injector, PLATFORM_ID, REQUEST, runInInjectionContext } from '@angular/core';
 import { HttpRequest, HttpResponse, type HttpEvent } from '@angular/common/http';
 import { createHostContext } from 'devframe/node';
@@ -104,6 +105,69 @@ describe('ssrMiddleware', () => {
     expect(res.sentHeaders['server-timing']).toMatch(/^db;dur=5, pangular;desc="/);
   });
 
+  it('merges with a Server-Timing value passed to writeHead in any case', async () => {
+    const middleware = createSsrMiddleware();
+    const server = createServer((req, res) => {
+      middleware(req, res, () => {});
+      res.writeHead(200, { 'Server-Timing': 'db;dur=5', 'Content-Type': 'text/html' });
+      res.end(HTML);
+    });
+    await new Promise<void>((resolve) => server.listen(0, '127.0.0.1', resolve));
+    try {
+      const { port } = server.address() as AddressInfo;
+      const response = await fetch(`http://127.0.0.1:${port}/`, {
+        headers: { accept: 'text/html' },
+      });
+      await response.text();
+      expect(response.headers.get('server-timing')).toMatch(/^db;dur=5, pangular;desc="/);
+    } finally {
+      await new Promise((resolve) => server.close(resolve));
+    }
+  });
+
+  it('merges with a Server-Timing value passed to writeHead as a raw array', async () => {
+    const middleware = createSsrMiddleware();
+    const raw = ['Server-Timing', 'db;dur=5', 'Content-Type', 'text/html'];
+    const server = createServer((req, res) => {
+      middleware(req, res, () => {});
+      res.writeHead(200, raw);
+      res.end(HTML);
+    });
+    await new Promise<void>((resolve) => server.listen(0, '127.0.0.1', resolve));
+    try {
+      const { port } = server.address() as AddressInfo;
+      const response = await fetch(`http://127.0.0.1:${port}/`, {
+        headers: { accept: 'text/html' },
+      });
+      await response.text();
+      expect(response.headers.get('server-timing')).toMatch(/^db;dur=5, pangular;desc="/);
+      expect(raw[1]).toBe('db;dur=5');
+    } finally {
+      await new Promise((resolve) => server.close(resolve));
+    }
+  });
+
+  it('adds Server-Timing to [name, value] pairs passed to writeHead', async () => {
+    const middleware = createSsrMiddleware();
+    const server = createServer((req, res) => {
+      middleware(req, res, () => {});
+      res.writeHead(200, [['Content-Type', 'text/html']] as unknown as string[]);
+      res.end(HTML);
+    });
+    await new Promise<void>((resolve) => server.listen(0, '127.0.0.1', resolve));
+    try {
+      const { port } = server.address() as AddressInfo;
+      const response = await fetch(`http://127.0.0.1:${port}/`, {
+        headers: { accept: 'text/html' },
+      });
+      await response.text();
+      expect(response.status).toBe(200);
+      expect(response.headers.get('server-timing')).toMatch(/^pangular;desc="/);
+    } finally {
+      await new Promise((resolve) => server.close(resolve));
+    }
+  });
+
   it('skips non-HTML requests, hub paths and when the http inspector is off', () => {
     const middleware = createSsrMiddleware({ skip: ['/__devframes/'] });
     const passed: string[] = [];
diff --git a/packages/devtools/src/ssr-middleware.ts b/packages/devtools/src/ssr-middleware.ts
index 2ee3abd8..c268f451 100644
--- a/packages/devtools/src/ssr-middleware.ts
+++ b/packages/devtools/src/ssr-middleware.ts
@@ -121,14 +121,35 @@ export function createSsrMiddleware(options: SsrMiddlewareOptions = {}) {
           timing.push(`resolve;dur=${sum((n) => n.resolvers?.ms)}`);
         }
         const existing = headerText(res.getHeader('server-timing'));
-        // A headers object passed to writeHead would override setHeader, so merge into it.
-        const headers = args.find(
-          (arg, i) => i > 0 && arg && typeof arg === 'object' && !Array.isArray(arg),
-        ) as OutgoingHttpHeaders | undefined;
-        const passed = headers ? headerText(headers['server-timing']) : undefined;
-        const value = [existing, passed, ...timing].filter(Boolean).join(', ');
-        if (headers && passed !== undefined) headers['server-timing'] = value;
-        else res.setHeader('server-timing', value);
+        const merge = (passed?: string) => [existing, passed, ...timing].filter(Boolean).join(', ');
+        // Header names are case-insensitive, so `Server-Timing` counts as the same header.
+        const isTiming = (name: unknown) => String(name).toLowerCase() === 'server-timing';
+        // Headers passed to writeHead would override setHeader, so merge into them. Node takes
+        // an object, a flat `[name, value, ...]` array or `[name, value]` pairs.
+        const slot = args.findIndex((arg, i) => i > 0 && arg && typeof arg === 'object');
+        const headers = args[slot];
+        if (Array.isArray(headers)) {
+          // Work on a copy so the caller's array is left as it was.
+          const raw = [...headers] as unknown[];
+          const pairs = Array.isArray(raw[0]);
+          const i = raw.findIndex((entry, n) =>
+            pairs ? isTiming((entry as unknown[])[0]) : n % 2 === 0 && isTiming(entry),
+          );
+          // Added to the array, not with setHeader: Node rejects pairs once a header is set.
+          if (i < 0)
+            raw.push(...(pairs ? [['server-timing', merge()]] : ['server-timing', merge()]));
+          else if (pairs) {
+            const [name, value] = raw[i] as [string, string | string[]];
+            raw[i] = [name, merge(headerText(value))];
+          } else raw[i + 1] = merge(headerText(raw[i + 1] as string | string[]));
+          args[slot] = raw;
+        } else {
+          const object = headers as OutgoingHttpHeaders | undefined;
+          const passedKey = object ? Object.keys(object).find(isTiming) : undefined;
+          const passed = passedKey ? headerText(object?.[passedKey]) : undefined;
+          if (object && passedKey && passed !== undefined) object[passedKey] = merge(passed);
+          else res.setHeader('server-timing', merge());
+        }
       }
       return (writeHead as (...a: unknown[]) => ServerResponse).apply(this, args);
     } as typeof res.writeHead;