diff --git a/dev-packages/e2e-tests/test-applications/node-fastify-5/package.json b/dev-packages/e2e-tests/test-applications/node-fastify-5/package.json
index e25531d0f575..a7a754d3f1a6 100644
--- a/dev-packages/e2e-tests/test-applications/node-fastify-5/package.json
+++ b/dev-packages/e2e-tests/test-applications/node-fastify-5/package.json
@@ -15,7 +15,7 @@
   "dependencies": {
     "@sentry/node": "latest || *",
     "@types/node": "^18.19.1",
-    "fastify": "5.3.2",
+    "fastify": "^5.7.0",
     "typescript": "5.6.3",
     "ts-node": "10.9.2"
   },
diff --git a/dev-packages/e2e-tests/test-applications/node-fastify-5/playwright.override.config.mjs b/dev-packages/e2e-tests/test-applications/node-fastify-5/playwright.override.config.mjs
index 31f2b913b58b..ee1f461c57c8 100644
--- a/dev-packages/e2e-tests/test-applications/node-fastify-5/playwright.override.config.mjs
+++ b/dev-packages/e2e-tests/test-applications/node-fastify-5/playwright.override.config.mjs
@@ -1,7 +1,7 @@
 import { getPlaywrightConfig } from '@sentry-internal/test-utils';
 
 const config = getPlaywrightConfig({
-  startCommand: `pnpm start`,
+  startCommand: `pnpm start:override`,
 });
 
 export default config;
diff --git a/dev-packages/e2e-tests/test-applications/node-fastify-5/src/app-handle-error-override.ts b/dev-packages/e2e-tests/test-applications/node-fastify-5/src/app-handle-error-override.ts
index 217201332e17..91f0353816bb 100644
--- a/dev-packages/e2e-tests/test-applications/node-fastify-5/src/app-handle-error-override.ts
+++ b/dev-packages/e2e-tests/test-applications/node-fastify-5/src/app-handle-error-override.ts
@@ -48,6 +48,11 @@ Sentry.setupFastifyErrorHandler(app, {
       return false;
     }
 
+    // @ts-ignore // Fastify V5 is not typed correctly
+    if (_request.routeOptions?.url?.includes('/test-error-ignored') && _reply.statusCode === 500) {
+      return false;
+    }
+
     return true;
   },
 });
@@ -130,10 +135,30 @@ app.get('/test-outgoing-http-external-disallowed', async function (req, res) {
   res.send(data);
 });
 
+// Regression test for https://github.com/fastify/fastify/issues/6409
+// The error diagnostic channel was always sending 200 unless explicitly changed.
+// This was fixed in Fastify 5.7.0
+app.register((childApp: F.FastifyInstance, _options: F.FastifyPluginOptions, next: (err?: Error) => void) => {
+  childApp.setErrorHandler((error: Error, _request: F.FastifyRequest, reply: F.FastifyReply) => {
+    reply.send({ ok: false });
+  });
+
+  childApp.get('/test-error-ignored', async function () {
+    throw new Error('This is an error that will not be captured');
+  });
+
+  next();
+});
+
 app.post('/test-post', function (req, res) {
   res.send({ status: 'ok', body: req.body });
 });
 
+app.get('/flush', async function (_req, res) {
+  await Sentry.flush();
+  res.send({ ok: true });
+});
+
 app.listen({ port: port });
 
 // A second app so we can test header propagation between external URLs
diff --git a/dev-packages/e2e-tests/test-applications/node-fastify-5/src/app.ts b/dev-packages/e2e-tests/test-applications/node-fastify-5/src/app.ts
index 83f7e53a45ce..610a42f6fc00 100644
--- a/dev-packages/e2e-tests/test-applications/node-fastify-5/src/app.ts
+++ b/dev-packages/e2e-tests/test-applications/node-fastify-5/src/app.ts
@@ -25,6 +25,11 @@ Sentry.init({
           return false;
         }
 
+        // @ts-ignore // Fastify V5 is not typed correctly
+        if (_request.routeOptions?.url?.includes('/test-error-ignored') && _reply.statusCode === 500) {
+          return false;
+        }
+
         return true;
       },
     }),
@@ -90,6 +95,21 @@ app.get('/test-error', async function (req, res) {
   res.send({ exceptionId });
 });
 
+// Regression test for https://github.com/fastify/fastify/issues/6409
+// The error diagnostic channel was always sending 200 unless explicitly changed.
+// This was fixed in Fastify 5.7.0
+app.register((childApp: F.FastifyInstance, _options: F.FastifyPluginOptions, next: (err?: Error) => void) => {
+  childApp.setErrorHandler((error: Error, _request: F.FastifyRequest, reply: F.FastifyReply) => {
+    reply.send({ ok: false });
+  });
+
+  childApp.get('/test-error-ignored', async function () {
+    throw new Error('This is an error that will not be captured');
+  });
+
+  next();
+});
+
 app.get('/test-error-not-captured', async function () {
   // This error will not be captured by Sentry
   throw new Error('This is an error that will not be captured');
@@ -127,6 +147,11 @@ app.post('/test-post', function (req, res) {
   res.send({ status: 'ok', body: req.body });
 });
 
+app.get('/flush', async function (_req, res) {
+  await Sentry.flush();
+  res.send({ ok: true });
+});
+
 app.listen({ port: port });
 
 // A second app so we can test header propagation between external URLs
diff --git a/dev-packages/e2e-tests/test-applications/node-fastify-5/tests/errors.test.ts b/dev-packages/e2e-tests/test-applications/node-fastify-5/tests/errors.test.ts
index a25f990748ea..8699b26d490b 100644
--- a/dev-packages/e2e-tests/test-applications/node-fastify-5/tests/errors.test.ts
+++ b/dev-packages/e2e-tests/test-applications/node-fastify-5/tests/errors.test.ts
@@ -1,5 +1,5 @@
 import { expect, test } from '@playwright/test';
-import { waitForError } from '@sentry-internal/test-utils';
+import { waitForError, waitForTransaction } from '@sentry-internal/test-utils';
 
 test('Sends correct error event', async ({ baseURL }) => {
   const errorEventPromise = waitForError('node-fastify-5', event => {
@@ -35,16 +35,54 @@ test('Sends correct error event', async ({ baseURL }) => {
 });
 
 test('Does not send error when shouldHandleError returns false', async ({ baseURL }) => {
-  const errorEventPromise = waitForError('node-fastify-5', event => {
-    return !event.type && event.exception?.values?.[0]?.value === 'This is an error that will not be captured';
+  let errorEventOccurred = false;
+
+  waitForError('node-fastify-5', event => {
+    if (!event.type && event.exception?.values?.[0]?.value === 'This is an error that will not be captured') {
+      errorEventOccurred = true;
+    }
+    return event?.transaction === 'GET /test-error-not-captured';
+  });
+
+  const transactionEventPromise = waitForTransaction('node-fastify-5', transactionEvent => {
+    return transactionEvent?.transaction === 'GET /test-error-not-captured';
+  });
+
+  const response = await fetch(`${baseURL}/test-error-not-captured`);
+
+  await transactionEventPromise;
+
+  const flushResponse = await fetch(`${baseURL}/flush`);
+
+  expect(response.status).toBe(500);
+  expect(flushResponse.status).toBe(200);
+  expect(errorEventOccurred).toBe(false);
+});
+
+// Regression test for https://github.com/fastify/fastify/issues/6409
+// The error diagnostic channel was always sending 200 unless explicitly changed.
+// This was fixed in Fastify 5.7.0
+test('Error in child plugin with rethrown error handler reports correct 500 status', async ({ baseURL }) => {
+  let errorEventOccurred = false;
+
+  waitForError('node-fastify-5', event => {
+    if (!event.type && event.exception?.values?.[0]?.value === 'This is an error that will not be captured') {
+      errorEventOccurred = true;
+    }
+    return event?.transaction === 'GET /test-error-ignored';
   });
 
-  errorEventPromise.then(() => {
-    throw new Error('This error should not be captured');
+  const transactionEventPromise = waitForTransaction('node-fastify-5', transactionEvent => {
+    return transactionEvent?.transaction === 'GET /test-error-ignored';
   });
 
-  await fetch(`${baseURL}/test-error-not-captured`);
+  const response = await fetch(`${baseURL}/test-error-ignored`);
+
+  await transactionEventPromise;
+
+  const flushResponse = await fetch(`${baseURL}/flush`);
 
-  // wait for a short time to ensure the error is not captured
-  await new Promise(resolve => setTimeout(resolve, 1000));
+  expect(response.status).toBe(500);
+  expect(flushResponse.status).toBe(200);
+  expect(errorEventOccurred).toBe(false);
 });