diff --git a/dev-packages/e2e-tests/test-applications/tanstackstart-react-sourcemaps/.gitignore b/dev-packages/e2e-tests/test-applications/tanstackstart-react-sourcemaps/.gitignore
new file mode 100644
index 000000000000..4187ae735082
--- /dev/null
+++ b/dev-packages/e2e-tests/test-applications/tanstackstart-react-sourcemaps/.gitignore
@@ -0,0 +1,11 @@
+node_modules
+.output
+.nitro
+.tanstack
+src/routeTree.gen.ts
+
+# Mock Sentry server artifacts
+.tmp_mock_uploads.json
+.tmp_chunks
+.tmp_build_stdout
+.tmp_build_stderr
diff --git a/dev-packages/e2e-tests/test-applications/tanstackstart-react-sourcemaps/assert-build.ts b/dev-packages/e2e-tests/test-applications/tanstackstart-react-sourcemaps/assert-build.ts
new file mode 100644
index 000000000000..2aa44e66f20e
--- /dev/null
+++ b/dev-packages/e2e-tests/test-applications/tanstackstart-react-sourcemaps/assert-build.ts
@@ -0,0 +1,191 @@
+import * as assert from 'assert/strict';
+import * as fs from 'fs';
+import * as path from 'path';
+import {
+  findDebugIdsWithConflictingSourcemaps,
+  findFilesWithForeignSourcemaps,
+  findFilesWithMultipleDebugIds,
+  findInjectedDebugIds,
+  getArtifactBundles,
+  getChunkUploadPosts,
+  getDebugIdPairs,
+  getSourcemapSources,
+  getSourcemaps,
+  loadMockServerResults,
+  startNodeServer,
+  type SourcemapEntry,
+} from '@sentry-internal/test-utils';
+
+const CLIENT_OUTPUT = path.join('.output', 'public');
+const SERVER_OUTPUT = path.join('.output', 'server');
+
+/** Both markers sit in comments, so bundlers strip them from the code but keep them in `sourcesContent`. */
+const CLIENT_MARKER = 'SOURCEMAP_MARKER_CLIENT';
+const SERVER_MARKER = 'SOURCEMAP_MARKER_SERVER';
+
+const DEBUG_ID_PATTERN = /sentry-dbid-([\da-f-]{36})/gi;
+
+function readDebugIds(file: string): string[] {
+  return [...fs.readFileSync(file, 'utf8').matchAll(DEBUG_ID_PATTERN)].map(match => (match[1] as string).toLowerCase());
+}
+
+function emittedJsFiles(dir: string): string[] {
+  return fs
+    .readdirSync(dir, { recursive: true, withFileTypes: true })
+    .filter(entry => entry.isFile() && /\.m?js$/.test(entry.name) && !entry.parentPath.includes('node_modules'))
+    .map(entry => path.join(entry.parentPath, entry.name));
+}
+
+/** In debug mode, the Sentry plugin logs each file it deletes after the upload. */
+const deletedFiles = new Set(
+  [...fs.readFileSync('.tmp_build_stdout', 'utf8').matchAll(/Deleting asset after upload: (.+)$/gm)].map(match =>
+    path.relative(fs.realpathSync('.'), match[1] as string),
+  ),
+);
+
+/** A chunk without mappable source, such as Nitro's runtime, gets a debug ID but no source map to upload. */
+function debugIdsOfChunksWithoutSourceMap(dir: string): Set<string> {
+  return new Set(
+    emittedJsFiles(dir)
+      .filter(file => !fs.existsSync(`${file}.map`) && !deletedFiles.has(`${file}.map`))
+      .flatMap(readDebugIds),
+  );
+}
+
+function sourceMapsIn(dir: string): string[] {
+  return fs
+    .readdirSync(dir, { recursive: true, withFileTypes: true })
+    .filter(entry => entry.isFile() && entry.name.endsWith('.map'))
+    .map(entry => path.join(entry.parentPath, entry.name));
+}
+
+// TODO: The uploader passes the emitted file paths to `glob` as patterns, so it skips a file whose path has glob
+// characters, like Nitro's `_libs/@tanstack/react-router+[...].mjs`. Remove this when the uploader matches paths literally.
+function debugIdsOfFilesWithGlobCharacters(dir: string): Set<string> {
+  return new Set(
+    emittedJsFiles(dir)
+      .filter(file => /[[\]{}()*?]/.test(path.relative(dir, file)))
+      .flatMap(readDebugIds),
+  );
+}
+
+const requests = loadMockServerResults();
+
+// --- The upload reached Sentry ---
+
+assert.ok(
+  getChunkUploadPosts(requests).some(request => request.bodySize > 0),
+  'Expected at least one chunk upload with a non-empty body',
+);
+
+const bundles = getArtifactBundles(requests);
+assert.ok(bundles.length > 0, 'Expected at least one artifact bundle with a manifest');
+
+const sourcemaps = getSourcemaps(bundles);
+
+/** The uploaded source maps of the files in an output directory, matched by debug ID. */
+function sourceMapsOf(outputDir: string): SourcemapEntry[] {
+  const debugIds = new Set(findInjectedDebugIds({ outputDir }));
+  return sourcemaps.filter(map => map.debugId && debugIds.has(map.debugId));
+}
+
+const clientSourceMaps = sourceMapsOf(CLIENT_OUTPUT);
+const serverSourceMaps = sourceMapsOf(SERVER_OUTPUT);
+const containsMarker = (maps: SourcemapEntry[], marker: string): boolean =>
+  maps.some(map => map.sourcemap.sourcesContent?.some(source => source?.includes(marker)));
+
+// Both markers are in the same route file, which client and server source maps embed completely. So each marker only
+// proves the upload of its side if the check only looks at the source maps of that side.
+assert.ok(containsMarker(clientSourceMaps, CLIENT_MARKER), 'Expected a client source map carrying the client source');
+assert.ok(containsMarker(serverSourceMaps, SERVER_MARKER), 'Expected a server source map carrying the server source');
+
+// --- Source paths match the files in the project ---
+
+const unresolvedSources = [...new Set(sourcemaps.flatMap(map => getSourcemapSources(map.sourcemap)))].filter(
+  source => source.startsWith('../') || path.isAbsolute(source) || /\?ts[rs]-/.test(source),
+);
+assert.deepEqual(unresolvedSources, [], 'Expected every uploaded source path to be relative to the project root');
+
+// --- Server source maps reach the original source ---
+
+// Nitro bundles the SSR output again. Its maps must lead through that output to the file the server function is in.
+const serverFunctionSources = serverSourceMaps
+  .filter(map => map.sourcemap.sourcesContent?.some(source => source?.includes(SERVER_MARKER)))
+  .flatMap(map => getSourcemapSources(map.sourcemap));
+assert.ok(
+  serverFunctionSources.includes('src/routes/index.tsx'),
+  `Expected a server source map to resolve to src/routes/index.tsx, got: ${serverFunctionSources.join(', ')}`,
+);
+
+// --- Debug IDs tie the shipped bundles to the uploaded maps ---
+
+const uploadedDebugIds = new Set(getDebugIdPairs(bundles).map(pair => pair.debugId.toLowerCase()));
+
+for (const outputDir of [CLIENT_OUTPUT, SERVER_OUTPUT]) {
+  const injectedDebugIds = findInjectedDebugIds({ outputDir });
+  assert.ok(injectedDebugIds.length > 0, `Expected debug IDs to be injected into ${outputDir}`);
+
+  const exemptDebugIds = new Set([
+    ...debugIdsOfChunksWithoutSourceMap(outputDir),
+    ...debugIdsOfFilesWithGlobCharacters(outputDir),
+  ]);
+  const unuploaded = injectedDebugIds.filter(debugId => !uploadedDebugIds.has(debugId) && !exemptDebugIds.has(debugId));
+  assert.deepEqual(unuploaded, [], `Expected every debug ID in ${outputDir} to have an uploaded source map`);
+
+  // A file of Nitro's output can carry the debug ID of the SSR chunk it was built from.
+  assert.deepEqual(
+    findFilesWithMultipleDebugIds({ outputDir }),
+    [],
+    `Expected at most one debug ID per file in ${outputDir}`,
+  );
+  assert.deepEqual(
+    findFilesWithForeignSourcemaps({ outputDir }, sourcemaps),
+    [],
+    `Expected the source map uploaded for each file in ${outputDir} to be generated for that file`,
+  );
+
+  console.log(`  ${outputDir}: ${injectedDebugIds.length} injected debug ID(s)`);
+}
+
+assert.deepEqual(
+  findDebugIdsWithConflictingSourcemaps(sourcemaps),
+  [],
+  'Expected every uploaded debug ID to resolve to exactly one source map',
+);
+
+// --- Sentry deletes the source maps it enabled after the upload ---
+
+for (const outputDir of [CLIENT_OUTPUT, SERVER_OUTPUT]) {
+  assert.deepEqual(sourceMapsIn(outputDir), [], `Expected no source maps in ${outputDir}`);
+}
+
+const clientChunkUrls = emittedJsFiles(CLIENT_OUTPUT).map(
+  file => `/${path.relative(CLIENT_OUTPUT, file).split(path.sep).join('/')}`,
+);
+assert.ok(clientChunkUrls.length > 0, `Expected client chunks in ${CLIENT_OUTPUT}`);
+
+const server = await startNodeServer({ entry: path.join(SERVER_OUTPUT, 'index.mjs'), port: 3030 });
+
+try {
+  // Without this, a server that serves no public files at all would pass the 404 checks.
+  const chunkStatus = (await fetch(`${server.baseUrl}${clientChunkUrls[0]}`)).status;
+  assert.equal(chunkStatus, 200, `Expected the server to serve the client chunk ${clientChunkUrls[0]}`);
+
+  // Nitro serves public files from a list it writes into the server bundle. A map deleted after that
+  // stays on the list, and Nitro answers its request with a 500 instead of a 404.
+  const responses = await Promise.all(
+    clientChunkUrls.map(async chunkUrl => ({
+      mapUrl: `${chunkUrl}.map`,
+      status: (await fetch(`${server.baseUrl}${chunkUrl}.map`)).status,
+    })),
+  );
+  assert.deepEqual(
+    responses.filter(({ status }) => status !== 404).map(({ mapUrl, status }) => `${mapUrl} → ${status}`),
+    [],
+    'Expected the server to answer each client source map URL with 404',
+  );
+} finally {
+  await server.close();
+}
+
+console.log('All source map assertions passed!');
diff --git a/dev-packages/e2e-tests/test-applications/tanstackstart-react-sourcemaps/package.json b/dev-packages/e2e-tests/test-applications/tanstackstart-react-sourcemaps/package.json
new file mode 100644
index 000000000000..e03631c63231
--- /dev/null
+++ b/dev-packages/e2e-tests/test-applications/tanstackstart-react-sourcemaps/package.json
@@ -0,0 +1,37 @@
+{
+  "name": "tanstackstart-react-sourcemaps",
+  "description": "E2E test app asserting what the TanStack Start SDK uploads to Sentry when Nitro builds the server.",
+  "private": true,
+  "type": "module",
+  "scripts": {
+    "build": "node start-mock-sentry-server.mjs & vite build > .tmp_build_stdout 2> .tmp_build_stderr; BUILD_EXIT=$?; kill %1 2>/dev/null; if [ $BUILD_EXIT -ne 0 ]; then cat .tmp_build_stdout; cat .tmp_build_stderr >&2; fi; exit $BUILD_EXIT",
+    "clean": "npx rimraf node_modules pnpm-lock.yaml .output",
+    "test:build": "pnpm install && pnpm build",
+    "test:assert": "pnpm ts-node --script-mode assert-build.ts"
+  },
+  "dependencies": {
+    "@sentry/tanstackstart-react": "file:../../packed/sentry-tanstackstart-react-packed.tgz",
+    "@tanstack/react-router": "1.170.44",
+    "@tanstack/react-start": "1.169.1",
+    "nitro": "latest",
+    "react": "^19.2.0",
+    "react-dom": "^19.2.0"
+  },
+  "devDependencies": {
+    "@playwright/test": "~1.63.0",
+    "@sentry-internal/test-utils": "link:../../../test-utils",
+    "@types/node": "^22.20.0",
+    "@types/react": "^19.2.0",
+    "@types/react-dom": "^19.2.0",
+    "@vitejs/plugin-react-swc": "^4.2.0",
+    "ts-node": "10.9.1",
+    "typescript": "^5.9.0",
+    "vite": "^8.3.0"
+  },
+  "volta": {
+    "extends": "../../package.json"
+  },
+  "sentryTest": {
+    "optional": true
+  }
+}
diff --git a/dev-packages/e2e-tests/test-applications/tanstackstart-react-sourcemaps/src/router.tsx b/dev-packages/e2e-tests/test-applications/tanstackstart-react-sourcemaps/src/router.tsx
new file mode 100644
index 000000000000..741d1aade5d5
--- /dev/null
+++ b/dev-packages/e2e-tests/test-applications/tanstackstart-react-sourcemaps/src/router.tsx
@@ -0,0 +1,4 @@
+import { createRouter } from '@tanstack/react-router';
+import { routeTree } from './routeTree.gen';
+
+export const getRouter = () => createRouter({ routeTree });
diff --git a/dev-packages/e2e-tests/test-applications/tanstackstart-react-sourcemaps/src/routes/__root.tsx b/dev-packages/e2e-tests/test-applications/tanstackstart-react-sourcemaps/src/routes/__root.tsx
new file mode 100644
index 000000000000..09432c62c40d
--- /dev/null
+++ b/dev-packages/e2e-tests/test-applications/tanstackstart-react-sourcemaps/src/routes/__root.tsx
@@ -0,0 +1,20 @@
+import type { ReactNode } from 'react';
+import { createRootRoute, HeadContent, Outlet, Scripts } from '@tanstack/react-router';
+
+export const Route = createRootRoute({
+  component: RootComponent,
+});
+
+function RootComponent(): ReactNode {
+  return (
+    <html>
+      <head>
+        <HeadContent />
+      </head>
+      <body>
+        <Outlet />
+        <Scripts />
+      </body>
+    </html>
+  );
+}
diff --git a/dev-packages/e2e-tests/test-applications/tanstackstart-react-sourcemaps/src/routes/index.tsx b/dev-packages/e2e-tests/test-applications/tanstackstart-react-sourcemaps/src/routes/index.tsx
new file mode 100644
index 000000000000..51012573f928
--- /dev/null
+++ b/dev-packages/e2e-tests/test-applications/tanstackstart-react-sourcemaps/src/routes/index.tsx
@@ -0,0 +1,21 @@
+import { createFileRoute } from '@tanstack/react-router';
+import { createServerFn } from '@tanstack/react-start';
+
+// SOURCEMAP_MARKER_CLIENT — a comment, so it is stripped from the emitted bundles but survives in `sourcesContent`.
+// `assert-build.ts` uses the markers to check that the original source reached the uploaded maps.
+const throwServerError = createServerFn().handler(async () => {
+  // SOURCEMAP_MARKER_SERVER
+  throw new Error('Server function error from the TanStack Start source map E2E app');
+});
+
+export const Route = createFileRoute('/')({
+  component: Home,
+});
+
+function Home() {
+  return (
+    <button type="button" onClick={() => throwServerError()}>
+      Throw server function error
+    </button>
+  );
+}
diff --git a/dev-packages/e2e-tests/test-applications/tanstackstart-react-sourcemaps/start-mock-sentry-server.mjs b/dev-packages/e2e-tests/test-applications/tanstackstart-react-sourcemaps/start-mock-sentry-server.mjs
new file mode 100644
index 000000000000..1680a6360165
--- /dev/null
+++ b/dev-packages/e2e-tests/test-applications/tanstackstart-react-sourcemaps/start-mock-sentry-server.mjs
@@ -0,0 +1,3 @@
+import { startMockSentryServer } from '@sentry-internal/test-utils';
+
+startMockSentryServer({ org: 'test-org' });
diff --git a/dev-packages/e2e-tests/test-applications/tanstackstart-react-sourcemaps/tsconfig.json b/dev-packages/e2e-tests/test-applications/tanstackstart-react-sourcemaps/tsconfig.json
new file mode 100644
index 000000000000..02faa5762420
--- /dev/null
+++ b/dev-packages/e2e-tests/test-applications/tanstackstart-react-sourcemaps/tsconfig.json
@@ -0,0 +1,16 @@
+{
+  "compilerOptions": {
+    "target": "ES2022",
+    "lib": ["ES2022", "DOM", "DOM.Iterable"],
+    "module": "ESNext",
+    "moduleResolution": "bundler",
+    "jsx": "react-jsx",
+    "strict": true,
+    "skipLibCheck": true,
+    "esModuleInterop": true,
+    "resolveJsonModule": true,
+    "noEmit": true
+  },
+  "include": ["**/*.ts", "**/*.tsx"],
+  "exclude": ["node_modules", ".output"]
+}
diff --git a/dev-packages/e2e-tests/test-applications/tanstackstart-react-sourcemaps/vite.config.ts b/dev-packages/e2e-tests/test-applications/tanstackstart-react-sourcemaps/vite.config.ts
new file mode 100644
index 000000000000..b43b2441b453
--- /dev/null
+++ b/dev-packages/e2e-tests/test-applications/tanstackstart-react-sourcemaps/vite.config.ts
@@ -0,0 +1,21 @@
+import { sentryTanstackStart } from '@sentry/tanstackstart-react/vite';
+import { tanstackStart } from '@tanstack/react-start/plugin/vite';
+import viteReact from '@vitejs/plugin-react-swc';
+import { nitro } from 'nitro/vite';
+import { defineConfig } from 'vite';
+
+export default defineConfig({
+  plugins: [
+    tanstackStart(),
+    nitro(),
+    viteReact(),
+    sentryTanstackStart({
+      sentryUrl: 'http://localhost:3032',
+      authToken: 'fake-auth-token',
+      org: 'test-org',
+      project: 'test-project',
+      release: { name: 'test-release' },
+      debug: true,
+    }),
+  ],
+});