diff --git a/.github/workflows/test.yml b/.github/workflows/test.yml
index 375d5fada..026ca0fb7 100644
--- a/.github/workflows/test.yml
+++ b/.github/workflows/test.yml
@@ -38,6 +38,10 @@ jobs:
         run: |
           pnpm exec vitest run ./packages/mermaid/src/diagrams/gantt/ganttDb.spec.ts --coverage
 
+      - name: Verify out-of-tree build with TypeScript
+        run: |
+          pnpm test:check:tsc
+
       - name: Upload Coverage to Codecov
         uses: codecov/codecov-action@e28ff129e5465c2c0dcc6f003fc735cb6ae0c673 # v4.5.0
         # Run step only pushes to develop and pull_requests
diff --git a/package.json b/package.json
index 21ae5360c..ff100f1f6 100644
--- a/package.json
+++ b/package.json
@@ -42,6 +42,7 @@
     "test": "pnpm lint && vitest run",
     "test:watch": "vitest --watch",
     "test:coverage": "vitest --coverage",
+    "test:check:tsc": "tsx scripts/tsc-check.ts",
     "prepare": "husky && pnpm build",
     "pre-commit": "lint-staged"
   },
diff --git a/packages/mermaid/src/rendering-util/render.ts b/packages/mermaid/src/rendering-util/render.ts
index e3118b5b7..8306bc15a 100644
--- a/packages/mermaid/src/rendering-util/render.ts
+++ b/packages/mermaid/src/rendering-util/render.ts
@@ -1,5 +1,6 @@
 import type { SVG } from 'mermaid/dist/diagram-api/types.js';
-import type { InternalHelpers, internalHelpers } from 'mermaid/dist/internals.js';
+import type { InternalHelpers } from 'mermaid/dist/internals.js';
+import { internalHelpers } from 'mermaid/dist/internals.js';
 import { log } from 'mermaid/dist/logger.js';
 import type { LayoutData } from './types.js';
 
diff --git a/scripts/tsc-check.ts b/scripts/tsc-check.ts
new file mode 100644
index 000000000..7a5ff50a9
--- /dev/null
+++ b/scripts/tsc-check.ts
@@ -0,0 +1,175 @@
+/**
+ * Verify the as-built tarballs can be imported into a fresh, out-of-tree TypeScript project.
+ */
+
+/* eslint-disable no-console */
+import { mkdtemp, mkdir, writeFile, readFile, readdir, copyFile, rm } from 'node:fs/promises';
+import { execFileSync } from 'child_process';
+import * as path from 'path';
+import { fileURLToPath } from 'url';
+import { tmpdir } from 'node:os';
+
+const __filename = fileURLToPath(import.meta.url); // get the resolved path to the file
+const __dirname = path.dirname(__filename); // get the name of the directory
+
+/**
+ * Packages to build and import
+ */
+const PACKAGES = {
+  mermaid: 'mermaid',
+  '@mermaid-js/layout-elk': 'mermaid-layout-elk',
+  // TODO: these don't import cleanly yet due to exotic tsconfig.json requirements
+  // '@mermaid-js/mermaid-zenuml': 'mermaid-zenuml',
+  // '@mermaid-js/parser': 'parser',
+};
+
+/**
+ * Files to create in the temporary package.
+ */
+const SRC = {
+  // a minimal description of a buildable package
+  'package.json': (tarballs: Record<string, string>) =>
+    JSON.stringify(
+      {
+        dependencies: tarballs,
+        scripts: { build: 'tsc -b --verbose' },
+        devDependencies: {
+          // these are somewhat-unexpectedly required, and a downstream would need
+          // to match the real `package.json` values
+          'type-fest': '*',
+          '@types/d3': '^7.4.3',
+          '@types/dompurify': '^3.0.5',
+          typescript: '*',
+        },
+      },
+      null,
+      2
+    ),
+  // a fairly strict TypeScript configuration
+  'tsconfig.json': () =>
+    JSON.stringify(
+      {
+        compilerOptions: {
+          allowSyntheticDefaultImports: true,
+          composite: true,
+          declaration: true,
+          esModuleInterop: true,
+          incremental: true,
+          lib: ['dom', 'es2020'],
+          module: 'esnext',
+          moduleResolution: 'node',
+          noEmitOnError: true,
+          noImplicitAny: true,
+          noUnusedLocals: true,
+          sourceMap: true,
+          target: 'es2020',
+          rootDir: './src',
+          outDir: './lib',
+          strict: true,
+          tsBuildInfoFile: 'lib/.tsbuildinfo',
+        },
+      },
+      null,
+      2
+    ),
+  // the simplest possible script: will everything even import?
+  'src/index.ts': (tarballs) => {
+    const imports: string[] = [];
+    const outputs: string[] = [];
+    let i = 0;
+    for (const pkg of Object.keys(tarballs)) {
+      imports.push(`import * as pkg_${i} from '${pkg}';`);
+      outputs.push(`console.log(pkg_${i});`);
+      i++;
+    }
+    return [...imports, ...outputs].join('\n');
+  },
+};
+
+/**
+ * Commands to run after source files are created.
+ *
+ * `npm` is used to detect any unwanted `pnpm`-specific runtime "features".
+ */
+const COMMANDS = [
+  ['npm', 'install'],
+  ['npm', 'run', 'build'],
+];
+
+/**
+ * Built files to expect after commands are executed.
+ */
+const LIB = ['lib/index.js', 'lib/index.js.map', 'lib/index.d.ts', 'lib/.tsbuildinfo'];
+
+/**
+ * Run a small out-of-tree build.
+ */
+async function main() {
+  console.warn('Checking out-of-tree TypeScript build using', Object.keys(PACKAGES).join('\n'));
+  const cwd = await mkdtemp(path.join(tmpdir(), 'mermaid-tsc-check-'));
+  console.warn('... creating temporary folder', cwd);
+  const tarballs = await buildTarballs(cwd);
+
+  for (const [filename, generate] of Object.entries(SRC)) {
+    const dest = path.join(cwd, filename);
+    await mkdir(path.dirname(dest), { recursive: true });
+    console.warn('... creating', dest);
+    const text = generate(tarballs);
+    await writeFile(dest, text);
+    console.info(text);
+  }
+
+  for (const argv of COMMANDS) {
+    console.warn('... in', cwd);
+    console.warn('>>>', ...argv);
+    execFileSync(argv[0], argv.slice(1), { cwd });
+  }
+
+  for (const lib of LIB) {
+    const checkLib = path.join(cwd, lib);
+    console.warn('... checking built file', checkLib);
+    await readFile(checkLib, 'utf-8');
+  }
+
+  console.warn('... deleting', cwd);
+  await rm(cwd, { recursive: true, force: true });
+  console.warn('... tsc-check OK for\n', Object.keys(PACKAGES).join('\n'));
+}
+
+/** Build all the tarballs. */
+async function buildTarballs(tmp: string): Promise<Record<string, string>> {
+  const dist = path.join(tmp, 'dist');
+  await mkdir(dist);
+  const promises: Promise<void>[] = [];
+  const tarballs: Record<string, string> = {};
+  for (const [pkg, srcPath] of Object.entries(PACKAGES)) {
+    promises.push(buildOneTarball(pkg, srcPath, dist, tarballs));
+  }
+  await Promise.all(promises);
+  return tarballs;
+}
+
+/** Build a single tarball. */
+async function buildOneTarball(
+  pkg: string,
+  srcPath: string,
+  dist: string,
+  tarballs: Record<string, string>
+): Promise<void> {
+  const cwd = await mkdtemp(path.join(dist, 'pack-'));
+  const pkgDir = path.join(__dirname, '../packages', srcPath);
+  const argv = ['pnpm', 'pack', '--pack-destination', cwd];
+  console.warn('>>>', ...argv);
+  execFileSync(argv[0], argv.slice(1), { cwd: pkgDir });
+  const built = await readdir(cwd);
+  const dest = path.join(dist, built[0]);
+  await copyFile(path.join(cwd, built[0]), dest);
+  await rm(cwd, { recursive: true, force: true });
+  tarballs[pkg] = dest;
+}
+
+void main().catch((err) => {
+  console.error(err);
+  console.error('!!! tsc-check FAIL: temp folder left in place. see logs above for failure notes');
+  process.exit(1);
+});