diff --git a/dev-packages/rollup-utils/npmHelpers.mjs b/dev-packages/rollup-utils/npmHelpers.mjs
index 0014616f8071..43e11e36f19c 100644
--- a/dev-packages/rollup-utils/npmHelpers.mjs
+++ b/dev-packages/rollup-utils/npmHelpers.mjs
@@ -94,6 +94,11 @@ export function makeBaseNPMConfig(options = {}) {
       // (We don't need it, so why waste the bytes?)
       freeze: false,
 
+      // Assume externals are ESM-shaped (`__esModule` + `.default`), which our own `@sentry/*`
+      // packages satisfy via `esModule: 'if-default-prop'`. This keeps `import * as x` a live
+      // reference to the real module rather than an `_interopNamespace` copy — instrumentation code
+      // relies on that to monkey-patch modules like `fs` in place. Packages that pull in bare-CJS
+      // third-party deps (no `.default`) override this per-module (see server-utils).
       interop: 'esModule',
     },
 
diff --git a/packages/nextjs/src/config/webpack.ts b/packages/nextjs/src/config/webpack.ts
index 94db1cca621d..30532b354360 100644
--- a/packages/nextjs/src/config/webpack.ts
+++ b/packages/nextjs/src/config/webpack.ts
@@ -433,7 +433,7 @@ export function constructWebpackConfigFunction({
 
     // Orchestrion code-transform loader — Node server runtime only, never the edge compilation
     if (runtime === 'server' && userSentryOptions._experimental?.useDiagnosticsChannelInjection) {
-      newConfig.plugins.push(sentryOrchestrionWebpackPlugin() as WebpackPluginInstance);
+      newConfig.plugins.push(sentryOrchestrionWebpackPlugin() as unknown as WebpackPluginInstance);
     }
 
     return newConfig;
diff --git a/packages/server-utils/rollup.npm.config.mjs b/packages/server-utils/rollup.npm.config.mjs
index 5f02614f716d..0ff1a23ba534 100644
--- a/packages/server-utils/rollup.npm.config.mjs
+++ b/packages/server-utils/rollup.npm.config.mjs
@@ -42,6 +42,14 @@ export default [
           exports: 'named',
           // set preserveModules to true because we don't want to bundle everything into one file.
           preserveModules: true,
+          // `@apm-js-collab/code-transformer-bundler-plugins` ships CJS entries as bare
+          // `module.exports = fn` with no `__esModule`/`.default`. The repo default
+          // `interop: 'esModule'` assumes ESM-shaped externals and would dereference a nonexistent
+          // `.default`, so a default import compiles to `codeTransformer.default(...)` → "not a
+          // function". Use 'auto' for just these so Rollup emits its interop helper. Scoped here (not
+          // repo-wide) because 'auto' also turns `import * as x` into a copy, which breaks in-place
+          // monkey-patching that other packages (e.g. the OTel fs instrumentation) depend on.
+          interop: id => (id?.startsWith('@apm-js-collab/code-transformer-bundler-plugins') ? 'auto' : 'esModule'),
         },
       },
     }),
diff --git a/packages/server-utils/src/orchestrion/bundler/webpack.ts b/packages/server-utils/src/orchestrion/bundler/webpack.ts
index 04eb56a2c6b9..860a1cc046bc 100644
--- a/packages/server-utils/src/orchestrion/bundler/webpack.ts
+++ b/packages/server-utils/src/orchestrion/bundler/webpack.ts
@@ -5,6 +5,7 @@ import { createRequire } from 'node:module';
 import { dirname } from 'node:path';
 import type { InstrumentationConfig } from '@apm-js-collab/code-transformer';
 import { SENTRY_INSTRUMENTATIONS } from '../config';
+import codeTransformer from '@apm-js-collab/code-transformer-bundler-plugins/webpack';
 
 // Both branches use `createRequire` (never alias the CJS `require`) so bundlers consuming this
 // module don't emit a "Critical dependency" warning.
@@ -46,10 +47,6 @@ export function getSentryInstrumentations(): InstrumentationConfig[] {
  * does NOT inject the `__SENTRY_ORCHESTRION__.bundler` marker — that would disable the runtime
  * module hook, which externalized packages still need (hybrid setup).
  */
-export function sentryOrchestrionWebpackPlugin(): unknown {
-  const mod = getOrchestrionRequire()('@apm-js-collab/code-transformer-bundler-plugins/webpack') as {
-    default?: (options: { instrumentations: InstrumentationConfig[] }) => unknown;
-  };
-  const codeTransformerWebpack = mod.default ?? (mod as unknown as NonNullable<typeof mod.default>);
-  return codeTransformerWebpack({ instrumentations: SENTRY_INSTRUMENTATIONS });
+export function sentryOrchestrionWebpackPlugin(): ReturnType<typeof codeTransformer> {
+  return codeTransformer({ instrumentations: SENTRY_INSTRUMENTATIONS });
 }