diff --git a/packages/server-utils/src/orchestrion/bundler/rollup.ts b/packages/server-utils/src/orchestrion/bundler/rollup.ts
index c71aa18938ab..394874a26fdd 100644
--- a/packages/server-utils/src/orchestrion/bundler/rollup.ts
+++ b/packages/server-utils/src/orchestrion/bundler/rollup.ts
@@ -1,12 +1,28 @@
 import codeTransformer from '@apm-js-collab/code-transformer-bundler-plugins/rollup';
-import type { NormalizedInputOptions, Plugin, PluginContext } from 'rollup';
+import type { ExternalOption, InputOptions, NormalizedInputOptions, Plugin, PluginContext } from 'rollup';
 
 export type { Plugin as RollupPlugin } from 'rollup';
 import { instrumentedModuleNames } from '../config';
 import type { PluginOptions } from './options';
-import { externalizedModulesWarning, orchestrionTransformOptions } from './options';
+import { externalEntryMatchesModule, externalizedModulesWarning, orchestrionTransformOptions } from './options';
 import { resolveOrchestrionRuntimeRequest, SNIPPET_IMPORT_SPECIFIER } from './resolve';
 
+/**
+ * Whether a raw (un-normalized) `external` input option marks `name` as
+ * external. String entries use the shared subpath-aware matching so a
+ * `'mysql/lib/...'` entry flags `mysql`, consistent with the esbuild and
+ * webpack plugins.
+ */
+function rawExternalMatchesModule(external: ExternalOption, name: string): boolean {
+  if (typeof external === 'function') {
+    return !!external(name, undefined, false);
+  }
+  const entries = Array.isArray(external) ? external : [external];
+  return entries.some(entry =>
+    typeof entry === 'string' ? externalEntryMatchesModule(entry, name) : entry.test(name),
+  );
+}
+
 /**
  * Rollup plugin that runs the orchestrion code transform on the bundled output.
  *
@@ -28,8 +44,17 @@ export function sentryOrchestrionPlugin(options: PluginOptions = {}): Plugin {
 
   const moduleNames = instrumentedModuleNames(options.instrumentations);
 
+  // Rolldown omits `external` from the normalized options passed to
+  // `buildStart` (function-typed options don't cross its Rust/JS boundary —
+  // rolldown/rolldown#1041), so capture the raw value for the probe below.
+  let rawExternal: ExternalOption | undefined;
+
   return {
     ...codeTransformer(orchestrionTransformOptions(options)),
+    options(inputOptions: InputOptions): null {
+      rawExternal = inputOptions.external;
+      return null;
+    },
     // The module-injected snippet imports `@sentry/server-utils` from INSIDE
     // transformed `node_modules` files. Under isolated installs (pnpm) that bare
     // specifier doesn't resolve from an instrumented package's location, so when
@@ -47,10 +72,15 @@ export function sentryOrchestrionPlugin(options: PluginOptions = {}): Plugin {
     },
     buildStart(this: PluginContext, rollupOptions: NormalizedInputOptions): void {
       // An externalized dependency never passes through the code transform, so
-      // its diagnostics_channel calls are silently never injected. By the time
-      // buildStart runs, Rollup has normalized `external` (string arrays,
-      // RegExps or user functions) into a single predicate we can probe.
-      const externalizedModules = moduleNames.filter(name => rollupOptions.external(name, undefined, false));
+      // its diagnostics_channel calls are silently never injected. Rollup has
+      // normalized `external` into a single predicate by the time buildStart
+      // runs; Rolldown doesn't provide it here at all, so probe the raw value
+      // captured in the `options` hook instead.
+      const externalizedModules = moduleNames.filter(name =>
+        typeof rollupOptions.external === 'function'
+          ? rollupOptions.external(name, undefined, false)
+          : rawExternal != null && rawExternalMatchesModule(rawExternal, name),
+      );
       if (externalizedModules.length > 0) {
         this.warn(externalizedModulesWarning(externalizedModules));
       }
diff --git a/packages/server-utils/test/orchestrion/rollup-plugin.test.ts b/packages/server-utils/test/orchestrion/rollup-plugin.test.ts
new file mode 100644
index 000000000000..6002e141a835
--- /dev/null
+++ b/packages/server-utils/test/orchestrion/rollup-plugin.test.ts
@@ -0,0 +1,49 @@
+import type { InputOptions, NormalizedInputOptions, PluginContext } from 'rollup';
+import { describe, expect, it, vi } from 'vitest';
+import { sentryOrchestrionPlugin } from '../../src/orchestrion/bundler/rollup';
+
+type OptionsHook = (this: unknown, inputOptions: InputOptions) => null;
+type BuildStartHook = (this: Pick<PluginContext, 'warn'>, rollupOptions: NormalizedInputOptions) => void;
+
+function runBuildStart(inputOptions: InputOptions, normalizedExternal?: NormalizedInputOptions['external']): string[] {
+  const plugin = sentryOrchestrionPlugin();
+  const warn = vi.fn();
+  (plugin.options as OptionsHook).call({}, inputOptions);
+  (plugin.buildStart as BuildStartHook).call({ warn }, { external: normalizedExternal } as NormalizedInputOptions);
+  return warn.mock.calls.map(call => call[0] as string);
+}
+
+describe('sentryOrchestrionPlugin (rollup) externalized-modules warning', () => {
+  it('warns via the normalized predicate when Rollup provides one', () => {
+    const warnings = runBuildStart({}, (source: string) => source === 'express');
+    expect(warnings).toHaveLength(1);
+    expect(warnings[0]).toContain('express');
+  });
+
+  describe('without a normalized predicate (Rolldown — rolldown/rolldown#1041)', () => {
+    it('does not crash and stays silent when nothing is externalized', () => {
+      expect(runBuildStart({ external: ['react'] })).toEqual([]);
+      expect(runBuildStart({})).toEqual([]);
+    });
+
+    it('warns for a raw string entry', () => {
+      const warnings = runBuildStart({ external: 'express' });
+      expect(warnings).toHaveLength(1);
+      expect(warnings[0]).toContain('express');
+    });
+
+    it('warns for raw array entries, including subpaths and RegExps', () => {
+      const warnings = runBuildStart({ external: ['react', 'mysql/lib/index.js', /^pg$/] });
+      expect(warnings).toHaveLength(1);
+      expect(warnings[0]).toContain('mysql');
+      expect(warnings[0]).toContain('pg');
+      expect(warnings[0]).not.toContain('react');
+    });
+
+    it('warns via a raw user function', () => {
+      const warnings = runBuildStart({ external: source => source === 'express' });
+      expect(warnings).toHaveLength(1);
+      expect(warnings[0]).toContain('express');
+    });
+  });
+});