diff --git a/packages/server-utils/src/orchestrion/bundler/esbuild.ts b/packages/server-utils/src/orchestrion/bundler/esbuild.ts
index be6f7bfca8ad..307e5e78a033 100644
--- a/packages/server-utils/src/orchestrion/bundler/esbuild.ts
+++ b/packages/server-utils/src/orchestrion/bundler/esbuild.ts
@@ -1,6 +1,16 @@
 import codeTransformer from '@apm-js-collab/code-transformer-bundler-plugins/esbuild';
+import { escapeStringForRegex } from '@sentry/core';
+import { instrumentedModuleNames } from '../config';
 import type { PluginOptions } from './options';
-import { orchestrionTransformOptions } from './options';
+import { externalEntryMatchesModule, externalizedModulesWarning, orchestrionTransformOptions } from './options';
+
+// esbuild `external` entries may contain `*` wildcards.
+function matchesEsbuildExternal(entry: string, moduleName: string): boolean {
+  if (entry.includes('*')) {
+    return new RegExp(`^${entry.split('*').map(escapeStringForRegex).join('.*')}$`).test(moduleName);
+  }
+  return externalEntryMatchesModule(entry, moduleName);
+}
 
 /**
  * esbuild plugin that runs the orchestrion code transform on the bundled output.
@@ -8,9 +18,8 @@ import { orchestrionTransformOptions } from './options';
  * Use when bundling a Node app with esbuild. For unbundled Node processes use the
  * runtime hook instead (`node --import @sentry/node/orchestrion app.js`).
  *
- * esbuild does not flatten nested `plugins` arrays, so this returns a single
- * plugin that strips instrumented packages from an `external` denylist before
- * delegating to the upstream transform.
+ * Instrumented packages marked as `external` never pass through the code
+ * transform, so a build warning is emitted for them.
  *
  * @example
  * ```ts
@@ -20,5 +29,21 @@ import { orchestrionTransformOptions } from './options';
  * ```
  */
 export function sentryOrchestrionPlugin(options: PluginOptions = {}): ReturnType<typeof codeTransformer> {
-  return codeTransformer(orchestrionTransformOptions(options));
+  const plugin = codeTransformer(orchestrionTransformOptions(options));
+  const moduleNames = instrumentedModuleNames(options.instrumentations);
+  const setup = plugin.setup;
+
+  return {
+    ...plugin,
+    setup(build): ReturnType<typeof setup> {
+      const external = build.initialOptions.external || [];
+      const externalizedModules = moduleNames.filter(name =>
+        external.some(entry => matchesEsbuildExternal(entry, name)),
+      );
+      if (externalizedModules.length > 0) {
+        build.onStart(() => ({ warnings: [{ text: externalizedModulesWarning(externalizedModules) }] }));
+      }
+      return setup(build);
+    },
+  };
 }
diff --git a/packages/server-utils/src/orchestrion/bundler/options.ts b/packages/server-utils/src/orchestrion/bundler/options.ts
index c536482df346..a3c194aae165 100644
--- a/packages/server-utils/src/orchestrion/bundler/options.ts
+++ b/packages/server-utils/src/orchestrion/bundler/options.ts
@@ -19,6 +19,30 @@ export type PluginOptions = {
   shouldInjectDiagnostics?: boolean;
 };
 
+/**
+ * Whether an "external" config entry covers an instrumented module: an exact
+ * package name (`'mysql'`) or a subpath (`'mysql/lib/...'`) — the transform may
+ * target exactly the file a subpath entry externalizes. Mirrors the matching in
+ * `withoutInstrumentedExternals`.
+ */
+export function externalEntryMatchesModule(entry: string, moduleName: string): boolean {
+  return entry === moduleName || entry.startsWith(`${moduleName}/`);
+}
+
+/**
+ * Warning emitted when a bundler config externalizes packages that orchestrion
+ * needs to transform. An externalized dependency is resolved from
+ * `node_modules` at runtime and never passes through the code transform, so
+ * its diagnostics_channel calls are silently never injected.
+ */
+export function externalizedModulesWarning(externalizedModules: string[]): string {
+  return (
+    `The following packages are marked as external in your bundler configuration but need to be bundled for Sentry ` +
+    `instrumentation to work: ${externalizedModules.join(', ')}. Remove them from your bundler's "external" ` +
+    `configuration, or use the Sentry Node SDK's runtime instrumentation instead.`
+  );
+}
+
 /**
  * The `@apm-js-collab/code-transformer-bundler-plugins` options shared by every
  * orchestrion bundler plugin.
diff --git a/packages/server-utils/src/orchestrion/bundler/rollup.ts b/packages/server-utils/src/orchestrion/bundler/rollup.ts
index 28b080802fee..d42abede972c 100644
--- a/packages/server-utils/src/orchestrion/bundler/rollup.ts
+++ b/packages/server-utils/src/orchestrion/bundler/rollup.ts
@@ -1,6 +1,8 @@
 import codeTransformer from '@apm-js-collab/code-transformer-bundler-plugins/rollup';
+import type { NormalizedInputOptions, PluginContext } from 'rollup';
+import { instrumentedModuleNames } from '../config';
 import type { PluginOptions } from './options';
-import { orchestrionTransformOptions } from './options';
+import { externalizedModulesWarning, orchestrionTransformOptions } from './options';
 
 /**
  * Rollup plugin that runs the orchestrion code transform on the bundled output.
@@ -16,5 +18,19 @@ import { orchestrionTransformOptions } from './options';
  * ```
  */
 export function sentryOrchestrionPlugin(options: PluginOptions = {}): ReturnType<typeof codeTransformer> {
-  return codeTransformer(orchestrionTransformOptions(options));
+  const moduleNames = instrumentedModuleNames(options.instrumentations);
+
+  return {
+    ...codeTransformer(orchestrionTransformOptions(options)),
+    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));
+      if (externalizedModules.length > 0) {
+        this.warn(externalizedModulesWarning(externalizedModules));
+      }
+    },
+  };
 }
diff --git a/packages/server-utils/src/orchestrion/bundler/vite.ts b/packages/server-utils/src/orchestrion/bundler/vite.ts
index 75ad1ab49855..0b586f0b682c 100644
--- a/packages/server-utils/src/orchestrion/bundler/vite.ts
+++ b/packages/server-utils/src/orchestrion/bundler/vite.ts
@@ -1,7 +1,8 @@
 import codeTransformer from '@apm-js-collab/code-transformer-bundler-plugins/vite';
+import type { ResolvedConfig } from 'vite';
 import { instrumentedModuleNames } from '../config';
 import type { PluginOptions } from './options';
-import { orchestrionTransformOptions } from './options';
+import { externalEntryMatchesModule, externalizedModulesWarning, orchestrionTransformOptions } from './options';
 
 /**
  * Vite plugin that runs the orchestrion code transform on the bundled output.
@@ -30,5 +31,21 @@ export function sentryOrchestrionPlugin(options: PluginOptions = {}): ReturnType
       // their additions.
       return { ssr: { noExternal: instrumentedModuleNames(options.instrumentations) } };
     },
+    configResolved(config: ResolvedConfig): void {
+      // Explicit `ssr.external` string entries take priority over `noExternal`
+      // in Vite, so they defeat the force-bundling above. (`ssr.external: true`
+      // does not — `noExternal` entries still win there.)
+      const external = config.ssr?.external;
+      if (!Array.isArray(external)) {
+        return;
+      }
+      const moduleNames = instrumentedModuleNames(options.instrumentations);
+      const externalizedModules = moduleNames.filter(name =>
+        external.some(entry => externalEntryMatchesModule(entry, name)),
+      );
+      if (externalizedModules.length > 0) {
+        config.logger.warn(`[Sentry] ${externalizedModulesWarning(externalizedModules)}`);
+      }
+    },
   };
 }
diff --git a/packages/server-utils/src/orchestrion/bundler/webpack.ts b/packages/server-utils/src/orchestrion/bundler/webpack.ts
index 7e2f584b1819..a20daf49b7b9 100644
--- a/packages/server-utils/src/orchestrion/bundler/webpack.ts
+++ b/packages/server-utils/src/orchestrion/bundler/webpack.ts
@@ -3,11 +3,12 @@
 
 import { createRequire } from 'node:module';
 import { dirname } from 'node:path';
+import type { Compiler } from 'webpack';
 import type { InstrumentationConfig } from '..';
-import { SENTRY_INSTRUMENTATIONS } from '../config';
+import { instrumentedModuleNames, SENTRY_INSTRUMENTATIONS } from '../config';
 import codeTransformerWebpack from '@apm-js-collab/code-transformer-bundler-plugins/webpack';
 import type { PluginOptions } from './options';
-import { orchestrionTransformOptions } from './options';
+import { externalEntryMatchesModule, externalizedModulesWarning, orchestrionTransformOptions } from './options';
 
 // Both branches use `createRequire` (never alias the CJS `require`) so bundlers consuming this
 // module don't emit a "Critical dependency" warning.
@@ -44,9 +45,47 @@ export function getSentryInstrumentations(): InstrumentationConfig[] {
   return SENTRY_INSTRUMENTATIONS;
 }
 
+// Handles the declarative `externals` shapes (string, RegExp, object, arrays
+// thereof). Function externals (e.g. webpack-node-externals) are skipped: they
+// may resolve asynchronously, so they can't be probed reliably here.
+function externalizedWebpackModules(externals: unknown, moduleNames: string[]): string[] {
+  const entries = Array.isArray(externals) ? externals : [externals];
+  return moduleNames.filter(name =>
+    entries.some(entry => {
+      if (typeof entry === 'string') {
+        return externalEntryMatchesModule(entry, name);
+      }
+      if (entry instanceof RegExp) {
+        return entry.test(name);
+      }
+      if (entry && typeof entry === 'object') {
+        return name in entry;
+      }
+      return false;
+    }),
+  );
+}
+
 /**
- * The code-transform webpack plugin, pre-fed the instrumentation config
+ * The code-transform webpack plugin, pre-fed the instrumentation config.
+ *
+ * Instrumented packages marked as `externals` never pass through the code
+ * transform, so a compilation warning is emitted for them.
  */
 export function sentryOrchestrionWebpackPlugin(options: PluginOptions = {}): ReturnType<typeof codeTransformerWebpack> {
-  return codeTransformerWebpack(orchestrionTransformOptions(options));
+  const plugin = codeTransformerWebpack(orchestrionTransformOptions(options));
+  const moduleNames = instrumentedModuleNames(options.instrumentations);
+  // The upstream plugin is a class instance, so `apply` is overridden in place
+  // rather than spread into a new object (which would lose prototype methods).
+  const apply = plugin.apply.bind(plugin);
+  plugin.apply = (compiler: Compiler): void => {
+    const externalizedModules = externalizedWebpackModules(compiler.options.externals, moduleNames);
+    if (externalizedModules.length > 0) {
+      compiler.hooks.thisCompilation.tap('SentryOrchestrionExternalsCheck', compilation => {
+        compilation.warnings.push(new compiler.webpack.WebpackError(externalizedModulesWarning(externalizedModules)));
+      });
+    }
+    apply(compiler);
+  };
+  return plugin;
 }
diff --git a/packages/server-utils/test/orchestrion/bundler.test.ts b/packages/server-utils/test/orchestrion/bundler.test.ts
index b4c309b9152c..f13209fba323 100644
--- a/packages/server-utils/test/orchestrion/bundler.test.ts
+++ b/packages/server-utils/test/orchestrion/bundler.test.ts
@@ -1,7 +1,140 @@
 import { existsSync } from 'node:fs';
 import { join } from 'node:path';
-import { describe, expect, it } from 'vitest';
-import { getTracingHooksDirectory } from '../../src/orchestrion/bundler/webpack';
+import type { OnStartResult, PluginBuild } from 'esbuild';
+import type { NormalizedInputOptions, PluginContext } from 'rollup';
+import type { ResolvedConfig } from 'vite';
+import type { Compiler } from 'webpack';
+import { describe, expect, it, vi } from 'vitest';
+import { sentryOrchestrionPlugin as esbuildPlugin } from '../../src/orchestrion/bundler/esbuild';
+import { sentryOrchestrionPlugin as rollupPlugin } from '../../src/orchestrion/bundler/rollup';
+import { sentryOrchestrionPlugin as vitePlugin } from '../../src/orchestrion/bundler/vite';
+import { getTracingHooksDirectory, sentryOrchestrionWebpackPlugin } from '../../src/orchestrion/bundler/webpack';
+
+// The upstream transform plugins are mocked so tests exercise only the hooks
+// added on top of them (the externalized-modules warnings).
+vi.mock('@apm-js-collab/code-transformer-bundler-plugins/esbuild', () => ({
+  default: () => ({ name: 'code-transformer', setup: vi.fn() }),
+}));
+vi.mock('@apm-js-collab/code-transformer-bundler-plugins/vite', () => ({
+  default: () => ({ name: 'code-transformer' }),
+}));
+vi.mock('@apm-js-collab/code-transformer-bundler-plugins/webpack', () => ({
+  default: () => ({ apply: vi.fn() }),
+}));
+
+describe('sentryOrchestrionPlugin (rollup)', () => {
+  // Mirrors what Rollup passes to buildStart: `external` is already normalized
+  // into a predicate function, regardless of how the user configured it.
+  function runBuildStart(external: (source: string) => boolean): ReturnType<typeof vi.fn> {
+    const warn = vi.fn();
+    const plugin = rollupPlugin();
+    (plugin.buildStart as (this: unknown, options: unknown) => void).call(
+      { warn } as unknown as PluginContext,
+      { external } as unknown as NormalizedInputOptions,
+    );
+    return warn;
+  }
+
+  it('warns when instrumented modules are externalized', () => {
+    const warn = runBuildStart(source => source === 'mysql' || source === 'pg');
+
+    expect(warn).toHaveBeenCalledTimes(1);
+    expect(warn).toHaveBeenCalledWith(expect.stringContaining('mysql, pg'));
+    expect(warn).toHaveBeenCalledWith(expect.stringContaining('need to be bundled'));
+  });
+
+  it('does not warn when no instrumented modules are externalized', () => {
+    const warn = runBuildStart(source => source === 'some-other-package');
+
+    expect(warn).not.toHaveBeenCalled();
+  });
+});
+
+describe('sentryOrchestrionPlugin (esbuild)', () => {
+  function runSetup(external: string[] | undefined): OnStartResult[] {
+    const onStartCallbacks: Array<() => OnStartResult> = [];
+    const build = {
+      initialOptions: { external },
+      onStart: (callback: () => OnStartResult) => onStartCallbacks.push(callback),
+    } as unknown as PluginBuild;
+    void esbuildPlugin().setup(build);
+    return onStartCallbacks.map(callback => callback());
+  }
+
+  it('warns when instrumented modules are externalized', () => {
+    const results = runSetup(['mysql', 'pg/lib/client']);
+
+    expect(results).toHaveLength(1);
+    expect(results[0]?.warnings?.[0]?.text).toContain('mysql, pg');
+  });
+
+  it('matches wildcard external patterns', () => {
+    const results = runSetup(['mysql*']);
+
+    expect(results).toHaveLength(1);
+    expect(results[0]?.warnings?.[0]?.text).toContain('mysql');
+    expect(results[0]?.warnings?.[0]?.text).toContain('mysql2');
+  });
+
+  it('does not warn when no instrumented modules are externalized', () => {
+    expect(runSetup(['lodash'])).toHaveLength(0);
+    expect(runSetup(undefined)).toHaveLength(0);
+  });
+});
+
+describe('sentryOrchestrionWebpackPlugin', () => {
+  function runApply(externals: unknown): Error[] {
+    const compilation = { warnings: [] as Error[] };
+    const compiler = {
+      options: { externals },
+      hooks: {
+        thisCompilation: { tap: (_name: string, callback: (compilation: unknown) => void) => callback(compilation) },
+      },
+      webpack: { WebpackError: Error },
+    } as unknown as Compiler;
+    sentryOrchestrionWebpackPlugin().apply(compiler);
+    return compilation.warnings;
+  }
+
+  it('warns for string, RegExp and object externals', () => {
+    expect(runApply(['mysql'])[0]?.message).toContain('mysql');
+    expect(runApply('mysql')[0]?.message).toContain('mysql');
+    expect(runApply([/^pg$/])[0]?.message).toContain('pg');
+    expect(runApply({ mysql: 'commonjs mysql' })[0]?.message).toContain('mysql');
+  });
+
+  it('does not warn for unrelated or function externals', () => {
+    expect(runApply(['lodash'])).toHaveLength(0);
+    expect(runApply(() => undefined)).toHaveLength(0);
+    expect(runApply(undefined)).toHaveLength(0);
+  });
+});
+
+describe('sentryOrchestrionPlugin (vite)', () => {
+  function runConfigResolved(ssrExternal: string[] | true | undefined): ReturnType<typeof vi.fn> {
+    const warn = vi.fn();
+    const plugin = vitePlugin();
+    (plugin.configResolved as (config: unknown) => void)({
+      ssr: { external: ssrExternal },
+      logger: { warn },
+    } as unknown as ResolvedConfig);
+    return warn;
+  }
+
+  it('warns when instrumented modules are listed in ssr.external', () => {
+    const warn = runConfigResolved(['mysql', 'lodash']);
+
+    expect(warn).toHaveBeenCalledTimes(1);
+    expect(warn).toHaveBeenCalledWith(expect.stringContaining('mysql'));
+  });
+
+  it('does not warn for ssr.external: true or unrelated entries', () => {
+    // `ssr.external: true` does not override the plugin's noExternal entries.
+    expect(runConfigResolved(true)).not.toHaveBeenCalled();
+    expect(runConfigResolved(['lodash'])).not.toHaveBeenCalled();
+    expect(runConfigResolved(undefined)).not.toHaveBeenCalled();
+  });
+});
 
 describe('getTracingHooksDirectory', () => {
   it('returns the tracing-hooks package directory with the runtime hook entry points', () => {