@@ -1,5 +1,7 @@
 ---
 "trigger.dev": patch
+"@trigger.dev/build": patch
+"@trigger.dev/core": patch
 ---
 
-Deploys now warn when a package is loaded with `createRequire()` but won't be available in the deployed image. The bundler can't follow `createRequire()` calls, so such a package is neither bundled nor installed, and previously this failed only at runtime with a confusing module-not-found error. The warning points at the exact file and line and suggests the `additionalPackages` build extension. Deploys also now surface the bundler's own warnings for your files (for example `require()` with a non-literal argument) instead of discarding them.
+`deploy` and `dev` now warn when a package is loaded with `createRequire()` but won't be available in the deployed image. The bundler can't follow `createRequire()` calls, so such a package is neither bundled nor installed, and previously this failed only at runtime in production with a confusing module-not-found error (`dev` works because your local `node_modules` exists, which made the failure deploy-only). The warning points at the exact file and line and shows the `additionalPackages` config that fixes it; packages declared via `additionalPackages` don't warn. Deploys also now surface the bundler's own warnings for your files (for example `require()` with a non-literal argument) instead of discarding them. Bundling output is unchanged.
@@ -19,6 +19,13 @@ export type AdditionalPackagesOptions = {
 export function additionalPackages(options: AdditionalPackagesOptions): BuildExtension {
   return {
     name: "additionalPackages",
+    installedPackagesForTarget(target) {
+      if (target === "dev") {
+        return [];
+      }
+
+      return options.packages.map((pkg) => parsePackageName(pkg).name);
+    },
     async onBuildStart(context) {
       if (context.target !== "deploy") {
         return;
 
@@ -7,15 +7,19 @@ import {
   createBuildManifestFromBundle,
   logBuildWarnings,
 } from "./bundle.js";
-import { CreateRequireCollector, createRequireUsageToWarning } from "./createRequireWarnings.js";
+import {
+  CreateRequireCollector,
+  createRequireUsageToWarning,
+  unavailableCreateRequireUsages,
+} from "./createRequireWarnings.js";
 import { bundleSkills } from "./bundleSkills.js";
 import {
   createBuildContext,
   notifyExtensionOnBuildComplete,
   notifyExtensionOnBuildStart,
   resolvePluginsForContext,
 } from "./extensions.js";
-import { createExternalsBuildExtension } from "./externals.js";
+import { createExternalsBuildExtension, deployExternalMatchers } from "./externals.js";
 import { tmpdir } from "node:os";
 import { mkdtemp, rm } from "node:fs/promises";
 import { join, relative, sep } from "node:path";
@@ -142,7 +146,9 @@ export async function buildWorker(options: BuildWorkerOptions) {
     const buildWarnings = collectDeployBuildWarnings(
       bundleResult,
       createRequireCollector,
-      buildManifest
+      buildManifest,
+      resolvedConfig,
+      options.forcedExternals
     );
 
     if (buildWarnings.length > 0) {
@@ -167,12 +173,15 @@ export async function buildWorker(options: BuildWorkerOptions) {
 /**
  * Deploy-only diagnostics: esbuild's own warnings scoped to the user's files,
  * plus packages loaded via createRequire() that end up neither bundled nor
- * installed in the image (i.e. not in the manifest's externals).
+ * installed in the image (not in the manifest's externals and not configured
+ * as an external anywhere).
  */
 function collectDeployBuildWarnings(
   bundleResult: BundleResult,
   createRequireCollector: CreateRequireCollector,
-  buildManifest: BuildManifest
+  buildManifest: BuildManifest,
+  resolvedConfig: ResolvedConfig,
+  forcedExternals: string[] = []
 ): esbuild.PartialMessage[] {
   const esbuildWarnings = bundleResult.warnings.filter(
     (warning) => warning.location?.file && !warning.location.file.includes("node_modules")
@@ -182,9 +191,11 @@ function collectDeployBuildWarnings(
     (buildManifest.externals ?? []).map((external) => external.name)
   );
 
-  const createRequireWarnings = createRequireCollector.usages
-    .filter((usage) => !installedPackages.has(usage.packageName))
-    .map(createRequireUsageToWarning);
+  const createRequireWarnings = unavailableCreateRequireUsages(
+    createRequireCollector.usages,
+    installedPackages,
+    deployExternalMatchers(resolvedConfig, forcedExternals)
+  ).map((usage) => createRequireUsageToWarning(usage, "deploy"));
 
   return [...esbuildWarnings, ...createRequireWarnings];
 }
 
@@ -8,6 +8,7 @@ import {
   createRequireUsageToWarning,
   packageNameForSpecifier,
   scanSourceForCreateRequire,
+  unavailableCreateRequireUsages,
 } from "./createRequireWarnings.js";
 
 describe("scanSourceForCreateRequire", () => {
@@ -225,15 +226,17 @@ export const mssql = createRequire(import.meta.url)("mssql");
 });
 
 describe("createRequireUsageToWarning", () => {
+  const usage = {
+    specifier: "mssql/lib/tedious",
+    packageName: "mssql",
+    file: "src/db.ts",
+    line: 12,
+    column: 20,
+    lineText: `const mssql = createRequire(import.meta.url)("mssql/lib/tedious");`,
+  };
+
   it("carries the concrete fix in a note", () => {
-    const warning = createRequireUsageToWarning({
-      specifier: "mssql/lib/tedious",
-      packageName: "mssql",
-      file: "src/db.ts",
-      line: 12,
-      column: 20,
-      lineText: `const mssql = createRequire(import.meta.url)("mssql/lib/tedious");`,
-    });
+    const warning = createRequireUsageToWarning(usage, "deploy");
 
     expect(warning.location).toMatchObject({ file: "src/db.ts", line: 12, column: 20 });
 
@@ -242,6 +245,48 @@ describe("createRequireUsageToWarning", () => {
     expect(note).toContain(`additionalPackages({ packages: ["mssql"] })`);
     expect(note).toContain("https://trigger.dev/docs/config/extensions/additionalPackages");
   });
+
+  it("explains that the failure is deploy-only when building for dev", () => {
+    const warning = createRequireUsageToWarning(usage, "dev");
+
+    expect(warning.text).toContain("works locally");
+    expect(warning.text).toContain("deploys of this code will fail at runtime");
+  });
+});
+
+describe("unavailableCreateRequireUsages", () => {
+  const usageFor = (specifier: string, packageName: string) => ({
+    specifier,
+    packageName,
+    file: "src/db.ts",
+    line: 1,
+    column: 0,
+    lineText: "",
+  });
+
+  it("keeps usages that are neither installed nor configured as external", () => {
+    const usages = [usageFor("mssql", "mssql")];
+
+    expect(unavailableCreateRequireUsages(usages, new Set(), [])).toHaveLength(1);
+  });
+
+  it("drops usages whose package is in the resolved externals", () => {
+    const usages = [usageFor("sharp", "sharp"), usageFor("mssql", "mssql")];
+
+    const result = unavailableCreateRequireUsages(usages, new Set(["sharp"]), []);
+
+    expect(result.map((u) => u.packageName)).toEqual(["mssql"]);
+  });
+
+  it("drops usages matching a configured external pattern", () => {
+    const usages = [usageFor("mssql/lib/tedious", "mssql"), usageFor("pg", "pg")];
+
+    const result = unavailableCreateRequireUsages(usages, new Set(), [
+      new RegExp(`^mssql(?:/[^'"]*)?$`),
+    ]);
+
+    expect(result.map((u) => u.packageName)).toEqual(["pg"]);
+  });
 });
 
 describe("packageNameForSpecifier", () => {
 
@@ -1,3 +1,4 @@
+import { BuildTarget } from "@trigger.dev/core/v3/schemas";
 import * as esbuild from "esbuild";
 import { readFile } from "node:fs/promises";
 import { builtinModules } from "node:module";
@@ -243,10 +244,37 @@ export class CreateRequireCollector {
   }
 }
 
-export function createRequireUsageToWarning(usage: CreateRequireUsage): esbuild.PartialMessage {
+/**
+ * Filters collected usages down to the ones that will actually be missing at
+ * runtime in the deployed image: not in the resolved externals (installed
+ * dependencies) and not matching any configured external.
+ */
+export function unavailableCreateRequireUsages(
+  usages: ReadonlyArray<CreateRequireUsage>,
+  installedPackages: Set<string>,
+  externalMatchers: RegExp[]
+): CreateRequireUsage[] {
+  return usages.filter(
+    (usage) =>
+      !installedPackages.has(usage.packageName) &&
+      !externalMatchers.some(
+        (matcher) => matcher.test(usage.packageName) || matcher.test(usage.specifier)
+      )
+  );
+}
+
+export function createRequireUsageToWarning(
+  usage: CreateRequireUsage,
+  target: BuildTarget
+): esbuild.PartialMessage {
+  const text =
+    target === "dev"
+      ? `"${usage.specifier}" is loaded with createRequire(). This works locally because your project's node_modules exists, but the package won't be available in the deployed image, so deploys of this code will fail at runtime. The bundler can't follow createRequire() calls, so "${usage.packageName}" is neither bundled into your code nor installed in the image.`
+      : `"${usage.specifier}" is loaded with createRequire() but won't be available in the deployed image, so loading it will fail at runtime. The bundler can't follow createRequire() calls, so "${usage.packageName}" is neither bundled into your code nor installed in the image.`;
+
   return {
     pluginName: "create-require-collector",
-    text: `"${usage.specifier}" is loaded with createRequire() but won't be available in the deployed image, so loading it will fail at runtime. The bundler can't follow createRequire() calls, so "${usage.packageName}" is neither bundled into your code nor installed in the image.`,
+    text,
     location: {
       file: usage.file,
       line: usage.line,
@@ -266,7 +294,7 @@ export function createRequireUsageToWarning(usage: CreateRequireUsage): esbuild.
     },
   });
 
-Alternatively, import the package statically so it gets bundled. Docs: https://trigger.dev/docs/config/extensions/additionalPackages`,
+Alternatively, replace the createRequire() call with a static import so the package is bundled. Docs: https://trigger.dev/docs/config/extensions/additionalPackages`,
       },
     ],
   };
 
@@ -414,6 +414,34 @@ function createExternalsCollector(
   };
 }
 
+/**
+ * Matchers for every package that will be available at runtime in the deployed
+ * image: configured externals (build.external, instrumented packages,
+ * extension-declared externals, server-forced externals) plus packages that
+ * extensions install into the image (e.g. additionalPackages). Diagnostics
+ * only, regardless of the target currently being built.
+ */
+export function deployExternalMatchers(
+  config: ResolvedConfig,
+  forcedExternal: string[] = []
+): RegExp[] {
+  const matchers = discoverMaybeExternals("deploy", config, forcedExternal).map(
+    (external) => external.filter
+  );
+
+  for (const buildExtension of config.build?.extensions ?? []) {
+    for (const packageName of buildExtension.installedPackagesForTarget?.("deploy") ?? []) {
+      const filter = makeExternalRegexp(packageName);
+
+      if (filter) {
+        matchers.push(filter);
+      }
+    }
+  }
+
+  return matchers;
+}
+
 type MaybeExternal = { raw: string; filter: RegExp };
 
 function discoverMaybeExternals(
 
@@ -14,6 +14,13 @@ export function esbuildPlugin(plugin: Plugin, options: RegisterPluginOptions = {
 export interface BuildExtension {
   name: string;
   externalsForTarget?: (target: BuildTarget) => string[] | undefined;
+  /**
+   * Package names this extension installs into the deployed image for the
+   * given target. Diagnostics only: the bundler ignores this, it just tells
+   * build warnings (e.g. the createRequire scan) the package will be
+   * available at runtime.
+   */
+  installedPackagesForTarget?: (target: BuildTarget) => string[] | undefined;
   onBuildStart?: (context: BuildContext) => Promise<void> | void;
   onBuildComplete?: (
     context: BuildContext,