diff --git a/packages/compat/src/compat-app-builder.ts b/packages/compat/src/compat-app-builder.ts
index be6ed8f9a..6f3bcca77 100644
--- a/packages/compat/src/compat-app-builder.ts
+++ b/packages/compat/src/compat-app-builder.ts
@@ -277,6 +277,7 @@ export class CompatAppBuilder {
       engines: engines.map((engine, index) => ({
         packageName: engine.package.name,
         root: index === 0 ? this.root : engine.package.root, // first engine is the app, which has been relocated to this.roto
+        fastbootFiles: {},
         activeAddons: [...engine.addons]
           .map(a => ({
             name: a.name,
diff --git a/packages/compat/tests/audit.test.ts b/packages/compat/tests/audit.test.ts
index c5c3030e8..ecebfe5a4 100644
--- a/packages/compat/tests/audit.test.ts
+++ b/packages/compat/tests/audit.test.ts
@@ -43,6 +43,7 @@ describe('audit', function () {
       engines: [
         {
           packageName: 'audit-this-app',
+          fastbootFiles: {},
           activeAddons: [],
           root: app.baseDir,
         },
diff --git a/packages/core/src/module-resolver.ts b/packages/core/src/module-resolver.ts
index 73131fc97..9d62ef551 100644
--- a/packages/core/src/module-resolver.ts
+++ b/packages/core/src/module-resolver.ts
@@ -3,6 +3,7 @@ import {
   emberVirtualPeerDeps,
   extensionsPattern,
   packageName as getPackageName,
+  packageName,
 } from '@embroider/shared-internals';
 import { dirname, resolve } from 'path';
 import { Package, V2Package, explicitRelative, RewrittenPackageCache } from '@embroider/shared-internals';
@@ -65,6 +66,7 @@ export interface Options {
 interface EngineConfig {
   packageName: string;
   activeAddons: { name: string; root: string }[];
+  fastbootFiles: { [appName: string]: { localFilename: string; shadowedFilename: string | undefined } };
   root: string;
 }
 
@@ -156,6 +158,8 @@ export class Resolver {
     request = this.handleFastbootCompat(request);
     request = this.handleGlobalsCompat(request);
     request = this.handleRenaming(request);
+    // we expect the specifier to be app relative at this point - must be after handleRenaming
+    request = this.handleAppFastboot(request);
     request = this.preHandleExternal(request);
 
     // this should probably stay the last step in beforeResolve, because it can
@@ -295,6 +299,46 @@ export class Resolver {
     return owningPackage;
   }
 
+  private handleAppFastboot<R extends ModuleRequest>(request: R): R {
+    let pkg = this.owningPackage(request.fromFile);
+
+    if (!pkg) {
+      return request;
+    }
+
+    if (packageName(request.specifier)) {
+      // not a relative request, and we're assuming all within-engine requests
+      // are relative by this point due to `v1 self-import` which happens
+      // earlier
+      return request;
+    }
+
+    let engineConfig = this.engineConfig(pkg.name);
+    if (engineConfig) {
+      for (let candidate of this.withResolvableExtensions(request.specifier)) {
+        let fastbootFile = engineConfig.fastbootFiles[candidate];
+        if (fastbootFile) {
+          if (fastbootFile.shadowedFilename) {
+            let { names } = describeExports(readFileSync(resolve(pkg.root, fastbootFile.shadowedFilename), 'utf8'), {});
+            return logTransition(
+              'shadowed app fastboot',
+              request,
+              request.virtualize(fastbootSwitch(candidate, resolve(pkg.root, 'package.json'), names))
+            );
+          } else {
+            return logTransition(
+              'unshadowed app fastboot',
+              request,
+              request.alias(fastbootFile.localFilename).rehome(resolve(pkg.root, 'package.json'))
+            );
+          }
+        }
+      }
+    }
+
+    return request;
+  }
+
   private handleFastbootCompat<R extends ModuleRequest>(request: R): R {
     let match = decodeFastbootSwitch(request.fromFile);
     if (!match) {
@@ -315,7 +359,22 @@ export class Resolver {
     let pkg = this.owningPackage(match.filename);
     if (pkg) {
       let rel = explicitRelative(pkg.root, match.filename);
-      let entry = this.getEntryFromMergeMap(rel, pkg.root);
+
+      let engineConfig = this.engineConfig(pkg.name);
+      if (engineConfig) {
+        let fastbootFile = engineConfig.fastbootFiles[rel];
+        if (fastbootFile && fastbootFile.shadowedFilename) {
+          let targetFile: string;
+          if (section === 'app-js') {
+            targetFile = fastbootFile.shadowedFilename;
+          } else {
+            targetFile = fastbootFile.localFilename;
+          }
+          return request.alias(targetFile).rehome(resolve(pkg.root, 'package.json'));
+        }
+      }
+
+      let entry = this.getEntryFromMergeMap(rel, pkg.root)?.entry;
       if (entry?.type === 'both') {
         return request.alias(entry[section].localPath).rehome(resolve(entry[section].packageRoot, 'package.json'));
       }
@@ -951,22 +1010,27 @@ export class Resolver {
     return logTransition('fallbackResolve final exit', request);
   }
 
-  private getEntryFromMergeMap(inEngineSpecifier: string, root: string): MergeEntry | undefined {
+  private getEntryFromMergeMap(
+    inEngineSpecifier: string,
+    root: string
+  ): { entry: MergeEntry; matched: string } | undefined {
     let entry: MergeEntry | undefined;
+    for (let candidate of this.withResolvableExtensions(inEngineSpecifier)) {
+      entry = this.mergeMap.get(root)?.get(candidate);
+      if (entry) {
+        return { entry, matched: candidate };
+      }
+    }
+  }
 
-    if (inEngineSpecifier.match(/\.(hbs|js|hbs\.js)$/)) {
-      entry = this.mergeMap.get(root)?.get(inEngineSpecifier);
+  private *withResolvableExtensions(filename: string): Generator<string, void, void> {
+    if (filename.match(/\.(hbs|js|hbs\.js)$/)) {
+      yield filename;
     } else {
-      // try looking up .hbs .js and .hbs.js in that order for specifiers without extenstions
-      ['.hbs', '.js', '.hbs.js'].forEach(ext => {
-        if (entry) {
-          return;
-        }
-
-        entry = this.mergeMap.get(root)?.get(`${inEngineSpecifier}${ext}`);
-      });
+      for (let ext of ['.hbs', '.js', '.hbs.js']) {
+        yield `${filename}${ext}`;
+      }
     }
-    return entry;
   }
 
   private searchAppTree<R extends ModuleRequest>(
@@ -974,29 +1038,31 @@ export class Resolver {
     engine: EngineConfig,
     inEngineSpecifier: string
   ): R | undefined {
-    let entry = this.getEntryFromMergeMap(inEngineSpecifier, engine.root);
+    let matched = this.getEntryFromMergeMap(inEngineSpecifier, engine.root);
 
-    switch (entry?.type) {
+    switch (matched?.entry.type) {
       case undefined:
         return undefined;
       case 'app-only':
-        return request.alias(entry['app-js'].localPath).rehome(resolve(entry['app-js'].packageRoot, 'package.json'));
+        return request
+          .alias(matched.entry['app-js'].localPath)
+          .rehome(resolve(matched.entry['app-js'].packageRoot, 'package.json'));
       case 'fastboot-only':
         return request
-          .alias(entry['fastboot-js'].localPath)
-          .rehome(resolve(entry['fastboot-js'].packageRoot, 'package.json'));
+          .alias(matched.entry['fastboot-js'].localPath)
+          .rehome(resolve(matched.entry['fastboot-js'].packageRoot, 'package.json'));
       case 'both':
         let foundAppJS = this.nodeResolve(
-          entry['app-js'].localPath,
-          resolve(entry['app-js'].packageRoot, 'package.json')
+          matched.entry['app-js'].localPath,
+          resolve(matched.entry['app-js'].packageRoot, 'package.json')
         );
         if (foundAppJS.type !== 'real') {
           throw new Error(
-            `${entry['app-js'].fromPackageName} declared ${inEngineSpecifier} in packageJSON.ember-addon.app-js, but that module does not exist`
+            `${matched.entry['app-js'].fromPackageName} declared ${inEngineSpecifier} in packageJSON.ember-addon.app-js, but that module does not exist`
           );
         }
         let { names } = describeExports(readFileSync(foundAppJS.filename, 'utf8'), {});
-        return request.virtualize(fastbootSwitch(request.specifier, request.fromFile, names));
+        return request.virtualize(fastbootSwitch(matched.matched, resolve(engine.root, 'package.json'), names));
     }
   }
 
diff --git a/tests/scenarios/compat-resolver-test.ts b/tests/scenarios/compat-resolver-test.ts
index f2390f275..5c18ef346 100644
--- a/tests/scenarios/compat-resolver-test.ts
+++ b/tests/scenarios/compat-resolver-test.ts
@@ -79,6 +79,7 @@ Scenarios.fromProject(() => new Project())
                 packageName: 'my-app',
                 root: app.dir,
                 activeAddons: [],
+                fastbootFiles: {},
               },
             ],
             modulePrefix: 'my-app',
diff --git a/tests/scenarios/core-resolver-test.ts b/tests/scenarios/core-resolver-test.ts
index 96bff154c..9f3646a46 100644
--- a/tests/scenarios/core-resolver-test.ts
+++ b/tests/scenarios/core-resolver-test.ts
@@ -52,6 +52,7 @@ Scenarios.fromProject(() => new Project())
         podModulePrefix?: string;
         renamePackages?: Record<string, string>;
         addonMeta?: Partial<AddonMeta>;
+        fastbootFiles?: { [appName: string]: { localFilename: string; shadowedFilename: string | undefined } };
       }
 
       let configure: (opts?: ConfigureOpts) => Promise<void>;
@@ -92,6 +93,7 @@ Scenarios.fromProject(() => new Project())
               {
                 packageName: 'my-app',
                 root: app.dir,
+                fastbootFiles: opts?.fastbootFiles ?? {},
                 activeAddons: [
                   {
                     name: 'my-addon',
@@ -569,6 +571,21 @@ Scenarios.fromProject(() => new Project())
             .to('./node_modules/my-addon/_fastboot_/hello-world.js');
         });
 
+        test(`resolves app fastboot-js`, async function () {
+          givenFiles({
+            './fastboot/hello-world.js': ``,
+            'app.js': `import "my-app/hello-world"`,
+          });
+
+          await configure({
+            fastbootFiles: {
+              './hello-world.js': { localFilename: './fastboot/hello-world.js', shadowedFilename: undefined },
+            },
+          });
+
+          expectAudit.module('./app.js').resolves('my-app/hello-world').to('./fastboot/hello-world.js');
+        });
+
         test(`file exists in both app-js and fastboot-js`, async function () {
           givenFiles({
             'node_modules/my-addon/_fastboot_/hello-world.js': `
@@ -616,6 +633,56 @@ Scenarios.fromProject(() => new Project())
           switcherModule.resolves('./fastboot').to('./node_modules/my-addon/_fastboot_/hello-world.js');
           switcherModule.resolves('./browser').to('./node_modules/my-addon/_app_/hello-world.js');
         });
+
+        test(`app and fastboot file exists`, async function () {
+          givenFiles({
+            'fastboot/hello-world.js': `
+              export function hello() { return 'fastboot'; }
+              export class Bonjour {}
+              export default function() {}
+              const value = 1;
+              export { value };
+              export const x = 2;
+            `,
+            'app/hello-world.js': `
+              export function hello() { return 'browser'; }
+              export class Bonjour {}
+              export default function() {}
+              const value = 1;
+              export { value };
+              export const x = 2;
+          `,
+            'app.js': `import "my-app/hello-world"`,
+          });
+
+          await configure({
+            fastbootFiles: {
+              './hello-world.js': {
+                localFilename: './fastboot/hello-world.js',
+                shadowedFilename: './app/hello-world.js',
+              },
+            },
+          });
+
+          let switcherModule = expectAudit.module('./app.js').resolves('my-app/hello-world').toModule();
+          switcherModule.codeEquals(`
+            import { macroCondition, getGlobalConfig, importSync } from '@embroider/macros';
+            let mod;
+            if (macroCondition(getGlobalConfig().fastboot?.isRunning)) {
+              mod = importSync("./fastboot");
+            } else {
+              mod = importSync("./browser");
+            }
+            export default mod.default;
+            export const hello = mod.hello;
+            export const Bonjour = mod.Bonjour;
+            export const value = mod.value;
+            export const x = mod.x;
+          `);
+
+          switcherModule.resolves('./fastboot').to('./fastboot/hello-world.js');
+          switcherModule.resolves('./browser').to('./app/hello-world.js');
+        });
       });
 
       Qmodule('legacy-addons', function () {