fix(compiler): improve error message when --emit specifies an uninstalled emitter

.chronus/changes/copilot-fix-emitter-not-found-error-message-2026-7-30-16-11-58.md

@@ -0,0 +1,7 @@
+---
+changeKind: fix
+packages:
+  - "@typespec/compiler"
+---
+
+Report better error message when specifying an emitter that is not installed with `--emit` flag

packages/compiler/src/core/messages.ts

@@ -805,6 +805,12 @@ const diagnostics = {
       default: paramMessage`onValidate failed with errors. ${"error"}`,
     },
   },
+  "emitter-not-found": {
+    severity: "error",
+    messages: {
+      default: paramMessage`Emitter "${"emitterPackage"}" not found. Make sure to install it with \`npm install ${"emitterPackage"}\`.`,
+    },
+  },
   "invalid-emitter": {
     severity: "error",
     messages: {

packages/compiler/src/core/program.ts

@@ -582,14 +582,25 @@ async function createProgram(
   async function loadLibrary(
     basedir: string,
     libraryNameOrPath: string,
+    context?: { isEmitter?: boolean },
   ): Promise<LibraryInstance | undefined> {
     const [resolution, diagnostics] = await resolveEmitterModuleAndEntrypoint(
       basedir,
       libraryNameOrPath,
     );
 
     if (resolution === undefined) {
-      program.reportDiagnostics(diagnostics);
+      if (context?.isEmitter && diagnostics.some((d) => d.code === "import-not-found")) {
+        program.reportDiagnostic(
+          createDiagnostic({
+            code: "emitter-not-found",
+            format: { emitterPackage: libraryNameOrPath },
+            target: NoTarget,
+          }),
+        );
+      } else {
+        program.reportDiagnostics(diagnostics);
+      }
       return undefined;
     }
     const { module, entrypoint } = resolution;
@@ -610,7 +621,7 @@ async function createProgram(
     emitterNameOrPath: string,
     emittersOptions: Record<string, EmitterOptions>,
   ): Promise<EmitterRef | undefined> {
-    const library = await loadLibrary(basedir, emitterNameOrPath);
+    const library = await loadLibrary(basedir, emitterNameOrPath, { isEmitter: true });
 
     if (library === undefined) {
       return undefined;

packages/compiler/test/core/emitter.test.ts

@@ -6,7 +6,7 @@ import {
   TypeSpecLibrary,
   TypeSpecLibraryDef,
 } from "../../src/index.js";
-import { expectDiagnosticEmpty } from "../../src/testing/expect.js";
+import { expectDiagnosticEmpty, expectDiagnostics } from "../../src/testing/expect.js";
 import { mockFile } from "../../src/testing/fs.js";
 import { Tester } from "../tester.js";
 
@@ -109,3 +109,16 @@ it("when using dry-run only call emitter with the capabilities", async () => {
   expect(emitter2.$onEmit).not.toHaveBeenCalled();
   expect(emitter3.$onEmit).not.toHaveBeenCalled();
 });
+
+it("reports emitter-not-found when emitter package is not installed", async () => {
+  const diagnostics = await Tester.diagnose("model Foo {}", {
+    compilerOptions: {
+      emit: ["not-installed-emitter"],
+    },
+  });
+
+  expectDiagnostics(diagnostics, {
+    code: "emitter-not-found",
+    message: `Emitter "not-installed-emitter" not found. Make sure to install it with \`npm install not-installed-emitter\`.`,
+  });
+});