Add linter warning for empty clients

.chronus/changes/copilot-add-linter-rules-for-empty-clients-2026-0-13-20-44-50.md

@@ -0,0 +1,7 @@
+---
+changeKind: feature
+packages:
+  - "@azure-tools/typespec-client-generator-core"
+---
+
+Add linter warning for empty clients (clients with no operations)

packages/typespec-client-generator-core/src/clients.ts

@@ -33,7 +33,7 @@ import {
   isSubscriptionId,
   updateWithApiVersionInformation,
 } from "./internal-utils.js";
-import { createDiagnostic } from "./lib.js";
+import { createDiagnostic, reportDiagnostic } from "./lib.js";
 import { createSdkMethods, getSdkMethodParameter } from "./methods.js";
 import { getCrossLanguageDefinitionId } from "./public-utils.js";
 import { getSdkBuiltInType, getSdkCredentialParameter, getTypeSpecBuiltInType } from "./types.js";
@@ -217,6 +217,17 @@ export function createSdkClientType<TServiceOperation extends SdkServiceOperatio
   sdkClientType.methods = diagnostics.pipe(
     createSdkMethods<TServiceOperation>(context, client, sdkClientType),
   );
+  // Check if the client is empty (has no methods and no children)
+  // Only emit diagnostic if client.type is defined (client has a source TypeSpec type to attach the diagnostic to)
+  if (sdkClientType.methods.length === 0 && !sdkClientType.children?.length && client.type) {
+    reportDiagnostic(context.program, {
+      code: "empty-client",
+      target: client.type,
+      format: {
+        name: sdkClientType.name,
+      },
+    });
+  }
   addDefaultClientParameters(context, sdkClientType);
   // update initialization model properties
 

packages/typespec-client-generator-core/src/lib.ts

@@ -141,6 +141,12 @@ export const $lib = createTypeSpecLibrary({
         default: paramMessage`Client "${"name"}" is not inside a service namespace. Use @client({service: MyServiceNS})`,
       },
     },
+    "empty-client": {
+      severity: "warning",
+      messages: {
+        default: paramMessage`Client "${"name"}" has no operations. Clients should contain at least one operation.`,
+      },
+    },
     "union-null": {
       severity: "warning",
       messages: {

packages/typespec-client-generator-core/test/decorators/client.test.ts

@@ -78,7 +78,9 @@ describe("@client", () => {
         `
         @client
         @service
-        @test namespace MyService;
+        @test namespace MyService {
+          op test(): void;
+        }
       `,
       )
       .toEmitDiagnostics([
@@ -96,7 +98,9 @@ describe("@client", () => {
         `
         @client({name: "MySDK"})
         @service
-        @test namespace MyService;
+        @test namespace MyService {
+          op test(): void;
+        }
       `,
       )
       .toEmitDiagnostics([
@@ -474,18 +478,22 @@ describe("@operationGroup", () => {
         @service(#{
           title: "DeviceUpdateClient",
         })
-        namespace Azure.IoT.DeviceUpdate;
+        namespace Azure.IoT.DeviceUpdate {
+          op test(): void;
+        }
       `,
       `
         @client({name: "DeviceUpdateClient", service: Azure.IoT.DeviceUpdate}, "python")
         namespace Customizations;
 
         @operationGroup("java")
         interface SubClientOnlyForJava {
+          op javaOp(): void;
         }
 
         @operationGroup("python")
         interface SubClientOnlyForPython {
+          op pythonOp(): void;
         }
       `,
     ];
@@ -514,16 +522,17 @@ describe("@operationGroup", () => {
       strictEqual(listOperationGroups(runner.context, client).length, 1);
     }
 
-    // csharp should have no client
+    // csharp should have one client (default from service namespace since no explicit @client for csharp)
     {
       const runner = await createSdkTestRunner({ emitterName: "@azure-tools/typespec-csharp" });
       const [_, diagnostics] = await runner.compileAndDiagnoseWithCustomization(
         testCode[0],
         testCode[1],
       );
       expectDiagnosticEmpty(diagnostics);
-      const client = listClients(runner.context);
-      strictEqual(client.length, 0);
+      const clients = listClients(runner.context);
+      strictEqual(clients.length, 1);
+      strictEqual(listOperationGroups(runner.context, clients[0]).length, 0);
     }
   });
 });
@@ -1461,3 +1470,153 @@ it("operations under namespace or interface without @client or @operationGroup",
   const operationGroup = operationGroups[0];
   strictEqual(listOperationsInOperationGroup(runner.context, operationGroup).length, 1);
 });
+
+describe("empty client diagnostic", () => {
+  it("should emit diagnostic for empty namespace client", async () => {
+    const diagnostics = await runner.diagnose(`
+      @client
+      @service
+      namespace MyService {
+      }
+    `);
+
+    expectDiagnostics(diagnostics, {
+      code: "@azure-tools/typespec-client-generator-core/empty-client",
+    });
+  });
+
+  it("should not emit diagnostic for client with operations", async () => {
+    const diagnostics = await runner.diagnose(`
+      @service
+      namespace MyService {
+        op test(): void;
+      }
+    `);
+
+    expectDiagnosticEmpty(diagnostics);
+  });
+
+  it("should not emit diagnostic for client with operation groups", async () => {
+    const diagnostics = await runner.diagnose(`
+      @service
+      namespace MyService {
+        @operationGroup
+        interface SubClient {
+          op test(): void;
+        }
+      }
+    `);
+
+    expectDiagnosticEmpty(diagnostics);
+  });
+
+  it("should emit diagnostic for empty interface client", async () => {
+    const diagnostics = await runner.diagnose(`
+      @service
+      namespace MyService {
+        op serviceOp(): void;
+
+        @client({service: MyService})
+        interface MyClient {
+        }
+      }
+    `);
+
+    expectDiagnostics(diagnostics, {
+      code: "@azure-tools/typespec-client-generator-core/empty-client",
+    });
+  });
+
+  it("should not emit diagnostic for interface client with operations", async () => {
+    const diagnostics = await runner.diagnose(`
+      @service
+      namespace MyService {
+        @route("/service") op serviceOp(): void;
+
+        @client({service: MyService})
+        interface MyClient {
+          @route("/test") op test(): void;
+        }
+      }
+    `);
+
+    expectDiagnosticEmpty(diagnostics);
+  });
+
+  it("should not emit diagnostic for target when operation is moved into it via @clientLocation", async () => {
+    // Target was originally empty but receives an operation via @clientLocation
+    // Source still has an operation, so no empty-client diagnostic
+    const diagnostics = await runner.diagnose(`
+      @service
+      namespace MyService {
+        interface Source {
+          @route("/stay")
+          op stayOp(): void;
+
+          @route("/moved")
+          @clientLocation(Target)
+          op movedOp(): void;
+        }
+
+        interface Target {
+        }
+      }
+    `);
+
+    expectDiagnosticEmpty(diagnostics);
+  });
+
+  it("should not emit diagnostic when @clientLocation moves all operations out of a non-explicit client", async () => {
+    // When all operations are moved out of a client that wasn't explicitly defined with @client/@operationGroup,
+    // the client is simply removed (not warned about) since it was auto-generated
+    const diagnostics = await runner.diagnose(`
+      @service
+      namespace MyService {
+        interface Source {
+          @route("/moved")
+          @clientLocation(Target)
+          op movedOp(): void;
+        }
+
+        interface Target {
+          @route("/target")
+          op targetOp(): void;
+        }
+      }
+    `);
+
+    expectDiagnosticEmpty(diagnostics);
+  });
+
+  it("should not emit diagnostic when @clientLocation moves all operations to root client", async () => {
+    // Source interface becomes empty and is removed since it wasn't explicitly defined
+    const diagnostics = await runner.diagnose(`
+      @service
+      namespace MyService {
+        interface Source {
+          @route("/moved")
+          @clientLocation(MyService)
+          op movedOp(): void;
+        }
+      }
+    `);
+
+    expectDiagnosticEmpty(diagnostics);
+  });
+
+  it("should not emit diagnostic when @clientLocation moves all operations to new operation group", async () => {
+    // Source interface becomes empty and is removed since it wasn't explicitly defined
+    const diagnostics = await runner.diagnose(`
+      @service
+      namespace MyService {
+        interface Source {
+          @route("/moved")
+          @clientLocation("NewTarget")
+          op movedOp(): void;
+        }
+      }
+    `);
+
+    expectDiagnosticEmpty(diagnostics);
+  });
+});

packages/typespec-client-generator-core/test/rules/require-client-suffix.test.ts

@@ -25,7 +25,9 @@ it("namespace doesn't end in client", async () => {
       `
       @client
       @service
-      namespace MyService;
+      namespace MyService {
+        op test(): void;
+      }
       `,
     )
     .toEmitDiagnostics([
@@ -43,7 +45,9 @@ it("explicit client name doesn't ends with Client", async () => {
       `
       @client({name: "MySDK"})
       @service
-      namespace MyService;
+      namespace MyService {
+        op test(): void;
+      }
       `,
     )
     .toEmitDiagnostics([
@@ -60,11 +64,14 @@ it("interface", async () => {
     .expect(
       `
       @service
-      namespace MyService;
+      namespace MyService {
+        op serviceOp(): void;
+      }
 
       namespace MyCustomizations {
         @client({service: MyService})
         interface MyInterface {
+          op test(): void;
         };
       }
       `,

packages/typespec-client-generator-core/test/validations/types.test.ts

@@ -13,7 +13,7 @@ it("no duplicate operation with @clientLocation", async () => {
     `
     @service
     namespace StorageService;
-      
+
     interface StorageTasks {
       @clientLocation("StorageTasksReport")
       @route("/list")
@@ -27,6 +27,8 @@ it("no duplicate operation with @clientLocation", async () => {
     `,
   );
 
+  // StorageTasks becomes empty because all operations are moved out via @clientLocation
+  // Since it wasn't explicitly defined with @client/@operationGroup, it's simply removed (no diagnostic)
   expectDiagnosticEmpty(diagnostics);
 });
 
@@ -128,7 +130,7 @@ it("duplicate operation with @clientLocation to new clients", async () => {
     `
     @service
     namespace Contoso.WidgetManager;
-      
+
     interface A {
       @clientLocation("B")
       @route("/a")
@@ -142,6 +144,7 @@ it("duplicate operation with @clientLocation to new clients", async () => {
     `,
   );
 
+  // A becomes empty and is removed since it wasn't explicitly defined with @client/@operationGroup
   expectDiagnostics(diagnostics, [
     {
       code: "@azure-tools/typespec-client-generator-core/duplicate-client-name",