Fix @actionSeparator decorator to accept Operation, Interface, and Namespace targets with hierarchy support

.chronus/changes/copilot-fix-62e88d88-0048-4ae7-8e70-1730bf31543c-2026-1-2-23-34-15.md

@@ -0,0 +1,7 @@
+---
+changeKind: fix
+packages:
+  - "@typespec/rest"
+---
+
+Fix @actionSeparator decorator to only accept Operation, Interface, and Namespace targets

packages/rest/README.md

@@ -51,13 +51,17 @@ Specify this operation is an action. (Scoped to a resource item /pets/{petId}/my
 
 Defines the separator string that is inserted before the action name in auto-generated routes for actions.
 
+When applied to a namespace, the separator applies to all action operations in that namespace and its sub-namespaces.
+When applied to an interface, the separator applies to all action operations in that interface and overrides any namespace-level separator.
+When applied to an operation, the separator applies only to that operation and overrides any interface or namespace-level separator.
+
 ```typespec
 @TypeSpec.Rest.actionSeparator(seperator: valueof "/" | ":" | "/:")
 ```
 
 ##### Target
 
-`Model | ModelProperty | Operation`
+`Operation | Interface | Namespace`
 
 ##### Parameters
 

packages/rest/generated-defs/TypeSpec.Rest.ts

@@ -4,6 +4,7 @@ import type {
   Interface,
   Model,
   ModelProperty,
+  Namespace,
   Operation,
 } from "@typespec/compiler";
 
@@ -58,11 +59,15 @@ export type SegmentOfDecorator = (
 /**
  * Defines the separator string that is inserted before the action name in auto-generated routes for actions.
  *
+ * When applied to a namespace, the separator applies to all action operations in that namespace and its sub-namespaces.
+ * When applied to an interface, the separator applies to all action operations in that interface and overrides any namespace-level separator.
+ * When applied to an operation, the separator applies only to that operation and overrides any interface or namespace-level separator.
+ *
  * @param seperator Seperator seperating the action segment from the rest of the url
  */
 export type ActionSeparatorDecorator = (
   context: DecoratorContext,
-  target: Model | ModelProperty | Operation,
+  target: Operation | Interface | Namespace,
   seperator: "/" | ":" | "/:",
 ) => DecoratorValidatorCallbacks | void;
 

packages/rest/lib/rest-decorators.tsp

@@ -38,10 +38,14 @@ extern dec segmentOf(target: Operation, type: Model);
 /**
  * Defines the separator string that is inserted before the action name in auto-generated routes for actions.
  *
+ * When applied to a namespace, the separator applies to all action operations in that namespace and its sub-namespaces.
+ * When applied to an interface, the separator applies to all action operations in that interface and overrides any namespace-level separator.
+ * When applied to an operation, the separator applies only to that operation and overrides any interface or namespace-level separator.
+ *
  * @param seperator Seperator seperating the action segment from the rest of the url
  */
 extern dec actionSeparator(
-  target: Model | ModelProperty | Operation,
+  target: Operation | Interface | Namespace,
   seperator: valueof "/" | ":" | "/:"
 );
 

packages/rest/src/rest.ts

@@ -5,6 +5,7 @@ import {
   Interface,
   Model,
   ModelProperty,
+  Namespace,
   Operation,
   Program,
   Scalar,
@@ -273,11 +274,11 @@ const actionSeparatorKey = createStateSymbol("actionSeparator");
  * `@actionSeparator` defines the separator string that is used to precede the action name
  *  in auto-generated actions.
  *
- * `@actionSeparator` can only be applied to model properties, operation parameters, or operations.
+ * `@actionSeparator` can only be applied to operations, interfaces, or namespaces.
  */
 export function $actionSeparator(
   context: DecoratorContext,
-  entity: Model | ModelProperty | Operation,
+  entity: Operation | Interface | Namespace,
   separator: "/" | ":" | "/:",
 ) {
   context.program.stateMap(actionSeparatorKey).set(entity, separator);
@@ -286,10 +287,64 @@ export function $actionSeparator(
 /**
  * @param program the TypeSpec program
  * @param entity the target entity
- * @returns the action separator string
+ * @returns the action separator string, checking the hierarchy: operation -> interface -> namespace
  */
 export function getActionSeparator(program: Program, entity: Type): string | undefined {
-  return program.stateMap(actionSeparatorKey).get(entity);
+  const stateMap = program.stateMap(actionSeparatorKey);
+
+  // First, check if the entity itself has an action separator
+  const directSeparator = stateMap.get(entity);
+  if (directSeparator !== undefined) {
+    return directSeparator;
+  }
+
+  // If entity is an operation, check its interface, then namespace
+  if (entity.kind === "Operation") {
+    // Check the interface
+    if (entity.interface) {
+      const interfaceSeparator = stateMap.get(entity.interface);
+      if (interfaceSeparator !== undefined) {
+        return interfaceSeparator;
+      }
+
+      // Check the namespace of the interface
+      if (entity.interface.namespace) {
+        return getNamespaceActionSeparator(program, entity.interface.namespace);
+      }
+    }
+
+    // Check the namespace directly if no interface
+    if (entity.namespace) {
+      return getNamespaceActionSeparator(program, entity.namespace);
+    }
+  }
+
+  // If entity is an interface, check its namespace
+  if (entity.kind === "Interface" && entity.namespace) {
+    return getNamespaceActionSeparator(program, entity.namespace);
+  }
+
+  return undefined;
+}
+
+/**
+ * Helper function to recursively check namespace hierarchy for action separator
+ */
+function getNamespaceActionSeparator(program: Program, namespace: Namespace): string | undefined {
+  const stateMap = program.stateMap(actionSeparatorKey);
+
+  // Check current namespace
+  const separator = stateMap.get(namespace);
+  if (separator !== undefined) {
+    return separator;
+  }
+
+  // Check parent namespace recursively
+  if (namespace.namespace) {
+    return getNamespaceActionSeparator(program, namespace.namespace);
+  }
+
+  return undefined;
 }
 
 /**

packages/rest/test/action-separator.test.ts

@@ -0,0 +1,232 @@
+import { expectDiagnostics } from "@typespec/compiler/testing";
+import { strictEqual } from "assert";
+import { describe, it } from "vitest";
+import { Tester, getRoutesFor } from "./test-host.js";
+
+describe("rest: @actionSeparator decorator", () => {
+  describe("valid targets", () => {
+    it("works on Operation and affects routing", async () => {
+      const routes = await getRoutesFor(`
+        @autoRoute
+        interface Things {
+          @action
+          @TypeSpec.Rest.actionSeparator(":")
+          @put op customAction(@segment("things") @path thingId: string): string;
+        }
+      `);
+
+      strictEqual(routes.length, 1);
+      strictEqual(routes[0].path, "/things/{thingId}:customAction");
+    });
+
+    it("accepts Interface as target without compilation errors", async () => {
+      // This test verifies that @actionSeparator can be applied to interfaces without errors
+      const diagnostics = await Tester.diagnose(`
+        @TypeSpec.Rest.actionSeparator(":")
+        interface TestInterface {
+          op test(): void;
+        }
+      `);
+
+      // No diagnostics means the decorator accepts interfaces as valid targets
+      strictEqual(diagnostics.length, 0);
+    });
+
+    it("accepts Namespace as target without compilation errors", async () => {
+      // This test verifies that @actionSeparator can be applied to namespaces without errors
+      const diagnostics = await Tester.diagnose(`
+        @TypeSpec.Rest.actionSeparator(":")
+        namespace TestNamespace {
+          op test(): void;
+        }
+      `);
+
+      // No diagnostics means the decorator accepts namespaces as valid targets
+      strictEqual(diagnostics.length, 0);
+    });
+
+    it("supports all separator values in routing", async () => {
+      const routes = await getRoutesFor(`
+        @autoRoute
+        interface Things {
+          @action
+          @TypeSpec.Rest.actionSeparator("/")
+          @put op action1(@segment("things") @path thingId: string): string;
+
+          @action
+          @TypeSpec.Rest.actionSeparator(":")
+          @put op action2(@segment("things") @path thingId: string): string;
+
+          @action
+          @TypeSpec.Rest.actionSeparator("/:")
+          @put op action3(@segment("things") @path thingId: string): string;
+        }
+      `);
+
+      strictEqual(routes.length, 3);
+      strictEqual(routes[0].path, "/things/{thingId}/action1");
+      strictEqual(routes[1].path, "/things/{thingId}:action2");
+      strictEqual(routes[2].path, "/things/{thingId}/:action3");
+    });
+  });
+
+  describe("hierarchy behavior", () => {
+    it("interface-level separator applies to all actions in interface", async () => {
+      const routes = await getRoutesFor(`
+        @autoRoute
+        @TypeSpec.Rest.actionSeparator(":")
+        interface Things {
+          @action
+          @put op action1(@segment("things") @path thingId: string): string;
+
+          @action
+          @put op action2(@segment("things") @path thingId: string): string;
+        }
+      `);
+
+      strictEqual(routes.length, 2);
+      strictEqual(routes[0].path, "/things/{thingId}:action1");
+      strictEqual(routes[1].path, "/things/{thingId}:action2");
+    });
+
+    it("namespace-level separator applies to all actions in namespace", async () => {
+      const routes = await getRoutesFor(`
+        @TypeSpec.Rest.actionSeparator(":")
+        namespace TestNs {
+          @autoRoute
+          interface Things {
+            @action
+            @put op action1(@segment("things") @path thingId: string): string;
+          }
+        }
+      `);
+
+      strictEqual(routes.length, 1);
+      strictEqual(routes[0].path, "/things/{thingId}:action1");
+    });
+
+    it("operation-level separator overrides interface-level separator", async () => {
+      const routes = await getRoutesFor(`
+        @autoRoute
+        @TypeSpec.Rest.actionSeparator(":")
+        interface Things {
+          @action
+          @TypeSpec.Rest.actionSeparator("/")
+          @put op action1(@segment("things") @path thingId: string): string;
+
+          @action
+          @put op action2(@segment("things") @path thingId: string): string;
+        }
+      `);
+
+      strictEqual(routes.length, 2);
+      strictEqual(routes[0].path, "/things/{thingId}/action1"); // Uses operation-level "/"
+      strictEqual(routes[1].path, "/things/{thingId}:action2"); // Uses interface-level ":"
+    });
+
+    it("interface-level separator overrides namespace-level separator", async () => {
+      const routes = await getRoutesFor(`
+        @TypeSpec.Rest.actionSeparator("/:")
+        namespace TestNs {
+          @autoRoute
+          @TypeSpec.Rest.actionSeparator(":")
+          interface Things {
+            @action
+            @put op action1(@segment("things") @path thingId: string): string;
+          }
+
+          @autoRoute
+          interface Other {
+            @action
+            @put op action2(@segment("other") @path otherId: string): string;
+          }
+        }
+      `);
+
+      strictEqual(routes.length, 2);
+      strictEqual(routes[0].path, "/things/{thingId}:action1"); // Uses interface-level ":"
+      strictEqual(routes[1].path, "/other/{otherId}/:action2"); // Uses namespace-level "/:"
+    });
+
+    it("namespace separator applies to subnamespaces", async () => {
+      const routes = await getRoutesFor(`
+        @TypeSpec.Rest.actionSeparator(":")
+        namespace Parent {
+          namespace Child {
+            @autoRoute
+            interface Things {
+              @action
+              @put op action1(@segment("things") @path thingId: string): string;
+            }
+          }
+        }
+      `);
+
+      strictEqual(routes.length, 1);
+      strictEqual(routes[0].path, "/things/{thingId}:action1"); // Uses parent namespace-level ":"
+    });
+
+    it("operation in namespace without interface uses namespace separator", async () => {
+      const routes = await getRoutesFor(`
+        @TypeSpec.Rest.actionSeparator(":")
+        namespace TestNs {
+          @autoRoute
+          @action
+          @put op action1(@segment("things") @path thingId: string): string;
+        }
+      `);
+
+      strictEqual(routes.length, 1);
+      strictEqual(routes[0].path, "/things/{thingId}:action1");
+    });
+  });
+
+  describe("invalid targets", () => {
+    it("rejects Model", async () => {
+      const diagnostics = await Tester.diagnose(`
+        @TypeSpec.Rest.actionSeparator(":")
+        model TestModel {
+          id: string;
+        }
+      `);
+
+      expectDiagnostics(diagnostics, [
+        {
+          code: "decorator-wrong-target",
+          message:
+            "Cannot apply @actionSeparator decorator to TestModel since it is not assignable to Operation | Interface | Namespace",
+        },
+      ]);
+    });
+
+    it("rejects ModelProperty", async () => {
+      const diagnostics = await Tester.diagnose(`
+        model TestModel {
+          @TypeSpec.Rest.actionSeparator(":")
+          id: string;
+        }
+      `);
+
+      expectDiagnostics(diagnostics, [
+        {
+          code: "decorator-wrong-target",
+          message:
+            /Cannot apply @actionSeparator decorator to .* since it is not assignable to Operation \| Interface \| Namespace/,
+        },
+      ]);
+    });
+
+    it("rejects invalid separator values", async () => {
+      const diagnostics = await Tester.diagnose(`
+        @TypeSpec.Rest.actionSeparator("invalid")
+        op test(): void;
+      `);
+
+      expectDiagnostics(diagnostics, [
+        {
+          code: "invalid-argument",
+        },
+      ]);
+    });
+  });
+});