feat(smithy-client): add isInstance method for ServiceException class

.changeset/warm-pans-cross.md

@@ -0,0 +1,5 @@
+---
+"@smithy/smithy-client": minor
+---
+
+feat: type check helper method to to know if something is an instance of the ServiceException class

packages/smithy-client/src/exceptions.spec.ts

@@ -34,6 +34,39 @@ it("ExceptionOptionType allows specifying message", () => {
   expect(exception.code).toBe("code");
 });
 
+describe("ServiceException.isInstance", () => {
+  it("should return true for valid ServiceException instances", () => {
+    const error = new ServiceException({
+      name: "Error",
+      $fault: "client",
+      $metadata: {},
+    });
+    expect(ServiceException.isInstance(error)).toBe(true);
+  });
+
+  it("should return true for valid duck-typed objects", () => {
+    const duckTyped = {
+      $fault: "server",
+      $metadata: {},
+    };
+    expect(ServiceException.isInstance(duckTyped)).toBe(true);
+  });
+
+  it("should return false for null or undefined", () => {
+    expect(ServiceException.isInstance(null)).toBe(false);
+    expect(ServiceException.isInstance(undefined)).toBe(false);
+  });
+
+  it("should return false for invalid $fault values", () => {
+    expect(ServiceException.isInstance({ $fault: "invalid", $metadata: {} })).toBe(false);
+  });
+
+  it("should return false for missing properties", () => {
+    expect(ServiceException.isInstance({ $fault: "client" })).toBe(false);
+    expect(ServiceException.isInstance({ $metadata: {} })).toBe(false);
+  });
+});
+
 describe("decorateServiceException", () => {
   const exception = new ServiceException({
     name: "Error",

packages/smithy-client/src/exceptions.ts

@@ -37,6 +37,19 @@ export class ServiceException extends Error implements SmithyException, Metadata
     this.$fault = options.$fault;
     this.$metadata = options.$metadata;
   }
+
+  /**
+   * Checks if a value is an instance of ServiceException (duck typed)
+   */
+  public static isInstance(value: unknown): value is ServiceException {
+    if (!value) return false;
+    const candidate = value as ServiceException;
+    return (
+      Boolean(candidate.$fault) &&
+      Boolean(candidate.$metadata) &&
+      (candidate.$fault === "client" || candidate.$fault === "server")
+    );
+  }
 }
 
 /**