allow ErrorOpts as first arg for no-param errors

index.d.ts

@@ -64,6 +64,7 @@ export type ErrorConstructor<Def extends ErrorDefinition> = (HasParams<
     }
   : {
       new (): ErrorInstance<Def>;
+      new (opts: ErrorOpts): ErrorInstance<Def>;
       new (message: string): ErrorInstance<Def>;
       new (message: string | undefined, opts: ErrorOpts): ErrorInstance<Def>;
     }) & {

index.js

@@ -143,6 +143,8 @@ export function createErrorClass(definition) {
 
   validateMessage(code, defaultMessage);
 
+  const hasTemplateParams = /\{\w+\}/.test(defaultMessage);
+
   const ErrorKlass = class extends Error {
     code = code;
     status = status;
@@ -155,9 +157,14 @@ export function createErrorClass(definition) {
       let params, cause;
 
       if (typeof messageOrParams === "object" && messageOrParams !== null) {
-        // First arg is params object: new Err(params) or new Err(params, opts)
-        params = messageOrParams;
-        cause = paramsOrOpts?.cause;
+        if (!hasTemplateParams && "cause" in messageOrParams) {
+          // No-param error: first arg is ErrorOpts
+          cause = messageOrParams.cause;
+        } else {
+          // First arg is params object: new Err(params) or new Err(params, opts)
+          params = messageOrParams;
+          cause = paramsOrOpts?.cause;
+        }
       } else if (typeof paramsOrOpts === "object" && paramsOrOpts !== null) {
         if (opts !== undefined) {
           // Three-arg form: new Err("msg", params, opts)

index.test-d.ts

@@ -240,6 +240,33 @@ expectError(
 expectType<"NOT_FOUND">(NotFound.code);
 expectType<"NotFound">(NotFound.name);
 
+// ──────────────────────────────────────────────
+// ErrorOpts as first parameter (no-param errors)
+// ──────────────────────────────────────────────
+
+const SimpleErr = createErrorClass({
+  code: "SIMPLE",
+  message: "Something failed",
+  status: 500,
+});
+
+// ErrorOpts as first arg — should compile for no-param errors
+new SimpleErr({ cause: new Error("root") });
+new SimpleErr({ cause: "string cause" });
+new SimpleErr({ cause: null });
+new SimpleErr({ cause: 42 });
+
+// Empty object also valid (cause is optional in ErrorOpts)
+new SimpleErr({});
+
+// Should NOT compile for parameterized errors (object treated as params, wrong shape)
+const ParamErrOpts = createErrorClass({
+  code: "PARAM_ERR",
+  message: "Missing {field}",
+  status: 400,
+});
+expectError(new ParamErrOpts({ cause: new Error() }));
+
 // ──────────────────────────────────────────────
 // Non-string template parameters
 // ──────────────────────────────────────────────

test/index.test.js

@@ -318,6 +318,57 @@ describe("createErrorClass", () => {
     });
   });
 
+  describe("ErrorOpts as first argument for no-param errors", () => {
+    const Err = createErrorClass({
+      code: "SIMPLE",
+      message: "Something failed",
+      status: 500,
+    });
+
+    it("accepts { cause } as first argument", () => {
+      const cause = new Error("root cause");
+      const err = new Err({ cause });
+      assert.equal(err.message, "Something failed");
+      assert.equal(err.cause, cause);
+    });
+
+    it("accepts { cause: null } preserving falsy cause", () => {
+      const err = new Err({ cause: null });
+      assert.equal(err.message, "Something failed");
+      assert.equal(err.cause, null);
+    });
+
+    it("accepts { cause: undefined } — no cause set", () => {
+      const err = new Err({ cause: undefined });
+      assert.equal(err.message, "Something failed");
+      assert.equal(err.cause, undefined);
+    });
+
+    it("accepts {} — no cause set, uses default message", () => {
+      const err = new Err({});
+      assert.equal(err.message, "Something failed");
+      assert.equal(err.cause, undefined);
+    });
+
+    it("does not affect parameterized errors", () => {
+      const ParamErr = createErrorClass({
+        code: "PARAM_ERR",
+        message: "Missing {field}",
+        status: 400,
+      });
+      // For parameterized errors, first object arg is always params
+      const err = new ParamErr({ field: "name" });
+      assert.equal(err.message, "Missing name");
+      assert.equal(err.cause, undefined);
+    });
+
+    it("cause via opts is non-enumerable", () => {
+      const err = new Err({ cause: new Error("root") });
+      const descriptor = Object.getOwnPropertyDescriptor(err, "cause");
+      assert.equal(descriptor.enumerable, false);
+    });
+  });
+
   describe("non-string template parameters", () => {
     it("interpolates number params", () => {
       const Err = createErrorClass({