diff --git a/changelog.md b/changelog.md
index 12062be..7f5b2f6 100644
--- a/changelog.md
+++ b/changelog.md
@@ -1,5 +1,12 @@
 # Changelog
 
+## 2025-04-29 - 2.1.1 - fix(Assertion)
+Improve chainability by fixing return types in assertion methods
+
+- Update runCheck method to explicitly return the correct chainable type for both async and sync assertions
+- Ensure customAssertion propagates the chainable Assertion instance
+- Refactor internal promise handling for clarity and consistency
+
 ## 2025-04-28 - 2.1.0 - feat(core)
 Add new matchers and improve negation messaging
 
diff --git a/ts/00_commitinfo_data.ts b/ts/00_commitinfo_data.ts
index cf906af..5e8b7c7 100644
--- a/ts/00_commitinfo_data.ts
+++ b/ts/00_commitinfo_data.ts
@@ -3,6 +3,6 @@
  */
 export const commitinfo = {
   name: '@push.rocks/smartexpect',
-  version: '2.1.0',
+  version: '2.1.1',
   description: 'A testing library to manage expectations in code, offering both synchronous and asynchronous assertion methods.'
 }
diff --git a/ts/smartexpect.classes.assertion.ts b/ts/smartexpect.classes.assertion.ts
index 84590ed..484a68e 100644
--- a/ts/smartexpect.classes.assertion.ts
+++ b/ts/smartexpect.classes.assertion.ts
@@ -203,7 +203,7 @@ export class Assertion<T = unknown> {
     return this;
   }
 
-  private runCheck(checkFunction: () => any) {
+  private runCheck(checkFunction: () => any): Assertion<T> | Promise<Assertion<T>> {
     const runDirectOrNegated = (checkFunction: () => any) => {
       if (!this.notSetting) {
         return checkFunction();
@@ -223,7 +223,7 @@ export class Assertion<T = unknown> {
     };
 
     if (this.executionMode === 'async') {
-      const done = plugins.smartpromise.defer();
+      const done = plugins.smartpromise.defer<Assertion<T>>();
       const isThenable = this.baseReference && typeof (this.baseReference as any).then === 'function';
       if (!isThenable) {
         done.reject(new Error(`Expected a Promise but received: ${this.formatValue(this.baseReference)}`));
@@ -241,41 +241,44 @@ export class Assertion<T = unknown> {
           (res: any) => {
             done.reject(new Error(`Expected Promise to reject but it resolved with ${this.formatValue(res)}`));
           },
-          (err: any) => {
-            this.baseReference = err;
-            try {
-              const ret = runDirectOrNegated(checkFunction);
-              done.resolve(ret);
-            } catch (e: any) {
-              done.reject(e);
+            (err: any) => {
+              this.baseReference = err;
+              try {
+                runDirectOrNegated(checkFunction);
+                done.resolve(this);
+              } catch (e: any) {
+                done.reject(e);
+              }
             }
-          }
         );
       } else {
         (this.baseReference as Promise<any>).then(
-          (res: any) => {
-            this.baseReference = res;
-            try {
-              const ret = runDirectOrNegated(checkFunction);
-              done.resolve(ret);
-            } catch (e: any) {
-              done.reject(e);
-            }
-          },
+            (res: any) => {
+              this.baseReference = res;
+              try {
+                runDirectOrNegated(checkFunction);
+                done.resolve(this);
+              } catch (e: any) {
+                done.reject(e);
+              }
+            },
           (err: any) => {
             done.reject(err);
           }
         );
       }
-      return done.promise;
+      // return a promise resolving to this for chaining
+      return done.promise.then(() => this);
     }
-    return runDirectOrNegated(checkFunction);
+    // sync: run and return this for chaining
+    runDirectOrNegated(checkFunction);
+    return this;
   }
 
   public customAssertion(
     assertionFunction: (value: any) => boolean,
     errorMessage: string | ((value: any) => string)
-  ) {
+  ): Assertion<T> | Promise<Assertion<T>> {
     // Prepare negation message based on the positive error template, if static
     if (typeof errorMessage === 'string') {
       this.negativeMessage = this.computeNegationMessage(errorMessage);
@@ -359,6 +362,7 @@ export class Assertion<T = unknown> {
   public toBeTypeofBoolean() { return this.type.toBeTypeofBoolean(); }
   public toBeTypeOf(typeName: string) { return this.type.toBeTypeOf(typeName); }
   public toBeDefined() { return this.type.toBeDefined(); }
+  
   // Namespaced matcher accessors
   /** String-specific matchers */
   public get string() {