diff --git a/examples/basic/test.d.ts b/examples/basic/test.d.ts
index 38cb63d..fe7a6e4 100644
--- a/examples/basic/test.d.ts
+++ b/examples/basic/test.d.ts
@@ -4,7 +4,7 @@ declare module "main" {
 
 declare module "xtp:test" {
   interface harness {
-    assert(name: PTR, value: I64);
+    assert(name: PTR, value: I64, reason: PTR);
     call(func: PTR, input: PTR): PTR;
     time(func: PTR, input: PTR): I64;
     group(name: PTR);
diff --git a/examples/basic/test.wasm b/examples/basic/test.wasm
index e2d81b1..2c945ef 100644
Binary files a/examples/basic/test.wasm and b/examples/basic/test.wasm differ
diff --git a/index.ts b/index.ts
index 2181e4c..767f4ce 100644
--- a/index.ts
+++ b/index.ts
@@ -106,18 +106,25 @@ export class Test {
     return Test.call(funcName, input).readBuffer();
   }
 
-  static assert(msg: string, value: boolean) {
+  static assert(name: string, value: boolean, reason: string) {
     // @ts-ignore: Memory
-    const a = Memory.fromString(msg);
-    assert(a.offset, !!value);
+    const a = Memory.fromString(name);
+    // @ts-ignore: Memory
+    const b = Memory.fromString(reason);
+    assert(a.offset, !!value, b.offset);
     a.free();
+    b.free();
   }
 
   static assertEqual(msg: string, x: unknown, y: unknown) {
-    Test.assert(msg, x === y);
+    const stack = new Error().stack;
+    stack.trim();
+    Test.assert(msg, x === y, `Expected ${x} === ${y}\n${stack}`);
   }
 
   static assertNotEqual(msg: string, x: unknown, y: unknown) {
-    Test.assert(msg, x !== y);
+    const stack = new Error().stack;
+    stack.trim();
+    Test.assert(msg, x !== y, `Expected ${x} !== ${y}\n${stack}`);
   }
 }
diff --git a/test.d.ts b/test.d.ts
index 38cb63d..fe7a6e4 100644
--- a/test.d.ts
+++ b/test.d.ts
@@ -4,7 +4,7 @@ declare module "main" {
 
 declare module "xtp:test" {
   interface harness {
-    assert(name: PTR, value: I64);
+    assert(name: PTR, value: I64, reason: PTR);
     call(func: PTR, input: PTR): PTR;
     time(func: PTR, input: PTR): I64;
     group(name: PTR);