diff --git a/packages/bruno-js/src/runtime/assert-runtime.js b/packages/bruno-js/src/runtime/assert-runtime.js
index ba752bf0d1d..3dd25f818f0 100644
--- a/packages/bruno-js/src/runtime/assert-runtime.js
+++ b/packages/bruno-js/src/runtime/assert-runtime.js
@@ -13,7 +13,12 @@ chai.use(function (chai, utils) {
   // Custom assertion for checking if a variable is JSON
   chai.Assertion.addProperty('json', function () {
     const obj = this._obj;
-    const isJson = typeof obj === 'object' && obj !== null && !Array.isArray(obj) && obj.constructor === Object;
+    // Use Object.prototype.toString instead of constructor check for cross-realm compatibility.
+    // Objects created inside Node's vm.createContext() have a different Object constructor,
+    // so obj.constructor === Object fails for objects passed via res.setBody() from scripts.
+    // Note: toString check is more permissive than constructor check — custom class instances
+    const isJson = typeof obj === 'object' && obj !== null && !Array.isArray(obj)
+      && Object.prototype.toString.call(obj) === '[object Object]';
 
     this.assert(isJson, `expected ${utils.inspect(obj)} to be JSON`, `expected ${utils.inspect(obj)} not to be JSON`);
   });
diff --git a/packages/bruno-js/src/sandbox/quickjs/shims/test.js b/packages/bruno-js/src/sandbox/quickjs/shims/test.js
index 9da224a3962..12c6e5058f0 100644
--- a/packages/bruno-js/src/sandbox/quickjs/shims/test.js
+++ b/packages/bruno-js/src/sandbox/quickjs/shims/test.js
@@ -58,6 +58,27 @@ const addBruShimToContext = (vm, __brunoTestResults) => {
       globalThis.test = Test(__brunoTestResults);
     `
   );
+
+  // Register custom chai assertion for isJson (expect(...).to.be.json)
+  // The bundled chai only exposes { expect, assert } — no Assertion class.
+  // Access the prototype through an expect() instance instead.
+  vm.evalCode(
+    `
+      (function() {
+        var proto = Object.getPrototypeOf(expect(null));
+        Object.defineProperty(proto, 'json', {
+          get: function () {
+            var obj = this._obj;
+            var isJson = typeof obj === 'object' && obj !== null && !Array.isArray(obj) &&
+              Object.prototype.toString.call(obj) === '[object Object]';
+            this.assert(isJson, 'expected #{this} to be JSON', 'expected #{this} not to be JSON');
+            return this;
+          },
+          configurable: true
+        });
+      })();
+    `
+  );
 };
 
 module.exports = addBruShimToContext;
diff --git a/packages/bruno-js/tests/runtime.spec.js b/packages/bruno-js/tests/runtime.spec.js
index 797e305982d..2ecc19f4e52 100644
--- a/packages/bruno-js/tests/runtime.spec.js
+++ b/packages/bruno-js/tests/runtime.spec.js
@@ -1,6 +1,7 @@
 const { describe, it, expect } = require('@jest/globals');
 const TestRuntime = require('../src/runtime/test-runtime');
 const ScriptRuntime = require('../src/runtime/script-runtime');
+const AssertRuntime = require('../src/runtime/assert-runtime');
 const Bru = require('../src/bru');
 const VarsRuntime = require('../src/runtime/vars-runtime');
 
@@ -258,4 +259,87 @@ describe('runtime', () => {
       expect(result.runtimeVariables.title).toBe('{{$randomFirstName}}');
     });
   });
+
+  describe('assert-runtime', () => {
+    const baseRequest = {
+      method: 'GET',
+      url: 'http://localhost:3000/',
+      headers: {},
+      data: undefined
+    };
+
+    const makeResponse = (data) => ({
+      status: 200,
+      statusText: 'OK',
+      data,
+      headers: {}
+    });
+
+    const runAssertions = (assertions, response, runtime = 'nodevm') => {
+      const assertRuntime = new AssertRuntime({ runtime });
+      return assertRuntime.runAssertions(assertions, { ...baseRequest }, response, {}, {}, process.env);
+    };
+
+    describe('isJson', () => {
+      it('should pass for a plain object', () => {
+        const results = runAssertions(
+          [{ name: 'res.body', value: 'isJson', enabled: true }],
+          makeResponse({ id: 1, name: 'test' })
+        );
+        expect(results[0].status).toBe('pass');
+      });
+
+      it('should pass for a nested object', () => {
+        const results = runAssertions(
+          [{ name: 'res.body', value: 'isJson', enabled: true }],
+          makeResponse({ user: { id: 1, tags: ['a', 'b'] } })
+        );
+        expect(results[0].status).toBe('pass');
+      });
+
+      it('should pass for objects from a different realm (e.g. after res.setBody in node-vm)', async () => {
+        const response = makeResponse({ id: 1, name: 'original' });
+
+        // res.setBody() inside node-vm creates a cross-realm object whose
+        // constructor is the VM's Object, not the host's Object
+        const scriptRuntime = new ScriptRuntime({ runtime: 'nodevm' });
+        await scriptRuntime.runResponseScript(
+          `res.setBody({ id: 2, name: 'updated' });`,
+          { ...baseRequest },
+          response,
+          {}, {}, '.', null, process.env
+        );
+
+        const results = runAssertions(
+          [{ name: 'res.body', value: 'isJson', enabled: true }],
+          response
+        );
+        expect(results[0].status).toBe('pass');
+      });
+
+      it('should fail for an array', () => {
+        const results = runAssertions(
+          [{ name: 'res.body', value: 'isJson', enabled: true }],
+          makeResponse([1, 2, 3])
+        );
+        expect(results[0].status).toBe('fail');
+      });
+
+      it('should fail for a string', () => {
+        const results = runAssertions(
+          [{ name: 'res.body', value: 'isJson', enabled: true }],
+          makeResponse('hello')
+        );
+        expect(results[0].status).toBe('fail');
+      });
+
+      it('should fail for null', () => {
+        const results = runAssertions(
+          [{ name: 'res.body', value: 'isJson', enabled: true }],
+          makeResponse(null)
+        );
+        expect(results[0].status).toBe('fail');
+      });
+    });
+  });
 });
diff --git a/packages/bruno-tests/collection/scripting/api/res/setBody/isJson after setBody.bru b/packages/bruno-tests/collection/scripting/api/res/setBody/isJson after setBody.bru
new file mode 100644
index 00000000000..1f858352a2a
--- /dev/null
+++ b/packages/bruno-tests/collection/scripting/api/res/setBody/isJson after setBody.bru	
@@ -0,0 +1,36 @@
+meta {
+  name: isJson after setBody
+  type: http
+  seq: 2
+}
+
+post {
+  url: {{host}}/api/echo/json
+  body: json
+  auth: none
+}
+
+body:json {
+  {
+    "hello": "bruno"
+  }
+}
+
+assert {
+  res.status: eq 200
+  res.body: isJson
+}
+
+script:post-response {
+  res.setBody({ id: 1, name: "updated", nested: { key: "value" } });
+}
+
+tests {
+  test("res.body should be json after setBody with object", function() {
+    const body = res.getBody();
+    expect(body).to.be.json;
+    expect(body.id).to.eql(1);
+    expect(body.name).to.eql("updated");
+    expect(body.nested.key).to.eql("value");
+  });
+}