diff --git a/bin/testObservability/cypress/index.js b/bin/testObservability/cypress/index.js
index bb898811..9cae1617 100644
--- a/bin/testObservability/cypress/index.js
+++ b/bin/testObservability/cypress/index.js
@@ -6,6 +6,34 @@ const STEP_KEYWORDS = ['given', 'when', 'then', 'and', 'but', '*'];
 let eventsQueue = [];
 let testRunStarted = false;
 
+/*
+ * Command args (command.attributes.args) and cy.log items are captured raw and can hold
+ * circular Cypress runtime objects (e.g. a config-like object whose `renderOptions.host`
+ * points back to itself). cy.task() JSON-serializes its payload to ship it from the browser
+ * to the Node plugin process, so a circular arg makes Cypress throw
+ * "Converting circular structure to JSON" and aborts the run. Decycle the payload before
+ * handing it to cy.task so o11y instrumentation can never break the customer's tests. [SDK-6016]
+ */
+const getCircularReplacer = () => {
+  const seen = new WeakSet();
+  return (key, value) => {
+    if (typeof value === 'object' && value !== null) {
+      if (seen.has(value)) return '[Circular]';
+      seen.add(value);
+    }
+    return value;
+  };
+};
+
+const sanitizeForTask = (data) => {
+  try {
+    return JSON.parse(JSON.stringify(data, getCircularReplacer()));
+  } catch (e) {
+    /* Never let serialization of o11y data break the user's test run (graceful degradation). */
+    return { serializationError: e && e.message ? e.message : String(e) };
+  }
+};
+
 const browserStackLog = (message) => {
 
   if (!Cypress.env('BROWSERSTACK_LOGS')) return;
@@ -208,7 +236,7 @@ Cypress.on('command:enqueued', (attrs) => {
   if (args.includes('test_observability_log') || args.includes('test_observability_command')) return;
   const message = args.reduce((result, logItem) => {
     if (typeof logItem === 'object') {
-      return [result, JSON.stringify(logItem)].join(' ');
+      return [result, JSON.stringify(logItem, getCircularReplacer())].join(' ');
     }
     return [result, logItem ? logItem.toString() : ''].join(' ');
   }, '');
@@ -309,7 +337,7 @@ beforeEach(() => {
 
   if (eventsQueue.length > 0) {
     eventsQueue.forEach(event => {
-      cy.task(event.task, event.data, event.options);
+      cy.task(event.task, sanitizeForTask(event.data), event.options);
     });
   }
   eventsQueue = [];
@@ -324,7 +352,7 @@ afterEach(function() {
 
   if (eventsQueue.length > 0) {
     eventsQueue.forEach(event => {
-      cy.task(event.task, event.data, event.options);
+      cy.task(event.task, sanitizeForTask(event.data), event.options);
     });
   }