Unmasking the Ghost in the Machine: How a Translation API Error Nearly Broke Our Cypress E2E Tests

The Invisible Enemy of End-to-End Testing: When Your App Works, But Your Tests Don’t

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Unmasking the Ghost in the Machine: How a Translation API Error Nearly Broke Our Cypress E2E Tests

The Invisible Enemy of End-to-End Testing: When Your App Works, But Your Tests Don’t

As a test automation engineer, there’s nothing quite as frustrating as a test suite that suddenly goes rogue. One day, everything’s green; the next, a sea of red. But what if the application itself seems perfectly fine in a real browser? What if the errors point to network issues that simply don’t exist for human users?

This was the perplexing scenario we faced with our critical Enterprise Resource Planning (ERP) system. Our E2E tests, powered by Cypress.io, were designed to give us unwavering confidence in every new deployment. The system supports multiple languages, a crucial feature for our global operations. Then, without warning, our tests started failing. Consistently. Mysteriously.

The Plot Thickens: “It Works on My Machine (Browser)!”

The first sign of trouble was a cascade of assertions failing because UI labels were displaying raw “translation keys” instead of their actual translated values. Imagine seeing “dashboard.title” instead of “Dashboard,” or “purchaseOrder.status” instead of “Purchase Order Status.” This immediately pointed to an internationalization (i18n) problem.

Digging into the Cypress network logs revealed the culprit: our translation API, LoadTranslationFile, was failing. The console was screaming:

Error loading translations: [object ProgressEvent]

Failed to load resource: net::ERR_INCOMPLETE_CHUNKED_ENCODING 200 (OK)

And even more perplexing:

Cypress console: Error: Parse Error: Expected HTTP/

Simultaneously, we noticed a barrage of 404 Not Found errors for seemingly innocuous static assets like SVG icons and PNG images, all loaded from paths containing __/assets/images/.

The immediate reaction from the team was universal bewilderment. “But it works in my browser!” became the mantra. Manual testing, even in the exact same environment (staging, dev), showed the application functioning flawlessly, with all translations loaded and all assets visible. The ghost was only appearing in Cypress.

The Automation Engineer’s Quest: Chasing the Phantom Bug

As the automation engineer, the ball was in my court. My initial thoughts gravitated towards Cypress-specific issues:

1. Language Race Conditions: Our application’s default language is Arabic, but our tests force English. Could there be a race condition where the app tries to load Arabic translations before Cypress injects the English preferences?

○ Attempted Solution: We meticulously set localStorage, cookies (culture, preferredLanguage), and even navigator.language within cy.visit’s onBeforeLoad hook. This ensures the browser environment is configured for English before the application’s JavaScript bundles even begin executing.

2. Flaky Network/Timing: Maybe Cypress’s network handling was too fast or too slow?

○ Attempted Solution: We implemented robust cy.intercept commands to ensure our translation API calls were always directed to English (culture=2). We also refined our cy.ensurePageIsReady command, which waits for network idle and DOM stability, to be even more resilient.

3. Cypress Promise Anti-Patterns: During debugging, Cypress threw a CypressError about returning promises from commands. This was a red herring, but it led to refactoring our command chaining to strictly adhere to Cypress’s queuing mechanism, ensuring cy.log and other commands were properly sequenced. (This was a good cleanup, but not the core fix for the network issue).

Despite these diligent efforts, the ERR_INCOMPLETE_CHUNKED_ENCODING persisted. The application was still failing to load translations, and the static asset 404s remained. It was clear: the problem wasn’t in how Cypress was making the request, but in what Cypress was receiving from the server.

Unmasking the True Culprit: A Server-Side HTTP Quirk

The ERR_INCOMPLETE_CHUNKED_ENCODING error, especially when paired with a 200 OK status, was the key. This isn’t a “resource not found” error; it’s a “resource found, but couldn’t be fully read/parsed” error.

The Technical Breakdown:

● HTTP Chunked Encoding (Transfer-Encoding: chunked): This is a standard way for web servers to send data in a stream, without knowing the total size of the response beforehand. The server sends data in “chunks,” each preceded by its size, and terminates with a zero-length chunk.

● Gzip Compression (Content-Encoding: gzip): This is a common method to compress the response body, making transfers faster.

● The Incompatibility: Modern web browsers are incredibly forgiving and robust. They can often recover from minor discrepancies in chunked encoding or handle slight network interruptions gracefully. However, Cypress’s internal proxy, which is built on Node.js’s HTTP client, tends to be stricter. If the server sends a response that is even slightly malformed (e.g., an incorrect chunk size, a missing chunk terminator, or a premature connection close), Cypress’s proxy will fail to reassemble the response correctly, leading to the Parse Error: Expected HTTP/ and the ERR_INCOMPLETE_CHUNKED_ENCODING.

● The Static Asset 404s: These likely stemmed from a similar, subtle server-side configuration issue. The __/assets path suggested a URL rewrite or alias on the web server (like Nginx or Apache) that wasn’t behaving identically when proxied by Cypress, leading to the assets not being found.

This meant the problem wasn’t with our Cypress tests, nor the frontend application’s request logic. The problem was squarely with how the backend API server or the hosting infrastructure (e.g., Nginx, API Gateway, Load Balancer) was serving these specific responses.

The Power of Collaboration: A Cross-Functional Solution

This complex issue highlighted the absolute necessity of cross-functional collaboration. My role shifted from “fix the tests” to “diagnose and provide clear evidence for the backend/DevOps teams.”

I prepared a detailed report for our team lead, backend developers, and DevOps/SRE team, outlining:

● The exact symptoms and impact on E2E automation.

● Precise Cypress network and console logs.

● A clear explanation of the ERR_INCOMPLETE_CHUNKED_ENCODING and 404s, emphasizing why they pointed to server-side issues.

● Proposed areas for their investigation:

○ Backend API: Reviewing the LoadTranslationFile API’s response generation, specifically its chunked encoding and gzip compression.

○ DevOps/SRE: Examining web server (Nginx/Apache) configurations, load balancer settings, and any URL rewrite rules for static assets and API responses.

The Cypress-Side Workaround: Unblocking Automation with Fixtures

While the backend and DevOps teams investigated the server-side root cause, we couldn’t halt our automation efforts. The solution was to aggressively mock the problematic API calls using Cypress fixtures.

How we did it:

1. Capture Valid Responses: We used a real browser’s developer tools to capture the complete, correctly formed JSON responses from the LoadTranslationFile API for both English and Arabic. These were saved as local JSON files (e.g., cypress/fixtures/translations/stage/en/finance.json).

2. Intercept and Serve Fixtures: We enhanced our interceptTranslationAPI command to dynamically serve these local fixtures based on the modules and culture parameters in the request URL. This completely bypassed the server’s problematic chunked encoding.

// Simplified interceptTranslationAPI logic

Cypress.Commands.add("interceptTranslationAPI", () => {  
 cy.intercept('GET', '*/LoadTranslationFile?modules=\*&culture=\*', (req) => {  
 const url = new URL(req.url);  
 const moduleNumber = url.searchParams.get('modules');  
 const culture = url.searchParams.get('culture');  
 const cultureFolder = culture === '2' ? 'en' : 'ar';  
 const fileName = moduleNumberToFileMap\[moduleNumber || ''\] || 'erp'; // Dynamic file name  
 const fixturePath = `translations/stage/${cultureFolder}/${fileName}.json`;  
 cy.readFile(`cypress/fixtures/${fixturePath}`).then((fixtureContent) => {  
 req.reply({ statusCode: 200, body: fixtureContent, headers: { 'Content-Type': 'application/json' } });  
 }).catch(() => {  
 req.reply({ statusCode: 200, body: {} }); // Fallback to empty JSON  
 });  
 }).as('mockTranslations');  
});

1. Mock Static Assets: Similarly, we intercepted the 404'ing SVG/PNG requests and served a tiny transparent GIF to prevent the errors.

// Simplified interceptStaticAssets logic  
Cypress.Commands.add("interceptStaticAssets", () => {  
 const transparentGif = 'data:image/gif;base64,R0lGODlhAQABAAD/ACwAAAAAAQABAAACADs=';  
 cy.intercept('GET', '\*\_\_/assets/images/\*\*/\*.svg', { statusCode: 200, body: transparentGif, headers: { 'Content-Type': 'image/gif' } }).as('mockSvg');  
 // … similar for .png, .webp  
});

4. Robust Navigation (navigateToERPModule): Our main navigation command was updated to orchestrate these intercepts, set initial language preferences (cookies, local storage, navigator.language via onBeforeLoad), handle dynamic Clientid headers, and manage login redirects gracefully.

// Core of navigateToERPModule  
Cypress.Commands.add("navigateToERPModule", (moduleExtension: string) => {  
 // … setup Clientid, set language preferences …  
 cy.interceptTranslationAPI(); // Set up intercepts  
 cy.interceptStaticAssets();  
 cy.visit(fullUrl, { /\* … headers, onBeforeLoad … \*/ })  
 .then(() => {  
 cy.wait('@mockTranslations', { timeout: 30000 }); // Wait for our mock  
 cy.ensurePageIsReady(); // Wait for other network/DOM stability  
 // … handle login redirects …  
 cy.ensureEnglishPageLoaded(isInventory); // Verify UI language  
 cy.verifyEnglishLanguage();  
 });  
});

The Outcome: Tests are Green, Learning is Rich

With these Cypress-side workarounds, our E2E test suite is back to green. We’ve successfully bypassed the server’s problematic responses, allowing us to continue validating critical business flows. More importantly, this experience served as a powerful reminder:

● E2E tests are not just for functional validation; they are an early warning system for subtle infrastructure issues. This problem would have gone unnoticed by manual testing alone.

● Understanding Cypress’s architecture (especially its proxy) is crucial. It behaves differently from a browser, which can expose real-world server quirks.

● Collaboration is key. A complex problem like this cannot be solved by one team in isolation. Clear, evidence-based communication between QA, Backend, and DevOps is paramount.

● Fixtures are your best friend for unstable APIs. When the backend is flaky or has compatibility issues with your test runner, mocking provides stability.

While the permanent server-side fix is still underway, our test automation pipeline remains robust, providing continuous feedback and ensuring the quality of our ERP system.

Have you encountered similar “ghost in the machine” network issues in your E2E testing? Share your experiences in the comments below!

By Mohamed Said Ibrahim on July 15, 2025.

Exported from Medium on October 2, 2025.