Most Common Communication Errors Between Frontend and Backend

July 8, 2025

This content originally appeared on DEV Community and was authored by Diego Liascovich

By Diego Liascovich

Full-Stack Developer | Microservices | Angular | Node.js

Effective communication between frontend and backend is essential for building robust and scalable applications. Yet, it’s common for mismatches and misunderstandings to arise, causing bugs, delays, and poor user experience.

In this post, we’ll explore the most frequent communication errors between frontend and backend teams (or codebases), and how to avoid them.

1. API Contract Mismatches

Structure Differences

The frontend expects a certain shape of data, but the backend sends something else.

Example:

// Frontend expects:
{ "username": "john_doe" }
// Backend sends:
{ "user_name": "john_doe" }

Missing or Outdated Documentation

APIs without reliable documentation lead to incorrect usage.

No Versioning

Backend changes an API response or behavior without versioning, breaking frontend compatibility.

2. Timing and Async Handling Errors

Timeouts

The backend takes too long to respond, and the frontend doesn’t handle the timeout gracefully.

Race Conditions

Multiple simultaneous requests lead to inconsistent UI or state because their order isn’t controlled.

3. Validation and Error Handling

Inconsistent Error Formats

Backend responses vary or don’t use proper HTTP status codes.

Bad Practice:
Returning 200 OK with an error in the response body.

Weak or Conflicting Validation

Missing validation in frontend or backend.
Validation rules differ on both sides (e.g., regex for emails).

4. Authentication & Authorization Errors

Unhandled Token Expiration

Frontend doesn’t detect when the token is expired (401 Unauthorized), and fails silently or behaves unpredictably.

Role Mismatches

Frontend allows actions that backend denies — or backend allows what frontend should restrict.

5. Data Format Issues

Date and Time Mismanagement

Timezone inconsistencies and formatting differences (ISO, UTC, etc.) can cause major bugs.

Incorrect Type Handling

Frontend sends a string, backend expects a number.

Array, `null`, or `undefined` Confusion

Inconsistent handling of optional or empty fields.

6. HTTP Method and Status Code Misuse

Wrong HTTP Verbs

Using GET for actions that should use POST, PUT, or DELETE.

Incorrect Status Codes

Returning 500 Internal Server Error for validation errors (should be 400 Bad Request).

7. CORS Issues

Cross-Origin Resource Sharing (CORS) misconfiguration can block frontend requests altogether.

8. Mocking vs Real Backend Discrepancies

Mocks Are Not Accurate

Frontend is built using mock data that doesn’t match the real backend’s behavior or structure.

Backend Doesn’t Test with Real Frontend

API changes may go unnoticed if not tested in full integration.

How to Avoid These Issues

Use OpenAPI/Swagger for auto-generated and up-to-date documentation.
Validate inputs/outputs using tools like Zod, Joi, or class-validator.
Share types between frontend and backend (e.g. using tRPC, GraphQL, or codegen tools).
Version your APIs to avoid breaking changes.
Write integration tests that include both frontend and backend layers.
Communicate early between teams and document assumptions.

Final Thoughts

Frontend-backend communication is like a contract — and like any contract, if it’s not well-defined, it leads to conflict. By improving documentation, validation, and collaboration, you can reduce bugs and ship faster with confidence.