Validation error details in Rest API

Introduction

Validation error details help you understand why your data was not accepted by an API. They tell you exactly what went wrong so you can fix it.

When you send data to a web service and it rejects your input.

When you want to show users clear messages about what they need to correct in a form.

When debugging why your API request failed.

When building an API that needs to tell clients what errors happened.

When logging errors to improve your app's data quality.

Syntax

Rest API

HTTP/1.1 400 Bad Request
Content-Type: application/json

{
  "errors": [
    {
      "field": "username",
      "message": "Username is required",
      "code": "required"
    },
    {
      "field": "email",
      "message": "Email format is invalid",
      "code": "invalid_format"
    }
  ]
}

The response usually uses HTTP status 400 for validation errors.

The body is often JSON with an array of error objects describing each problem.

Examples

This example shows one error about a password being too short.

Rest API

{
  "errors": [
    {"field": "password", "message": "Password too short", "code": "min_length"}
  ]
}

This example shows two errors: one for age type and one for missing email.

Rest API

{
  "errors": [
    {"field": "age", "message": "Age must be a number", "code": "type_error"},
    {"field": "email", "message": "Email is missing", "code": "required"}
  ]
}

Sample Program

This simple Flask API checks if username and email are valid. If not, it returns validation error details in JSON with status 400.

Rest API

from flask import Flask, request, jsonify

app = Flask(__name__)

@app.route('/register', methods=['POST'])
def register():
    data = request.json
    errors = []

    if not data.get('username'):
        errors.append({"field": "username", "message": "Username is required", "code": "required"})
    if not data.get('email') or '@' not in data.get('email', ''):
        errors.append({"field": "email", "message": "Email format is invalid", "code": "invalid_format"})

    if errors:
        return jsonify({"errors": errors}), 400

    return jsonify({"message": "User registered successfully"}), 200

if __name__ == '__main__':
    app.run(debug=True)

Important Notes

Always use clear and specific messages so users know how to fix errors.

Include the field name so clients can highlight the exact input with problems.

Use standard HTTP status codes like 400 for validation errors.

Summary

Validation error details explain what input is wrong and why.

They help users and developers fix data problems quickly.

Usually sent as JSON with error messages and field names.

Practice

1. What is the main purpose of validation error details in a REST API response?

easy

A. To explain which input fields are wrong and why

B. To speed up the server response time

C. To encrypt the data sent to the client

D. To log user activity for analytics

Solution

1. Step 1: Understand validation errors

   Validation errors tell us what input data is incorrect or missing.

2. Step 2: Purpose of error details

   Error details explain which fields caused the problem and why, helping fix input.

3. Final Answer:

   To explain which input fields are wrong and why -> Option A

4. Quick Check:

   Validation error details = Explain input errors [OK]

Hint: Validation errors show what and why input is wrong [OK]

Common Mistakes:

• Thinking error details speed up server
• Confusing error details with encryption
• Assuming error details are for logging only

2. Which of the following is the correct JSON structure for validation error details in a REST API?

easy

A. {"data": {"email": "[e-mail]"}}

B. ["email", "password"]

C. {"status": 200, "message": "Success"}

D. {"errors": {"email": "Invalid format", "password": "Too short"}}

Solution

1. Step 1: Identify error details format

   Error details usually use a JSON object with field names as keys and error messages as values.

2. Step 2: Check options

   {"errors": {"email": "Invalid format", "password": "Too short"}} shows a JSON object with "errors" key and field-specific messages, which is correct.

3. Final Answer:

   {"errors": {"email": "Invalid format", "password": "Too short"}} -> Option D

4. Quick Check:

   Validation errors = JSON object with field messages [OK]

Hint: Look for JSON object with field names and error messages [OK]

Common Mistakes:

• Using arrays instead of objects for errors
• Confusing success response with error details
• Missing field names in error messages

3. Given this JSON error response:

{"errors": {"username": "Required field", "age": "Must be a number"}}

What message should the client show for the 'age' field?

medium

A. Must be a number

B. Invalid email format

C. Required field

D. Password too short

Solution

1. Step 1: Read the JSON error response

   The error for "age" is "Must be a number".

2. Step 2: Match the message to the field

   The client should show the message exactly as given for the "age" field.

3. Final Answer:

   Must be a number -> Option A

4. Quick Check:

   Field 'age' error = Must be a number [OK]

Hint: Match field name to its error message in JSON [OK]

Common Mistakes:

• Mixing error messages between fields
• Showing unrelated error messages
• Ignoring the JSON structure

4. You receive this validation error JSON:

{"error": "Invalid input", "fields": ["email", "password"]}

What is wrong with this error detail format?

medium

A. It correctly shows error details

B. It uses a list instead of mapping fields to messages

C. It uses wrong HTTP status code

D. It encrypts error messages

Solution

1. Step 1: Analyze error detail format

   The "fields" key has a list of field names but no messages explaining the errors.

2. Step 2: Identify correct format

   Validation error details should map each field to a specific error message, not just list fields.

3. Final Answer:

   It uses a list instead of mapping fields to messages -> Option B

4. Quick Check:

   Error details need field-message pairs [OK]

Hint: Error details need field names paired with messages [OK]

Common Mistakes:

• Listing fields without error explanations
• Assuming any error JSON is correct
• Ignoring the need for descriptive messages

5. You want to design a REST API validation error response that shows multiple errors per field. Which JSON structure correctly supports this?

hard

A. {"errors": ["email: Required", "password: Too short"]}

B. {"error": "Multiple errors found"}

C. {"errors": {"email": ["Required", "Invalid format"], "password": ["Too short"]}}

D. {"fields": ["email", "password"]}

Solution

1. Step 1: Understand multiple errors per field

   To show many errors for one field, use a list (array) of messages for that field.

2. Step 2: Check JSON structures

   {"errors": {"email": ["Required", "Invalid format"], "password": ["Too short"]}} uses a dictionary with field keys and lists of error messages, which fits the need.

3. Final Answer:

   {"errors": {"email": ["Required", "Invalid format"], "password": ["Too short"]}} -> Option C

4. Quick Check:

   Multiple errors per field = list of messages [OK]

Hint: Use lists inside error fields to show multiple messages [OK]

Common Mistakes:

• Using flat lists without field keys
• Not supporting multiple messages per field
• Using vague error messages without details