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M
20
Easy Fix
MongoDB · Database Server & Atlas
MongoDB server reported: Illegal Operation.
Quick Answer
Error code 20 on your MongoDB Database Server & Atlas means: MongoDB server reported: Illegal Operation.. This is usually fixable — see the step-by-step guide below.
Affected Models
Applies to all models
MONGO-ATLAS-DEDICATEDMONGO-ATLAS-SERVERLESSMONGO-COMM-6.0MONGO-COMM-7.0MONGO-COMM-8.0MONGO-ENT-6.0MONGO-ENT-7.0MONGO-ENT-8.0
Likely Causes
  1. MongoDB returned code 20 (IllegalOperation) while processing this request path.
  2. Client and server protocol expectations diverged for this operation.
  3. Request payload, command shape, or field constraints violated the API/engine contract.
How to Fix It
1
Review logs/telemetry for code 20 (IllegalOperation) and capture the exact command and parameters that triggered it.
2
Reproduce with a minimal payload to isolate which attribute is rejected.
3
Add client-side guards and contract tests so this validation pattern is blocked pre-deploy.
4
Validate request schema, field types, and identifiers against current server rules.
5
Add monitoring for recurring IllegalOperation events so future incidents are detected and triaged faster.
Pro Tip
This is a common issue that most users can resolve without specialized tools. Try the first step before moving on to more complex solutions.
Related Error Codes
12456
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