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M
66
Easy Fix
MongoDB · Database Server & Atlas
MongoDB server reported: Immutable Field.
Quick Answer
Error code 66 on your MongoDB Database Server & Atlas means: MongoDB server reported: Immutable Field.. 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 66 (ImmutableField) while processing this request path.
  2. Request payload, command shape, or field constraints violated the API/engine contract.
  3. Object identifiers, namespaces, or index metadata did not match current server state.
How to Fix It
1
Review logs/telemetry for code 66 (ImmutableField) and capture the exact command and parameters that triggered it.
2
Add client-side guards and contract tests so this validation pattern is blocked pre-deploy.
3
Validate request schema, field types, and identifiers against current server rules.
4
Reproduce with a minimal payload to isolate which attribute is rejected.
5
Add monitoring for recurring ImmutableField 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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