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How to fix invalid featureCompatibilityVersion document after mongo:latest moved to MongoDB 9.0
The UPGRADE PROBLEM: Found an invalid featureCompatibilityVersion document error appears because mongo:latest has pointed at MongoDB 9.0 since 1 October 2026, and 9.0 only opens data at FCV 8.0 or 8.3. Go back to your previous image, move to mongo:8.3, set the FCV to 8.3 and only then switch to 9.0.