Salesforce deploy "FIELD_INTEGRITY_EXCEPTION" in CI

A field change violates an integrity rule: the new type or attribute is incompatible with existing data, a dependent picklist, or another field. Salesforce refuses the change to protect data consistency.

What this error means

The deploy fails with "FIELD_INTEGRITY_EXCEPTION" and a message about an invalid field type change, controlling field, or incompatible attribute.

Deploy failed.
Type        Name              Problem
CustomField Account.Tier__c   FIELD_INTEGRITY_EXCEPTION: There's a problem with this field, which might be a circular reference or a field-level security issue.

Common causes

An unsupported field type change

Changing a field to an incompatible type (or altering length/precision in a way data cannot satisfy) violates field integrity.

A broken dependent picklist or controlling field

A dependent picklist referencing a controlling field that does not match, or field-level security inconsistencies, trigger the exception.

How to fix it

Make the change compatible with existing data

Read which field and attribute the error names.
Adjust the field definition to a compatible type or split the change into safe steps.
Ensure dependent picklist controlling-field mappings are valid.

Deploy controlling and dependent fields together

When a dependent picklist is involved, deploy the controlling field and its valueSettings in the same set so the relationship is consistent.

Terminal

sf project deploy start --target-org ci-org \
  --source-dir force-app/main/default/objects

How to prevent it

Avoid incompatible field type changes; add a new field and migrate instead.
Deploy controlling and dependent picklist metadata together.
Validate field changes against an org with representative data.