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---
name: semantic-layer-change-review
description: "Use when modifying dbt metrics or semantic models to ensure mathematical correctness and backwards compatibility."
---
# Semantic Layer Change Review
Changes to the semantic layer directly impact dashboards and business reporting. A silent drift in a metric definition destroys trust.
## Review Protocol
1. **Identify the Change Type**:
- **Addition**: Safe. (Adding a new metric or dimension).
- **Deprecation**: Requires communication. (Removing a metric).
- **Modification**: High Risk. (Changing the SQL expression, aggregation, or filters of an existing metric).
2. **Evaluate Mathematical Soundness**:
- Are we averaging an average?
- Are we summing a distinct count?
- Does adding this dimension cause a fan-out that inflates the metric?
3. **Check Backwards Compatibility**:
- If an existing metric's logic is changed, you MUST flag it. The recommended path is to use dbt's metric versioning or create a new metric (e.g., `revenue_v2`) rather than silently altering historical numbers.
4. **Verify Entity Mapping**:
- Ensure `entities` (primary/foreign keys) match the granularity of the underlying semantic model.
## Anti-Pattern
Do not approve a pull request that changes the `expr` of a core metric without explicitly confirming the business requested the restatement of historical data.
