---
title: "Reason code"
description: "A stable, human-readable statement of a principal factor behind an adverse decision, mapped from the factors the model actually scored and versioned with the model; required in substance by US…"
canonical: https://aigovernanceengineer.com/glossary/reason-code
author: "Jorge García Aibar"
license: "CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/)"
doi: https://doi.org/10.5281/zenodo.22956197
version: "0.5.0"
updated: 2026-09-25
---

# Reason code

A stable, human-readable statement of a principal factor behind an adverse decision, mapped from the factors the model actually scored and versioned with the model; required in substance by US adverse-action rules [1].

- Developed in: [ch. 16, Credit: adverse-action notices and reason codes](https://aigovernanceengineer.com/bok/fairness-and-explainability#credit-adverse-action-notices-and-reason-codes); [ch. 05, Pattern: Decision Notice & Contest Path](https://aigovernanceengineer.com/patterns/decision-notice-contest-path); [ch. 05, Pattern: Explanation Artefact](https://aigovernanceengineer.com/patterns/explanation-artefact)
- Chapters: [ch. 05, Patterns](https://aigovernanceengineer.com/bok/patterns) · [ch. 16, Fairness & XAI](https://aigovernanceengineer.com/bok/fairness-and-explainability) · [ch. 20, Existing Law](https://aigovernanceengineer.com/bok/existing-law)
- In the glossary chapter: https://aigovernanceengineer.com/bok/glossary#t-reason-code

## Sources

[1] 12 CFR 1002.9 (Regulation B, notifications) (1002.9(b)(2) statement of specific principal reasons for adverse action; Supplement I commentary; text as of 2026-09-01). eCFR. 2026-09-01. https://www.ecfr.gov/current/title-12/chapter-X/part-1002/section-1002.9 (verified: primary)