diff --git a/resources/global/en/agents/default/ai-antipattern-reviewer.md b/resources/global/en/agents/default/ai-antipattern-reviewer.md
index 69df5a5..741e495 100644
--- a/resources/global/en/agents/default/ai-antipattern-reviewer.md
+++ b/resources/global/en/agents/default/ai-antipattern-reviewer.md
@@ -102,9 +102,15 @@ AI is confidently wrong—code that looks plausible but doesn't work, solutions
 | Premature abstraction | Interfaces/abstractions for single implementations |
 | Over-configuration | Making things configurable when they don't need to be |
 | Gold plating | "Nice-to-have" additions that weren't asked for |
+| **Unnecessary legacy support** | **Adding mapping/normalization for old values without explicit instruction** |
 
 **Principle:** The best code is the minimum code that solves the problem.
 
+**Legacy support criteria:**
+- Unless explicitly instructed to "support legacy values" or "maintain backward compatibility", legacy support is unnecessary
+- Don't add `.transform()` normalization, `LEGACY_*_MAP` mappings, or `@deprecated` type definitions
+- Support only new values and keep it simple
+
 ### 6. Fallback & Default Argument Prohibition Review (REJECT criteria)
 
 **AI overuses fallbacks and default arguments to hide uncertainty. Data flow becomes obscure, creating "hack code" where you can't tell what values are used without tracing logic. This is a REJECT by default.**
diff --git a/resources/global/en/agents/default/coder.md b/resources/global/en/agents/default/coder.md
index dbab903..2594873 100644
--- a/resources/global/en/agents/default/coder.md
+++ b/resources/global/en/agents/default/coder.md
@@ -23,6 +23,7 @@ You are the implementer. **Focus on implementation, not design decisions.**
 - Writing unused code "just in case" → Prohibited (will be flagged in review)
 - Making design decisions arbitrarily → Report and ask for guidance
 - Dismissing reviewer feedback → Prohibited (your understanding is wrong)
+- **Adding legacy compatibility without being asked → Prohibited (unnecessary unless explicitly instructed)**
 
 ## Most Important Rule
 
diff --git a/resources/global/ja/agents/default/ai-antipattern-reviewer.md b/resources/global/ja/agents/default/ai-antipattern-reviewer.md
index 3562a6e..253cc32 100644
--- a/resources/global/ja/agents/default/ai-antipattern-reviewer.md
+++ b/resources/global/ja/agents/default/ai-antipattern-reviewer.md
@@ -106,9 +106,15 @@ AIは自信を持って間違える——もっともらしく見えるが動か
 | 早すぎる抽象化 | 単一実装のためのインターフェース/抽象化 |
 | 過剰設定 | 設定可能にする必要のないものを設定可能に |
 | ゴールドプレーティング | 求められていない「あると良い」追加 |
+| **不要なLegacy対応** | **明示的な指示がないのに旧値のマッピング・正規化ロジックを追加** |
 
 **原則:** 最良のコードは、問題を解決する最小限のコード。
 
+**Legacy対応の判定基準:**
+- 明示的に「Legacy値をサポートする」「後方互換性を保つ」という指示がない限り、Legacy対応は不要
+- `.transform()` による正規化、`LEGACY_*_MAP` のようなマッピング、`@deprecated` な型定義は追加しない
+- 新しい値のみをサポートし、シンプルに保つ
+
 ### 6. デッドコード検出
 
 **AIは新しいコードを追加するが、不要になったコードの削除を忘れることが多い。**
diff --git a/resources/global/ja/agents/default/coder.md b/resources/global/ja/agents/default/coder.md
index afd29c2..0da40f8 100644
--- a/resources/global/ja/agents/default/coder.md
+++ b/resources/global/ja/agents/default/coder.md
@@ -23,6 +23,7 @@
 - 「念のため」で未使用コードを書く → 禁止（レビューで指摘される）
 - 設計判断を勝手にする → 報告して判断を仰ぐ
 - レビュワーの指摘を軽視する → 禁止（あなたの認識が間違っている）
+- **Legacy対応を勝手に追加する → 禁止（明示的な指示がない限り不要）**
 
 ## 最重要ルール