diff --git a/AGENTS.md b/AGENTS.md
index 1146979..d4e7732 100644
--- a/AGENTS.md
+++ b/AGENTS.md
@@ -85,7 +85,20 @@ When analyzing code, refer to [ARCHITECTURE.md](./ARCHITECTURE.md) for:
 2. **Use** existing service patterns and interfaces
 3. **Follow** Single Responsibility Principle
 4. **Test** with mockable interfaces
-5. **Update** documentation when adding major features
+5. **Update [ARCHITECTURE.md](./ARCHITECTURE.md) when making architectural changes**
+
+## Important: Keep Documentation Updated
+
+**When making architectural changes, always update [ARCHITECTURE.md](./ARCHITECTURE.md):**
+- Adding/removing domain models (Account, Transaction, Receipt, etc.)
+- Adding/removing services or changing their responsibilities
+- Modifying database schema, relationships, or constraints
+- Adding new workflows or processes
+- Changing design patterns or conventions
+- Adding new pages or major features
+- Modifying security or performance considerations
+
+This shared documentation ensures both Claude Code and Codex CLI have accurate context for future work.
 
 ## Reference Links
 
diff --git a/CLAUDE.md b/CLAUDE.md
index 1064938..4082c5d 100644
--- a/CLAUDE.md
+++ b/CLAUDE.md
@@ -95,7 +95,20 @@ dotnet ef database update
 2. Make changes to models, services, or pages
 3. Test locally
 4. Create database migration if schema changed
-5. Commit with descriptive message
+5. **Update [ARCHITECTURE.md](./ARCHITECTURE.md) if architecture changes** (new models, services, workflows, etc.)
+6. Commit with descriptive message
+
+## Important: Keep Documentation Updated
+
+**When making architectural changes, always update [ARCHITECTURE.md](./ARCHITECTURE.md):**
+- Adding/removing domain models
+- Adding/removing services or changing their responsibilities
+- Modifying database schema or relationships
+- Adding new workflows or processes
+- Changing design patterns or conventions
+- Adding new pages or major features
+
+This ensures both Claude Code and Codex CLI have accurate, up-to-date context.
 
 ## Configuration