diff --git a/CHANGELOG.md b/CHANGELOG.md
index d8b9b3fb..a037894a 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -27,6 +27,27 @@ Each entry follows this format:
 ## [Unreleased]
 
 ### Added
+- **Phase 0: Foundation & Credit System - Initial Implementation**
+  - Updated `CREDIT_COSTS` constants to Phase 0 format with new operations
+    - Added new credit costs: `linking` (8 credits), `optimization` (1 credit per 200 words), `site_structure_generation` (50 credits), `site_page_generation` (20 credits)
+    - Maintained backward compatibility with legacy operation names (`ideas`, `content`, `images`, `reparse`)
+  - Enhanced `CreditService` with `get_credit_cost()` method for dynamic cost calculation
+    - Supports variable costs based on operation type and amount (word count, etc.)
+    - Updated `check_credits()` and `deduct_credits()` to support both legacy `required_credits` parameter and new `operation_type`/`amount` parameters
+    - Maintained full backward compatibility with existing code
+  - Created `AccountModuleSettings` model for module enable/disable functionality
+    - One settings record per account (get_or_create pattern)
+    - Enable/disable flags for all 8 modules: `planner_enabled`, `writer_enabled`, `thinker_enabled`, `automation_enabled`, `site_builder_enabled`, `linker_enabled`, `optimizer_enabled`, `publisher_enabled`
+    - Helper method `is_module_enabled(module_name)` for easy module checking
+  - Added `AccountModuleSettingsSerializer` and `AccountModuleSettingsViewSet`
+    - API endpoint: `/api/v1/system/settings/account-modules/`
+    - Custom action: `check/(?P<module_name>[^/.]+)` to check if a specific module is enabled
+    - Automatic account assignment on create
+    - Unified API Standard v1.0 compliant
+  - **Affected Areas**: Billing module (`constants.py`, `services.py`), System module (`settings_models.py`, `settings_serializers.py`, `settings_views.py`, `urls.py`)
+  - **Documentation**: See `docs/planning/phases/PHASE-0-FOUNDATION-CREDIT-SYSTEM.md` for complete details
+  - **Impact**: Foundation for credit-only system and module-based feature access control
+
 - **Planning Documents Organization**: Organized architecture and implementation planning documents
   - Created `docs/planning/` directory for all planning documents
   - Moved `IGNY8-HOLISTIC-ARCHITECTURE-PLAN.md` to `docs/planning/`