diff --git a/AGENTS.md b/AGENTS.md
index d039bbb..e2f6b58 100644
--- a/AGENTS.md
+++ b/AGENTS.md
@@ -65,14 +65,16 @@ auto-dev-test-target-2-python/
 │       ├── hello.py
 │       ├── string_utils.py
 │       ├── math_utils.py
-│       └── collection_utils.py
+│       ├── collection_utils.py
+│       └── constants.py
 ├── tests/
 │   ├── __init__.py
 │   ├── conftest.py
 │   ├── test_hello.py
 │   ├── test_string_utils.py
 │   ├── test_math_utils.py
-│   └── test_collection_utils.py
+│   ├── test_collection_utils.py
+│   └── test_constants.py
 ├── docs/
 │   └── CHANGELOG.md
 ├── comms/
diff --git a/README.md b/README.md
index 8b892c7..5303466 100644
--- a/README.md
+++ b/README.md
@@ -6,3 +6,4 @@
 - **string_utils** — String manipulation utilities: slugify, truncate, case conversion (snake_case, camelCase, PascalCase), and more.
 - **math_utils** — Math utilities: clamp, lerp, safe_divide, remap, and more.
 - **collection_utils** — Collection utilities: chunk, flatten, unique_by, group_by, partition, compact.
+- **constants** — Package-level constants (DEFAULT_HELLO_TARGET, etc.).
diff --git a/docs/CHANGELOG.md b/docs/CHANGELOG.md
index f5aa608..c885e50 100644
--- a/docs/CHANGELOG.md
+++ b/docs/CHANGELOG.md
@@ -2,6 +2,13 @@
 
 All notable changes to this project will be documented in this file.
 
+## [v004] - 2026-05-12
+
+### Added
+- `constants` module with `DEFAULT_HELLO_TARGET: str = "world"` package-level constant (BL-018)
+- `test_constants.py` with value, type, and importability assertions
+- Module re-export in `__init__.py`
+
 ## [v002] - 2026-03-06
 
 ### Added