diff --git a/dot_claude/rules/development-principles.md b/dot_claude/rules/development-principles.md
index 1529c8c..082f704 100644
--- a/dot_claude/rules/development-principles.md
+++ b/dot_claude/rules/development-principles.md
@@ -6,8 +6,7 @@ alwaysApply: true
 
 ## Test-Driven Development
 
-- Write tests first, then implementation
-- Red → Green → Refactor cycle
+- See `tdd.md` for detailed TDD rules
 
 ## Simplicity
 
diff --git a/dot_claude/rules/tdd.md b/dot_claude/rules/tdd.md
new file mode 100644
index 0000000..7afde17
--- /dev/null
+++ b/dot_claude/rules/tdd.md
@@ -0,0 +1,34 @@
+---
+alwaysApply: true
+---
+
+# Test-Driven Development
+
+## Red-Green-Refactor Cycle
+
+1. **Red**: Write a failing test that describes the desired behavior
+2. Confirm the test fails for the expected reason
+3. **Green**: Write the minimal implementation to make the test pass
+4. Confirm all tests pass
+5. **Refactor**: Improve code quality while keeping tests green
+
+## Rules
+
+- Never write production code without a corresponding failing test
+- One test = one behavior. Keep incremental steps small
+- Run tests after each step; show Red before Green
+- Do not skip the Red step — a test that has never failed proves nothing
+
+## When Uncertain
+
+- Ask the user about test location, framework, or naming conventions rather than guessing
+- If multiple test patterns exist in the project, ask which to follow
+
+## Exceptions
+
+TDD does not apply to contexts without test frameworks:
+
+- Dotfiles configuration and templates
+- CI/CD pipeline definitions
+- Shell scripts without a test harness
+- Declarative config files (JSON, YAML, TOML)