diff --git a/docs_build/dev/ProjectInstructions/README.txt b/docs_build/dev/ProjectInstructions/README.txt
index 214ab9acb..1276f0695 100644
--- a/docs_build/dev/ProjectInstructions/README.txt
+++ b/docs_build/dev/ProjectInstructions/README.txt
@@ -43,3 +43,4 @@ READ THIS FIRST
 
 Addendum index:
 - Canonical Repository Structure: project-instructions/addendums/canonical-repository-structure.md
+- Test Structure Standardization: project-instructions/addendums/test-structure-standardization.md
diff --git a/project-instructions/addendums/test-structure-standardization.md b/project-instructions/addendums/test-structure-standardization.md
new file mode 100644
index 000000000..997b16730
--- /dev/null
+++ b/project-instructions/addendums/test-structure-standardization.md
@@ -0,0 +1,33 @@
+# Test Structure Standardization
+
+## Purpose
+
+Standardize testing locations and ensure independent tool validation.
+
+## Canonical Test Structure
+
+Tool tests:
+- tests/toolbox/{tool-name}/
+
+Engine tests:
+- tests/engine/{feature-name}/
+
+API tests:
+- tests/api/{feature-name}/
+
+Server tests:
+- tests/server/{feature-name}/
+
+Shared JavaScript tests:
+- tests/js/shared/
+
+Regression tests:
+- tests/regression/
+
+## Rules
+
+- Every tool must be independently testable.
+- Regression tests do not replace tool tests.
+- Tool tests validate tool functionality.
+- Regression tests validate platform behavior.
+- New tests follow the canonical structure.