diff --git a/README.md b/README.md
index b6c394e..005f03c 100644
--- a/README.md
+++ b/README.md
@@ -20,6 +20,15 @@ graph LR
 ```
 <!-- topology:end -->
 
+## API
+
+### `run_tests(ctx)`
+**Syntax:** `M.run_tests(ctx)` -> void
+
+**Description:** Test-runner entrypoint. Executes 17 assertions against
+`lib-core.input`. Called automatically by the engine's
+`--test-libs=` orchestrator. Exits via `engine.exit(engine.test.failures())`.
+
 ## Coverage Statement
 
 Per Fault-Injection-Audit-Convention (2026-05-16):