diff --git a/CHANGELOG.md b/CHANGELOG.md
index 1a90c2322..60251d589 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -35,6 +35,15 @@
   with `shlex.split()` and `shell=False` for defense-in-depth command injection
   prevention, consistent with the existing pattern in
   `cli_plan_context_commands_steps.py`.  (#734)
+- Added ACMS Backend Abstraction Layer (BAL) protocol definitions and
+  in-memory stub implementations. Defines `TextBackend`, `VectorBackend`,
+  and `GraphBackend` protocols with frozen result dataclasses (`TextResult`,
+  `VectorResult`, `GraphResult`). In-memory stubs (`InMemoryTextBackend`,
+  `InMemoryVectorBackend`, `InMemoryGraphBackend`) validate arguments and
+  return empty results for development and testing. Backends registered as
+  configurable singletons in the DI container with provider selection via
+  `override_providers()`. Includes Behave BDD tests (35 scenarios), Robot
+  Framework smoke tests, ASV benchmarks, and reference documentation. (#498)
 - Added BuiltinAdapter class and MCP automatic resource slot creation.
   BuiltinAdapter wraps register_file_tools/register_git_tools/register_subplan_tool
   into a unified adapter interface. McpAdapter.infer_resource_slots() analyzes