diff --git a/CHANGELOG.md b/CHANGELOG.md
index 7345aefe6..965726b7c 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -6,6 +6,7 @@ Changed `wf10_batch.robot` to be less likely to create files, and
 `plan_generation_graph.robot` to give more test answers.
 
 ## [Unreleased]
+- **fix(a2a): regression tests for stale cleveragents.acp removal** (#5566): Added two Behave BDD scenarios verifying that `cleveragents.acp` is not importable (raises `ImportError`) and that `src/cleveragents/acp/` does not exist in the source tree. These guard against regression of the `__pycache__`-based import that allowed the removed ACP module to still be loaded from bytecode after the v3.6.0 rename to `a2a`.
 - **Virtual Resource Type Base Class** (#8610): Implemented `VirtualResource` base class with two example concrete implementations (`MetricResource`, `APIEndpointResource`) for abstract/computed resources that are derived rather than mapped to physical files. Virtual resources are computed on demand via a `compute_fn` callable. Includes Behave BDD scenarios in `features/resource_virtual_types.feature` exercising construction, computation, name validation, kwargs passthrough, exception handling, string representation, and subclassing. Resource names are validated against `^[a-zA-Z][a-zA-Z0-9_-]*$` (must start with a letter; alphanumeric, hyphens, and underscores otherwise).
 - **test(e2e): restore complete M2 acceptance test** (#11191): Restored the truncated M2 full actor compiler and LLM integration e2e acceptance test to its complete 10-step form. Added dynamic LLM provider selection via `Resolve LLM Actor` (falls back to Anthropic when OpenAI is unavailable or quota-exhausted), replacing hardcoded `gpt-4` / `openai/gpt-4` references in the actor config and action YAML. Added explicit return-code validation (`Should Be Equal As Integers ${r_actor.rc} 0`) for the actor registration step.
 - **docs(a2a): ACP to A2A migration guide** (#10230): Added migration guide documenting how to upgrade from the ACP module to the A2A module introduced in v3.6.0, including symbol renames, field renames, operation-name mappings, and YAML configuration updates.
diff --git a/features/a2a_acp_module_removed.feature b/features/a2a_acp_module_removed.feature
new file mode 100644
index 000000000..d560b3ec9
--- /dev/null
+++ b/features/a2a_acp_module_removed.feature
@@ -0,0 +1,13 @@
+Feature: ACP module is not importable
+  As a CleverAgents developer
+  I want to ensure the old ACP module is completely removed
+  So that the v3.6.0 deliverable #1 requirement is met: "No acp references in public API"
+
+  Scenario: Importing cleveragents.acp raises ImportError
+    When I attempt to import the cleveragents.acp module
+    Then an ImportError should be raised
+    And the error message should indicate the module does not exist
+
+  Scenario: The acp directory does not exist in the source tree
+    When I check the source tree for the acp directory
+    Then the src/cleveragents/acp directory should not exist
diff --git a/features/steps/a2a_acp_module_removed_steps.py b/features/steps/a2a_acp_module_removed_steps.py
new file mode 100644
index 000000000..8068561af
--- /dev/null
+++ b/features/steps/a2a_acp_module_removed_steps.py
@@ -0,0 +1,72 @@
+"""Step definitions for a2a_acp_module_removed.feature.
+
+Tests that the old ACP module is completely removed and not importable,
+ensuring v3.6.0 deliverable #1 compliance: "No acp references in public API".
+"""
+
+from __future__ import annotations
+
+import importlib
+import sys
+from pathlib import Path
+from typing import Any
+
+from behave import then, when
+
+
+@when("I attempt to import the cleveragents.acp module")
+def step_attempt_import_acp(context: Any) -> None:
+    """Attempt to import cleveragents.acp and capture any ImportError."""
+    context.import_error = None
+    try:
+        # Remove from sys.modules if it exists to ensure fresh import
+        if "cleveragents.acp" in sys.modules:
+            del sys.modules["cleveragents.acp"]
+
+        # Attempt to import
+        importlib.import_module("cleveragents.acp")
+        context.import_succeeded = True
+    except ImportError as e:
+        context.import_error = e
+        context.import_succeeded = False
+
+
+@then("an ImportError should be raised")
+def step_check_import_error(context: Any) -> None:
+    """Verify that an ImportError was raised."""
+    assert context.import_error is not None, (
+        "Expected ImportError to be raised when importing cleveragents.acp, "
+        "but import succeeded"
+    )
+    assert not context.import_succeeded, "Expected import to fail, but it succeeded"
+
+
+@then("the error message should indicate the module does not exist")
+def step_check_error_message(context: Any) -> None:
+    """Verify that the error message indicates the module doesn't exist."""
+    error_msg = str(context.import_error)
+    assert "cleveragents.acp" in error_msg or "No module named" in error_msg, (
+        f"Expected error message to mention 'cleveragents.acp' or 'No module named', "
+        f"but got: {error_msg}"
+    )
+
+
+@when("I check the source tree for the acp directory")
+def step_check_acp_directory(context: Any) -> None:
+    """Check if the acp directory exists in the source tree."""
+    # Get the path to the cleveragents package
+    import cleveragents
+
+    cleveragents_path = Path(cleveragents.__file__).parent
+    acp_path = cleveragents_path / "acp"
+
+    context.acp_directory_exists = acp_path.exists()
+    context.acp_path = acp_path
+
+
+@then("the src/cleveragents/acp directory should not exist")
+def step_check_acp_not_exists(context: Any) -> None:
+    """Verify that the acp directory does not exist."""
+    assert not context.acp_directory_exists, (
+        f"Expected acp directory to not exist, but found it at: {context.acp_path}"
+    )