diff --git a/features/a2a_facade_optional_param_validation.feature b/features/a2a_facade_optional_param_validation.feature
new file mode 100644
index 000000000..eb3230ea1
--- /dev/null
+++ b/features/a2a_facade_optional_param_validation.feature
@@ -0,0 +1,122 @@
+Feature: A2A facade optional parameter validation
+  As a developer using the A2A local facade
+  I want optional parameters to be validated before being passed to services
+  So that invalid inputs are rejected early with clear error messages
+
+  # -------------------------------------------------------------------
+  # _handle_registry_list_tools — namespace validation
+  # -------------------------------------------------------------------
+
+  Scenario: list_tools with valid namespace succeeds
+    Given an opt-val facade with a mock ToolRegistry
+    When I dispatch opt-val operation "registry.list_tools" with params {"namespace": "local"}
+    Then the opt-val response status should be "ok"
+    And the opt-val response data should contain key "tools"
+
+  Scenario: list_tools with None namespace succeeds
+    Given an opt-val facade with a mock ToolRegistry
+    When I dispatch opt-val operation "registry.list_tools" with params {}
+    Then the opt-val response status should be "ok"
+    And the opt-val response data should contain key "tools"
+
+  Scenario: list_tools with empty string namespace returns error
+    Given an opt-val facade with a mock ToolRegistry
+    When I dispatch opt-val operation "registry.list_tools" with params {"namespace": ""}
+    Then the opt-val response status should be "error"
+    And the opt-val response error message should contain "namespace must be a non-empty string"
+
+  Scenario: list_tools with non-string namespace returns error
+    Given an opt-val facade with a mock ToolRegistry
+    When I dispatch opt-val operation "registry.list_tools" with params {"namespace": 42}
+    Then the opt-val response status should be "error"
+    And the opt-val response error message should contain "namespace must be a string or None"
+
+  Scenario: list_tools with no registry returns empty tools list
+    Given an opt-val facade with no services
+    When I dispatch opt-val operation "registry.list_tools" with params {"namespace": "local"}
+    Then the opt-val response status should be "ok"
+    And the opt-val response data should contain key "tools"
+
+  # -------------------------------------------------------------------
+  # _handle_registry_list_resources — type_name validation
+  # -------------------------------------------------------------------
+
+  Scenario: list_resources with valid type_name succeeds
+    Given an opt-val facade with a mock ResourceRegistryService
+    When I dispatch opt-val operation "registry.list_resources" with params {"type_name": "git-checkout"}
+    Then the opt-val response status should be "ok"
+    And the opt-val response data should contain key "resources"
+
+  Scenario: list_resources with None type_name succeeds
+    Given an opt-val facade with a mock ResourceRegistryService
+    When I dispatch opt-val operation "registry.list_resources" with params {}
+    Then the opt-val response status should be "ok"
+    And the opt-val response data should contain key "resources"
+
+  Scenario: list_resources with empty string type_name returns error
+    Given an opt-val facade with a mock ResourceRegistryService
+    When I dispatch opt-val operation "registry.list_resources" with params {"type_name": ""}
+    Then the opt-val response status should be "error"
+    And the opt-val response error message should contain "type_name must be a non-empty string"
+
+  Scenario: list_resources with non-string type_name returns error
+    Given an opt-val facade with a mock ResourceRegistryService
+    When I dispatch opt-val operation "registry.list_resources" with params {"type_name": 99}
+    Then the opt-val response status should be "error"
+    And the opt-val response error message should contain "type_name must be a string or None"
+
+  Scenario: list_resources with no service returns empty resources list
+    Given an opt-val facade with no services
+    When I dispatch opt-val operation "registry.list_resources" with params {"type_name": "git-checkout"}
+    Then the opt-val response status should be "ok"
+    And the opt-val response data should contain key "resources"
+
+  # -------------------------------------------------------------------
+  # _handle_plan_create — arguments validation
+  # -------------------------------------------------------------------
+
+  Scenario: plan.create with valid dict arguments succeeds
+    Given an opt-val facade with a mock PlanLifecycleService
+    When I dispatch opt-val operation "plan.create" with params {"action_name": "build", "arguments": {"target": "all"}}
+    Then the opt-val response status should be "ok"
+    And the opt-val response data key "status" should equal "created"
+
+  Scenario: plan.create with None arguments succeeds
+    Given an opt-val facade with a mock PlanLifecycleService
+    When I dispatch opt-val operation "plan.create" with params {"action_name": "build"}
+    Then the opt-val response status should be "ok"
+    And the opt-val response data key "status" should equal "created"
+
+  Scenario: plan.create with non-dict arguments returns error
+    Given an opt-val facade with a mock PlanLifecycleService
+    When I dispatch opt-val operation "plan.create" with params {"action_name": "build", "arguments": "not-a-dict"}
+    Then the opt-val response status should be "error"
+    And the opt-val response error message should contain "arguments must be a dict or None"
+
+  # -------------------------------------------------------------------
+  # _handle_plan_create — created_by validation
+  # -------------------------------------------------------------------
+
+  Scenario: plan.create with valid created_by succeeds
+    Given an opt-val facade with a mock PlanLifecycleService
+    When I dispatch opt-val operation "plan.create" with params {"action_name": "build", "created_by": "alice"}
+    Then the opt-val response status should be "ok"
+    And the opt-val response data key "status" should equal "created"
+
+  Scenario: plan.create with None created_by succeeds
+    Given an opt-val facade with a mock PlanLifecycleService
+    When I dispatch opt-val operation "plan.create" with params {"action_name": "build"}
+    Then the opt-val response status should be "ok"
+    And the opt-val response data key "status" should equal "created"
+
+  Scenario: plan.create with empty string created_by returns error
+    Given an opt-val facade with a mock PlanLifecycleService
+    When I dispatch opt-val operation "plan.create" with params {"action_name": "build", "created_by": ""}
+    Then the opt-val response status should be "error"
+    And the opt-val response error message should contain "created_by must be a non-empty string"
+
+  Scenario: plan.create with non-string created_by returns error
+    Given an opt-val facade with a mock PlanLifecycleService
+    When I dispatch opt-val operation "plan.create" with params {"action_name": "build", "created_by": 123}
+    Then the opt-val response status should be "error"
+    And the opt-val response error message should contain "created_by must be a string or None"
diff --git a/features/steps/a2a_facade_optional_param_validation_steps.py b/features/steps/a2a_facade_optional_param_validation_steps.py
new file mode 100644
index 000000000..e1f142419
--- /dev/null
+++ b/features/steps/a2a_facade_optional_param_validation_steps.py
@@ -0,0 +1,177 @@
+"""Step definitions for a2a_facade_optional_param_validation.feature.
+
+Covers input validation for optional parameters in A2A facade handlers:
+
+| Handler                        | Parameter   | Validation                          |
+|--------------------------------|-------------|-------------------------------------|
+| _handle_registry_list_tools    | namespace   | non-empty string or None            |
+| _handle_registry_list_resources| type_name   | non-empty string or None            |
+| _handle_plan_create            | arguments   | dict or None                        |
+| _handle_plan_create            | created_by  | non-empty string or None            |
+"""
+
+from __future__ import annotations
+
+import json
+from typing import Any
+from unittest.mock import MagicMock
+
+from behave import given, then, use_step_matcher, when
+from behave.runner import Context
+
+try:
+    from cleveragents.a2a.facade import A2aLocalFacade
+    from cleveragents.a2a.models import A2aRequest
+except ImportError:
+    pass  # a2a module not available
+
+use_step_matcher("re")
+
+
+# ---------------------------------------------------------------------------
+# Mock helpers
+# ---------------------------------------------------------------------------
+
+
+class _MockPlanIdentity:
+    """Minimal PlanIdentity stub."""
+
+    def __init__(self, plan_id: str = "MOCK-PLAN-001") -> None:
+        self.plan_id = plan_id
+
+
+class _MockPlan:
+    """Minimal plan stub."""
+
+    def __init__(self, plan_id: str = "MOCK-PLAN-001") -> None:
+        self.identity = _MockPlanIdentity(plan_id)
+        self.phase = MagicMock()
+        self.phase.value = "strategize"
+        self.state = MagicMock()
+        self.state.value = "queued"
+
+
+class _MockToolSpec:
+    """Minimal ToolSpec stub."""
+
+    def __init__(self, name: str, description: str) -> None:
+        self.name = name
+        self.description = description
+
+
+class _MockResource:
+    """Minimal resource stub."""
+
+    def __init__(self, resource_id: str, name: str, resource_type_name: str) -> None:
+        self.resource_id = resource_id
+        self.name = name
+        self.resource_type_name = resource_type_name
+
+
+def _build_mock_plan_lifecycle_service() -> MagicMock:
+    svc = MagicMock()
+    svc.use_action.return_value = _MockPlan()
+    return svc
+
+
+def _build_mock_tool_registry() -> MagicMock:
+    registry = MagicMock()
+    registry.list_tools.return_value = [
+        _MockToolSpec("local/tool-a", "Tool A"),
+    ]
+    return registry
+
+
+def _build_mock_resource_registry_service() -> MagicMock:
+    svc = MagicMock()
+    svc.list_resources.return_value = [
+        _MockResource("RES-001", "my-repo", "git-checkout"),
+    ]
+    return svc
+
+
+# ---------------------------------------------------------------------------
+# Given steps
+# ---------------------------------------------------------------------------
+
+
+@given(r"an opt-val facade with no services")
+def step_ov_facade_no_services(context: Context) -> None:
+    context.ov_facade = A2aLocalFacade()
+
+
+@given(r"an opt-val facade with a mock ToolRegistry")
+def step_ov_facade_tool_registry(context: Context) -> None:
+    context.ov_facade = A2aLocalFacade(
+        services={"tool_registry": _build_mock_tool_registry()}
+    )
+
+
+@given(r"an opt-val facade with a mock ResourceRegistryService")
+def step_ov_facade_resource_registry(context: Context) -> None:
+    context.ov_facade = A2aLocalFacade(
+        services={"resource_registry_service": _build_mock_resource_registry_service()}
+    )
+
+
+@given(r"an opt-val facade with a mock PlanLifecycleService")
+def step_ov_facade_plan_lifecycle(context: Context) -> None:
+    context.ov_facade = A2aLocalFacade(
+        services={"plan_lifecycle_service": _build_mock_plan_lifecycle_service()}
+    )
+
+
+# ---------------------------------------------------------------------------
+# When steps
+# ---------------------------------------------------------------------------
+
+
+@when(
+    r'I dispatch opt-val operation "(?P<operation>[^"]+)" '
+    r"with params (?P<params_json>.+)"
+)
+def step_ov_dispatch(context: Context, operation: str, params_json: str) -> None:
+    params: dict[str, Any] = json.loads(params_json)
+    request = A2aRequest(method=operation, params=params)
+    context.ov_response = context.ov_facade.dispatch(request)
+
+
+# ---------------------------------------------------------------------------
+# Then steps
+# ---------------------------------------------------------------------------
+
+
+@then(r'the opt-val response status should be "(?P<status>[^"]+)"')
+def step_ov_response_status(context: Context, status: str) -> None:
+    assert (context.ov_response.error is None) == (status == "ok"), (
+        f"Expected status '{status}', got error={context.ov_response.error}"
+    )
+
+
+@then(r'the opt-val response data should contain key "(?P<key>[^"]+)"')
+def step_ov_data_has_key(context: Context, key: str) -> None:
+    assert key in context.ov_response.result, (
+        f"Expected key '{key}' in response data, "
+        f"got: {list(context.ov_response.result.keys())}"
+    )
+
+
+@then(
+    r'the opt-val response data key "(?P<key>[^"]+)" should equal "(?P<value>[^"]+)"'
+)
+def step_ov_data_key_equals(context: Context, key: str, value: str) -> None:
+    actual = context.ov_response.result.get(key)
+    assert str(actual) == value, f"Expected data['{key}'] = '{value}', got '{actual}'"
+
+
+@then(r'the opt-val response error message should contain "(?P<fragment>[^"]+)"')
+def step_ov_error_message_contains(context: Context, fragment: str) -> None:
+    assert context.ov_response.error is not None, "No error in response"
+    assert fragment in context.ov_response.error.message, (
+        f"Expected message containing '{fragment}', "
+        f"got '{context.ov_response.error.message}'"
+    )
+
+
+# Reset step matcher to parse (default) so subsequent step files are not affected
+use_step_matcher("parse")
diff --git a/src/cleveragents/a2a/facade.py b/src/cleveragents/a2a/facade.py
index d535111fe..66952d15a 100644
--- a/src/cleveragents/a2a/facade.py
+++ b/src/cleveragents/a2a/facade.py
@@ -539,10 +539,19 @@ class A2aLocalFacade:
         action_name = params.get("action_name", "")
         if not action_name:
             raise ValueError("action_name is required")
+        arguments = params.get("arguments")
+        if arguments is not None and not isinstance(arguments, dict):
+            raise TypeError("arguments must be a dict or None")
+        created_by = params.get("created_by")
+        if created_by is not None:
+            if not isinstance(created_by, str):
+                raise TypeError("created_by must be a string or None")
+            if not created_by:
+                raise ValueError("created_by must be a non-empty string if provided")
         plan = svc.use_action(
             action_name=action_name,
-            arguments=params.get("arguments"),
-            created_by=params.get("created_by"),
+            arguments=arguments,
+            created_by=created_by,
         )
         return {"plan_id": plan.identity.plan_id, "status": "created"}
 
@@ -610,6 +619,11 @@ class A2aLocalFacade:
         if registry is None:
             return {"tools": []}
         namespace: str | None = params.get("namespace")
+        if namespace is not None:
+            if not isinstance(namespace, str):
+                raise TypeError("namespace must be a string or None")
+            if not namespace:
+                raise ValueError("namespace must be a non-empty string if provided")
         specs = registry.list_tools(namespace=namespace)
         return {
             "tools": [{"name": s.name, "description": s.description} for s in specs],
@@ -620,6 +634,11 @@ class A2aLocalFacade:
         if svc is None:
             return {"resources": []}
         type_name: str | None = params.get("type_name")
+        if type_name is not None:
+            if not isinstance(type_name, str):
+                raise TypeError("type_name must be a string or None")
+            if not type_name:
+                raise ValueError("type_name must be a non-empty string if provided")
         resources = svc.list_resources(type_name=type_name)
         return {
             "resources": [