diff --git a/CONTRIBUTORS.md b/CONTRIBUTORS.md
index 9d758483c..8b384231f 100644
--- a/CONTRIBUTORS.md
+++ b/CONTRIBUTORS.md
@@ -130,3 +130,4 @@ Below are some specific details of individual PR contributions.
 * HAL 9000 has contributed the PyYAML security hardening fix (PR #11017 / issue #11012): added `pyyaml>=6.0.3` as an explicit runtime dependency in `pyproject.toml` to mitigate CVE-2025-8045, replacing the previous implicit transitive-only dependency chain that left YAML config loading vulnerable to silent supply-chain breakage from upstream dependency changes.
 * HAL 9000 has contributed the plan explain structured alternatives format fix (PR #11090): updated `_build_explain_dict()` in `src/cleveragents/cli/commands/plan.py` to convert the `alternatives_considered` list into structured objects with `index` (1-based), `description`, and `chosen` fields in the `alternatives` output key, aligning the `agents plan explain` output with the spec-required format.
 * HAL 9000 has contributed the plan tree JSON/YAML spec-compliant envelope fix (issue #11041): wrapped `agents plan tree` JSON and YAML output in the spec-required command envelope (`command`, `status`, `exit_code`, `data`, `timing`, `messages`), updated BDD step definitions to validate envelope structure, and removed the `@tdd_expected_fail` tag from the previously-failing JSON tree format test (issue #4254).
+* HAL 9000 has contributed the a2a session_id validation fix (PR #11098 / issue #9250): moved the session_id validation guard to the top of `_handle_session_close()` in `A2aLocalFacade`, closing the validation bypass path where empty or null session IDs could slip through to devcontainer cleanup when `SessionService` was not wired.
diff --git a/features/a2a_facade_coverage.feature b/features/a2a_facade_coverage.feature
index 6609bf92f..9ca9287ae 100644
--- a/features/a2a_facade_coverage.feature
+++ b/features/a2a_facade_coverage.feature
@@ -20,11 +20,27 @@ Feature: A2A local facade coverage — uncovered handler and edge-case paths
     Then the facade-cov response status should be "ok"
     And the facade-cov response data key "status" should equal "closed"
 
-  Scenario: Session close with empty session_id and no service
+  # -------------------------------------------------------------------
+  # Session close — session_id validation guard (lines 321-324)
+  # -------------------------------------------------------------------
+
+  @tdd_issue @tdd_issue_9250
+  Scenario: Session close with empty session_id and no service raises ValueError error
     Given a facade-cov facade with no services
     When I dispatch facade-cov operation "session.close" with params {"session_id": ""}
-    Then the facade-cov response status should be "ok"
-    And the facade-cov response data key "status" should equal "closed"
+    Then the facade-cov response status should be "error"
+
+  @tdd_issue @tdd_issue_9250
+  Scenario: Session close with missing session_id key and no service raises ValueError error
+    Given a facade-cov facade with no services
+    When I dispatch facade-cov operation "session.close" with params {}
+    Then the facade-cov response status should be "error"
+
+  @tdd_issue @tdd_issue_9250
+  Scenario: Session close with empty session_id and wired service raises ValueError error
+    Given a facade-cov facade with a mock SessionService
+    When I dispatch facade-cov operation "session.close" with params {"session_id": ""}
+    Then the facade-cov response status should be "error"
 
   # -------------------------------------------------------------------
   # Plan cancel — with service wired (lines 501-502)
diff --git a/features/a2a_facade_wiring.feature b/features/a2a_facade_wiring.feature
index cf27d1eac..c2ee8f207 100644
--- a/features/a2a_facade_wiring.feature
+++ b/features/a2a_facade_wiring.feature
@@ -34,6 +34,15 @@ Feature: A2A local facade wiring to live services
     When I dispatch wired operation "session.close" with params {}
     Then the wired response status should be "error"
 
+  # ---------------------------------------------------------------
+  # Validate session_id at entry — no service scenario (PR #9250)
+  # ---------------------------------------------------------------
+
+  Scenario: session.close without session_id and no service returns error
+    Given a wired A2aLocalFacade with no services
+    When I dispatch wired operation "session.close" with params {"session_id": ""}
+    Then the wired response status should be "error"
+
   # ---------------------------------------------------------------
   # Plan lifecycle wiring
   # ---------------------------------------------------------------
diff --git a/features/consolidated_misc.feature b/features/consolidated_misc.feature
index e8e21a0a5..575371f6a 100644
--- a/features/consolidated_misc.feature
+++ b/features/consolidated_misc.feature
@@ -42,7 +42,7 @@ Feature: Consolidated Misc
 
   Scenario: Dispatch session.close returns status closed
     Given a new A2aLocalFacade with no services
-    When I dispatch operation "session.close" with params {}
+    When I dispatch operation "session.close" with params {"session_id": "01M6SM0KESESS0N00000000000"}
     Then the response status should be "ok"
     And response data key "status" equals "closed"
 
@@ -563,7 +563,7 @@ Feature: Consolidated Misc
   Scenario: M6 smoke A2A session close returns closed status
     Given a m6 smoke test runner
     And a m6 smoke A2A local facade
-    When I m6 smoke dispatch "session.close" with params {}
+    When I m6 smoke dispatch "session.close" with params {"session_id": "01M6SM0KESESS0N00000000000"}
     Then the m6 smoke response status should be "ok"
     And the m6 smoke response data "status" should equal "closed"
 
diff --git a/features/m6_autonomy_acceptance.feature b/features/m6_autonomy_acceptance.feature
index d2a832996..ae38e7698 100644
--- a/features/m6_autonomy_acceptance.feature
+++ b/features/m6_autonomy_acceptance.feature
@@ -35,7 +35,7 @@ Feature: M6 autonomy acceptance smoke tests
     And the m6 smoke response data should contain key "status"
 
   Scenario: M6 smoke A2A session close returns closed status
-    When I m6 smoke dispatch "session.close" with params {}
+    When I m6 smoke dispatch "session.close" with params {"session_id": "01M6SM0KESESS0N00000000000"}
     Then the m6 smoke response status should be "ok"
     And the m6 smoke response data "status" should equal "closed"
 
diff --git a/features/steps/a2a_facade_coverage_steps.py b/features/steps/a2a_facade_coverage_steps.py
index d2413bd4b..3a0c7d100 100644
--- a/features/steps/a2a_facade_coverage_steps.py
+++ b/features/steps/a2a_facade_coverage_steps.py
@@ -297,6 +297,14 @@ def step_fc_data_no_key(context: Context, key: str) -> None:
     )
 
 
+@then(r"the facade-cov response error should contain 'session_id is required'")
+def step_fc_error_contains_session_id(context: Context) -> None:
+    assert context.fc_response.error is not None, "Expected an error in the response"
+    assert "session_id is required" in context.fc_response.error.message, (
+        f"Expected error containing 'session_id is required', got: {context.fc_response.error.message}"
+    )
+
+
 @then(r"the facade-cov response error should not be None")
 def step_fc_error_not_none(context: Context) -> None:
     assert context.fc_response.error is not None, "Expected an error in the response"
diff --git a/robot/helper_m6_autonomy_acceptance.py b/robot/helper_m6_autonomy_acceptance.py
index a0239448c..63627b72c 100644
--- a/robot/helper_m6_autonomy_acceptance.py
+++ b/robot/helper_m6_autonomy_acceptance.py
@@ -48,8 +48,11 @@ def facade_session() -> None:
     resp_create = facade.dispatch(A2aRequest(method="session.create", params={}))
     assert resp_create.result is not None, f"Expected ok, got {resp_create.status}"
     assert "session_id" in resp_create.result
+    session_id = resp_create.result["session_id"]
 
-    resp_close = facade.dispatch(A2aRequest(method="session.close", params={}))
+    resp_close = facade.dispatch(
+        A2aRequest(method="session.close", params={"session_id": session_id})
+    )
     assert resp_close.result is not None
     assert resp_close.result["status"] == "closed"
 
diff --git a/src/cleveragents/a2a/facade.py b/src/cleveragents/a2a/facade.py
index fcafae1b1..f5d27ce56 100644
--- a/src/cleveragents/a2a/facade.py
+++ b/src/cleveragents/a2a/facade.py
@@ -364,6 +364,13 @@ class A2aLocalFacade:
     def _handle_session_close(self, params: dict[str, Any]) -> dict[str, Any]:
         session_id = params.get("session_id", "")
 
+        # Validate session_id before any cleanup or service operations.
+        # This prevents best-effort devcontainer cleanup from running on
+        # an invalid or omitted session identifier, which could trigger
+        # incorrect container lifecycle operations on wrong/unknown sessions.
+        if not session_id:
+            raise ValueError("session_id is required")
+
         svc = self._session_service
         if svc is None:
             # R7-F4 fix: still run container cleanup even without a
@@ -371,8 +378,6 @@ class A2aLocalFacade:
             self._cleanup_session_devcontainers(session_id)
             return {"status": "closed"}
 
-        if not session_id:
-            raise ValueError("session_id is required")
         svc.delete(session_id)
 
         # R7-F4 fix: run container cleanup after session deletion.
@@ -503,10 +508,9 @@ class A2aLocalFacade:
         """Best-effort stop of devcontainers associated with a session.
 
         Failures are logged but never propagated so session close always
-        succeeds.
+        succeeds. ``session_id`` is guaranteed non-empty by the entry
+        validation in :meth:`_handle_session_close`.
         """
-        if not session_id:
-            return
         try:
             from cleveragents.application.services.cleanup_service import (
                 CleanupService,