diff --git a/workspace-server/internal/handlers/cross_tenant_isolation_test.go b/workspace-server/internal/handlers/cross_tenant_isolation_test.go
index 10cd91ba9..8289ddf10 100644
--- a/workspace-server/internal/handlers/cross_tenant_isolation_test.go
+++ b/workspace-server/internal/handlers/cross_tenant_isolation_test.go
@@ -294,8 +294,9 @@ func TestProxyA2A_CrossTenant_RoutingDenied(t *testing.T) {
 	// A URL exists for the target; the guard must deny BEFORE it is used.
 	mr.Set(fmt.Sprintf("ws:%s:url", target), "http://localhost:1")
 
-	// CanCommunicate: both root-level (parent_id NULL) → its weak "root-level
-	// siblings" rule ALLOWS this. The org guard must catch it afterward.
+	// Post-#1955: CanCommunicate no longer has the root-sibling bypass.
+	// Both root-level (parent_id NULL) but unrelated org roots → hierarchy
+	// check DENIES with 403 BEFORE the org-scope guard or resolveAgentURL.
 	mock.ExpectQuery("SELECT id, parent_id FROM workspaces WHERE id = ").
 		WithArgs(caller).
 		WillReturnRows(sqlmock.NewRows([]string{"id", "parent_id"}).AddRow(caller, nil))
@@ -303,15 +304,6 @@ func TestProxyA2A_CrossTenant_RoutingDenied(t *testing.T) {
 		WithArgs(target).
 		WillReturnRows(sqlmock.NewRows([]string{"id", "parent_id"}).AddRow(target, nil))
 
-	// #1953 org-scope guard: caller resolves to org-a-root, target to org-b-root
-	// → different orgs → 403. (Each org root resolves to itself.)
-	mock.ExpectQuery("WITH RECURSIVE org_chain AS").
-		WithArgs(caller).
-		WillReturnRows(sqlmock.NewRows([]string{"root_id"}).AddRow(caller))
-	mock.ExpectQuery("WITH RECURSIVE org_chain AS").
-		WithArgs(target).
-		WillReturnRows(sqlmock.NewRows([]string{"root_id"}).AddRow(target))
-
 	w := httptest.NewRecorder()
 	c, _ := gin.CreateTestContext(w)
 	c.Params = gin.Params{{Key: "id", Value: target}}
@@ -329,8 +321,8 @@ func TestProxyA2A_CrossTenant_RoutingDenied(t *testing.T) {
 	if err := json.Unmarshal(w.Body.Bytes(), &resp); err != nil {
 		t.Fatalf("body not JSON: %v", err)
 	}
-	if msg, _ := resp["error"].(string); !strings.Contains(msg, "different org") {
-		t.Errorf("expected cross-org denial message, got %v", resp["error"])
+	if msg, _ := resp["error"].(string); !strings.Contains(msg, "cannot communicate") {
+		t.Errorf("expected hierarchy denial message, got %v", resp["error"])
 	}
 	if err := mock.ExpectationsWereMet(); err != nil {
 		t.Errorf("unmet sqlmock expectations: %v", err)
diff --git a/workspace-server/internal/handlers/delegation_executor_integration_test.go b/workspace-server/internal/handlers/delegation_executor_integration_test.go
index 193ca385d..dd7d5cfa8 100644
--- a/workspace-server/internal/handlers/delegation_executor_integration_test.go
+++ b/workspace-server/internal/handlers/delegation_executor_integration_test.go
@@ -55,6 +55,7 @@ import (
 const integrationTestDelegationID = "del-159-test-integration"
 const integrationTestSourceID = "aaaaaaaa-aaaa-aaaa-aaaa-aaaaaaaaaaaa"
 const integrationTestTargetID = "bbbbbbbb-bbbb-bbbb-bbbb-bbbbbbbbbbbb"
+const integrationTestParentID = "cccccccc-cccc-cccc-cccc-cccccccccccc"
 
 // rawHTTPServer starts a TCP listener, serves one HTTP response, and closes.
 // It runs in a background goroutine so the test can proceed immediately after
diff --git a/workspace-server/internal/handlers/handlers_additional_test.go b/workspace-server/internal/handlers/handlers_additional_test.go
index 9916f0469..64c2af2d4 100644
--- a/workspace-server/internal/handlers/handlers_additional_test.go
+++ b/workspace-server/internal/handlers/handlers_additional_test.go
@@ -576,13 +576,14 @@ func TestDiscover_TargetOffline(t *testing.T) {
 	setupTestRedis(t)
 	handler := NewDiscoveryHandler()
 
-	// Both root-level, access allowed
+	// Share a parent so communication is allowed under post-#1955 rules
+	sharedParent := "ws-parent"
 	mock.ExpectQuery("SELECT id, parent_id FROM workspaces WHERE id =").
 		WithArgs("ws-caller").
-		WillReturnRows(sqlmock.NewRows([]string{"id", "parent_id"}).AddRow("ws-caller", nil))
+		WillReturnRows(sqlmock.NewRows([]string{"id", "parent_id"}).AddRow("ws-caller", sharedParent))
 	mock.ExpectQuery("SELECT id, parent_id FROM workspaces WHERE id =").
 		WithArgs("ws-off").
-		WillReturnRows(sqlmock.NewRows([]string{"id", "parent_id"}).AddRow("ws-off", nil))
+		WillReturnRows(sqlmock.NewRows([]string{"id", "parent_id"}).AddRow("ws-off", sharedParent))
 
 	// Name + runtime lookup (discovery now queries both)
 	mock.ExpectQuery("SELECT COALESCE").
@@ -622,13 +623,14 @@ func TestCheckAccess_SiblingsAllowed(t *testing.T) {
 	setupTestRedis(t)
 	handler := NewDiscoveryHandler()
 
-	// Both root-level siblings → allowed
+	// Share a parent so communication is allowed under post-#1955 rules
+	sharedParent := "ws-parent"
 	mock.ExpectQuery("SELECT id, parent_id FROM workspaces WHERE id =").
 		WithArgs("ws-a").
-		WillReturnRows(sqlmock.NewRows([]string{"id", "parent_id"}).AddRow("ws-a", nil))
+		WillReturnRows(sqlmock.NewRows([]string{"id", "parent_id"}).AddRow("ws-a", sharedParent))
 	mock.ExpectQuery("SELECT id, parent_id FROM workspaces WHERE id =").
 		WithArgs("ws-b").
-		WillReturnRows(sqlmock.NewRows([]string{"id", "parent_id"}).AddRow("ws-b", nil))
+		WillReturnRows(sqlmock.NewRows([]string{"id", "parent_id"}).AddRow("ws-b", sharedParent))
 
 	w := httptest.NewRecorder()
 	c, _ := gin.CreateTestContext(w)
diff --git a/workspace-server/internal/handlers/handlers_extended_test.go b/workspace-server/internal/handlers/handlers_extended_test.go
index 59cd59e6e..0e0fecaa3 100644
--- a/workspace-server/internal/handlers/handlers_extended_test.go
+++ b/workspace-server/internal/handlers/handlers_extended_test.go
@@ -374,14 +374,14 @@ func TestExtended_DiscoverWithCallerID(t *testing.T) {
 	handler := NewDiscoveryHandler()
 
 	// CanCommunicate needs to look up both workspaces
-	// Caller: root-level (no parent)
+	// Share a parent so communication is allowed under post-#1955 rules
+	sharedParent := "ws-parent"
 	mock.ExpectQuery("SELECT id, parent_id FROM workspaces WHERE id =").
 		WithArgs("ws-caller").
-		WillReturnRows(sqlmock.NewRows([]string{"id", "parent_id"}).AddRow("ws-caller", nil))
-	// Target: also root-level (no parent) — root-level siblings are allowed
+		WillReturnRows(sqlmock.NewRows([]string{"id", "parent_id"}).AddRow("ws-caller", sharedParent))
 	mock.ExpectQuery("SELECT id, parent_id FROM workspaces WHERE id =").
 		WithArgs("ws-target").
-		WillReturnRows(sqlmock.NewRows([]string{"id", "parent_id"}).AddRow("ws-target", nil))
+		WillReturnRows(sqlmock.NewRows([]string{"id", "parent_id"}).AddRow("ws-target", sharedParent))
 
 	// Discover handler looks up workspace name + runtime
 	mock.ExpectQuery("SELECT COALESCE").
@@ -515,13 +515,14 @@ func TestExtended_CheckAccess(t *testing.T) {
 	handler := NewDiscoveryHandler()
 
 	// CanCommunicate will look up both workspaces
-	// Both root-level — should be allowed
+	// Share a parent so communication is allowed under post-#1955 rules
+	sharedParent := "ws-parent"
 	mock.ExpectQuery("SELECT id, parent_id FROM workspaces WHERE id =").
 		WithArgs("ws-a").
-		WillReturnRows(sqlmock.NewRows([]string{"id", "parent_id"}).AddRow("ws-a", nil))
+		WillReturnRows(sqlmock.NewRows([]string{"id", "parent_id"}).AddRow("ws-a", sharedParent))
 	mock.ExpectQuery("SELECT id, parent_id FROM workspaces WHERE id =").
 		WithArgs("ws-b").
-		WillReturnRows(sqlmock.NewRows([]string{"id", "parent_id"}).AddRow("ws-b", nil))
+		WillReturnRows(sqlmock.NewRows([]string{"id", "parent_id"}).AddRow("ws-b", sharedParent))
 
 	w := httptest.NewRecorder()
 	c, _ := gin.CreateTestContext(w)
diff --git a/workspace-server/internal/registry/access.go b/workspace-server/internal/registry/access.go
index 56a9c419e..8b07624b5 100644
--- a/workspace-server/internal/registry/access.go
+++ b/workspace-server/internal/registry/access.go
@@ -99,11 +99,6 @@ func CanCommunicate(callerID, targetID string) bool {
 		*caller.ParentID == *target.ParentID {
 		return true
 	}
-	// Root-level siblings — both have no parent
-	if caller.ParentID == nil && target.ParentID == nil {
-		return true
-	}
-
 	// Direct parent → child (fast path; avoids the ancestor walk)
 	if target.ParentID != nil && caller.ID == *target.ParentID {
 		return true
diff --git a/workspace-server/internal/registry/access_test.go b/workspace-server/internal/registry/access_test.go
index b01a14691..ec20b28bd 100644
--- a/workspace-server/internal/registry/access_test.go
+++ b/workspace-server/internal/registry/access_test.go
@@ -63,14 +63,16 @@ func TestCanCommunicate_Siblings(t *testing.T) {
 	}
 }
 
-func TestCanCommunicate_RootSiblings(t *testing.T) {
+func TestCanCommunicate_Denied_RootSiblings(t *testing.T) {
 	mock := setupMockDB(t)
-	// Both at root level (no parent)
+	// Two unrelated org roots (both parent_id = NULL) must NOT communicate.
+	// This is the regression test for #1955: without an org_id column, two
+	// root workspaces have no shared ancestor, so they must be denied.
 	expectLookup(mock, "ws-a", nil)
 	expectLookup(mock, "ws-b", nil)
 
-	if !CanCommunicate("ws-a", "ws-b") {
-		t.Error("root-level siblings should communicate")
+	if CanCommunicate("ws-a", "ws-b") {
+		t.Error("unrelated root-level workspaces should NOT communicate")
 	}
 }