diff --git a/tests/e2e/test_api.sh b/tests/e2e/test_api.sh
index 770e627c8..26d443e8e 100644
--- a/tests/e2e/test_api.sh
+++ b/tests/e2e/test_api.sh
@@ -73,7 +73,15 @@ else
 fi
 
 # Test 4: Create workspace B (needs bearer — tokens now exist in DB)
-R=$(acurl -X POST "$BASE/workspaces" -H "Content-Type: application/json" -d '{"name":"Summarizer Agent","tier":1,"runtime":"external","external":true}')
+# #1953 cross-tenant isolation: Summarizer is created as a CHILD of Echo so the
+# two live in the SAME org (Echo is the org root; Summarizer hangs off it via
+# parent_id). The peer-discovery tests below assert same-org peer enumeration
+# (Echo sees its child, the child sees its parent). Previously both were created
+# parent_id=NULL — two DISTINCT org roots — and "peers" only listed each other
+# via the `WHERE parent_id IS NULL` branch that returned every tenant's org root.
+# That branch WAS the cross-tenant leak (#1953) and is now removed, so two org
+# roots no longer see each other; the assertions must run inside one org.
+R=$(acurl -X POST "$BASE/workspaces" -H "Content-Type: application/json" -d "{\"name\":\"Summarizer Agent\",\"tier\":1,\"runtime\":\"external\",\"external\":true,\"parent_id\":\"$ECHO_ID\"}")
 check "POST /workspaces (create summarizer)" '"status":"awaiting_agent"' "$R"
 SUM_ID=$(echo "$R" | python3 -c "import sys,json; print(json.load(sys.stdin)['id'])")
 
@@ -133,21 +141,23 @@ check "Heartbeat updated uptime" '"uptime_seconds":120' "$R"
 R=$(curl -s "$BASE/registry/discover/$ECHO_ID")
 check "GET /registry/discover/:id (missing caller rejected)" 'X-Workspace-ID header is required' "$R"
 
-# Test 12: Discover (from sibling — allowed)
+# Test 12: Discover (from same-org child — allowed)
 R=$(curl -s "$BASE/registry/discover/$ECHO_ID" -H "X-Workspace-ID: $SUM_ID" -H "Authorization: Bearer $SUM_TOKEN")
-check "GET /registry/discover/:id (sibling)" '"url"' "$R"
+check "GET /registry/discover/:id (same-org)" '"url"' "$R"
 
-# Test 13: Peers (root siblings see each other)
+# Test 13: Peers — same-org parent/child see each other (#1953). Echo is the org
+# root and lists its child Summarizer; Summarizer lists its parent Echo. A
+# cross-org workspace would NOT appear here (see cross_tenant_isolation_test.go).
 R=$(curl -s "$BASE/registry/$ECHO_ID/peers" -H "Authorization: Bearer $ECHO_TOKEN")
 check "GET /registry/:id/peers (has summarizer)" '"Summarizer' "$R"
 
 R=$(curl -s "$BASE/registry/$SUM_ID/peers" -H "Authorization: Bearer $SUM_TOKEN")
 check "GET /registry/:id/peers (has echo)" '"Echo Agent"' "$R"
 
-# Test 14: Check access (root siblings)
+# Test 14: Check access (same-org parent↔child — allowed)
 R=$(curl -s -X POST "$BASE/registry/check-access" -H "Content-Type: application/json" \
   -d "{\"caller_id\":\"$ECHO_ID\",\"target_id\":\"$SUM_ID\"}")
-check "POST /registry/check-access (siblings allowed)" '"allowed":true' "$R"
+check "POST /registry/check-access (same-org allowed)" '"allowed":true' "$R"
 
 # Test 15: PATCH workspace (update position)
 R=$(acurl -X PATCH "$BASE/workspaces/$ECHO_ID" -H "Content-Type: application/json" -d '{"x":100,"y":200}')
@@ -289,32 +299,40 @@ R=$(curl -s "$BASE/workspaces" -H "Authorization: Bearer $ECHO_TOKEN")
 check "current_task in list response" '"current_task"' "$R"
 
 # Test 21: Delete
-R=$(acurl -X DELETE "$BASE/workspaces/$ECHO_ID?confirm=true" \
-  -H "Authorization: Bearer $ECHO_TOKEN" \
-  -H "X-Confirm-Name: Echo Agent v2")
-check "DELETE /workspaces/:id" '"status":"removed"' "$R"
-
-R=$(curl -s "$BASE/workspaces" -H "Authorization: Bearer $SUM_TOKEN")
-COUNT=$(echo "$R" | python3 -c "import sys,json; print(len(json.load(sys.stdin)))")
-check "List after delete (count=1)" "1" "$COUNT"
-
-# Test 22: Bundle round-trip — export → delete → import → verify same config
-echo ""
-echo "--- Bundle Round-Trip Test ---"
-
-# Export the summarizer workspace (#165 / PR #167 — admin-gated)
+# #1953: Summarizer is now a CHILD of Echo (same-org, for the peer-discovery
+# tests above). DELETE on the *parent* (Echo) cascade-removes its descendants
+# (CascadeDelete walks the recursive `parent_id` CTE), so deleting Echo first
+# would also remove Summarizer and the "one survives" assertion would see 0.
+# Delete the CHILD (Summarizer) here instead: a child delete does NOT cascade
+# upward, so the parent Echo survives and count=1 holds. The bundle round-trip
+# below needs Summarizer's exported config, so capture it BEFORE this delete.
 BUNDLE=$(curl -s "$BASE/bundles/export/$SUM_ID" -H "Authorization: Bearer $SUM_TOKEN")
 check "GET /bundles/export/:id" '"name":"Summarizer Agent"' "$BUNDLE"
-
-# Capture original config for comparison
 ORIG_NAME=$(echo "$BUNDLE" | python3 -c "import sys,json; print(json.load(sys.stdin)['name'])")
 ORIG_TIER=$(echo "$BUNDLE" | python3 -c "import sys,json; print(json.load(sys.stdin)['tier'])")
 
-# Delete the workspace — use SUM_TOKEN (per-workspace) for WorkspaceAuth
-# and ADMIN_TOKEN for the AdminAuth layer.
-R=$(curl -s -X DELETE "$BASE/workspaces/$SUM_ID?confirm=true" \
+R=$(acurl -X DELETE "$BASE/workspaces/$SUM_ID?confirm=true" \
   -H "Authorization: Bearer $SUM_TOKEN" \
   -H "X-Confirm-Name: Summarizer Agent")
+check "DELETE /workspaces/:id" '"status":"removed"' "$R"
+
+# Parent Echo must survive a child delete — list as Echo and expect count=1.
+R=$(curl -s "$BASE/workspaces" -H "Authorization: Bearer $ECHO_TOKEN")
+COUNT=$(echo "$R" | python3 -c "import sys,json; print(len(json.load(sys.stdin)))")
+check "List after delete (count=1)" "1" "$COUNT"
+
+# Test 22: Bundle round-trip — export → delete → import → verify same config.
+# Summarizer's bundle was captured above; now delete the parent Echo (the only
+# remaining workspace) so the import lands in a clean org, then re-import the
+# Summarizer bundle.
+echo ""
+echo "--- Bundle Round-Trip Test ---"
+
+# Delete the remaining parent Echo — use ECHO_TOKEN (per-workspace) for
+# WorkspaceAuth and ADMIN_TOKEN for the AdminAuth layer.
+R=$(acurl -X DELETE "$BASE/workspaces/$ECHO_ID?confirm=true" \
+  -H "Authorization: Bearer $ECHO_TOKEN" \
+  -H "X-Confirm-Name: Echo Agent v2")
 check "Delete before re-import" '"status":"removed"' "$R"
 
 # After deleting both workspaces, all per-workspace tokens are revoked.
diff --git a/workspace-server/internal/handlers/a2a_proxy.go b/workspace-server/internal/handlers/a2a_proxy.go
index 46aa78f66..04f52af0a 100644
--- a/workspace-server/internal/handlers/a2a_proxy.go
+++ b/workspace-server/internal/handlers/a2a_proxy.go
@@ -375,6 +375,30 @@ func (h *WorkspaceHandler) proxyA2ARequest(ctx context.Context, workspaceID stri
 				Response: gin.H{"error": "access denied: workspaces cannot communicate per hierarchy rules"},
 			}
 		}
+
+		// #1953 cross-tenant isolation. CanCommunicate alone does NOT enforce
+		// org boundaries: its "root-level siblings — both have no parent" rule
+		// treats every tenant's org root as a sibling, so a caller that is an
+		// org root could resolve and route a2a to another tenant's org root
+		// (and resolveAgentURL accepts ANY workspace id with no org check).
+		// Gate on the SAME parent_id-chain org scoping the OFFSEC-015 broadcast
+		// fix uses: reject before resolveAgentURL when caller and target are in
+		// different orgs. Fail-closed — a DB error denies cross-org routing.
+		ok, err := sameOrg(ctx, db.DB, callerID, workspaceID)
+		if err != nil {
+			log.Printf("ProxyA2A: org-scope check failed %s → %s: %v — denying", callerID, workspaceID, err)
+			return 0, nil, &proxyA2AError{
+				Status:   http.StatusForbidden,
+				Response: gin.H{"error": "access denied: org isolation check failed"},
+			}
+		}
+		if !ok {
+			log.Printf("ProxyA2A: cross-org routing denied %s → %s (#1953)", callerID, workspaceID)
+			return 0, nil, &proxyA2AError{
+				Status:   http.StatusForbidden,
+				Response: gin.H{"error": "access denied: target workspace is in a different org"},
+			}
+		}
 	}
 
 	// Budget enforcement: reject A2A calls when the workspace has exceeded its
diff --git a/workspace-server/internal/handlers/a2a_proxy_test.go b/workspace-server/internal/handlers/a2a_proxy_test.go
index 2e997d168..6a254a7f5 100644
--- a/workspace-server/internal/handlers/a2a_proxy_test.go
+++ b/workspace-server/internal/handlers/a2a_proxy_test.go
@@ -437,6 +437,10 @@ func TestProxyA2A_CallerIDPropagated(t *testing.T) {
 		WithArgs("ws-target").
 		WillReturnRows(sqlmock.NewRows([]string{"id", "parent_id"}).AddRow("ws-target", "ws-parent"))
 
+	// #1953 cross-tenant guard: same-org check after CanCommunicate. Both
+	// workspaces resolve to the same org root → routing allowed.
+	mockSameOrg(mock, "ws-caller", "ws-target", true)
+
 	expectBudgetCheck(mock, "ws-target")
 
 	// Expect activity log with source_id set
@@ -465,6 +469,24 @@ func TestProxyA2A_CallerIDPropagated(t *testing.T) {
 	}
 }
 
+// mockSameOrg sets up the two org-root recursive-CTE expectations that the
+// #1953 cross-tenant guard in proxyA2ARequest runs after CanCommunicate passes.
+// sameOrg=true returns the SAME root_id for both caller and target (same tenant);
+// sameOrg=false returns different root_ids (cross-tenant → routing must be denied).
+func mockSameOrg(mock sqlmock.Sqlmock, caller, target string, sameOrg bool) {
+	callerRoot := "org-root-shared"
+	targetRoot := "org-root-shared"
+	if !sameOrg {
+		targetRoot = "org-root-other-tenant"
+	}
+	mock.ExpectQuery("WITH RECURSIVE org_chain AS").
+		WithArgs(caller).
+		WillReturnRows(sqlmock.NewRows([]string{"root_id"}).AddRow(callerRoot))
+	mock.ExpectQuery("WITH RECURSIVE org_chain AS").
+		WithArgs(target).
+		WillReturnRows(sqlmock.NewRows([]string{"root_id"}).AddRow(targetRoot))
+}
+
 // mockCanCommunicate sets up sqlmock expectations for CanCommunicate(caller, target).
 // allowed=true sets up rows that satisfy the access policy (siblings under same parent).
 // allowed=false sets up rows that don't (different parents).
@@ -659,6 +681,9 @@ func TestProxyA2A_CallerIDDerivedFromBearer(t *testing.T) {
 		WithArgs("ws-target").
 		WillReturnRows(sqlmock.NewRows([]string{"id", "parent_id"}).AddRow("ws-target", "ws-parent"))
 
+	// 3b. #1953 cross-tenant guard — same org root → routing allowed.
+	mockSameOrg(mock, "ws-caller", "ws-target", true)
+
 	expectBudgetCheck(mock, "ws-target")
 
 	// 4. activity_logs INSERT — verify source_id arg is the derived ws-caller
diff --git a/workspace-server/internal/handlers/cross_tenant_isolation_test.go b/workspace-server/internal/handlers/cross_tenant_isolation_test.go
new file mode 100644
index 000000000..10cd91ba9
--- /dev/null
+++ b/workspace-server/internal/handlers/cross_tenant_isolation_test.go
@@ -0,0 +1,427 @@
+package handlers
+
+// cross_tenant_isolation_test.go — #1953 regression tests.
+//
+// Three workspace-server paths historically derived an "org-root sibling set"
+// as `WHERE parent_id IS NULL`, which matches EVERY tenant's org root (the
+// workspaces table has no org_id column) → cross-tenant data exposure:
+//
+//  1. GET /registry/:id/peers   (discovery.Peers)
+//  2. MCP toolListPeers          (mcp_tools.toolListPeers)
+//  3. a2a routing                (a2a_proxy.proxyA2ARequest → resolveAgentURL)
+//
+// These tests assert that a workspace in a DIFFERENT org is never returned as a
+// peer and that a2a refuses to resolve/route to a workspace outside the caller's
+// org, while same-org peers/targets still work. They reuse the SAME parent_id-
+// chain org scoping the OFFSEC-015 broadcast fix introduced (org_scope.go).
+
+import (
+	"bytes"
+	"context"
+	"database/sql"
+	"encoding/json"
+	"fmt"
+	"net/http"
+	"net/http/httptest"
+	"strings"
+	"testing"
+	"time"
+
+	"git.moleculesai.app/molecule-ai/molecule-core/workspace-server/internal/db"
+	"github.com/DATA-DOG/go-sqlmock"
+	"github.com/gin-gonic/gin"
+)
+
+// dbHandleForTest returns the global sqlmock-backed *sql.DB that setupTestDB
+// installs, for tests that need to hand a *sql.DB to a component (e.g.
+// MCPHandler.database, sameOrg) rather than relying on the package-global.
+func dbHandleForTest() *sql.DB { return db.DB }
+
+// peerColsForIsolation matches queryPeerMaps' SELECT column set.
+var peerColsForIsolation = []string{
+	"id", "name", "role", "tier", "status", "agent_card", "url", "parent_id", "active_tasks",
+}
+
+// -------------------------------------------------------------------------
+// Path 1: GET /registry/:id/peers — discovery.Peers
+// -------------------------------------------------------------------------
+
+// TestPeers_CrossTenant_OrgRootNotLeaked is the core #1953 regression for the
+// discovery path. The caller is an org root (parent_id IS NULL). Pre-fix the
+// handler ran `SELECT ... WHERE w.parent_id IS NULL AND w.id != $1`, returning
+// every OTHER tenant's org root as a "sibling" peer. Post-fix an org-root caller
+// issues NO sibling query — its only peers are its own children. If the handler
+// regressed and issued the cross-tenant sibling query, sqlmock would report an
+// unexpected query (the expectation below is intentionally NOT registered) and
+// the test fails.
+func TestPeers_CrossTenant_OrgRootNotLeaked(t *testing.T) {
+	mock := setupTestDB(t)
+	setupTestRedis(t)
+	handler := NewDiscoveryHandler()
+
+	// Behavioural leak test: register the OLD leaky `parent_id IS NULL` sibling
+	// query so that IF the handler still issues it, it returns another tenant's
+	// org root (org-b-root). The fix removes that query for an org-root caller,
+	// so org-b-root must never appear in the output. Unordered matching makes
+	// the leaky-sibling expectation optional — the fix simply never consumes it.
+	mock.MatchExpectationsInOrder(false)
+
+	caller := "org-a-root" // parent_id IS NULL — an org root for tenant A
+
+	// parent_id lookup → NULL (caller is an org root)
+	mock.ExpectQuery("SELECT parent_id FROM workspaces WHERE id =").
+		WithArgs(caller).
+		WillReturnRows(sqlmock.NewRows([]string{"parent_id"}).AddRow(nil))
+
+	// LEAKY sibling query (pre-fix). Returns a DIFFERENT tenant's org root.
+	// The fix must NOT issue this query; if it does, org-b-root leaks into the
+	// peer list and the output assertion below fails.
+	mock.ExpectQuery("SELECT w.id, w.name.*WHERE w.parent_id IS NULL AND w.id != \\$1").
+		WithArgs(caller).
+		WillReturnRows(sqlmock.NewRows(peerColsForIsolation).
+			AddRow("org-b-root", "Org B Root", "lead", 0, "online", []byte("null"), "http://b-root", nil, 0))
+
+	// Children query — caller's own org-A children only. Return one child.
+	mock.ExpectQuery("SELECT w.id, w.name.*WHERE w.parent_id = \\$1 AND w.id != \\$2").
+		WithArgs(caller, caller).
+		WillReturnRows(sqlmock.NewRows(peerColsForIsolation).
+			AddRow("org-a-child", "Org A Child", "worker", 1, "online", []byte("null"), "http://a-child", caller, 0))
+
+	w := httptest.NewRecorder()
+	c, _ := gin.CreateTestContext(w)
+	c.Params = gin.Params{{Key: "id", Value: caller}}
+	c.Request = httptest.NewRequest("GET", "/registry/"+caller+"/peers", nil)
+
+	handler.Peers(c)
+
+	if w.Code != http.StatusOK {
+		t.Fatalf("expected 200, got %d: %s", w.Code, w.Body.String())
+	}
+
+	var peers []map[string]interface{}
+	if err := json.Unmarshal(w.Body.Bytes(), &peers); err != nil {
+		t.Fatalf("failed to parse response: %v", err)
+	}
+
+	// The other-tenant org root must NEVER appear; only the same-org child.
+	for _, p := range peers {
+		if id, _ := p["id"].(string); id == "org-b-root" {
+			t.Fatalf("cross-tenant leak (#1953): org-b-root appeared in org-a-root's peer list: %v", peers)
+		}
+	}
+	if len(peers) != 1 {
+		t.Fatalf("expected exactly 1 peer (same-org child), got %d: %v", len(peers), peers)
+	}
+	// NOTE: ExpectationsWereMet is intentionally NOT asserted — the leaky
+	// sibling expectation is deliberately left unconsumed by the fixed path.
+}
+
+// TestPeers_SameOrg_SiblingsStillWork is the positive companion: a non-root
+// child caller still sees its same-org siblings, children, and parent. This
+// guards against the fix over-scoping and breaking legitimate intra-org
+// discovery.
+func TestPeers_SameOrg_SiblingsStillWork(t *testing.T) {
+	mock := setupTestDB(t)
+	setupTestRedis(t)
+	handler := NewDiscoveryHandler()
+
+	caller := "org-a-child-1"
+	parent := "org-a-root"
+
+	mock.ExpectQuery("SELECT parent_id FROM workspaces WHERE id =").
+		WithArgs(caller).
+		WillReturnRows(sqlmock.NewRows([]string{"parent_id"}).AddRow(parent))
+
+	// Siblings — scoped to the shared parent (one tenant).
+	mock.ExpectQuery("SELECT w.id, w.name.*WHERE w.parent_id = \\$1 AND w.id != \\$2").
+		WithArgs(parent, caller).
+		WillReturnRows(sqlmock.NewRows(peerColsForIsolation).
+			AddRow("org-a-child-2", "Org A Sibling", "worker", 1, "online", []byte("null"), "http://a-sib", parent, 0))
+
+	// Children — none.
+	mock.ExpectQuery("SELECT w.id, w.name.*WHERE w.parent_id = \\$1 AND w.id != \\$2 AND w.status").
+		WithArgs(caller, caller).
+		WillReturnRows(sqlmock.NewRows(peerColsForIsolation))
+
+	// Parent.
+	mock.ExpectQuery("SELECT w.id, w.name.*WHERE w.id = \\$1 AND w.id != \\$2 AND w.status").
+		WithArgs(parent, caller).
+		WillReturnRows(sqlmock.NewRows(peerColsForIsolation).
+			AddRow(parent, "Org A Root", "lead", 0, "online", []byte("null"), "http://a-root", nil, 0))
+
+	w := httptest.NewRecorder()
+	c, _ := gin.CreateTestContext(w)
+	c.Params = gin.Params{{Key: "id", Value: caller}}
+	c.Request = httptest.NewRequest("GET", "/registry/"+caller+"/peers", nil)
+
+	handler.Peers(c)
+
+	if w.Code != http.StatusOK {
+		t.Fatalf("expected 200, got %d: %s", w.Code, w.Body.String())
+	}
+	var peers []map[string]interface{}
+	if err := json.Unmarshal(w.Body.Bytes(), &peers); err != nil {
+		t.Fatalf("failed to parse response: %v", err)
+	}
+	// Sibling + parent = 2 same-org peers.
+	if len(peers) != 2 {
+		t.Fatalf("expected 2 same-org peers (sibling + parent), got %d: %v", len(peers), peers)
+	}
+	names := map[string]bool{}
+	for _, p := range peers {
+		names[fmt.Sprint(p["name"])] = true
+	}
+	if !names["Org A Sibling"] || !names["Org A Root"] {
+		t.Errorf("expected same-org sibling + parent in peer list, got %v", names)
+	}
+	if err := mock.ExpectationsWereMet(); err != nil {
+		t.Errorf("unmet sqlmock expectations: %v", err)
+	}
+}
+
+// -------------------------------------------------------------------------
+// Path 2: MCP toolListPeers — mcp_tools.toolListPeers
+// -------------------------------------------------------------------------
+
+// mcpPeerCols matches toolListPeers' SELECT column set.
+var mcpPeerCols = []string{"id", "name", "role", "status", "tier"}
+
+// TestToolListPeers_CrossTenant_OrgRootNotLeaked is the #1953 regression for
+// the MCP path. Same shape as the discovery test: an org-root caller must NOT
+// enumerate other tenants' org roots. The cross-tenant `parent_id IS NULL`
+// sibling query is intentionally not registered, so if it runs sqlmock fails.
+func TestToolListPeers_CrossTenant_OrgRootNotLeaked(t *testing.T) {
+	mock := setupTestDB(t)
+	mock.MatchExpectationsInOrder(false)
+	h := &MCPHandler{database: dbHandleForTest()}
+
+	caller := "org-a-root"
+
+	mock.ExpectQuery("SELECT parent_id FROM workspaces WHERE id =").
+		WithArgs(caller).
+		WillReturnRows(sqlmock.NewRows([]string{"parent_id"}).AddRow(nil))
+
+	// LEAKY sibling query (pre-fix). Returns another tenant's org root. The fix
+	// must NOT issue this for an org-root caller; if it does, org-b-root leaks
+	// into the output and the assertion below fails. Left optional via
+	// unordered matching, so the fixed path simply never consumes it.
+	mock.ExpectQuery("WHERE w.parent_id IS NULL AND w.id != \\$1").
+		WithArgs(caller).
+		WillReturnRows(sqlmock.NewRows(mcpPeerCols).
+			AddRow("org-b-root", "Org B Root", "lead", "online", 0))
+
+	// Children — caller's own org-A children only.
+	mock.ExpectQuery("WHERE w.parent_id = \\$1 AND w.status").
+		WithArgs(caller).
+		WillReturnRows(sqlmock.NewRows(mcpPeerCols).
+			AddRow("org-a-child", "Org A Child", "worker", "online", 1))
+
+	out, err := h.toolListPeers(context.Background(), caller)
+	if err != nil {
+		t.Fatalf("toolListPeers returned error: %v", err)
+	}
+	if strings.Contains(out, "org-b-root") || strings.Contains(out, "Org B Root") {
+		t.Fatalf("cross-tenant leak (#1953): another tenant's org root appeared in toolListPeers output:\n%s", out)
+	}
+	if !strings.Contains(out, "org-a-child") {
+		t.Errorf("same-org child missing from toolListPeers output:\n%s", out)
+	}
+	// ExpectationsWereMet intentionally NOT asserted — leaky sibling expectation
+	// is deliberately left unconsumed by the fixed path.
+}
+
+// TestToolListPeers_SameOrg_SiblingsStillWork — positive companion for the MCP
+// path: a non-root child still enumerates its same-org siblings + children + parent.
+func TestToolListPeers_SameOrg_SiblingsStillWork(t *testing.T) {
+	mock := setupTestDB(t)
+	h := &MCPHandler{database: dbHandleForTest()}
+
+	caller := "org-a-child-1"
+	parent := "org-a-root"
+
+	mock.ExpectQuery("SELECT parent_id FROM workspaces WHERE id =").
+		WithArgs(caller).
+		WillReturnRows(sqlmock.NewRows([]string{"parent_id"}).AddRow(parent))
+
+	// Siblings — scoped to shared parent.
+	mock.ExpectQuery("WHERE w.parent_id = \\$1 AND w.id != \\$2 AND w.status").
+		WithArgs(parent, caller).
+		WillReturnRows(sqlmock.NewRows(mcpPeerCols).
+			AddRow("org-a-child-2", "Org A Sibling", "worker", "online", 1))
+
+	// Children — none.
+	mock.ExpectQuery("WHERE w.parent_id = \\$1 AND w.status").
+		WithArgs(caller).
+		WillReturnRows(sqlmock.NewRows(mcpPeerCols))
+
+	// Parent.
+	mock.ExpectQuery("WHERE w.id = \\$1 AND w.status").
+		WithArgs(parent).
+		WillReturnRows(sqlmock.NewRows(mcpPeerCols).
+			AddRow(parent, "Org A Root", "lead", "online", 0))
+
+	out, err := h.toolListPeers(context.Background(), caller)
+	if err != nil {
+		t.Fatalf("toolListPeers returned error: %v", err)
+	}
+	if !strings.Contains(out, "Org A Sibling") || !strings.Contains(out, "Org A Root") {
+		t.Errorf("expected same-org sibling + parent in toolListPeers output:\n%s", out)
+	}
+	if err := mock.ExpectationsWereMet(); err != nil {
+		t.Errorf("unmet sqlmock expectations: %v", err)
+	}
+}
+
+// -------------------------------------------------------------------------
+// Path 3: a2a routing — a2a_proxy.proxyA2ARequest / resolveAgentURL
+// -------------------------------------------------------------------------
+
+// TestProxyA2A_CrossTenant_RoutingDenied is the #1953 regression for a2a
+// routing. Caller and target are both org roots (parent_id IS NULL) belonging
+// to DIFFERENT tenants. Pre-fix, CanCommunicate's "root-level siblings" rule
+// waved this through and resolveAgentURL routed to the foreign tenant. Post-fix
+// the org-scope guard resolves each to a different org root and returns 403
+// BEFORE resolveAgentURL/dispatch.
+func TestProxyA2A_CrossTenant_RoutingDenied(t *testing.T) {
+	mock := setupTestDB(t)
+	mr := setupTestRedis(t)
+	broadcaster := newTestBroadcaster()
+	handler := NewWorkspaceHandler(broadcaster, nil, "http://localhost:8080", t.TempDir())
+
+	caller := "org-a-root"
+	target := "org-b-root" // different tenant
+
+	// 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.
+	mock.ExpectQuery("SELECT id, parent_id FROM workspaces WHERE id = ").
+		WithArgs(caller).
+		WillReturnRows(sqlmock.NewRows([]string{"id", "parent_id"}).AddRow(caller, nil))
+	mock.ExpectQuery("SELECT id, parent_id FROM workspaces WHERE id = ").
+		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}}
+	body := `{"method":"message/send","params":{"message":{"role":"user","parts":[{"text":"cross-tenant"}]}}}`
+	c.Request = httptest.NewRequest("POST", "/workspaces/"+target+"/a2a", bytes.NewBufferString(body))
+	c.Request.Header.Set("Content-Type", "application/json")
+	c.Request.Header.Set("X-Workspace-ID", caller)
+
+	handler.ProxyA2A(c)
+
+	if w.Code != http.StatusForbidden {
+		t.Fatalf("expected 403 for cross-tenant a2a routing, got %d: %s", w.Code, w.Body.String())
+	}
+	var resp map[string]interface{}
+	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 err := mock.ExpectationsWereMet(); err != nil {
+		t.Errorf("unmet sqlmock expectations: %v", err)
+	}
+}
+
+// TestResolveAgentURL_CrossTenant_RejectedViaSameOrg is a direct unit test of
+// the sameOrg primitive that gates resolveAgentURL: a target in a different org
+// must be reported as NOT same-org, so the a2a guard rejects it before
+// resolveAgentURL is ever called.
+func TestResolveAgentURL_CrossTenant_RejectedViaSameOrg(t *testing.T) {
+	mock := setupTestDB(t)
+
+	caller := "org-a-root"
+	target := "org-b-root"
+
+	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))
+
+	ok, err := sameOrg(context.Background(), dbHandleForTest(), caller, target)
+	if err != nil {
+		t.Fatalf("sameOrg returned unexpected error: %v", err)
+	}
+	if ok {
+		t.Errorf("expected cross-tenant workspaces to be reported as DIFFERENT orgs, got sameOrg=true")
+	}
+	if err := mock.ExpectationsWereMet(); err != nil {
+		t.Errorf("unmet sqlmock expectations: %v", err)
+	}
+}
+
+// TestProxyA2A_SameOrg_RoutingAllowed — positive companion for a2a: two
+// same-org siblings route successfully (mirrors TestProxyA2A_CallerIDPropagated
+// but named to document the #1953 same-org allow path).
+func TestProxyA2A_SameOrg_RoutingAllowed(t *testing.T) {
+	mock := setupTestDB(t)
+	mr := setupTestRedis(t)
+	allowLoopbackForTest(t)
+	broadcaster := newTestBroadcaster()
+	handler := NewWorkspaceHandler(broadcaster, nil, "http://localhost:8080", t.TempDir())
+	waitForHandlerAsyncBeforeDBCleanup(t, handler)
+
+	caller := "org-a-child-1"
+	target := "org-a-child-2"
+	parent := "org-a-root"
+
+	agentServer := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.Header().Set("Content-Type", "application/json")
+		fmt.Fprint(w, `{"jsonrpc":"2.0","id":"1","result":{}}`)
+	}))
+	defer agentServer.Close()
+	mr.Set(fmt.Sprintf("ws:%s:url", target), agentServer.URL)
+
+	// CanCommunicate — siblings under shared parent.
+	mock.ExpectQuery("SELECT id, parent_id FROM workspaces WHERE id = ").
+		WithArgs(caller).
+		WillReturnRows(sqlmock.NewRows([]string{"id", "parent_id"}).AddRow(caller, parent))
+	mock.ExpectQuery("SELECT id, parent_id FROM workspaces WHERE id = ").
+		WithArgs(target).
+		WillReturnRows(sqlmock.NewRows([]string{"id", "parent_id"}).AddRow(target, parent))
+
+	// #1953 org guard — both resolve to the same org root → allowed.
+	mock.ExpectQuery("WITH RECURSIVE org_chain AS").
+		WithArgs(caller).
+		WillReturnRows(sqlmock.NewRows([]string{"root_id"}).AddRow(parent))
+	mock.ExpectQuery("WITH RECURSIVE org_chain AS").
+		WithArgs(target).
+		WillReturnRows(sqlmock.NewRows([]string{"root_id"}).AddRow(parent))
+
+	expectBudgetCheck(mock, target)
+	mock.ExpectExec("INSERT INTO activity_logs").WillReturnResult(sqlmock.NewResult(0, 1))
+
+	w := httptest.NewRecorder()
+	c, _ := gin.CreateTestContext(w)
+	c.Params = gin.Params{{Key: "id", Value: target}}
+	body := `{"method":"message/send","params":{"message":{"role":"user","parts":[{"text":"same-org"}]}}}`
+	c.Request = httptest.NewRequest("POST", "/workspaces/"+target+"/a2a", bytes.NewBufferString(body))
+	c.Request.Header.Set("Content-Type", "application/json")
+	c.Request.Header.Set("X-Workspace-ID", caller)
+
+	handler.ProxyA2A(c)
+	time.Sleep(50 * time.Millisecond) // allow the async logA2ASuccess INSERT to flush
+
+	if w.Code != http.StatusOK {
+		t.Fatalf("expected 200 for same-org a2a routing, got %d: %s", w.Code, w.Body.String())
+	}
+	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 ed41a8da4..193ca385d 100644
--- a/workspace-server/internal/handlers/delegation_executor_integration_test.go
+++ b/workspace-server/internal/handlers/delegation_executor_integration_test.go
@@ -140,7 +140,14 @@ func buildHTTPResponse(statusCode int, body string) []byte {
 }
 
 // setupIntegrationFixtures inserts the rows executeDelegation requires:
-//   - workspaces: source and target (siblings, parent_id=NULL so CanCommunicate=true)
+//   - workspaces: source (org root) + target as its CHILD, so both live in the
+//     SAME org. CanCommunicate=true (parent↔child) AND the #1953 sameOrg() guard
+//     in proxyA2ARequest passes (both resolve to the same org root). A real
+//     delegation happens INSIDE one org. (Previously both were parent_id=NULL —
+//     two DISTINCT org roots — which only "communicated" via CanCommunicate's
+//     root-sibling rule; #1953 added a sameOrg() guard that now denies routing
+//     between two org roots as cross-tenant, so the success-path tests below
+//     must use a same-org source/target pair.)
 //   - activity_logs: the 'delegate' row that updateDelegationStatus UPDATE will find
 //   - delegations: the ledger row that recordLedgerStatus will UPDATE
 //
@@ -148,13 +155,14 @@ func buildHTTPResponse(statusCode int, body string) []byte {
 func setupIntegrationFixtures(t *testing.T, conn *sql.DB) func() {
 	t.Helper()
 	ctx, cancel := context.WithTimeout(context.Background(), 10*time.Second)
+	sourceID := integrationTestSourceID // org root (parent_id NULL); target hangs off it
 	for _, ws := range []struct {
 		id       string
 		name     string
 		parentID *string
 	}{
 		{integrationTestSourceID, "test-source", nil},
-		{integrationTestTargetID, "test-target", nil},
+		{integrationTestTargetID, "test-target", &sourceID}, // child of source → same org
 	} {
 		if _, err := conn.ExecContext(ctx,
 			`INSERT INTO workspaces (id, name, parent_id) VALUES ($1::uuid, $2, $3) ON CONFLICT (id) DO NOTHING`,
@@ -510,6 +518,94 @@ func TestIntegration_ExecuteDelegation_RedisDown_FallsBackToDB(t *testing.T) {
 	}
 }
 
+// TestIntegration_SameOrg_RealCTE_ResolvesAncestorChain is the regression gate
+// for the org_scope.go recursive-CTE bug (#1953 follow-up). The sqlmock unit
+// tests feed sameOrg() a pre-computed root_id row, so they CANNOT catch a wrong
+// CTE — they assume it already returns the right value. Only a real Postgres
+// run exercises orgRootSubtreeCTE itself.
+//
+// The bug: the CTE carried `id AS root_id` from the recursive SEED, so a
+// non-root workspace resolved to ITSELF instead of its topmost ancestor. That
+// made sameOrg() return false for two genuinely same-org workspaces and 403 a
+// legitimate same-org a2a route (over-block). This test seeds a real
+// root → child → grandchild chain plus a separate org root, and asserts:
+//   - every node in the chain resolves to the SAME org root (root, child, grandchild)
+//   - two workspaces in the same chain are sameOrg (incl. grandchild ↔ root)
+//   - a workspace in a DIFFERENT chain is NOT sameOrg (cross-tenant stays closed)
+func TestIntegration_SameOrg_RealCTE_ResolvesAncestorChain(t *testing.T) {
+	conn := integrationDB(t)
+
+	const (
+		rootA       = "11111111-1111-1111-1111-111111111111"
+		childA      = "22222222-2222-2222-2222-222222222222"
+		grandchildA = "33333333-3333-3333-3333-333333333333"
+		rootB       = "44444444-4444-4444-4444-444444444444"
+	)
+	ctx, cancel := context.WithTimeout(context.Background(), 10*time.Second)
+	defer cancel()
+
+	t.Cleanup(func() {
+		c2, cancel2 := context.WithTimeout(context.Background(), 5*time.Second)
+		defer cancel2()
+		// Delete leaf-first to respect the parent_id self-FK.
+		for _, id := range []string{grandchildA, childA, rootA, rootB} {
+			conn.ExecContext(c2, `DELETE FROM workspaces WHERE id = $1`, id)
+		}
+	})
+
+	// Insert parent-before-child to satisfy the self-referential FK.
+	seed := []struct {
+		id, name string
+		parent   *string
+	}{
+		{rootA, "org-a-root", nil},
+		{childA, "org-a-child", strPtr(rootA)},
+		{grandchildA, "org-a-grandchild", strPtr(childA)},
+		{rootB, "org-b-root", nil},
+	}
+	for _, s := range seed {
+		if _, err := conn.ExecContext(ctx,
+			`INSERT INTO workspaces (id, name, parent_id) VALUES ($1::uuid, $2, $3) ON CONFLICT (id) DO NOTHING`,
+			s.id, s.name, s.parent); err != nil {
+			t.Fatalf("seed %s: %v", s.name, err)
+		}
+	}
+
+	// Every node in chain A must resolve to rootA via the REAL CTE.
+	for _, id := range []string{rootA, childA, grandchildA} {
+		got, err := orgRootID(ctx, conn, id)
+		if err != nil {
+			t.Fatalf("orgRootID(%s): %v", id, err)
+		}
+		if got != rootA {
+			t.Errorf("orgRootID(%s) = %q, want rootA %q (CTE must walk to topmost ancestor)", id, got, rootA)
+		}
+	}
+
+	// Same-org positives — including the grandchild↔root pair that the buggy
+	// CTE got wrong.
+	for _, pair := range [][2]string{{childA, grandchildA}, {rootA, grandchildA}, {rootA, childA}} {
+		ok, err := sameOrg(ctx, conn, pair[0], pair[1])
+		if err != nil {
+			t.Fatalf("sameOrg(%s,%s): %v", pair[0], pair[1], err)
+		}
+		if !ok {
+			t.Errorf("sameOrg(%s,%s) = false, want true (same org chain)", pair[0], pair[1])
+		}
+	}
+
+	// Cross-org negative — isolation must stay closed.
+	for _, pair := range [][2]string{{rootA, rootB}, {grandchildA, rootB}, {childA, rootB}} {
+		ok, err := sameOrg(ctx, conn, pair[0], pair[1])
+		if err != nil {
+			t.Fatalf("sameOrg(%s,%s): %v", pair[0], pair[1], err)
+		}
+		if ok {
+			t.Errorf("sameOrg(%s,%s) = true, want false (different orgs — cross-tenant must stay denied)", pair[0], pair[1])
+		}
+	}
+}
+
 // extractHostPort parses "http://127.0.0.1:PORT/" and returns "127.0.0.1:PORT".
 func extractHostPort(rawURL string) string {
 	// Simple parse: strip "http://" prefix and trailing slash.
diff --git a/workspace-server/internal/handlers/delegation_test.go b/workspace-server/internal/handlers/delegation_test.go
index 223b8de77..c71454639 100644
--- a/workspace-server/internal/handlers/delegation_test.go
+++ b/workspace-server/internal/handlers/delegation_test.go
@@ -1059,13 +1059,25 @@ func expectExecuteDelegationBase(mock sqlmock.Sqlmock) {
 		WillReturnResult(sqlmock.NewResult(0, 1))
 
 	// CanCommunicate: getWorkspaceRef(source) + getWorkspaceRef(target).
-	// Both are root-level workspaces (parent_id=NULL) → root-level siblings → allowed.
+	// Source and target are siblings under one shared parent (one tenant) →
+	// CanCommunicate allowed. (#1953: they must NOT both be parent_id=NULL —
+	// two distinct org roots are now treated as DIFFERENT orgs and routing
+	// between them is denied. A real delegation happens inside one org.)
 	mock.ExpectQuery("SELECT id, parent_id FROM workspaces WHERE id = ").
 		WithArgs(testDeliverySourceID).
-		WillReturnRows(sqlmock.NewRows([]string{"id", "parent_id"}).AddRow(testDeliverySourceID, nil))
+		WillReturnRows(sqlmock.NewRows([]string{"id", "parent_id"}).AddRow(testDeliverySourceID, "ws-org-root-159"))
 	mock.ExpectQuery("SELECT id, parent_id FROM workspaces WHERE id = ").
 		WithArgs(testDeliveryTargetID).
-		WillReturnRows(sqlmock.NewRows([]string{"id", "parent_id"}).AddRow(testDeliveryTargetID, nil))
+		WillReturnRows(sqlmock.NewRows([]string{"id", "parent_id"}).AddRow(testDeliveryTargetID, "ws-org-root-159"))
+
+	// #1953 cross-tenant guard: same-org check after CanCommunicate. Both
+	// resolve to the same org root → routing allowed.
+	mock.ExpectQuery("WITH RECURSIVE org_chain AS").
+		WithArgs(testDeliverySourceID).
+		WillReturnRows(sqlmock.NewRows([]string{"root_id"}).AddRow("ws-org-root-159"))
+	mock.ExpectQuery("WITH RECURSIVE org_chain AS").
+		WithArgs(testDeliveryTargetID).
+		WillReturnRows(sqlmock.NewRows([]string{"root_id"}).AddRow("ws-org-root-159"))
 
 	// resolveAgentURL: test callers always set the URL in Redis (mr.Set ws:{id}:url),
 	// so resolveAgentURL gets a cache hit and never falls back to DB.
diff --git a/workspace-server/internal/handlers/discovery.go b/workspace-server/internal/handlers/discovery.go
index f97408ae7..ce679b29d 100644
--- a/workspace-server/internal/handlers/discovery.go
+++ b/workspace-server/internal/handlers/discovery.go
@@ -237,7 +237,17 @@ func (h *DiscoveryHandler) Peers(c *gin.Context) {
 
 	var peers []map[string]interface{}
 
-	// Siblings
+	// Siblings — workspaces sharing the caller's parent.
+	//
+	// #1953 cross-tenant isolation: the OLD code's else-branch handled the
+	// org-root caller (parent_id IS NULL) by returning EVERY workspace with
+	// parent_id IS NULL — i.e. every other tenant's org root, since the
+	// workspaces table has no org_id column. That leaked peer identities/URLs
+	// across tenants. An org root has no siblings inside its own org (each
+	// tenant is a distinct org root), so the org-root caller now gets an empty
+	// sibling set; its real peers are its children, returned below. Only the
+	// parent_id-bound branch enumerates siblings, and that is already scoped to
+	// one parent (one tenant).
 	if parentID.Valid {
 		siblings, _ := queryPeerMaps(`
 			SELECT w.id, w.name, COALESCE(w.role, ''), w.tier, w.status,
@@ -246,14 +256,6 @@ func (h *DiscoveryHandler) Peers(c *gin.Context) {
 			FROM workspaces w WHERE w.parent_id = $1 AND w.id != $2 AND w.status != 'removed'`,
 			parentID.String, workspaceID)
 		peers = append(peers, siblings...)
-	} else {
-		siblings, _ := queryPeerMaps(`
-			SELECT w.id, w.name, COALESCE(w.role, ''), w.tier, w.status,
-				   COALESCE(w.agent_card, 'null'::jsonb), COALESCE(w.url, ''),
-				   w.parent_id, w.active_tasks
-			FROM workspaces w WHERE w.parent_id IS NULL AND w.id != $1 AND w.status != 'removed'`,
-			workspaceID)
-		peers = append(peers, siblings...)
 	}
 
 	// Children — exclude self defensively. A child row whose parent_id
diff --git a/workspace-server/internal/handlers/discovery_test.go b/workspace-server/internal/handlers/discovery_test.go
index d803226a6..be855323a 100644
--- a/workspace-server/internal/handlers/discovery_test.go
+++ b/workspace-server/internal/handlers/discovery_test.go
@@ -223,10 +223,10 @@ func TestPeers_RootWorkspace_NoPeers(t *testing.T) {
 
 	peerCols := []string{"id", "name", "role", "tier", "status", "agent_card", "url", "parent_id", "active_tasks"}
 
-	// Siblings (other root-level workspaces) — none
-	mock.ExpectQuery("SELECT w.id, w.name.*WHERE w.parent_id IS NULL AND w.id != \\$1").
-		WithArgs("ws-root-alone").
-		WillReturnRows(sqlmock.NewRows(peerCols))
+	// #1953: an org-root caller (parent_id IS NULL) now issues NO sibling
+	// query at all. The old `WHERE w.parent_id IS NULL` sibling read returned
+	// EVERY tenant's org root (cross-tenant leak); an org root has no siblings
+	// inside its own org, so the handler skips the sibling read entirely.
 
 	// Children — none. #383 added explicit `w.id != $2` self-filter.
 	mock.ExpectQuery("SELECT w.id, w.name.*WHERE w.parent_id = \\$1 AND w.id != \\$2").
diff --git a/workspace-server/internal/handlers/handlers_extended_test.go b/workspace-server/internal/handlers/handlers_extended_test.go
index 5b82e7fc6..fdd86d489 100644
--- a/workspace-server/internal/handlers/handlers_extended_test.go
+++ b/workspace-server/internal/handlers/handlers_extended_test.go
@@ -453,6 +453,14 @@ func TestExtended_DiscoverMissingHeader(t *testing.T) {
 
 // ---------- TestPeers (Extended) ----------
 
+// TestExtended_Peers verifies a root-level (org-root) workspace's peer view.
+//
+// #1953: previously a root-level caller issued `WHERE w.parent_id IS NULL`
+// for siblings, which returned EVERY other tenant's org root as a "peer"
+// (cross-tenant leak, since the workspaces table has no org_id column). After
+// the fix an org root has no cross-tenant siblings; its only peers are its own
+// children. This test asserts the child is returned and that NO sibling query
+// is issued (no `parent_id IS NULL` read).
 func TestExtended_Peers(t *testing.T) {
 	mock := setupTestDB(t)
 	setupTestRedis(t)
@@ -463,17 +471,14 @@ func TestExtended_Peers(t *testing.T) {
 		WithArgs("ws-peer").
 		WillReturnRows(sqlmock.NewRows([]string{"parent_id"}).AddRow(nil))
 
-	// Expect root-level siblings query (parent IS NULL, excluding self)
-	mock.ExpectQuery("SELECT w.id, w.name").
-		WithArgs("ws-peer").
-		WillReturnRows(sqlmock.NewRows([]string{"id", "name", "role", "tier", "status", "agent_card", "url", "parent_id", "active_tasks"}).
-			AddRow("ws-sibling", "Sibling Agent", "worker", 1, "online", []byte("null"), "http://localhost:9001", nil, 0))
+	// NO root-level sibling query is issued for an org-root caller anymore.
 
-	// Expect children query (workspaces with parent_id = ws-peer, excluding self)
-	// Query now binds (parent_id, self_id) for the self-filter guard added in #383.
+	// Children query (workspaces with parent_id = ws-peer, excluding self).
+	// Query binds (parent_id, self_id) for the self-filter guard added in #383.
 	mock.ExpectQuery("SELECT w.id, w.name").
 		WithArgs("ws-peer", "ws-peer").
-		WillReturnRows(sqlmock.NewRows([]string{"id", "name", "role", "tier", "status", "agent_card", "url", "parent_id", "active_tasks"}))
+		WillReturnRows(sqlmock.NewRows([]string{"id", "name", "role", "tier", "status", "agent_card", "url", "parent_id", "active_tasks"}).
+			AddRow("ws-child", "Child Agent", "worker", 1, "online", []byte("null"), "http://localhost:9001", "ws-peer", 0))
 
 	// No parent query since workspace is root-level
 
@@ -493,10 +498,10 @@ func TestExtended_Peers(t *testing.T) {
 		t.Fatalf("failed to parse response: %v", err)
 	}
 	if len(resp) != 1 {
-		t.Fatalf("expected 1 peer, got %d", len(resp))
+		t.Fatalf("expected 1 peer (the child), got %d", len(resp))
 	}
-	if resp[0]["name"] != "Sibling Agent" {
-		t.Errorf("expected peer name 'Sibling Agent', got %v", resp[0]["name"])
+	if resp[0]["name"] != "Child Agent" {
+		t.Errorf("expected peer name 'Child Agent', got %v", resp[0]["name"])
 	}
 
 	if err := mock.ExpectationsWereMet(); err != nil {
diff --git a/workspace-server/internal/handlers/mcp_tools.go b/workspace-server/internal/handlers/mcp_tools.go
index cadc07963..247071007 100644
--- a/workspace-server/internal/handlers/mcp_tools.go
+++ b/workspace-server/internal/handlers/mcp_tools.go
@@ -97,7 +97,15 @@ func (h *MCPHandler) toolListPeers(ctx context.Context, workspaceID string) (str
 
 	const cols = `SELECT w.id, w.name, COALESCE(w.role,''), w.status, w.tier`
 
-	// Siblings
+	// Siblings — workspaces sharing the caller's parent.
+	//
+	// #1953 cross-tenant isolation: the OLD else-branch returned every
+	// workspace with parent_id IS NULL when the caller was itself an org root,
+	// i.e. every other tenant's org root (the workspaces table has no org_id
+	// column). That leaked peer identities across tenants via MCP list_peers.
+	// An org root has no siblings inside its own org, so the org-root caller
+	// now gets no siblings; its peers are its children, enumerated below. Only
+	// the parent_id-bound branch enumerates siblings, scoped to one tenant.
 	if parentID.Valid {
 		rows, err := h.database.QueryContext(ctx,
 			cols+` FROM workspaces w WHERE w.parent_id = $1 AND w.id != $2 AND w.status != 'removed'`,
@@ -107,15 +115,6 @@ func (h *MCPHandler) toolListPeers(ctx context.Context, workspaceID string) (str
 				log.Printf("MCP toolListPeers: sibling scan error: %v", scanErr)
 			}
 		}
-	} else {
-		rows, err := h.database.QueryContext(ctx,
-			cols+` FROM workspaces w WHERE w.parent_id IS NULL AND w.id != $1 AND w.status != 'removed'`,
-			workspaceID)
-		if err == nil {
-			if scanErr := scanPeers(rows); scanErr != nil {
-				log.Printf("MCP toolListPeers: sibling scan error: %v", scanErr)
-			}
-		}
 	}
 
 	// Children
diff --git a/workspace-server/internal/handlers/org_scope.go b/workspace-server/internal/handlers/org_scope.go
new file mode 100644
index 000000000..a9ccb4957
--- /dev/null
+++ b/workspace-server/internal/handlers/org_scope.go
@@ -0,0 +1,104 @@
+package handlers
+
+// org_scope.go — cross-tenant isolation helpers (#1953).
+//
+// The `workspaces` table has no `org_id` column; an "org" is the subtree of
+// workspaces reachable through the `parent_id` chain from a single org root
+// (a row with parent_id IS NULL). Several code paths historically computed an
+// org-root sibling set as `WHERE parent_id IS NULL`, which matches EVERY
+// tenant's org root and therefore leaks peer metadata / routing across tenants.
+//
+// This file centralises the org-scoping primitive so peer discovery, the MCP
+// list_peers tool, and a2a routing all derive "the caller's org" the SAME way
+// the OFFSEC-015 broadcast fix (commit 5a05302c, workspace_broadcast.go) does:
+// a recursive CTE that walks the parent_id chain up to the org root. Keeping
+// the CTE in one place means there is a single, testable source of truth for
+// tenant isolation rather than four hand-copied queries that can drift.
+//
+// NOTE: this is the parent_id-chain scoping that the broadcast fix already
+// ships. It is deliberately NOT an `org_id` column — adding that column is a
+// separate architecture decision pending CTO sign-off. See #1953.
+
+import (
+	"context"
+	"database/sql"
+	"errors"
+)
+
+// errNoOrgRoot is returned by orgRootID when the workspace id has no row (and
+// therefore no resolvable org root). Callers translate this into a 404/not-found
+// at their own layer; it is distinct from a transient DB error so a missing
+// workspace never gets treated as "belongs to every org".
+var errNoOrgRoot = errors.New("org root not found for workspace")
+
+// orgRootSubtreeCTE is the recursive CTE — identical in shape to the OFFSEC-015
+// broadcast fix — that walks UP the parent_id chain from a single workspace to
+// its org root. The org root is the row on the chain whose parent_id IS NULL.
+//
+//	$1 = workspace id to resolve
+//
+// The recursive member walks UP the parent_id chain: each step joins to the row
+// whose id is the current row's parent_id. The topmost ancestor is the single
+// chain row with parent_id IS NULL — and THAT row's own `id` is the org root.
+//
+// We select that parentless row's `id` (aliased root_id). We must NOT carry a
+// fixed `id AS root_id` from the recursive seed: that value is just the input
+// workspace id, so a non-root caller (e.g. a child delegating to a sibling)
+// would resolve to ITSELF instead of its org root, and sameOrg() would wrongly
+// report two genuinely same-org workspaces as different orgs and 403 a
+// legitimate a2a route. A workspace that already IS an org root has a one-row
+// chain whose id == itself, so it correctly resolves to itself.
+const orgRootSubtreeCTE = `
+	WITH RECURSIVE org_chain AS (
+		SELECT id, parent_id
+		FROM workspaces
+		WHERE id = $1
+		UNION ALL
+		SELECT w.id, w.parent_id
+		FROM workspaces w
+		JOIN org_chain c ON w.id = c.parent_id
+	)
+	SELECT id AS root_id FROM org_chain WHERE parent_id IS NULL LIMIT 1
+`
+
+// orgRootID resolves the org root of `workspaceID` by walking the parent_id
+// chain via orgRootSubtreeCTE. Returns errNoOrgRoot when the workspace (or its
+// chain) yields no org root row, and the underlying error on any DB failure.
+//
+// This is the SAME lookup the broadcast handler performs inline; the three
+// leak paths in #1953 call this instead of re-deriving "the org" from
+// `parent_id IS NULL` (which spans all tenants).
+func orgRootID(ctx context.Context, database *sql.DB, workspaceID string) (string, error) {
+	var root string
+	err := database.QueryRowContext(ctx, orgRootSubtreeCTE, workspaceID).Scan(&root)
+	if errors.Is(err, sql.ErrNoRows) {
+		return "", errNoOrgRoot
+	}
+	if err != nil {
+		return "", err
+	}
+	if root == "" {
+		return "", errNoOrgRoot
+	}
+	return root, nil
+}
+
+// sameOrg reports whether workspaces `a` and `b` share an org root, i.e. they
+// belong to the same tenant. Used by a2a routing to reject resolving/dispatching
+// to a workspace id outside the caller's org. Fail-CLOSED: any lookup error or
+// missing org root yields (false, err) so a DB hiccup denies cross-tenant
+// routing rather than allowing it.
+func sameOrg(ctx context.Context, database *sql.DB, a, b string) (bool, error) {
+	if a == b {
+		return true, nil
+	}
+	rootA, err := orgRootID(ctx, database, a)
+	if err != nil {
+		return false, err
+	}
+	rootB, err := orgRootID(ctx, database, b)
+	if err != nil {
+		return false, err
+	}
+	return rootA == rootB, nil
+}