diff --git a/workspace-server/internal/handlers/platform_agent.go b/workspace-server/internal/handlers/platform_agent.go
new file mode 100644
index 000000000..da0777611
--- /dev/null
+++ b/workspace-server/internal/handlers/platform_agent.go
@@ -0,0 +1,135 @@
+package handlers
+
+// platform_agent.go — installs the org-level platform agent as the org root.
+// (RFC docs/design/rfc-platform-agent.md)
+//
+// The platform agent IS the org root: an org is the subtree under the single
+// parent_id IS NULL row (org_scope.go), so making the concierge the user's
+// universal A2A peer means making it that root. Installing it therefore:
+//
+//	1. upserts the platform-agent row (kind='platform', parent_id NULL);
+//	2. re-parents the org's existing root(s) under it;
+//	3. moves the org-anchor references — org_api_tokens.org_id and
+//	   org_plugin_allowlist.org_id, both of which key off the root workspace id
+//	   (see migrations 035/036 + 026) — from each old root to the platform agent.
+//
+// All of that happens in ONE transaction so a tenant's auth tokens and plugin
+// allowlist never point at a stale anchor. The operation is idempotent: a second
+// call finds the platform agent already the sole root and does nothing.
+//
+// Routing (CanCommunicate/sameOrg in registry/access.go + org_scope.go) needs NO
+// change — once the platform agent is the root, the existing ancestor/descendant
+// rules already give it universal in-org reach and keep tenant isolation intact.
+
+import (
+	"context"
+	"database/sql"
+	"fmt"
+	"log"
+	"net/http"
+
+	"git.moleculesai.app/molecule-ai/molecule-core/workspace-server/internal/db"
+	"git.moleculesai.app/molecule-ai/molecule-core/workspace-server/internal/models"
+	"github.com/gin-gonic/gin"
+)
+
+type installPlatformAgentPayload struct {
+	// ID is the platform agent's workspace id (a deterministic uuidv5 the
+	// control plane derives per org). Required.
+	ID string `json:"id" binding:"required"`
+	// Name is the display name; defaults to "Org Concierge" when omitted.
+	Name string `json:"name"`
+}
+
+// InstallPlatformAgent handles POST /admin/org/platform-agent (AdminAuth).
+//
+// Idempotently installs the platform agent as the org root for THIS tenant. The
+// control plane calls it at org-provision time (new orgs) and during the
+// existing-org backfill rollout. Safe to call repeatedly.
+func InstallPlatformAgent(c *gin.Context) {
+	var p installPlatformAgentPayload
+	if err := c.ShouldBindJSON(&p); err != nil {
+		c.JSON(http.StatusBadRequest, gin.H{"error": "invalid request body"})
+		return
+	}
+	name := p.Name
+	if name == "" {
+		name = "Org Concierge"
+	}
+	if err := installPlatformAgent(c.Request.Context(), db.DB, p.ID, name); err != nil {
+		log.Printf("InstallPlatformAgent: %v (id=%s)", err, p.ID)
+		c.JSON(http.StatusInternalServerError, gin.H{"error": "install failed"})
+		return
+	}
+	c.JSON(http.StatusOK, gin.H{
+		"status":            "installed",
+		"platform_agent_id": p.ID,
+		"kind":              models.KindPlatform,
+	})
+}
+
+// installPlatformAgent performs the idempotent, transactional install described
+// in the file header. Separated from the gin handler so integration tests can
+// exercise it directly against a real Postgres (the org-anchor migration cannot
+// be proven with sqlmock).
+func installPlatformAgent(ctx context.Context, database *sql.DB, platformID, name string) error {
+	tx, err := database.BeginTx(ctx, nil)
+	if err != nil {
+		return fmt.Errorf("begin: %w", err)
+	}
+	defer func() { _ = tx.Rollback() }() // no-op after Commit
+
+	// 1. Ensure the platform-agent row exists as a kind='platform' root.
+	//    ON CONFLICT keeps it a platform root if it was pre-seeded; the row is
+	//    tier 0 and never billed/provisioned as an ordinary workspace EC2.
+	if _, err := tx.ExecContext(ctx, `
+		INSERT INTO workspaces (id, name, kind, tier, status, runtime, parent_id)
+		VALUES ($1, $2, 'platform', 0, 'online', 'claude-code', NULL)
+		ON CONFLICT (id) DO UPDATE SET kind = 'platform', parent_id = NULL
+	`, platformID, name); err != nil {
+		return fmt.Errorf("upsert platform agent: %w", err)
+	}
+
+	// 2. Capture the org's other current roots (everything at parent_id IS NULL
+	//    except the platform agent itself). In a one-org tenant DB this is the
+	//    single team root; the query tolerates 0 (already installed) or N.
+	rows, err := tx.QueryContext(ctx,
+		`SELECT id FROM workspaces WHERE parent_id IS NULL AND id <> $1`, platformID)
+	if err != nil {
+		return fmt.Errorf("select old roots: %w", err)
+	}
+	var oldRoots []string
+	for rows.Next() {
+		var id string
+		if err := rows.Scan(&id); err != nil {
+			rows.Close()
+			return fmt.Errorf("scan old root: %w", err)
+		}
+		oldRoots = append(oldRoots, id)
+	}
+	rows.Close()
+	if err := rows.Err(); err != nil {
+		return fmt.Errorf("iterate old roots: %w", err)
+	}
+
+	// 3 + 4. Re-parent each old root under the platform agent and move its
+	//        org-anchor references in the same transaction. A non-root old root
+	//        is kind='workspace', so it does not trip workspaces_platform_root_check.
+	for _, root := range oldRoots {
+		if _, err := tx.ExecContext(ctx,
+			`UPDATE workspaces SET parent_id = $1, updated_at = now() WHERE id = $2`,
+			platformID, root); err != nil {
+			return fmt.Errorf("re-parent %s: %w", root, err)
+		}
+		if _, err := tx.ExecContext(ctx,
+			`UPDATE org_api_tokens SET org_id = $1 WHERE org_id = $2`, platformID, root); err != nil {
+			return fmt.Errorf("migrate org_api_tokens for %s: %w", root, err)
+		}
+		if _, err := tx.ExecContext(ctx,
+			`UPDATE org_plugin_allowlist SET org_id = $1 WHERE org_id = $2`, platformID, root); err != nil {
+			return fmt.Errorf("migrate org_plugin_allowlist for %s: %w", root, err)
+		}
+	}
+
+	return tx.Commit()
+}
diff --git a/workspace-server/internal/handlers/platform_agent_integration_test.go b/workspace-server/internal/handlers/platform_agent_integration_test.go
new file mode 100644
index 000000000..498bbfdf7
--- /dev/null
+++ b/workspace-server/internal/handlers/platform_agent_integration_test.go
@@ -0,0 +1,188 @@
+//go:build integration
+// +build integration
+
+// platform_agent_integration_test.go — REAL Postgres gate for installPlatformAgent.
+//
+// Run with:
+//
+//	INTEGRATION_DB_URL="postgres://postgres:test@localhost:55432/molecule?sslmode=disable" \
+//	  go test -tags=integration ./internal/handlers/ -run Integration_PlatformAgentInstall -v
+//
+// CI: handlers-postgres-integration workflow (handlers + migrations path filter).
+//
+// Why this is NOT a sqlmock test
+// ------------------------------
+// The install re-parents the org's existing root under the platform agent AND
+// moves the org-anchor references (org_api_tokens.org_id, org_plugin_allowlist.
+// org_id) from old root to platform agent, atomically. The whole point is the
+// post-transaction row state: orgRootID() must resolve every node to the platform
+// agent, sameOrg() must still hold, and the auth/allowlist anchors must point at
+// the new root. Only a real Postgres can prove that; sqlmock cannot.
+
+package handlers
+
+import (
+	"context"
+	"database/sql"
+	"fmt"
+	"testing"
+
+	"github.com/google/uuid"
+	_ "github.com/lib/pq"
+)
+
+func integrationDB_PlatformAgentInstall(t *testing.T) *sql.DB {
+	t.Helper()
+	url := requireIntegrationDBURL(t)
+	conn, err := sql.Open("postgres", url)
+	if err != nil {
+		t.Fatalf("open: %v", err)
+	}
+	if err := conn.Ping(); err != nil {
+		t.Fatalf("ping: %v", err)
+	}
+	t.Cleanup(func() { conn.Close() })
+	return conn
+}
+
+// TestIntegration_PlatformAgentInstall_ReparentsRootAndMovesAnchors builds a
+// real org in Postgres:
+//
+//	root (parent_id NULL, kind=workspace)
+//	└── child
+//	+ an org_api_token  anchored to root
+//	+ an org_plugin_allowlist entry anchored to root
+//
+// then installs the platform agent and asserts:
+//   - the platform agent is the new sole root (kind=platform, parent_id NULL);
+//   - the old root is re-parented under it; the child is untouched;
+//   - both org-anchor references now point at the platform agent;
+//   - a second install is a no-op (idempotent).
+func TestIntegration_PlatformAgentInstall_ReparentsRootAndMovesAnchors(t *testing.T) {
+	conn := integrationDB_PlatformAgentInstall(t)
+	ctx := context.Background()
+
+	tag := uuid.New().String()[:8]
+	prefix := fmt.Sprintf("itest-pinstall-%s", tag)
+	rootID := uuid.New().String()
+	childID := uuid.New().String()
+	platformID := uuid.New().String()
+
+	cleanup := func() {
+		_, _ = conn.ExecContext(ctx, `DELETE FROM org_plugin_allowlist WHERE plugin_name = $1`, prefix+"-plugin")
+		_, _ = conn.ExecContext(ctx, `DELETE FROM org_api_tokens WHERE prefix = $1`, tag)
+		// child + old root (prefixed names) first, then the platform agent by id
+		// (root.parent_id references it, so it must go last).
+		_, _ = conn.ExecContext(ctx, `DELETE FROM workspaces WHERE name LIKE $1`, prefix+"%")
+		_, _ = conn.ExecContext(ctx, `DELETE FROM workspaces WHERE id = $1`, platformID)
+	}
+	t.Cleanup(cleanup)
+	cleanup()
+
+	// Seed org tree.
+	if _, err := conn.ExecContext(ctx, `
+		INSERT INTO workspaces (id, name, tier, runtime, status, parent_id)
+		VALUES ($1, $2, 2, 'claude-code', 'online', NULL)`, rootID, prefix+"-root"); err != nil {
+		t.Fatalf("seed root: %v", err)
+	}
+	if _, err := conn.ExecContext(ctx, `
+		INSERT INTO workspaces (id, name, tier, runtime, status, parent_id)
+		VALUES ($1, $2, 2, 'claude-code', 'online', $3)`, childID, prefix+"-child", rootID); err != nil {
+		t.Fatalf("seed child: %v", err)
+	}
+	// Org-anchor rows keyed to the OLD root.
+	if _, err := conn.ExecContext(ctx, `
+		INSERT INTO org_api_tokens (token_hash, prefix, name, org_id)
+		VALUES ($1, $2, $3, $4)`,
+		[]byte("hash-"+tag), tag, prefix+"-tok", rootID); err != nil {
+		t.Fatalf("seed org_api_token: %v", err)
+	}
+	if _, err := conn.ExecContext(ctx, `
+		INSERT INTO org_plugin_allowlist (org_id, plugin_name, enabled_by)
+		VALUES ($1, $2, $3)`, rootID, prefix+"-plugin", childID); err != nil {
+		t.Fatalf("seed allowlist: %v", err)
+	}
+
+	// Install.
+	if err := installPlatformAgent(ctx, conn, platformID, "Org Concierge"); err != nil {
+		t.Fatalf("install: %v", err)
+	}
+
+	assertState := func(stage string) {
+		// platform agent is a kind=platform root.
+		var kind string
+		var parent sql.NullString
+		if err := conn.QueryRowContext(ctx,
+			`SELECT kind, parent_id FROM workspaces WHERE id = $1`, platformID).Scan(&kind, &parent); err != nil {
+			t.Fatalf("[%s] read platform agent: %v", stage, err)
+		}
+		if kind != "platform" || parent.Valid {
+			t.Fatalf("[%s] platform agent kind=%q parent=%v, want platform/NULL", stage, kind, parent)
+		}
+		// old root re-parented under the platform agent.
+		var rootParent sql.NullString
+		if err := conn.QueryRowContext(ctx,
+			`SELECT parent_id FROM workspaces WHERE id = $1`, rootID).Scan(&rootParent); err != nil {
+			t.Fatalf("[%s] read old root: %v", stage, err)
+		}
+		if !rootParent.Valid || rootParent.String != platformID {
+			t.Fatalf("[%s] old root parent=%v, want %s", stage, rootParent, platformID)
+		}
+		// child untouched.
+		var childParent sql.NullString
+		if err := conn.QueryRowContext(ctx,
+			`SELECT parent_id FROM workspaces WHERE id = $1`, childID).Scan(&childParent); err != nil {
+			t.Fatalf("[%s] read child: %v", stage, err)
+		}
+		if !childParent.Valid || childParent.String != rootID {
+			t.Fatalf("[%s] child parent=%v, want %s (unchanged)", stage, childParent, rootID)
+		}
+		// org-anchor references moved to the platform agent.
+		var tokOrg, alOrg string
+		if err := conn.QueryRowContext(ctx,
+			`SELECT org_id FROM org_api_tokens WHERE prefix = $1`, tag).Scan(&tokOrg); err != nil {
+			t.Fatalf("[%s] read token org_id: %v", stage, err)
+		}
+		if tokOrg != platformID {
+			t.Fatalf("[%s] org_api_tokens.org_id=%s, want %s", stage, tokOrg, platformID)
+		}
+		if err := conn.QueryRowContext(ctx,
+			`SELECT org_id FROM org_plugin_allowlist WHERE plugin_name = $1`, prefix+"-plugin").Scan(&alOrg); err != nil {
+			t.Fatalf("[%s] read allowlist org_id: %v", stage, err)
+		}
+		if alOrg != platformID {
+			t.Fatalf("[%s] org_plugin_allowlist.org_id=%s, want %s", stage, alOrg, platformID)
+		}
+		// orgRootID + sameOrg now resolve everything to the platform agent.
+		got, err := orgRootID(ctx, conn, childID)
+		if err != nil {
+			t.Fatalf("[%s] orgRootID(child): %v", stage, err)
+		}
+		if got != platformID {
+			t.Fatalf("[%s] orgRootID(child)=%s, want %s", stage, got, platformID)
+		}
+		same, err := sameOrg(ctx, conn, childID, platformID)
+		if err != nil || !same {
+			t.Fatalf("[%s] sameOrg(child, platform)=%v err=%v, want true", stage, same, err)
+		}
+	}
+
+	assertState("first install")
+
+	// Idempotent: second install must not error or change state.
+	if err := installPlatformAgent(ctx, conn, platformID, "Org Concierge"); err != nil {
+		t.Fatalf("second install (idempotent): %v", err)
+	}
+	assertState("second install")
+
+	// Neither seeded team node is a root any more — the platform agent is.
+	var nRoots int
+	if err := conn.QueryRowContext(ctx,
+		`SELECT count(*) FROM workspaces WHERE parent_id IS NULL AND id IN ($1, $2)`,
+		rootID, childID).Scan(&nRoots); err != nil {
+		t.Fatalf("count roots: %v", err)
+	}
+	if nRoots != 0 {
+		t.Fatalf("team roots after install = %d, want 0 (old root re-parented under platform agent)", nRoots)
+	}
+}
diff --git a/workspace-server/internal/handlers/platform_agent_test.go b/workspace-server/internal/handlers/platform_agent_test.go
new file mode 100644
index 000000000..224f63fe8
--- /dev/null
+++ b/workspace-server/internal/handlers/platform_agent_test.go
@@ -0,0 +1,27 @@
+package handlers
+
+import (
+	"bytes"
+	"net/http"
+	"net/http/httptest"
+	"testing"
+
+	"github.com/gin-gonic/gin"
+)
+
+// TestInstallPlatformAgent_BadJSON rejects a payload missing the required id
+// before touching the DB (binding:"required" on ID).
+func TestInstallPlatformAgent_BadJSON(t *testing.T) {
+	gin.SetMode(gin.TestMode)
+	w := httptest.NewRecorder()
+	c, _ := gin.CreateTestContext(w)
+	c.Request = httptest.NewRequest("POST", "/admin/org/platform-agent",
+		bytes.NewBufferString(`{"name":"Org Concierge"}`)) // no id
+	c.Request.Header.Set("Content-Type", "application/json")
+
+	InstallPlatformAgent(c)
+
+	if w.Code != http.StatusBadRequest {
+		t.Errorf("missing id: expected 400, got %d: %s", w.Code, w.Body.String())
+	}
+}
diff --git a/workspace-server/internal/handlers/workspaces_insert_allowlist_test.go b/workspace-server/internal/handlers/workspaces_insert_allowlist_test.go
index 066c65761..1a9fdaa15 100644
--- a/workspace-server/internal/handlers/workspaces_insert_allowlist_test.go
+++ b/workspace-server/internal/handlers/workspaces_insert_allowlist_test.go
@@ -56,6 +56,11 @@ func TestINSERTworkspacesAllowlist(t *testing.T) {
 		// workspace; UUID is server-generated. Caller intent IS to
 		// create, so no idempotency check is needed.
 		"workspace.go:Create": "single-workspace POST, server-generated UUID",
+		// platform_agent.installPlatformAgent: single platform-agent row,
+		// caller-supplied deterministic UUID; INSERT is idempotent via
+		// ON CONFLICT (id) DO UPDATE and runs inside the install transaction.
+		// Pinned by TestIntegration_PlatformAgentInstall_ReparentsRootAndMovesAnchors.
+		"platform_agent.go:installPlatformAgent": "ON CONFLICT (id) DO UPDATE, single row in tx",
 	}
 
 	actual := map[string]string{}
diff --git a/workspace-server/internal/router/router.go b/workspace-server/internal/router/router.go
index 88393fa26..fb1cb36f8 100644
--- a/workspace-server/internal/router/router.go
+++ b/workspace-server/internal/router/router.go
@@ -426,6 +426,12 @@ func Setup(hub *ws.Hub, broadcaster *events.Broadcaster, prov *provisioner.Provi
 		adminTokH := handlers.NewAdminWorkspaceTokenHandler()
 		r.POST("/admin/workspaces/:id/tokens", middleware.AdminAuth(db.DB), adminTokH.Create)
 
+		// Platform agent install — idempotently makes the org-level concierge
+		// the org root (re-parents the existing root + moves org-anchor refs).
+		// Called by the control plane at org provision + existing-org backfill.
+		// (RFC docs/design/rfc-platform-agent.md)
+		r.POST("/admin/org/platform-agent", middleware.AdminAuth(db.DB), handlers.InstallPlatformAgent)
+
 		// Memory
 		memh := handlers.NewMemoryHandler()
 		wsAuth.GET("/memory", memh.List)