diff --git a/src/lib/core/ports.ts b/src/lib/core/ports.ts
index 99829908cb..cdcf05a897 100644
--- a/src/lib/core/ports.ts
+++ b/src/lib/core/ports.ts
@@ -54,6 +54,8 @@ export const VLLM_PORT = parsePort("NEMOCLAW_VLLM_PORT", 8000);
 export const OLLAMA_PORT = parsePort("NEMOCLAW_OLLAMA_PORT", 11434);
 /** Ollama auth proxy port (default 11435, override via NEMOCLAW_OLLAMA_PROXY_PORT). */
 export const OLLAMA_PROXY_PORT = parsePort("NEMOCLAW_OLLAMA_PROXY_PORT", 11435);
+/** Hermes OpenAI-compatible API port (manifest `forward_ports[1]` / start.sh `PUBLIC_PORT`); reserved — never a valid dashboard port, for any agent. (#4984) */
+export const HERMES_OPENAI_API_PORT = 8642;
 /** Bedrock Runtime adapter port (default 11436, override via NEMOCLAW_BEDROCK_RUNTIME_ADAPTER_PORT). */
 export const BEDROCK_RUNTIME_ADAPTER_PORT = parsePort(
   "NEMOCLAW_BEDROCK_RUNTIME_ADAPTER_PORT",
diff --git a/src/lib/onboard.ts b/src/lib/onboard.ts
index 08d65d27d0..535c90c5c6 100644
--- a/src/lib/onboard.ts
+++ b/src/lib/onboard.ts
@@ -481,6 +481,8 @@ const {
   preflightDashboardPortRangeAvailability,
   resolveCreateSandboxDashboardPort,
 } = require("./onboard/dashboard-port") as typeof import("./onboard/dashboard-port");
+const { assertDashboardPortNotReserved, buildRequiredPreflightPorts } =
+  require("./onboard/preflight-ports") as typeof import("./onboard/preflight-ports");
 const { tryCleanupOrphanedDashboardForward } =
   require("./onboard/orphaned-dashboard-forward") as typeof import("./onboard/orphaned-dashboard-forward");
 const { destroyGatewayForReuse } =
@@ -1750,22 +1752,14 @@ async function preflight(
   // skip the dashboard port check entirely — ensureDashboardForward will
   // find a free port.
   const dashboardPortToCheck = _preflightDashboardPort ?? null;
-  const requiredPorts = [
-    {
-      port: GATEWAY_PORT,
-      label: "OpenShell gateway",
-      envVar: "NEMOCLAW_GATEWAY_PORT",
-    },
-    ...(dashboardPortToCheck !== null
-      ? [
-          {
-            port: dashboardPortToCheck,
-            label: `${cliDisplayName()} dashboard`,
-            envVar: "NEMOCLAW_DASHBOARD_PORT",
-          },
-        ]
-      : []),
-  ];
+  // #4984 — fail fast on an explicit reserved dashboard port; deferred paths
+  // (CHAT_UI_URL / persisted) are caught at createSandbox.
+  assertDashboardPortNotReserved(dashboardPortToCheck);
+  const requiredPorts = buildRequiredPreflightPorts({
+    gatewayPort: GATEWAY_PORT,
+    dashboardPort: dashboardPortToCheck,
+    dashboardLabel: `${cliDisplayName()} dashboard`,
+  });
   for (const { port, label, envVar } of requiredPorts) {
     const portCheckOptions =
       port === GATEWAY_PORT ? dockerDriverGatewayEnv.getGatewayPortCheckOptions() : undefined;
diff --git a/src/lib/onboard/dashboard-preflight-ports.test.ts b/src/lib/onboard/dashboard-preflight-ports.test.ts
new file mode 100644
index 0000000000..502ce911e4
--- /dev/null
+++ b/src/lib/onboard/dashboard-preflight-ports.test.ts
@@ -0,0 +1,52 @@
+// SPDX-FileCopyrightText: Copyright (c) 2026 NVIDIA CORPORATION & AFFILIATES. All rights reserved.
+// SPDX-License-Identifier: Apache-2.0
+
+import { describe, expect, it, vi } from "vitest";
+import { HERMES_OPENAI_API_PORT } from "../core/ports";
+import { assertDashboardPortNotReserved, buildRequiredPreflightPorts } from "./preflight-ports";
+
+describe("buildRequiredPreflightPorts", () => {
+  it("returns the gateway only when no dashboard port is requested (auto-allocation)", () => {
+    expect(
+      buildRequiredPreflightPorts({
+        gatewayPort: 8080,
+        dashboardPort: null,
+        dashboardLabel: "NemoClaw dashboard",
+      }),
+    ).toEqual([{ port: 8080, label: "OpenShell gateway", envVar: "NEMOCLAW_GATEWAY_PORT" }]);
+  });
+
+  it("includes the dashboard port when one is explicitly requested", () => {
+    expect(
+      buildRequiredPreflightPorts({
+        gatewayPort: 8080,
+        dashboardPort: 18789,
+        dashboardLabel: "NemoClaw dashboard",
+      }),
+    ).toEqual([
+      { port: 8080, label: "OpenShell gateway", envVar: "NEMOCLAW_GATEWAY_PORT" },
+      { port: 18789, label: "NemoClaw dashboard", envVar: "NEMOCLAW_DASHBOARD_PORT" },
+    ]);
+  });
+});
+
+describe("assertDashboardPortNotReserved (#4984)", () => {
+  it("rejects the reserved Hermes API port 8642 via fail()", () => {
+    const fail = vi.fn((message: string): never => {
+      throw new Error(message);
+    });
+    expect(() => assertDashboardPortNotReserved(HERMES_OPENAI_API_PORT, fail)).toThrow(
+      "[SECURITY] Invalid dashboard port 8642 - reserved for the Hermes OpenAI-compatible API",
+    );
+    expect(fail).toHaveBeenCalledOnce();
+  });
+
+  it("allows a normal dashboard port and a null (auto-allocated) port", () => {
+    const fail = vi.fn((message: string): never => {
+      throw new Error(message);
+    });
+    expect(() => assertDashboardPortNotReserved(18789, fail)).not.toThrow();
+    expect(() => assertDashboardPortNotReserved(null, fail)).not.toThrow();
+    expect(fail).not.toHaveBeenCalled();
+  });
+});
diff --git a/src/lib/onboard/hermes-dashboard.test.ts b/src/lib/onboard/hermes-dashboard.test.ts
index f0061980e1..400a198a8a 100644
--- a/src/lib/onboard/hermes-dashboard.test.ts
+++ b/src/lib/onboard/hermes-dashboard.test.ts
@@ -67,7 +67,7 @@ describe("onboard Hermes dashboard helpers", () => {
         env: { NEMOCLAW_DASHBOARD_PORT: "8642" },
       }),
     ).toThrow(
-      "[SECURITY] Invalid Hermes dashboard port 8642 - reserved for the Hermes OpenAI-compatible API",
+      "[SECURITY] Invalid dashboard port 8642 - reserved for the Hermes OpenAI-compatible API",
     );
   });
 
@@ -96,7 +96,7 @@ describe("onboard Hermes dashboard helpers", () => {
         env: {},
       }),
     ).toThrow(
-      "[SECURITY] Invalid Hermes dashboard port 8642 - reserved for the Hermes OpenAI-compatible API",
+      "[SECURITY] Invalid dashboard port 8642 - reserved for the Hermes OpenAI-compatible API",
     );
   });
 
@@ -110,14 +110,19 @@ describe("onboard Hermes dashboard helpers", () => {
     ).not.toThrow();
   });
 
-  it("does not apply the #4984 reserved-port guard to non-Hermes agents", () => {
-    expect(
+  it("rejects the reserved Hermes API port 8642 even for non-Hermes agents (#4984)", () => {
+    // A non-hermes onboard that binds 8642 squats the host port and silently
+    // breaks a later `nemoclaw onboard` of hermes (whose API forwards 8642),
+    // so the reserved-port guard is host-side and agent-agnostic.
+    expect(() =>
       resolveHermesDashboardOnboardState({
         agentName: "openclaw",
         effectivePort: 18789,
         env: { NEMOCLAW_DASHBOARD_PORT: "8642" },
       }),
-    ).toEqual({ config: null, enabled: false });
+    ).toThrow(
+      "[SECURITY] Invalid dashboard port 8642 - reserved for the Hermes OpenAI-compatible API",
+    );
   });
 
   it("rolls back and fails when an opted-in dashboard forward cannot start", () => {
diff --git a/src/lib/onboard/hermes-dashboard.ts b/src/lib/onboard/hermes-dashboard.ts
index daf6b971ad..b89ee29449 100644
--- a/src/lib/onboard/hermes-dashboard.ts
+++ b/src/lib/onboard/hermes-dashboard.ts
@@ -9,11 +9,10 @@ import {
   type HermesDashboardConfig,
   readHermesDashboardConfig,
 } from "../hermes-dashboard";
+import { HERMES_OPENAI_API_PORT } from "../core/ports";
+import { RESERVED_HERMES_DASHBOARD_PORT_MESSAGE } from "./preflight-ports";
 import type { SandboxEntry } from "../state/registry";
 
-/** Hermes OpenAI-compatible API port (manifest `forward_ports[1]` / start.sh `PUBLIC_PORT`); reserved — never a dashboard port. (#4984) */
-const HERMES_OPENAI_API_PORT = 8642;
-
 export interface HermesDashboardOnboardState {
   config: HermesDashboardConfig | null;
   enabled: boolean;
@@ -32,24 +31,24 @@ export function resolveHermesDashboardOnboardState({
   env: NodeJS.ProcessEnv;
   fail?: (message: string) => never;
 }): HermesDashboardOnboardState {
-  if (agentName !== "hermes") return { config: null, enabled: false };
-
-  // #4984 — reject the reserved Hermes API port as the dashboard port, host-side,
-  // before any sandbox is built. Check both the resolved effectivePort (covers
-  // --control-ui-port / CHAT_UI_URL / persisted port) and the raw env override,
-  // which the host otherwise silently drops so effectivePort never shows it.
-  // Message mirrors agents/hermes/start.sh:164.
+  // #4984 — reject the reserved Hermes API port (HERMES_OPENAI_API_PORT) as the
+  // dashboard port for ANY agent, before any sandbox is built. Check both the
+  // resolved effectivePort (covers --control-ui-port / CHAT_UI_URL / persisted)
+  // and the raw env override, which the host otherwise silently drops so
+  // effectivePort never shows it. Message mirrors agents/hermes/start.sh:164.
   const rawDashboardPort = env.NEMOCLAW_DASHBOARD_PORT?.trim();
   const requestedDashboardPort = rawDashboardPort ? Number(rawDashboardPort) : undefined;
   if (
     effectivePort === HERMES_OPENAI_API_PORT ||
     requestedDashboardPort === HERMES_OPENAI_API_PORT
   ) {
-    const message = `[SECURITY] Invalid Hermes dashboard port ${HERMES_OPENAI_API_PORT} - reserved for the Hermes OpenAI-compatible API`;
+    const message = RESERVED_HERMES_DASHBOARD_PORT_MESSAGE;
     if (fail) return fail(message);
     throw new Error(message);
   }
 
+  if (agentName !== "hermes") return { config: null, enabled: false };
+
   let config: HermesDashboardConfig;
   try {
     config = readHermesDashboardConfig(env);
diff --git a/src/lib/onboard/preflight-ports.ts b/src/lib/onboard/preflight-ports.ts
new file mode 100644
index 0000000000..ec81471786
--- /dev/null
+++ b/src/lib/onboard/preflight-ports.ts
@@ -0,0 +1,55 @@
+// SPDX-FileCopyrightText: Copyright (c) 2026 NVIDIA CORPORATION & AFFILIATES. All rights reserved.
+// SPDX-License-Identifier: Apache-2.0
+
+import { HERMES_OPENAI_API_PORT } from "../core/ports";
+
+/** Agent-neutral rejection message when {@link HERMES_OPENAI_API_PORT} is requested as a dashboard port; shared by both #4984 guards. */
+export const RESERVED_HERMES_DASHBOARD_PORT_MESSAGE = `[SECURITY] Invalid dashboard port ${HERMES_OPENAI_API_PORT} - reserved for the Hermes OpenAI-compatible API`;
+
+export interface PreflightPort {
+  port: number;
+  label: string;
+  envVar: string;
+}
+
+/**
+ * Build the preflight required-ports list: the OpenShell gateway always, plus
+ * the dashboard port when an explicit one is requested (auto-allocation skips
+ * it). Extracted from onboard.ts so the reserved-port guard can live in a
+ * submodule. (#4984)
+ */
+export function buildRequiredPreflightPorts(opts: {
+  gatewayPort: number;
+  dashboardPort: number | null;
+  dashboardLabel: string;
+}): PreflightPort[] {
+  return [
+    { port: opts.gatewayPort, label: "OpenShell gateway", envVar: "NEMOCLAW_GATEWAY_PORT" },
+    ...(opts.dashboardPort !== null
+      ? [
+          {
+            port: opts.dashboardPort,
+            label: opts.dashboardLabel,
+            envVar: "NEMOCLAW_DASHBOARD_PORT",
+          },
+        ]
+      : []),
+  ];
+}
+
+/**
+ * Reject the reserved {@link HERMES_OPENAI_API_PORT} as a dashboard port at
+ * preflight (any agent) so onboarding fails fast at [1/8], before any sandbox.
+ * Mirrors the createSandbox guard in resolveHermesDashboardOnboardState. (#4984)
+ */
+export function assertDashboardPortNotReserved(
+  dashboardPort: number | null,
+  fail: (message: string) => never = (message) => {
+    console.error(`  ${message}`);
+    process.exit(1);
+  },
+): void {
+  if (dashboardPort === HERMES_OPENAI_API_PORT) {
+    fail(RESERVED_HERMES_DASHBOARD_PORT_MESSAGE);
+  }
+}