diff --git a/src/lib/onboard/hermes-dashboard.test.ts b/src/lib/onboard/hermes-dashboard.test.ts
index 2bed3cfb7b..8c6749a3d4 100644
--- a/src/lib/onboard/hermes-dashboard.test.ts
+++ b/src/lib/onboard/hermes-dashboard.test.ts
@@ -36,8 +36,9 @@ describe("onboard Hermes dashboard helpers", () => {
 
   it("tracks registry drift for enabled dashboard settings", () => {
     const state = resolveHermesDashboardOnboardState({
+      // 18789 = realistic resolved dashboard port; 8642 is the reserved API port (#4984).
       agentName: "hermes",
-      effectivePort: 8642,
+      effectivePort: 18789,
       env: {
         NEMOCLAW_HERMES_DASHBOARD: "1",
         NEMOCLAW_HERMES_DASHBOARD_PORT: "9120",
@@ -58,6 +59,63 @@ describe("onboard Hermes dashboard helpers", () => {
     ).toBe(true);
   });
 
+  it("rejects NEMOCLAW_DASHBOARD_PORT set to the reserved Hermes API port 8642 (#4984)", () => {
+    expect(() =>
+      resolveHermesDashboardOnboardState({
+        agentName: "hermes",
+        effectivePort: 18789,
+        env: { NEMOCLAW_DASHBOARD_PORT: "8642" },
+      }),
+    ).toThrow("[SECURITY] Invalid Hermes dashboard port 8642 - reserved for the Hermes OpenAI-compatible API");
+  });
+
+  it("routes the #4984 rejection through fail() so onboarding exits non-zero", () => {
+    const fail = vi.fn((message: string): never => {
+      throw new Error(message);
+    });
+    expect(() =>
+      resolveHermesDashboardOnboardState({
+        agentName: "hermes",
+        effectivePort: 18789,
+        env: { NEMOCLAW_DASHBOARD_PORT: " 8642 " },
+        fail,
+      }),
+    ).toThrow(/reserved for the Hermes OpenAI-compatible API/);
+    expect(fail).toHaveBeenCalledOnce();
+  });
+
+  it("rejects a resolved dashboard port of 8642 from --control-ui-port / CHAT_UI_URL even when raw env is empty (#4984)", () => {
+    // --control-ui-port / CHAT_UI_URL / persisted port can resolve effectivePort to
+    // 8642 with the raw env unset; the host guard must still reject before build.
+    expect(() =>
+      resolveHermesDashboardOnboardState({
+        agentName: "hermes",
+        effectivePort: 8642,
+        env: {},
+      }),
+    ).toThrow("[SECURITY] Invalid Hermes dashboard port 8642 - reserved for the Hermes OpenAI-compatible API");
+  });
+
+  it("accepts a non-reserved NEMOCLAW_DASHBOARD_PORT for Hermes (#4984)", () => {
+    expect(() =>
+      resolveHermesDashboardOnboardState({
+        agentName: "hermes",
+        effectivePort: 18789,
+        env: { NEMOCLAW_DASHBOARD_PORT: "18790" },
+      }),
+    ).not.toThrow();
+  });
+
+  it("does not apply the #4984 reserved-port guard to non-Hermes agents", () => {
+    expect(
+      resolveHermesDashboardOnboardState({
+        agentName: "openclaw",
+        effectivePort: 18789,
+        env: { NEMOCLAW_DASHBOARD_PORT: "8642" },
+      }),
+    ).toEqual({ config: null, enabled: false });
+  });
+
   it("rolls back and fails when an opted-in dashboard forward cannot start", () => {
     const rollback = vi.fn();
     const fail = vi.fn((message: string): never => {
@@ -65,8 +123,9 @@ describe("onboard Hermes dashboard helpers", () => {
     });
     const ensure = createHermesDashboardForwardEnsurer({
       state: resolveHermesDashboardOnboardState({
+        // 18789 = realistic resolved dashboard port; 8642 is now reserved (#4984).
         agentName: "hermes",
-        effectivePort: 8642,
+        effectivePort: 18789,
         env: { NEMOCLAW_HERMES_DASHBOARD: "1" },
       }),
       ensureForward: vi.fn(() => false),
diff --git a/src/lib/onboard/hermes-dashboard.ts b/src/lib/onboard/hermes-dashboard.ts
index d74dfa5b95..90911c2aad 100644
--- a/src/lib/onboard/hermes-dashboard.ts
+++ b/src/lib/onboard/hermes-dashboard.ts
@@ -11,6 +11,9 @@ import {
 } from "../hermes-dashboard";
 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;
@@ -31,6 +34,22 @@ export function resolveHermesDashboardOnboardState({
 }): 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.
+  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`;
+    if (fail) return fail(message);
+    throw new Error(message);
+  }
+
   let config: HermesDashboardConfig;
   try {
     config = readHermesDashboardConfig(env);