diff --git a/src/agents/mcp-stdio-transport.test.ts b/src/agents/mcp-stdio-transport.test.ts
index 806445ae165..94853d76d70 100644
--- a/src/agents/mcp-stdio-transport.test.ts
+++ b/src/agents/mcp-stdio-transport.test.ts
@@ -6,6 +6,7 @@ import { OpenClawStdioClientTransport } from "./mcp-stdio-transport.js";
 
 const spawnMock = vi.hoisted(() => vi.fn());
 const killProcessTreeMock = vi.hoisted(() => vi.fn());
+const signalProcessTreeMock = vi.hoisted(() => vi.fn());
 
 vi.mock("node:child_process", async () => ({
   ...(await vi.importActual<typeof import("node:child_process")>("node:child_process")),
@@ -14,6 +15,7 @@ vi.mock("node:child_process", async () => ({
 
 vi.mock("../process/kill-tree.js", () => ({
   killProcessTree: killProcessTreeMock,
+  signalProcessTree: signalProcessTreeMock,
 }));
 
 class MockChildProcess extends EventEmitter {
@@ -29,6 +31,7 @@ describe("OpenClawStdioClientTransport", () => {
     vi.useRealTimers();
     spawnMock.mockReset();
     killProcessTreeMock.mockReset();
+    signalProcessTreeMock.mockReset();
   });
 
   it("starts stdio MCP servers in a disposable process group on POSIX", async () => {
@@ -88,6 +91,30 @@ describe("OpenClawStdioClientTransport", () => {
     await closing;
   });
 
+  it("force-SIGKILLs synchronously when killProcessTree's grace expires (#86412)", async () => {
+    vi.useFakeTimers();
+    const child = new MockChildProcess();
+    spawnMock.mockReturnValue(child);
+
+    const transport = new OpenClawStdioClientTransport({ command: "npx" });
+    const started = transport.start();
+    child.emit("spawn");
+    await started;
+
+    const closing = transport.close();
+    await vi.advanceTimersByTimeAsync(2000);
+    expect(killProcessTreeMock).toHaveBeenCalledWith(4321);
+    expect(signalProcessTreeMock).not.toHaveBeenCalled();
+
+    // killProcessTree's SIGKILL is .unref()'d (#86412); close() force-SIGKILLs synchronously instead.
+    await vi.advanceTimersByTimeAsync(2000);
+    expect(signalProcessTreeMock).toHaveBeenCalledWith(4321, "SIGKILL");
+
+    child.exitCode = 0;
+    child.emit("close", 0);
+    await closing;
+  });
+
   it("does not kill the process tree when graceful stdio close exits", async () => {
     vi.useFakeTimers();
     const child = new MockChildProcess();
diff --git a/src/agents/mcp-stdio-transport.ts b/src/agents/mcp-stdio-transport.ts
index d99a7ac5aed..4308ce2013a 100644
--- a/src/agents/mcp-stdio-transport.ts
+++ b/src/agents/mcp-stdio-transport.ts
@@ -5,7 +5,7 @@ import { getDefaultEnvironment } from "@modelcontextprotocol/sdk/client/stdio.js
 import { ReadBuffer, serializeMessage } from "@modelcontextprotocol/sdk/shared/stdio.js";
 import type { Transport } from "@modelcontextprotocol/sdk/shared/transport.js";
 import type { JSONRPCMessage } from "@modelcontextprotocol/sdk/types.js";
-import { killProcessTree } from "../process/kill-tree.js";
+import { killProcessTree, signalProcessTree } from "../process/kill-tree.js";
 import { prepareOomScoreAdjustedSpawn } from "../process/linux-oom-score.js";
 
 export type OpenClawStdioServerParameters = {
@@ -17,6 +17,7 @@ export type OpenClawStdioServerParameters = {
 };
 
 const CLOSE_TIMEOUT_MS = 2000;
+const SIGKILL_REAP_TIMEOUT_MS = 500;
 
 function delay(ms: number) {
   return new Promise<void>((resolve) => {
@@ -125,6 +126,11 @@ export class OpenClawStdioClientTransport implements Transport {
       if (processToClose.exitCode === null && processToClose.pid) {
         killProcessTree(processToClose.pid);
         await Promise.race([closePromise, delay(CLOSE_TIMEOUT_MS)]);
+        if (processToClose.exitCode === null && processToClose.pid) {
+          // SIGKILL synchronously: killProcessTree's setTimeout is .unref()'d and races shutdown (#86412).
+          signalProcessTree(processToClose.pid, "SIGKILL");
+          await Promise.race([closePromise, delay(SIGKILL_REAP_TIMEOUT_MS)]);
+        }
       }
     }
     this.readBuffer.clear();