diff --git a/packages/agent-core/src/harness/env/kill-tree.ts b/packages/agent-core/src/harness/env/kill-tree.ts
index 4d0d9193c308..11a87f6f5539 100644
--- a/packages/agent-core/src/harness/env/kill-tree.ts
+++ b/packages/agent-core/src/harness/env/kill-tree.ts
@@ -4,6 +4,7 @@ import { readFileSync } from "node:fs";
 
 const DEFAULT_GRACE_MS = 3000;
 const MAX_GRACE_MS = 60_000;
+const TASKKILL_COMPLETION_TIMEOUT_MS = 3000;
 
 export type KillProcessTreeOptions = {
   graceMs?: number;
@@ -64,20 +65,22 @@ export function killProcessTree(pid: number, opts?: KillProcessTreeOptions): voi
 export function signalProcessTree(
   pid: number,
   signal: "SIGTERM" | "SIGKILL",
-  opts?: { detached?: boolean },
+  opts?: { detached?: boolean; onComplete?: () => void },
 ): void {
   if (!Number.isFinite(pid) || pid <= 0) {
+    opts?.onComplete?.();
     return;
   }
 
   if (process.platform === "win32") {
-    signalProcessTreeWindows(pid, signal);
+    void signalProcessTreeWindowsAndWait(pid, signal).then(opts?.onComplete);
     return;
   }
 
   const useGroupKill =
     opts?.detached === true || (opts?.detached !== false && isProcessGroupLeader(pid));
   signalProcessTreeUnix(pid, signal, useGroupKill);
+  opts?.onComplete?.();
 }
 
 function normalizeGraceMs(value?: number): number {
@@ -166,17 +169,32 @@ function signalProcessTreeUnix(
   }
 }
 
-function runTaskkill(args: string[]): void {
-  try {
-    const child = spawn("taskkill", args, {
-      stdio: "ignore",
-      detached: true,
-      windowsHide: true,
-    });
-    child.once("error", () => {});
-  } catch {
-    // Ignore taskkill spawn failures.
-  }
+function runTaskkill(args: string[]): Promise<void> {
+  return new Promise((resolve) => {
+    let settled = false;
+    const finish = () => {
+      if (settled) {
+        return;
+      }
+      settled = true;
+      clearTimeout(completionTimer);
+      resolve();
+    };
+    const completionTimer = setTimeout(finish, TASKKILL_COMPLETION_TIMEOUT_MS);
+    completionTimer.unref?.();
+    try {
+      const child = spawn("taskkill", args, {
+        stdio: "ignore",
+        detached: true,
+        windowsHide: true,
+      });
+      child.once("error", finish);
+      child.once("close", finish);
+    } catch {
+      // Ignore taskkill spawn failures.
+      finish();
+    }
+  });
 }
 
 function killProcessTreeWindows(pid: number, graceMs: number): void {
@@ -191,7 +209,14 @@ function killProcessTreeWindows(pid: number, graceMs: number): void {
 }
 
 function signalProcessTreeWindows(pid: number, signal: "SIGTERM" | "SIGKILL"): void {
+  void signalProcessTreeWindowsAndWait(pid, signal);
+}
+
+function signalProcessTreeWindowsAndWait(
+  pid: number,
+  signal: "SIGTERM" | "SIGKILL",
+): Promise<void> {
   const args =
     signal === "SIGKILL" ? ["/F", "/T", "/PID", String(pid)] : ["/T", "/PID", String(pid)];
-  runTaskkill(args);
+  return runTaskkill(args);
 }
diff --git a/src/process/kill-tree.test.ts b/src/process/kill-tree.test.ts
index f47a3b181ed6..8508a3a12732 100644
--- a/src/process/kill-tree.test.ts
+++ b/src/process/kill-tree.test.ts
@@ -317,6 +317,41 @@ describe("killProcessTree", () => {
     });
   });
 
+  it("on Windows exposes taskkill completion", async () => {
+    const taskkillChild = new EventEmitter();
+    spawnMock.mockReturnValueOnce(taskkillChild);
+
+    await withMockedPlatform("win32", async () => {
+      const completed = vi.fn();
+      signalProcessTree(8989, "SIGKILL", { onComplete: completed });
+      await Promise.resolve();
+      expect(completed).not.toHaveBeenCalled();
+
+      taskkillChild.emit("close", 0);
+      await Promise.resolve();
+
+      expect(completed).toHaveBeenCalledOnce();
+      expectTaskkillCall(0, ["/F", "/T", "/PID", "8989"]);
+    });
+  });
+
+  it("on Windows bounds taskkill completion when no event arrives", async () => {
+    const taskkillChild = new EventEmitter();
+    spawnMock.mockReturnValueOnce(taskkillChild);
+
+    await withMockedPlatform("win32", async () => {
+      const completed = vi.fn();
+      signalProcessTree(9090, "SIGKILL", { onComplete: completed });
+
+      await vi.advanceTimersByTimeAsync(2_999);
+      expect(completed).not.toHaveBeenCalled();
+
+      await vi.advanceTimersByTimeAsync(1);
+      expect(completed).toHaveBeenCalledOnce();
+      expectTaskkillCall(0, ["/F", "/T", "/PID", "9090"]);
+    });
+  });
+
   it("on Windows force-kills synchronously without delayed taskkill", async () => {
     await withMockedPlatform("win32", async () => {
       killProcessTree(9999, { force: true });
diff --git a/src/process/supervisor/adapters/child.test.ts b/src/process/supervisor/adapters/child.test.ts
index 27b1745b89c9..b32e637fade0 100644
--- a/src/process/supervisor/adapters/child.test.ts
+++ b/src/process/supervisor/adapters/child.test.ts
@@ -15,7 +15,11 @@ import {
 const { spawnWithFallbackMock, signalProcessTreeMock, createWindowsOutputDecoderMock } = vi.hoisted(
   () => ({
     spawnWithFallbackMock: vi.fn(),
-    signalProcessTreeMock: vi.fn(),
+    signalProcessTreeMock: vi.fn(
+      (_pid: number, _signal: string, opts?: { onComplete?: () => void }) => {
+        opts?.onComplete?.();
+      },
+    ),
     createWindowsOutputDecoderMock: vi.fn(() => ({
       decode: (chunk: Buffer | string) => (Buffer.isBuffer(chunk) ? chunk.toString("utf8") : chunk),
       flush: () => "",
@@ -202,13 +206,16 @@ describe("createChildAdapter", () => {
     }
 
     adapter.kill();
+    await Promise.resolve();
 
     // Detachment flag is now passed to signalProcessTree so it knows whether
     // it can safely group-kill via -pid. (#71662)
     const expectedDetached = process.platform !== "win32" && !process.env.OPENCLAW_SERVICE_MARKER;
-    expect(signalProcessTreeMock).toHaveBeenCalledWith(4321, "SIGKILL", {
-      detached: expectedDetached,
-    });
+    expect(signalProcessTreeMock).toHaveBeenCalledWith(
+      4321,
+      "SIGKILL",
+      expect.objectContaining({ detached: expectedDetached }),
+    );
     expect(killMock).toHaveBeenCalledWith("SIGKILL");
   });
 
@@ -228,8 +235,13 @@ describe("createChildAdapter", () => {
     });
 
     adapter.kill();
+    await Promise.resolve();
 
-    expect(signalProcessTreeMock).toHaveBeenCalledWith(8888, "SIGKILL", { detached: false });
+    expect(signalProcessTreeMock).toHaveBeenCalledWith(
+      8888,
+      "SIGKILL",
+      expect.objectContaining({ detached: false }),
+    );
     expect(killMock).toHaveBeenCalledWith("SIGKILL");
   });
 
@@ -238,7 +250,12 @@ describe("createChildAdapter", () => {
     try {
       const { adapter, killMock } = await createAdapterHarness({ pid: 9999 });
       adapter.kill();
-      expect(signalProcessTreeMock).toHaveBeenCalledWith(9999, "SIGKILL", { detached: false });
+      await Promise.resolve();
+      expect(signalProcessTreeMock).toHaveBeenCalledWith(
+        9999,
+        "SIGKILL",
+        expect.objectContaining({ detached: false }),
+      );
       expect(killMock).toHaveBeenCalledWith("SIGKILL");
     } finally {
       delete process.env.OPENCLAW_SERVICE_MARKER;
@@ -375,7 +392,106 @@ describe("createChildAdapter", () => {
     expect(killMock).toHaveBeenCalledWith("SIGKILL");
   });
 
-  it("settles wait from exit state on Windows even when close never arrives", async () => {
+  it("waits for Windows tree-kill completion before forced stream settlement", async () => {
+    vi.useFakeTimers();
+    setPlatform("win32");
+    let resolveTreeKill: (() => void) | undefined;
+    signalProcessTreeMock.mockImplementationOnce(
+      (_pid: number, _signal: string, opts?: { onComplete?: () => void }) => {
+        resolveTreeKill = opts?.onComplete;
+      },
+    );
+
+    const stub = createStubChild(9753);
+    spawnWithFallbackMock.mockResolvedValue({ child: stub.child, usedFallback: false });
+    const adapter = await createChildAdapter({
+      argv: ["node", "-e", "setInterval(() => {}, 1000)"],
+      stdinMode: "pipe-closed",
+    });
+    const settled = vi.fn();
+    void adapter.wait().then(settled);
+
+    adapter.kill("SIGKILL");
+    stub.emitExit(null, "SIGKILL");
+    await vi.advanceTimersByTimeAsync(1_000);
+    expect(settled).not.toHaveBeenCalled();
+    expect(stub.child.stdout?.destroyed).toBe(false);
+    expect(stub.child.stderr?.destroyed).toBe(false);
+
+    resolveTreeKill?.();
+    await Promise.resolve();
+    await vi.advanceTimersByTimeAsync(249);
+    expect(settled).not.toHaveBeenCalled();
+
+    await vi.advanceTimersByTimeAsync(1);
+    expect(settled).toHaveBeenCalledWith({ code: null, signal: "SIGKILL" });
+    expect(stub.child.stdout?.destroyed).toBe(true);
+    expect(stub.child.stderr?.destroyed).toBe(true);
+  });
+
+  it("blocks Windows child close until tree-kill completion", async () => {
+    vi.useFakeTimers();
+    setPlatform("win32");
+    let resolveTreeKill: (() => void) | undefined;
+    signalProcessTreeMock.mockImplementationOnce(
+      (_pid: number, _signal: string, opts?: { onComplete?: () => void }) => {
+        resolveTreeKill = opts?.onComplete;
+      },
+    );
+
+    const stub = createStubChild(9754);
+    spawnWithFallbackMock.mockResolvedValue({ child: stub.child, usedFallback: false });
+    const adapter = await createChildAdapter({
+      argv: ["node", "-e", "setInterval(() => {}, 1000)"],
+      stdinMode: "pipe-closed",
+    });
+    const settled = vi.fn();
+    void adapter.wait().then(settled);
+
+    adapter.kill("SIGKILL");
+    stub.emitExit(null, "SIGKILL");
+    stub.emitClose(null, "SIGKILL");
+    await vi.advanceTimersByTimeAsync(1_000);
+    expect(settled).not.toHaveBeenCalled();
+
+    resolveTreeKill?.();
+    await Promise.resolve();
+    await vi.advanceTimersByTimeAsync(250);
+    expect(settled).toHaveBeenCalledWith({ code: null, signal: "SIGKILL" });
+  });
+
+  it("blocks drained Windows streams until tree-kill completion", async () => {
+    vi.useFakeTimers();
+    setPlatform("win32");
+    let resolveTreeKill: (() => void) | undefined;
+    signalProcessTreeMock.mockImplementationOnce(
+      (_pid: number, _signal: string, opts?: { onComplete?: () => void }) => {
+        resolveTreeKill = opts?.onComplete;
+      },
+    );
+
+    const stub = createStubChild(9755);
+    spawnWithFallbackMock.mockResolvedValue({ child: stub.child, usedFallback: false });
+    const adapter = await createChildAdapter({
+      argv: ["node", "-e", "setInterval(() => {}, 1000)"],
+      stdinMode: "pipe-closed",
+    });
+    const settled = vi.fn();
+    void adapter.wait().then(settled);
+
+    adapter.kill("SIGKILL");
+    stub.emitExit(null, "SIGKILL");
+    stub.child.stdout?.emit("end");
+    stub.child.stderr?.emit("end");
+    await Promise.resolve();
+    expect(settled).not.toHaveBeenCalled();
+
+    resolveTreeKill?.();
+    await vi.advanceTimersByTimeAsync(0);
+    expect(settled).toHaveBeenCalledWith({ code: null, signal: "SIGKILL" });
+  });
+
+  it("preserves descendant output after ordinary Windows child exit", async () => {
     vi.useFakeTimers();
     setPlatform("win32");
 
@@ -391,6 +507,10 @@ describe("createChildAdapter", () => {
       });
       return { ...stub, adapter: adapterLocal };
     })();
+    const stdout = vi.fn();
+    const stderr = vi.fn();
+    adapter.onStdout(stdout);
+    adapter.onStderr(stderr);
 
     const settled = vi.fn();
     void adapter.wait().then((result) => {
@@ -398,13 +518,45 @@ describe("createChildAdapter", () => {
     });
 
     emitExit(0, null);
-    child.stdout?.emit("end");
-    child.stderr?.emit("end");
-    await vi.advanceTimersByTimeAsync(300);
+    await vi.advanceTimersByTimeAsync(1_000);
+    expect(settled).not.toHaveBeenCalled();
+    expect(child.stdout?.destroyed).toBe(false);
+    expect(child.stderr?.destroyed).toBe(false);
 
+    const stdoutPipe = child.stdout as PassThrough;
+    const stderrPipe = child.stderr as PassThrough;
+    stdoutPipe.write("late stdout");
+    stderrPipe.write("late stderr");
+    stdoutPipe.end();
+    stderrPipe.end();
+    await vi.runAllTimersAsync();
+
+    expect(stdout).toHaveBeenCalledWith("late stdout");
+    expect(stderr).toHaveBeenCalledWith("late stderr");
     expect(settled).toHaveBeenCalledWith({ code: 0, signal: null });
   });
 
+  it("settles ordinary Windows exit when streams drain before exit and close is missing", async () => {
+    setPlatform("win32");
+    const stub = createStubChild(9756);
+    spawnWithFallbackMock.mockResolvedValue({ child: stub.child, usedFallback: false });
+    const adapter = await createChildAdapter({
+      argv: ["node", "-e", "process.exit(0)"],
+      stdinMode: "pipe-closed",
+    });
+    const waitPromise = adapter.wait();
+    const settled = vi.fn();
+    void waitPromise.then(settled);
+
+    stub.child.stdout?.emit("end");
+    stub.child.stderr?.emit("end");
+    await Promise.resolve();
+    expect(settled).not.toHaveBeenCalled();
+
+    stub.emitExit(0, null);
+    await expect(waitPromise).resolves.toEqual({ code: 0, signal: null });
+  });
+
   it("disables detached mode in service-managed runtime", async () => {
     process.env.OPENCLAW_SERVICE_MARKER = "openclaw";
 
diff --git a/src/process/supervisor/adapters/child.ts b/src/process/supervisor/adapters/child.ts
index 2adac1357224..6e067b9758aa 100644
--- a/src/process/supervisor/adapters/child.ts
+++ b/src/process/supervisor/adapters/child.ts
@@ -19,7 +19,7 @@ import type { ManagedRunStdin, SpawnProcessAdapter } from "../types.js";
 import { toStringEnv } from "./env.js";
 
 const FORCE_KILL_WAIT_FALLBACK_MS = 4000;
-const WINDOWS_CLOSE_STATE_SETTLE_TIMEOUT_MS = 250;
+const FORCED_WINDOWS_CLOSE_SETTLE_MS = 250;
 const WINDOWS_PACKAGE_MANAGER_SHIMS = ["npm", "pnpm", "yarn", "npx"] as const;
 
 function resolveChildInvocation(params: {
@@ -252,8 +252,11 @@ export async function createChildAdapter(params: {
   let rejectWait: ((reason?: unknown) => void) | null = null;
   let waitPromise: Promise<{ code: number | null; signal: NodeJS.Signals | null }> | null = null;
   let forceKillWaitFallbackTimer: NodeJS.Timeout | null = null;
+  let forcedWindowsCloseTimer: NodeJS.Timeout | null = null;
+  let hardKillRequested = false;
+  let windowsTreeKillCompleted = false;
   let childExitState: { code: number | null; signal: NodeJS.Signals | null } | null = null;
-  let windowsCloseFallbackTimer: NodeJS.Timeout | null = null;
+  let childCloseState: { code: number | null; signal: NodeJS.Signals | null } | null = null;
   let stdoutDrained = child.stdout == null;
   let stderrDrained = child.stderr == null;
 
@@ -265,12 +268,12 @@ export async function createChildAdapter(params: {
     forceKillWaitFallbackTimer = null;
   };
 
-  const clearWindowsCloseFallbackTimer = () => {
-    if (!windowsCloseFallbackTimer) {
+  const clearForcedWindowsCloseTimer = () => {
+    if (!forcedWindowsCloseTimer) {
       return;
     }
-    clearTimeout(windowsCloseFallbackTimer);
-    windowsCloseFallbackTimer = null;
+    clearTimeout(forcedWindowsCloseTimer);
+    forcedWindowsCloseTimer = null;
   };
 
   const settleWait = (value: { code: number | null; signal: NodeJS.Signals | null }) => {
@@ -278,7 +281,7 @@ export async function createChildAdapter(params: {
       return;
     }
     clearForceKillWaitFallback();
-    clearWindowsCloseFallbackTimer();
+    clearForcedWindowsCloseTimer();
     waitResult = value;
     if (resolveWait) {
       const resolve = resolveWait;
@@ -293,7 +296,7 @@ export async function createChildAdapter(params: {
       return;
     }
     clearForceKillWaitFallback();
-    clearWindowsCloseFallbackTimer();
+    clearForcedWindowsCloseTimer();
     waitError = error;
     if (rejectWait) {
       const reject = rejectWait;
@@ -325,9 +328,32 @@ export async function createChildAdapter(params: {
     };
   };
 
+  const scheduleForcedWindowsCloseSettlement = () => {
+    if (
+      process.platform !== "win32" ||
+      !hardKillRequested ||
+      !windowsTreeKillCompleted ||
+      childExitState == null ||
+      forcedWindowsCloseTimer
+    ) {
+      return;
+    }
+    const exitState = childExitState;
+    forcedWindowsCloseTimer = setTimeout(() => {
+      child.stdout?.destroy();
+      child.stderr?.destroy();
+      settleWait(resolveObservedExitState(exitState));
+    }, FORCED_WINDOWS_CLOSE_SETTLE_MS);
+    forcedWindowsCloseTimer.unref?.();
+  };
+
+  const isWindowsHardKillSettlementBlocked = () =>
+    process.platform === "win32" && hardKillRequested && !windowsTreeKillCompleted;
+
   const maybeSettleAfterWindowsExit = () => {
     if (
       process.platform !== "win32" ||
+      isWindowsHardKillSettlementBlocked() ||
       childExitState == null ||
       !stdoutDrained ||
       !stderrDrained
@@ -337,17 +363,6 @@ export async function createChildAdapter(params: {
     settleWait(resolveObservedExitState(childExitState));
   };
 
-  const scheduleWindowsCloseFallback = () => {
-    if (process.platform !== "win32") {
-      return;
-    }
-    clearWindowsCloseFallbackTimer();
-    windowsCloseFallbackTimer = setTimeout(() => {
-      maybeSettleAfterWindowsExit();
-    }, WINDOWS_CLOSE_STATE_SETTLE_TIMEOUT_MS);
-    windowsCloseFallbackTimer.unref?.();
-  };
-
   child.stdout?.once("end", () => {
     stdoutDrained = true;
     maybeSettleAfterWindowsExit();
@@ -370,10 +385,16 @@ export async function createChildAdapter(params: {
   });
   child.once("exit", (code, signal) => {
     childExitState = { code, signal };
-    scheduleWindowsCloseFallback();
+    scheduleForcedWindowsCloseSettlement();
+    maybeSettleAfterWindowsExit();
   });
   child.once("close", (code, signal) => {
-    settleWait(resolveObservedExitState({ code, signal }));
+    childCloseState = { code, signal };
+    childExitState ??= childCloseState;
+    if (isWindowsHardKillSettlementBlocked()) {
+      return;
+    }
+    settleWait(resolveObservedExitState(childCloseState));
   });
 
   const wait = async () => {
@@ -416,19 +437,40 @@ export async function createChildAdapter(params: {
   const signalProcessTreeForChild = (pid: number, signal: "SIGTERM" | "SIGKILL") => {
     signalProcessTree(pid, signal, { detached: childIsDetached });
   };
+  const signalProcessTreeForChildAndWait = (pid: number, signal: "SIGTERM" | "SIGKILL") =>
+    new Promise<void>((resolve) => {
+      signalProcessTree(pid, signal, { detached: childIsDetached, onComplete: resolve });
+    });
   const kill = (signal?: NodeJS.Signals) => {
     const pid = child.pid ?? undefined;
     if (signal === undefined || signal === "SIGKILL") {
+      hardKillRequested = true;
+      scheduleForcedWindowsCloseSettlement();
       if (pid) {
-        // Pass through whether the child is actually detached. Without this,
-        // `signalProcessTree` group-kills via `-pid` and takes out the gateway's
-        // own process group along with the child. (#71662)
-        signalProcessTreeForChild(pid, "SIGKILL");
-      }
-      try {
-        child.kill("SIGKILL");
-      } catch {
-        // ignore kill errors
+        // Let the tree owner traverse the live root before directly killing it.
+        // On Windows, killing the root first can make `taskkill /T` lose the
+        // descendant relationship. (#71662)
+        void signalProcessTreeForChildAndWait(pid, "SIGKILL").then(() => {
+          try {
+            child.kill("SIGKILL");
+          } catch {
+            // ignore kill errors
+          }
+          windowsTreeKillCompleted = true;
+          if (childCloseState) {
+            settleWait(resolveObservedExitState(childCloseState));
+            return;
+          }
+          maybeSettleAfterWindowsExit();
+          scheduleForcedWindowsCloseSettlement();
+        });
+      } else {
+        windowsTreeKillCompleted = true;
+        try {
+          child.kill("SIGKILL");
+        } catch {
+          // ignore kill errors
+        }
       }
       scheduleForceKillWaitFallback("SIGKILL");
       return;
@@ -446,7 +488,7 @@ export async function createChildAdapter(params: {
 
   const dispose = () => {
     clearForceKillWaitFallback();
-    clearWindowsCloseFallbackTimer();
+    clearForcedWindowsCloseTimer();
     child.removeAllListeners();
   };
 
diff --git a/src/process/supervisor/supervisor.test.ts b/src/process/supervisor/supervisor.test.ts
index 28b32dbbcedb..a3392f73913a 100644
--- a/src/process/supervisor/supervisor.test.ts
+++ b/src/process/supervisor/supervisor.test.ts
@@ -1,6 +1,7 @@
 // Process supervisor tests cover lifecycle, restart, and termination behavior.
 import { performance } from "node:perf_hooks";
 import { afterEach, beforeEach, describe, expect, it, vi } from "vitest";
+import { mockProcessPlatform } from "../../test-utils/vitest-spies.js";
 import type { SpawnProcessAdapter } from "./types.js";
 
 const { createChildAdapterMock, createPtyAdapterMock } = vi.hoisted(() => ({
@@ -161,14 +162,45 @@ describe("process supervisor", () => {
     await vi.advanceTimersByTimeAsync(5);
 
     const exit = await exitPromise;
-    expect(adapter.killMock).toHaveBeenCalledWith("SIGTERM");
-    await vi.advanceTimersByTimeAsync(5_000);
-    expect(adapter.killMock).not.toHaveBeenCalledWith("SIGKILL");
+    const expectedTimeoutSignal = process.platform === "win32" ? "SIGKILL" : "SIGTERM";
+    expect(adapter.killMock).toHaveBeenCalledWith(expectedTimeoutSignal);
+    if (process.platform !== "win32") {
+      await vi.advanceTimersByTimeAsync(5_000);
+      expect(adapter.killMock).not.toHaveBeenCalledWith("SIGKILL");
+    }
     expect(exit.reason).toBe("no-output-timeout");
     expect(exit.noOutputTimedOut).toBe(true);
     expect(exit.timedOut).toBe(true);
   });
 
+  it("coalesces overlapping Windows deadline cancellation while hard kill is pending", async () => {
+    vi.useFakeTimers();
+    mockProcessPlatform("win32");
+    const adapter = createStubChildAdapter();
+    createChildAdapterMock.mockResolvedValue(adapter);
+
+    const supervisor = createProcessSupervisor();
+    const run = await spawnChild(supervisor, {
+      sessionId: "s-windows-timeout-overlap",
+      argv: createSilentIdleArgv(),
+      timeoutMs: 20,
+      noOutputTimeoutMs: 5,
+      stdinMode: "pipe-closed",
+    });
+    const exitPromise = run.wait();
+
+    await vi.advanceTimersByTimeAsync(5);
+    expect(adapter.killMock).toHaveBeenCalledTimes(1);
+    expect(adapter.killMock).toHaveBeenCalledWith("SIGKILL");
+
+    await vi.advanceTimersByTimeAsync(15);
+    expect(adapter.killMock).toHaveBeenCalledTimes(1);
+
+    adapter.settle(null, "SIGKILL");
+    const exit = await exitPromise;
+    expect(exit.reason).toBe("no-output-timeout");
+  });
+
   it("escalates cancellation to SIGKILL when graceful shutdown does not settle", async () => {
     vi.useFakeTimers();
     const adapter = createStubChildAdapter({
@@ -263,7 +295,9 @@ describe("process supervisor", () => {
     await vi.advanceTimersByTimeAsync(1);
 
     const exit = await exitPromise;
-    expect(adapter.killMock).toHaveBeenCalledWith("SIGTERM");
+    expect(adapter.killMock).toHaveBeenCalledWith(
+      process.platform === "win32" ? "SIGKILL" : "SIGTERM",
+    );
     expect(exit.reason).toBe("overall-timeout");
     expect(exit.timedOut).toBe(true);
   });
diff --git a/src/process/supervisor/supervisor.ts b/src/process/supervisor/supervisor.ts
index f8c57d165bec..9f05b50a0017 100644
--- a/src/process/supervisor/supervisor.ts
+++ b/src/process/supervisor/supervisor.ts
@@ -145,6 +145,7 @@ export function createProcessSupervisor(): ProcessSupervisor {
     let timeoutTimer: NodeJS.Timeout | null = null;
     let noOutputTimer: NodeJS.Timeout | null = null;
     let forceKillTimer: NodeJS.Timeout | null = null;
+    let cancelRequested = false;
     const captureOutput = input.captureOutput !== false;
     const maxCapturedOutputChars = clampCapturedOutputChars(input.maxCapturedOutputChars);
 
@@ -227,8 +228,19 @@ export function createProcessSupervisor(): ProcessSupervisor {
         }
       };
 
-      cancelAdapter = (_reason: TerminationReason) => {
-        if (settled || forceKillTimer) {
+      cancelAdapter = (reason: TerminationReason) => {
+        if (settled || cancelRequested) {
+          return;
+        }
+        cancelRequested = true;
+        // Windows has no catchable SIGTERM equivalent: the adapter implements it
+        // with asynchronous taskkill, so waiting the cleanup grace only delays an
+        // already-expired deadline before the same forced tree termination.
+        if (
+          process.platform === "win32" &&
+          (reason === "overall-timeout" || reason === "no-output-timeout")
+        ) {
+          adapter.kill("SIGKILL");
           return;
         }
         adapter.kill("SIGTERM");