diff --git a/src/relay/pty-handler.test.ts b/src/relay/pty-handler.test.ts
index 332828408..c90e87480 100644
--- a/src/relay/pty-handler.test.ts
+++ b/src/relay/pty-handler.test.ts
@@ -1018,6 +1018,88 @@ describe('PtyHandler', () => {
     expect(mockKill).toHaveBeenCalledWith('SIGTERM')
   })
 
+  // Why: node-pty's Windows agent throws "Signals not supported on windows."
+  // for any signal argument. killPtyProcess drops the signal on win32 — cover
+  // every call site so a future regression cannot reintroduce signal args.
+  describe('kills PTY without a signal on Windows', () => {
+    async function withWindowsPlatform(fn: () => Promise<void>): Promise<void> {
+      const originalPlatform = process.platform
+      Object.defineProperty(process, 'platform', { configurable: true, value: 'win32' })
+      try {
+        await fn()
+      } finally {
+        Object.defineProperty(process, 'platform', {
+          configurable: true,
+          value: originalPlatform
+        })
+      }
+    }
+
+    function mockKillablePty(): ReturnType<typeof vi.fn> {
+      const mockKill = vi.fn()
+      mockPtySpawn.mockReturnValue({
+        ...mockPtyInstance,
+        kill: mockKill,
+        onData: vi.fn(),
+        onExit: vi.fn()
+      })
+      return mockKill
+    }
+
+    function expectBareKills(mockKill: ReturnType<typeof vi.fn>, times: number): void {
+      expect(mockKill).toHaveBeenCalledTimes(times)
+      expect(mockKill.mock.calls.every((args) => args.length === 0)).toBe(true)
+    }
+
+    it('on graceful shutdown', async () => {
+      await withWindowsPlatform(async () => {
+        const mockKill = mockKillablePty()
+        await dispatcher.callRequest('pty.spawn', {})
+        await dispatcher.callRequest('pty.shutdown', { id: 'pty-1', immediate: false })
+        expectBareKills(mockKill, 1)
+      })
+    })
+
+    it('on immediate shutdown', async () => {
+      await withWindowsPlatform(async () => {
+        const mockKill = mockKillablePty()
+        await dispatcher.callRequest('pty.spawn', {})
+        await dispatcher.callRequest('pty.shutdown', { id: 'pty-1', immediate: true })
+        expectBareKills(mockKill, 1)
+      })
+    })
+
+    it('on stale-spawn cleanup including the SIGKILL fallback', async () => {
+      await withWindowsPlatform(async () => {
+        const mockKill = mockKillablePty()
+        await dispatcher.callRequest('pty.spawn', {}, { isStale: () => true })
+        expectBareKills(mockKill, 1)
+        vi.advanceTimersByTime(5000)
+        expectBareKills(mockKill, 2)
+      })
+    })
+
+    it('on graceful-shutdown SIGKILL fallback', async () => {
+      await withWindowsPlatform(async () => {
+        const mockKill = mockKillablePty()
+        await dispatcher.callRequest('pty.spawn', {})
+        await dispatcher.callRequest('pty.shutdown', { id: 'pty-1', immediate: false })
+        expectBareKills(mockKill, 1)
+        vi.advanceTimersByTime(5000)
+        expectBareKills(mockKill, 2)
+      })
+    })
+
+    it('on dispose', async () => {
+      await withWindowsPlatform(async () => {
+        const mockKill = mockKillablePty()
+        await dispatcher.callRequest('pty.spawn', {})
+        handler.dispose()
+        expectBareKills(mockKill, 1)
+      })
+    })
+  })
+
   it('flushes pending PTY output before immediate shutdown cleanup', async () => {
     let dataCallback: ((data: string) => void) | undefined
     const mockKill = vi.fn()
diff --git a/src/relay/pty-handler.ts b/src/relay/pty-handler.ts
index 965d8cbdc..d7dd99500 100644
--- a/src/relay/pty-handler.ts
+++ b/src/relay/pty-handler.ts
@@ -85,6 +85,18 @@ type ManagedStartupCommand = {
   timer: ReturnType<typeof setTimeout> | null
 }
 
+// Why: node-pty's Windows agent throws "Signals not supported on windows." for
+// any signal argument. ConPTY/winpty has no signal semantics — a bare kill()
+// force-terminates the child — so drop the signal on Windows and forward it
+// (SIGTERM graceful vs SIGKILL force) on POSIX.
+function killPtyProcess(pty: IPty, signal: string): void {
+  if (process.platform === 'win32') {
+    pty.kill()
+    return
+  }
+  pty.kill(signal)
+}
+
 function disposeManagedPty(managed: ManagedPty): void {
   if (managed.disposed) {
     return
@@ -710,11 +722,11 @@ export class PtyHandler {
       // response is discarded and no renderer can own this PTY. Shut it down
       // immediately so it does not linger as an unreachable remote shell.
       this.releaseStartupCommand(managed)
-      term.kill('SIGTERM')
+      killPtyProcess(term, 'SIGTERM')
       managed.killTimer = setTimeout(() => {
         const still = this.ptys.get(id)
         if (still && !still.disposed) {
-          still.pty.kill('SIGKILL')
+          killPtyProcess(still.pty, 'SIGKILL')
           // Why: stale-spawn cleanup has no client who will ever attach. If
           // SIGKILL's onExit is missed (kernel edge case, uninterruptible
           // sleep), the managed entry + ptmx fd would leak forever. Dispose
@@ -839,7 +851,7 @@ export class PtyHandler {
     if (immediate) {
       this.releaseStartupCommand(managed)
       this.flushPtyOutput(id)
-      managed.pty.kill('SIGKILL')
+      killPtyProcess(managed.pty, 'SIGKILL')
       // Why: SIGKILL has already reaped the child; release the ptmx fd on the
       // same tick. Deferring to onExit leaves a window where the fd is live
       // with a dead child. Idempotent via the disposed guard — if onExit fires
@@ -858,7 +870,7 @@ export class PtyHandler {
       this.clearPtyFlowState(id)
     } else {
       this.releaseStartupCommand(managed)
-      managed.pty.kill('SIGTERM')
+      killPtyProcess(managed.pty, 'SIGTERM')
 
       // Why: Some processes ignore SIGTERM (e.g. a hung child, a custom signal
       // handler). Without a SIGKILL fallback the PTY process would leak and the
@@ -872,7 +884,7 @@ export class PtyHandler {
       managed.killTimer = setTimeout(() => {
         const still = this.ptys.get(id)
         if (still && !still.disposed) {
-          still.pty.kill('SIGKILL')
+          killPtyProcess(still.pty, 'SIGKILL')
           this.flushPtyOutput(id)
           // Why: emit pty.exit BEFORE disposeManagedPty sets disposed=true.
           // The natural onExit short-circuits on `managed.disposed`, so
@@ -1140,7 +1152,7 @@ export class PtyHandler {
       // ptmx fd release via disposeManagedPty is synchronous, so there is
       // no graceful-shutdown window to preserve at this point.
       try {
-        managed.pty.kill('SIGKILL')
+        killPtyProcess(managed.pty, 'SIGKILL')
       } catch {
         /* child may already be dead */
       }