diff --git a/apps/server/src/orchestration-v2/Adapters/AcpAdapterV2.test.ts b/apps/server/src/orchestration-v2/Adapters/AcpAdapterV2.test.ts
index dff370bd61101..2e8630309ff04 100644
--- a/apps/server/src/orchestration-v2/Adapters/AcpAdapterV2.test.ts
+++ b/apps/server/src/orchestration-v2/Adapters/AcpAdapterV2.test.ts
@@ -52,6 +52,7 @@ import * as Stream from "effect/Stream";
 import * as Scope from "effect/Scope";
 import * as TestClock from "effect/testing/TestClock";
 import * as ChildProcessSpawner from "effect/process/ChildProcessSpawner";
+import * as ChildProcess from "effect/process/ChildProcess";
 import * as EffectAcpErrors from "effect-acp/errors";
 import type * as EffectAcpProtocol from "effect-acp/protocol";
 import type * as EffectAcpSchema from "effect-acp/compat";
@@ -515,6 +516,71 @@ function makeMockRuntime(input: {
     });
 }
 
+it.live("does not detach an owned Windows ACP process and retains PID tree cleanup", () => {
+  let finalizerPid = 0;
+  let spawnedPid = 0;
+  return Effect.gen(function* () {
+    const spawner = yield* ChildProcessSpawner.ChildProcessSpawner;
+    const path = yield* Path.Path;
+    const terminated: number[] = [];
+    const runtime = yield* AcpSessionRuntime.make({
+      spawn: {
+        command: process.execPath,
+        args: [
+          yield* path.fromFileUrl(new URL("../../../scripts/acp-mock-agent.ts", import.meta.url)),
+        ],
+      },
+      cwd: process.cwd(),
+      clientInfo: { name: "test", version: "1" },
+      ownDetachedProcessGroup: true,
+      processGroupPlatform: "win32",
+      windowsProcessTreeTerminator: (pid) =>
+        Effect.sync(() => {
+          terminated.push(pid);
+        }),
+    }).pipe(
+      Effect.provideService(HostProcess.Platform, "win32"),
+      Effect.provideService(
+        ChildProcessSpawner.ChildProcessSpawner,
+        ChildProcessSpawner.make((command) => {
+          if (!ChildProcess.isStandardCommand(command))
+            return Effect.die("Expected standard command");
+          if (command.command === "taskkill") {
+            finalizerPid = Number(command.args[1]);
+            // Exercise the Windows finalizer while terminating the mock child on any host.
+            return spawner.spawn(
+              ChildProcess.make(process.execPath, ["-e", `process.kill(${spawnedPid})`]),
+            );
+          }
+          assert.equal(command.command, process.execPath);
+          assert.isFalse(command.options.detached);
+          return spawner.spawn(command).pipe(
+            Effect.tap((child) =>
+              Effect.sync(() => {
+                spawnedPid = Number(child.pid);
+              }),
+            ),
+          );
+        }),
+      ),
+    );
+    if (runtime.terminateProcessGroup === undefined)
+      return yield* Effect.die("Missing tree cleanup");
+    yield* runtime.terminateProcessGroup;
+    assert.deepEqual(terminated, [spawnedPid]);
+    assert.isAbove(spawnedPid, 0);
+  }).pipe(
+    Effect.provide(layerTest),
+    Effect.scoped,
+    Effect.tap(() =>
+      Effect.sync(() => {
+        assert.isAbove(spawnedPid, 0);
+        assert.equal(finalizerPid, spawnedPid);
+      }),
+    ),
+  );
+});
+
 const isProviderAdapterProtocolError = Schema.is(ProviderAdapter.ProviderAdapterProtocolError);
 
 const makePromptInactivityHarness = Effect.fnUntraced(function* (input: {
diff --git a/packages/provider-acp/src/server/AcpSessionRuntime.ts b/packages/provider-acp/src/server/AcpSessionRuntime.ts
index ecbe61e342746..9e42b5845af4d 100644
--- a/packages/provider-acp/src/server/AcpSessionRuntime.ts
+++ b/packages/provider-acp/src/server/AcpSessionRuntime.ts
@@ -1244,6 +1244,7 @@ export const make = Effect.fn("AcpSessionRuntime.make")(function* (
 > {
   const random = yield* SecureRandom.SecureRandom;
   const spawner = yield* ChildProcessSpawner.ChildProcessSpawner;
+  const hostPlatform = yield* HostProcess.Platform;
   const runtimeScope = yield* Scope.fork(yield* Scope.Scope);
   const eventQueue = yield* Queue.unbounded<AcpSessionRuntimeEvent>();
   const modeStateRef = yield* Ref.make<AcpSessionModeState | undefined>(undefined);
@@ -1455,7 +1456,7 @@ export const make = Effect.fn("AcpSessionRuntime.make")(function* (
         extendEnv: options.spawn.extendEnv ?? true,
         ...(options.ownDetachedProcessGroup === undefined
           ? {}
-          : { detached: options.ownDetachedProcessGroup }),
+          : { detached: options.ownDetachedProcessGroup && hostPlatform !== "win32" }),
         shell: containedSpawnCommand.shell,
       }),
     )