diff --git a/apps/server/src/orchestration-v2/Adapters/AcpAdapterV2.test.ts b/apps/server/src/orchestration-v2/Adapters/AcpAdapterV2.test.ts
index 02d6f64c0be66..28b1897e58d81 100644
--- a/apps/server/src/orchestration-v2/Adapters/AcpAdapterV2.test.ts
+++ b/apps/server/src/orchestration-v2/Adapters/AcpAdapterV2.test.ts
@@ -53,6 +53,7 @@ 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";
@@ -571,6 +572,36 @@ function makeMockRuntime(input: {
     });
 }
 
+it.effect("detaches owned ACP processes on POSIX hosts but not on Windows", () =>
+  Effect.gen(function* () {
+    for (const [platform, expected] of [
+      ["win32", false],
+      ["linux", true],
+    ] as const) {
+      let detached: boolean | undefined;
+      yield* AcpSessionRuntime.make({
+        spawn: { command: "acp-agent", args: [] },
+        cwd: process.cwd(),
+        clientInfo: { name: "test", version: "1" },
+        ownDetachedProcessGroup: true,
+        processGroupPlatform: platform,
+      }).pipe(
+        Effect.provideService(HostProcess.Platform, platform),
+        Effect.provideService(
+          ChildProcessSpawner.ChildProcessSpawner,
+          ChildProcessSpawner.make((command) => {
+            if (ChildProcess.isStandardCommand(command)) detached = command.options.detached;
+            return Effect.die("spawn stopped by test");
+          }),
+        ),
+        Effect.scoped,
+        Effect.exit,
+      );
+      assert.equal(detached, expected, platform);
+    }
+  }).pipe(Effect.provide(layerTest)),
+);
+
 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 b216d6cc88b47..c603dd937fcaf 100644
--- a/packages/provider-acp/src/server/AcpSessionRuntime.ts
+++ b/packages/provider-acp/src/server/AcpSessionRuntime.ts
@@ -1255,6 +1255,7 @@ 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);
@@ -1464,9 +1465,11 @@ export const make = Effect.fn("AcpSessionRuntime.make")(function* (
         ...(options.spawn.cwd ? { cwd: options.spawn.cwd } : {}),
         ...(spawnEnvironment ? { env: spawnEnvironment } : {}),
         extendEnv: options.spawn.extendEnv ?? true,
+        // Windows `detached` gives the child its own console, which breaks stdio pipes through
+        // `.cmd` shims; taskkill /T cleans up the tree there without it.
         ...(options.ownDetachedProcessGroup === undefined
           ? {}
-          : { detached: options.ownDetachedProcessGroup }),
+          : { detached: options.ownDetachedProcessGroup && hostPlatform !== "win32" }),
         shell: containedSpawnCommand.shell,
       }),
     )
@@ -1628,7 +1631,6 @@ export const make = Effect.fn("AcpSessionRuntime.make")(function* (
     Effect.uninterruptible(terminateOwnedProcessGroupImpl),
   );
   if (options.ownDetachedProcessGroup === true) {
-    const hostPlatform = yield* HostProcess.Platform;
     const forceTerminateOwnedProcessGroup =
       hostPlatform === "win32"
         ? terminateWindowsProcessTreeWithTaskkill(Number(child.pid)).pipe(