diff --git a/crates/tui/plugins/computer-use/src/backends/win32.mjs b/crates/tui/plugins/computer-use/src/backends/win32.mjs
index d73b8bb8e8..18a2057adc 100644
--- a/crates/tui/plugins/computer-use/src/backends/win32.mjs
+++ b/crates/tui/plugins/computer-use/src/backends/win32.mjs
@@ -10,30 +10,6 @@ import crypto from "node:crypto";
 import { spawn } from "node:child_process";
 import { run, runOk, ExecError, tryJson } from "../exec.mjs";
 
-function ps(script, opts = {}) {
-  const encoded = Buffer.from(script, "utf16le").toString("base64");
-  return run("powershell.exe", ["-NoProfile", "-NonInteractive", "-EncodedCommand", encoded], {
-    timeoutMs: opts.timeoutMs ?? 25_000,
-    maxBuffer: 32 * 1024 * 1024,
-  });
-}
-
-/** ps() but truthful: timeout, nonzero exit, and spawn failure all throw. */
-async function psOk(script, opts = {}) {
-  const r = await ps(script, opts);
-  if (r.timedOut) throw new ExecError(`powershell timed out after ${opts.timeoutMs ?? 25_000}ms`, r);
-  if (r.code !== 0) throw new ExecError(`powershell.exe exited ${r.code}: ${(r.stderr || r.stdout).trim().slice(0, 300)}`, r);
-  return r;
-}
-
-async function psJson(script, opts = {}) {
-  const r = await psOk(script, opts);
-  const out = r.stdout.trim();
-  const j = tryJson(out, null);
-  if (!j) throw new ExecError(`powershell did not return JSON: ${(r.stderr || out).trim().slice(0, 300)}`, r);
-  return j;
-}
-
 const USER32 = `
 using System;
 using System.Runtime.InteropServices;
@@ -79,7 +55,36 @@ const MODVK = { ctrl: 0x11, control: 0x11, alt: 0x12, shift: 0x10, win: 0x5b, me
 // (Add-Type re-definition is tolerated through -ErrorAction SilentlyContinue).
 const USER32_PRELUDE = `Add-Type -TypeDefinition @'\n${USER32}\n'@ -ErrorAction SilentlyContinue;`;
 
-export function create() {
+export function create(opts = {}) {
+  // Allow tests (and other embedders) to inject a runner so no real
+  // powershell.exe is spawned. Production uses the imported runner.
+  const injectedRun = opts.exec && typeof opts.exec.run === "function" ? opts.exec.run : null;
+  const runner = injectedRun ?? run;
+
+  async function ps(script, o = {}) {
+    const encoded = Buffer.from(script, "utf16le").toString("base64");
+    return runner("powershell.exe", ["-NoProfile", "-NonInteractive", "-EncodedCommand", encoded], {
+      timeoutMs: o.timeoutMs ?? 25_000,
+      maxBuffer: 32 * 1024 * 1024,
+    });
+  }
+
+  /** ps() but truthful: timeout, nonzero exit, and spawn failure all throw. */
+  async function psOk(script, o = {}) {
+    const r = await ps(script, o);
+    if (r.timedOut) throw new ExecError(`powershell timed out after ${o.timeoutMs ?? 25_000}ms`, r);
+    if (r.code !== 0) throw new ExecError(`powershell.exe exited ${r.code}: ${(r.stderr || r.stdout).trim().slice(0, 300)}`, r);
+    return r;
+  }
+
+  async function psJson(script, o = {}) {
+    const r = await psOk(script, o);
+    const out = r.stdout.trim();
+    const j = tryJson(out, null);
+    if (!j) throw new ExecError(`powershell did not return JSON: ${(r.stderr || out).trim().slice(0, 300)}`, r);
+    return j;
+  }
+
   let lastRaster = null;
   let recording = null; // {id, pid, file, startedAt, mode}
 
diff --git a/crates/tui/plugins/computer-use/tests/win32.test.mjs b/crates/tui/plugins/computer-use/tests/win32.test.mjs
new file mode 100644
index 0000000000..59b31a029a
--- /dev/null
+++ b/crates/tui/plugins/computer-use/tests/win32.test.mjs
@@ -0,0 +1,65 @@
+// Win32 backend tests that run on ANY host via the injectable runner seam
+// (`create({ exec })`): the fake runner captures the PowerShell scripts so the
+// generated commands can be asserted directly, and no real powershell.exe is
+// ever spawned. These pin the failure-truthful and self-contained-action
+// behavior merged in #5903 without needing a Windows host or a
+// fake-powershell.exe-on-PATH fixture.
+import { test } from "node:test";
+import assert from "node:assert/strict";
+
+function decodeScript(args) {
+  const i = args.indexOf("-EncodedCommand");
+  if (i === -1) return null;
+  return Buffer.from(args[i + 1], "base64").toString("utf16le");
+}
+
+function mockExec({ fail = false } = {}) {
+  const calls = [];
+  const run = async (_cmd, args) => {
+    calls.push({ script: decodeScript(args) });
+    if (fail) return { code: 1, stdout: "", stderr: "simulated powershell failure" };
+    return { code: 0, stdout: '{"ok": true}\n', stderr: "" };
+  };
+  return { run, calls };
+}
+
+test("win32: actions run through an injected runner (no powershell needed)", async () => {
+  const { run, calls } = mockExec();
+  const mod = await import("../src/backends/win32.mjs");
+  const b = mod.create({ exec: { run } });
+  const r = await b.left_click({ target: { x: 5, y: 6 } });
+  assert.equal(r.action_sent, true);
+  assert.ok(calls.length >= 1, "the injected runner must receive the action command");
+});
+
+test("win32: input actions fail truthfully on a nonzero exit", async () => {
+  const { run } = mockExec({ fail: true });
+  const mod = await import("../src/backends/win32.mjs");
+  const b = mod.create({ exec: { run } });
+  await assert.rejects(() => b.left_click({ target: { x: 1, y: 2 } }), /exited 1/);
+  await assert.rejects(() => b.left_mouse_down({ target: { x: 1, y: 2 } }), /exited 1/);
+});
+
+test("win32: targeted left_mouse_down both moves and presses, self-contained", async () => {
+  const { run, calls } = mockExec();
+  const mod = await import("../src/backends/win32.mjs");
+  const b = mod.create({ exec: { run } });
+  await b.left_mouse_down({ target: { x: 12, y: 34 } });
+  const script = calls.at(-1).script;
+  // Self-contained: the User32 P/Invoke type travels with the action.
+  assert.ok(script.includes("public static class User32"), "action must define User32 in its own process");
+  assert.ok(script.includes("SetCursorPos(12, 34)"), "must move the cursor to the target");
+  assert.ok(script.includes("LEFTDOWN"), "must press the left button");
+});
+
+test("win32: every User32 action carries the type prelude in-process", async () => {
+  const { run, calls } = mockExec();
+  const mod = await import("../src/backends/win32.mjs");
+  const b = mod.create({ exec: { run } });
+  await b.mouse_move({ target: { x: 3, y: 4 } });
+  await b.key({ text: "a" });
+  assert.ok(calls.length >= 2, "two actions should have run through the injected runner");
+  for (const call of calls) {
+    assert.ok(call.script.includes("public static class User32"), "each action must redefine User32 in-process");
+  }
+});