fix(tool): stop the socket gate rejecting every tool call on Windows

The tool gate ran access() on DAEMON_SOCKET_PATH, which on Windows is the named
pipe \\.\pipe\pi-browser-daemon. Named pipes are not filesystem entries, so
access() always fails there and every browser_* call returned "Browser harness
not initialized" no matter how healthy the daemon was. spawn.ts already skipped
the identical check for this reason; that knowledge is now one shared
daemonSocketMayExist() used by both call sites, with platform and path as
parameters so the branch is testable off-platform.

Report: docs/superpowers/reports/2026-07-26-bug-audit.md#f1

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

3 files changed, 113 insertions, 15 deletions

diff --git a/src/daemon/spawn.ts b/src/daemon/spawn.ts
index d051fcc..165abcd 100644
--- a/src/daemon/spawn.ts
+++ b/src/daemon/spawn.ts
@@ -33,22 +33,36 @@ const cleanupStaleSocket = (): void => {
 };
 
 /**
+ * Whether the daemon endpoint could plausibly exist, judged without connecting.
+ *
+ * On POSIX a missing socket file means the daemon definitely isn't running, so
+ * callers can short-circuit. Windows named pipes are NOT filesystem entries —
+ * access() always fails on `\\.\pipe\…` even while the daemon is serving it — so
+ * on Windows the answer is always "maybe" and the caller must probe instead.
+ *
+ * Platform and path are parameters so the branch is testable off-platform.
+ */
+export const daemonSocketMayExist = async (
+  platform: NodeJS.Platform = process.platform,
+  socketPath: string = DAEMON_SOCKET_PATH,
+): Promise<boolean> => {
+  if (platform === "win32") return true;
+  try {
+    await access(socketPath);
+    return true;
+  } catch {
+    return false;
+  }
+};
+
+/**
  * Check whether the daemon is running by probing its Unix socket.
  * A file-existence check is unreliable — a crashed daemon leaves a stale socket.
  * We connect, send a register, wait for the registered ack, then disconnect.
  * On failure, clean up the stale socket so ensureDaemon can spawn a fresh one.
  */
 export const isDaemonRunning = async (timeoutMs = 2_000): Promise<boolean> => {
-  // On POSIX, a missing socket file means the daemon definitely isn't running,
-  // so we can short-circuit. Windows named pipes are not filesystem entries —
-  // access() always fails there — so skip the pre-check and probe directly.
-  if (!isWindows) {
-    try {
-      await access(DAEMON_SOCKET_PATH);
-    } catch {
-      return false;
-    }
-  }
+  if (!(await daemonSocketMayExist())) return false;
 
   return new Promise((resolve) => {
     let settled = false;
diff --git a/src/util/tool.ts b/src/util/tool.ts
index e4bb5ba..76087bc 100644
--- a/src/util/tool.ts
+++ b/src/util/tool.ts
@@ -8,8 +8,7 @@ import type {
   ToolRenderResultOptions,
 } from "@mariozechner/pi-coding-agent";
 import type { Component } from "@mariozechner/pi-tui";
-import { access } from "node:fs/promises";
-import { DAEMON_SOCKET_PATH } from "../daemon/protocol";
+import { daemonSocketMayExist } from "../daemon/spawn";
 import type { BrowserClient } from "../client";
 import type { Result } from "./result";
 
@@ -141,9 +140,7 @@ export const registerBrowserTool = (
         // Check daemon socket exists before attempting browser control.
         // If missing, the user hasn't run /browser-setup yet. Return a clear
         // error so the agent knows to direct the user to initialize.
-        try {
-          await access(DAEMON_SOCKET_PATH);
-        } catch {
+        if (!(await daemonSocketMayExist())) {
           return toToolResult(
             {
               success: false,
diff --git a/test/util/daemon-socket-test.ts b/test/util/daemon-socket-test.ts
new file mode 100644
index 0000000..eee0500
--- /dev/null
+++ b/test/util/daemon-socket-test.ts
@@ -0,0 +1,87 @@
+import { test, describe } from "node:test";
+import assert from "node:assert/strict";
+import { fileURLToPath } from "node:url";
+import { Type } from "typebox";
+import type { ExtensionAPI, ToolDefinition } from "@mariozechner/pi-coding-agent";
+import type { TSchema } from "typebox";
+import { daemonSocketMayExist } from "../../src/daemon/spawn";
+import { type AnyBrowserToolDefinition, registerBrowserTool } from "../../src/util/tool";
+import type { BrowserClient } from "../../src/client";
+import { ok } from "../../src/util/result";
+
+const WINDOWS_PIPE = "\\\\.\\pipe\\pi-browser-daemon";
+const MISSING_SOCKET = "/pi-browser-harness/definitely/not/here.sock";
+const thisFile = fileURLToPath(import.meta.url);
+
+describe("daemon socket presence check", () => {
+  test("a Windows named pipe is never judged absent from the filesystem", async () => {
+    assert.equal(await daemonSocketMayExist("win32", WINDOWS_PIPE), true);
+  });
+
+  test("a missing POSIX socket path is judged absent", async () => {
+    assert.equal(await daemonSocketMayExist("linux", MISSING_SOCKET), false);
+  });
+
+  test("an existing POSIX path is judged present", async () => {
+    assert.equal(await daemonSocketMayExist("linux", thisFile), true);
+  });
+
+  test("darwin uses the filesystem like linux", async () => {
+    assert.equal(await daemonSocketMayExist("darwin", MISSING_SOCKET), false);
+    assert.equal(await daemonSocketMayExist("darwin", thisFile), true);
+  });
+});
+
+type GateProbe = { readonly ran: ReadonlyArray<string>; readonly text: string };
+
+const runGate = async (ensureAlive: boolean | undefined): Promise<GateProbe> => {
+  const ran: string[] = [];
+  const client = {
+    mutationMutex: () => ({ acquire: async () => () => {} }),
+    ensureAlive: async () => ok(undefined),
+  } as unknown as BrowserClient;
+  const def = {
+    name: "browser_probe",
+    label: "Probe",
+    description: "probe",
+    promptSnippet: "probe",
+    promptGuidelines: [],
+    parameters: Type.Object({}),
+    concurrency: "parallel",
+    ...(ensureAlive !== undefined ? { ensureAlive } : {}),
+    handler: async () => {
+      ran.push("handler");
+      return ok({ text: "done" });
+    },
+  } as AnyBrowserToolDefinition;
+
+  let registered: ToolDefinition<TSchema> | undefined;
+  const pi = {
+    registerTool: (td: ToolDefinition<TSchema>) => {
+      registered = td;
+    },
+  } as unknown as ExtensionAPI;
+  registerBrowserTool(pi, client, def);
+  assert.ok(registered, "tool was not registered");
+  const result = await registered.execute?.("id", {}, undefined, undefined, undefined as never);
+  const first = result?.content[0];
+  return { ran, text: first !== undefined && first.type === "text" ? first.text : "" };
+};
+
+describe("tool gate on the daemon socket", () => {
+  test("ensureAlive:false bypasses the gate entirely", async () => {
+    const probe = await runGate(false);
+    assert.deepEqual([...probe.ran], ["handler"]);
+    assert.equal(probe.text, "done");
+  });
+
+  test("a gated tool agrees with the socket check about reaching the handler", async () => {
+    const reachable = await daemonSocketMayExist();
+    const probe = await runGate(undefined);
+    if (reachable) assert.deepEqual([...probe.ran], ["handler"]);
+    else {
+      assert.deepEqual([...probe.ran], []);
+      assert.match(probe.text, /not initialized/);
+    }
+  });
+});