@@ -399,11 +399,43 @@ export interface InstallResult {
   readonly error?: string
 }
 
+/** Terminate a spawned shell and everything it started. */
+function killTree(child: ReturnType<typeof spawn>): void {
+  const pid = child.pid
+  if (pid === undefined) return
+  if (process.platform === "win32") {
+    try {
+      // No process groups on Windows; taskkill walks the tree instead.
+      spawn("taskkill", ["/pid", String(pid), "/T", "/F"], { stdio: "ignore" })
+    } catch {
+      child.kill()
+    }
+    return
+  }
+  try {
+    // Negative pid targets the whole process group created by `detached`.
+    process.kill(-pid, "SIGTERM")
+  } catch {
+    try {
+      child.kill("SIGTERM")
+    } catch {
+      // Already gone.
+    }
+  }
+}
+
 function runNpm(args: string[], cwd: string, timeoutMs: number): Promise<{ code: number; output: string }> {
   return new Promise((resolve) => {
     // npm ships as a shell script on POSIX and a .cmd on Windows; `shell: true`
-    // lets the platform resolve whichever is present on PATH.
-    const child = spawn("npm", args, { cwd, shell: true, stdio: ["ignore", "pipe", "pipe"] })
+    // lets the platform resolve whichever is present on PATH. `detached` puts
+    // the shell in its own process group on POSIX so a timeout can take the
+    // whole group down — see killTree below.
+    const child = spawn("npm", args, {
+      cwd,
+      shell: true,
+      stdio: ["ignore", "pipe", "pipe"],
+      detached: process.platform !== "win32",
+    })
     let output = ""
     let settled = false
     const finish = (code: number) => {
@@ -413,7 +445,12 @@ function runNpm(args: string[], cwd: string, timeoutMs: number): Promise<{ code:
       resolve({ code, output: output.trim() })
     }
     const timer = setTimeout(() => {
-      child.kill()
+      // Killing the child alone leaves npm running: with `shell: true` the child
+      // is the shell, and npm is its descendant. An orphaned install keeps
+      // writing to the shared driver directory long after this promise settles,
+      // which the in-flight map cannot prevent — it serializes promises, not
+      // processes.
+      killTree(child)
       output += `\nTimed out after ${Math.round(timeoutMs / 1000)}s.`
       finish(124)
     }, timeoutMs)
 
@@ -578,3 +578,21 @@ describe("shellQuote on Windows", () => {
     expect(shellQuote("C:\\Users\\x\\drivers", "win32")).toBe("C:\\Users\\x\\drivers")
   })
 })
+
+describe("manual-install hints are copy-pasteable", () => {
+  test("every printed --prefix is quoted", () => {
+    // The failure branch of warehouse_install_driver was the one site that
+    // interpolated the directory raw, and it is the branch a user reads
+    // precisely when the automatic install has just failed them.
+    process.env["ALTIMATE_DRIVER_DIR"] = "/Users/John Doe/Library/drivers"
+
+    const err = new DriverNotInstalledError("snowflake", DRIVER_PACKAGES.snowflake, [])
+    const prefixes = [...err.message.matchAll(/--prefix (\S+)/g)].map((m) => m[1]!)
+
+    expect(prefixes.length).toBeGreaterThan(0)
+    for (const prefix of prefixes) {
+      // An unquoted path with a space splits, and npm receives the wrong prefix.
+      expect(prefix.startsWith("'") || prefix.startsWith('"')).toBe(true)
+    }
+  })
+})
@@ -2,6 +2,7 @@ import z from "zod"
 import { Tool } from "../../tool/tool"
 import {
   DRIVER_PACKAGES,
+  shellQuote,
   loadOptionalDriver,
   driverInstallDir,
   driverLabel,
@@ -96,7 +97,7 @@ export const WarehouseInstallDriverTool = Tool.define("warehouse_install_driver"
         output:
           `Could not install the ${label} driver (${packages}).\n` +
           `${result.error}\n\n` +
-          `Install it manually with:\n  npm install --prefix ${result.dir} ${packages}`,
+          `Install it manually with:\n  npm install --prefix ${shellQuote(result.dir)} ${packages}`,
       }
     }