diff --git a/internal/health/installs.go b/internal/health/installs.go
index c671374..ea2a3de 100644
--- a/internal/health/installs.go
+++ b/internal/health/installs.go
@@ -80,13 +80,46 @@ func HomebrewInstall(self string) (path, version string) {
 
 // logosVersion is what a logos at path answers to --version. Bounded, because
 // the file may be any program with that name.
+//
+// It runs in a home of its own with no vault named. Every logos does its
+// startup work before it reads its arguments, and a 0.4.x logos's startup
+// adopts ~/brain — it writes the vault pointer and renames .brain — so asking
+// an old Homebrew install its version changed the user's vault. A version
+// answer needs none of the user's state.
 func logosVersion(path string) (string, bool) {
+	home, err := os.MkdirTemp("", "logos-probe-")
+	if err != nil {
+		return "", false
+	}
+	defer os.RemoveAll(home)
 	ctx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
 	defer cancel()
-	out, err := exec.CommandContext(ctx, path, "--version").Output()
+	cmd := exec.CommandContext(ctx, path, "--version")
+	cmd.Env = probeEnv(home)
+	out, err := cmd.Output()
 	fields := strings.Fields(string(out))
 	if err != nil || len(fields) < 2 || fields[0] != "logos" {
 		return "", false
 	}
 	return strings.TrimPrefix(fields[1], "v"), true
 }
+
+// probeEnv is this process's environment with home, config and data dirs
+// pointed at home and every LOGOS_ and BRAIN_ variable removed: those name a
+// vault, a runtime or a host to wire, and the probe must reach none of them.
+func probeEnv(home string) []string {
+	homeVars := map[string]bool{"HOME": true, "USERPROFILE": true, "APPDATA": true, "LOCALAPPDATA": true,
+		"XDG_CONFIG_HOME": true, "XDG_DATA_HOME": true, "XDG_STATE_HOME": true, "XDG_CACHE_HOME": true}
+	var env []string
+	for _, kv := range os.Environ() {
+		k, _, _ := strings.Cut(kv, "=")
+		if homeVars[strings.ToUpper(k)] || strings.HasPrefix(k, "LOGOS_") || strings.HasPrefix(k, "BRAIN_") {
+			continue
+		}
+		env = append(env, kv)
+	}
+	for k := range homeVars {
+		env = append(env, k+"="+home)
+	}
+	return env
+}
diff --git a/internal/health/installs_test.go b/internal/health/installs_test.go
index 01126c4..b39d10f 100644
--- a/internal/health/installs_test.go
+++ b/internal/health/installs_test.go
@@ -84,3 +84,38 @@ func TestOneHomebrewInstallFirstOnPathReportsNothing(t *testing.T) {
 		t.Errorf("a single install was reported as two: %+v", c)
 	}
 }
+
+// Doctor asks another install its version by running it, and every logos does
+// its startup work before it reads its arguments. A 0.4.x logos's startup
+// adopts ~/brain, writing the vault pointer and renaming .brain, so a 0.5.0
+// doctor next to an old Homebrew logos changed the user's vault by looking.
+func TestAskingAnotherInstallItsVersionCannotTouchTheUsersHomeOrVault(t *testing.T) {
+	prefix := t.TempDir()
+	cellar := filepath.Join(prefix, "Cellar", "logos-mcp", "0.4.9", "bin", "logos")
+	brewAt(t, prefix, "0.4.9")
+	// The old install's startup, as far as this test cares: it writes into
+	// whatever home and vault its environment names, then answers.
+	script := "#!/bin/sh\n" +
+		"mkdir -p \"$HOME/brain\" && touch \"$HOME/brain/adopted\"\n" +
+		"[ -n \"$LOGOS_VAULT\" ] && touch \"$LOGOS_VAULT/touched\"\n" +
+		"[ -n \"$BRAIN_VAULT\" ] && touch \"$BRAIN_VAULT/touched\"\n" +
+		"echo \"logos v0.4.9 darwin/arm64\"\n"
+	if err := os.WriteFile(cellar, []byte(script), 0o755); err != nil {
+		t.Fatal(err)
+	}
+	home, vault := t.TempDir(), t.TempDir()
+	t.Setenv("HOME", home)
+	t.Setenv("LOGOS_VAULT", vault)
+	t.Setenv("BRAIN_VAULT", vault)
+	self := fakeLogos(t, filepath.Join(t.TempDir(), ".local", "bin", "logos"), "0.5.0")
+
+	c := CheckOtherInstall(self, "v0.5.0")
+	if !strings.Contains(c.Detail, "0.4.9") {
+		t.Fatalf("the Homebrew install's version was not read: %+v", c)
+	}
+	for _, p := range []string{filepath.Join(home, "brain"), filepath.Join(vault, "touched")} {
+		if _, err := os.Stat(p); err == nil {
+			t.Errorf("running the other install to read its version wrote %s", p)
+		}
+	}
+}