diff --git a/internal/proc/path_other.go b/internal/proc/path_other.go
index c02e98be..d00799bd 100644
--- a/internal/proc/path_other.go
+++ b/internal/proc/path_other.go
@@ -166,25 +166,51 @@ func shellEnv() (string, map[string]string) {
 	}
 	ctx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
 	defer cancel()
-	// interactive too, since many put PATH in .zshrc/.bashrc; the marker
-	// tells the answer apart from whatever the profile prints, and a NUL
-	// (which no variable can hold) parts the values: PATH first, then each
-	// of agentenv.Vars in order
-	const mark = "__magpie_path__"
-	format, args := mark+"%s", ` "$PATH"`
+	// interactive too, since many put PATH in .zshrc/.bashrc
+	cmd := CommandContext(ctx, sh, "-ilc", shellProbe(sh))
+	cmd.Stdin = nil
+	out, _ := cmd.Output()
+	return parseShellEnv(string(out), shellMark)
+}
+
+// shellMark tells shellEnv's answer apart from whatever the profile prints.
+const shellMark = "__magpie_path__"
+
+// shellProbe is the command the login shell sh runs to print its PATH and
+// agentenv.Vars, between two marks. A NUL (which no variable can hold)
+// parts the values: PATH first, then each of agentenv.Vars in order.
+//
+// A POSIX shell (zsh, bash, fish too) expands "$PATH" itself. nushell
+// doesn't: a double-quoted string is literal there, so it printed "$PATH",
+// "$CLAUDE_CONFIG_DIR", ... and magpie set each variable to its own name
+// (#738: then Claude sign-in failed with "mkdir $CLAUDE_CONFIG_DIR:
+// read-only file system", and the account lapsed minutes later, Claude
+// Code's sign-in looked for under that folder). nushell has no POSIX
+// quoting to ask with, and its PATH is a list, so it is asked to run
+// /bin/sh with the same printf: nushell hands it the environment as a
+// child sees it, PATH joined back into one string, and sh expands the
+// references. The outer command is in nushell's single quotes, where a
+// backslash is a backslash, and the printf format in sh's double quotes,
+// where \0 reaches printf as it is.
+func shellProbe(sh string) string {
+	format, args := shellMark+"%s", ` "$PATH"`
 	for _, v := range agentenv.Vars {
 		format += `\0%s`
 		args += ` "$` + v + `"`
 	}
-	cmd := CommandContext(ctx, sh, "-ilc", "printf '"+format+mark+"'"+args)
-	cmd.Stdin = nil
-	out, _ := cmd.Output()
-	return parseShellEnv(string(out), mark)
+	if filepath.Base(sh) == "nu" {
+		return `^/bin/sh -c 'printf "` + format + shellMark + `"` + args + `'`
+	}
+	return "printf '" + format + shellMark + "'" + args
 }
 
 // parseShellEnv reads shellEnv's answer out of what the shell printed. The
 // variables are nil when their count isn't agentenv.Vars' (a printf that
-// doesn't know \0); PATH alone is still taken then.
+// doesn't know \0); PATH alone is still taken then. A value that is the
+// variable's own reference ("$CODEX_HOME") is a shell that printed the
+// text instead of expanding it (nushell before shellProbe knew it; a shell
+// with other quoting still), not a folder: it counts as unset, as does a
+// PATH of "$PATH".
 func parseShellEnv(s, mark string) (string, map[string]string) {
 	i := strings.Index(s, mark)
 	if i < 0 {
@@ -196,6 +222,9 @@ func parseShellEnv(s, mark string) (string, map[string]string) {
 		return "", nil
 	}
 	f := strings.Split(s[:j], "\x00")
+	if f[0] == "$PATH" {
+		f[0] = ""
+	}
 	if len(f) != len(agentenv.Vars)+1 {
 		if len(f) == 1 {
 			return f[0], nil
@@ -204,6 +233,9 @@ func parseShellEnv(s, mark string) (string, map[string]string) {
 	}
 	vars := make(map[string]string, len(agentenv.Vars))
 	for k, v := range agentenv.Vars {
+		if f[k+1] == "$"+v {
+			f[k+1] = ""
+		}
 		vars[v] = f[k+1]
 	}
 	return f[0], vars
diff --git a/internal/proc/path_other_test.go b/internal/proc/path_other_test.go
index f72b84ea..7ad0952a 100644
--- a/internal/proc/path_other_test.go
+++ b/internal/proc/path_other_test.go
@@ -9,6 +9,8 @@ import (
 	"strings"
 	"testing"
 	"time"
+
+	"github.com/yetone/magpie/internal/agentenv"
 )
 
 // A desktop app with launchd's PATH finds a claude installed under a custom
@@ -82,3 +84,60 @@ func TestUserPathTakesAgentVars(t *testing.T) {
 		t.Fatalf("PATH lost: %q", os.Getenv("PATH"))
 	}
 }
+
+// A login shell that doesn't expand "$CLAUDE_CONFIG_DIR" prints the
+// reference itself. That is no folder: the variable counts as unset,
+// rather than magpie setting CLAUDE_CONFIG_DIR to "$CLAUDE_CONFIG_DIR" and
+// then looking for Claude Code's sign-in under it (#738).
+func TestParseShellEnvUnexpanded(t *testing.T) {
+	s := shellMark + "$PATH"
+	for _, v := range agentenv.Vars {
+		s += "\x00$" + v
+	}
+	p, vars := parseShellEnv("hello from the profile\n"+s+shellMark, shellMark)
+	if p != "" {
+		t.Fatalf("PATH = %q, want none", p)
+	}
+	if len(vars) != len(agentenv.Vars) {
+		t.Fatalf("%d variables, want %d", len(vars), len(agentenv.Vars))
+	}
+	for v, got := range vars {
+		if got != "" {
+			t.Fatalf("%s = %q, want none", v, got)
+		}
+	}
+}
+
+// nushell is asked in its own way, and answers with its PATH and the
+// variables a child of it gets, as a POSIX shell does; where nushell isn't
+// installed, the command's shape is all that can be checked.
+func TestShellEnvNushell(t *testing.T) {
+	probe := shellProbe("/opt/homebrew/bin/nu")
+	if !strings.HasPrefix(probe, "^/bin/sh -c 'printf \""+shellMark+"%s") || !strings.HasSuffix(probe, `"$`+agentenv.Vars[len(agentenv.Vars)-1]+`"'`) {
+		t.Fatalf("nushell's probe: %q", probe)
+	}
+	if strings.Contains(probe, `'`+shellMark) || strings.Count(probe, "'") != 2 {
+		t.Fatalf("nushell's probe must hold sh's command in one pair of single quotes: %q", probe)
+	}
+	nu, err := exec.LookPath("nu")
+	if err != nil {
+		t.Skip("nushell isn't installed")
+	}
+	t.Setenv("CODEX_HOME", "/from/the/environment")
+	t.Setenv("CLAUDE_CONFIG_DIR", "")
+	os.Unsetenv("CLAUDE_CONFIG_DIR")
+	out, err := exec.Command(nu, "-ilc", shellProbe(nu)).Output()
+	if err != nil {
+		t.Fatalf("nu: %v", err)
+	}
+	p, vars := parseShellEnv(string(out), shellMark)
+	if p == "" || strings.Contains(p, "$PATH") {
+		t.Fatalf("PATH from nushell: %q", p)
+	}
+	if got := vars["CODEX_HOME"]; got != "/from/the/environment" {
+		t.Fatalf("CODEX_HOME = %q: nushell's environment didn't reach sh", got)
+	}
+	if got := vars["CLAUDE_CONFIG_DIR"]; got != "" {
+		t.Fatalf("CLAUDE_CONFIG_DIR = %q, want none", got)
+	}
+}