diff --git a/.github/wiki/Plugin-Security.md b/.github/wiki/Plugin-Security.md
index 7ebb5e6ea..080614b21 100644
--- a/.github/wiki/Plugin-Security.md
+++ b/.github/wiki/Plugin-Security.md
@@ -2,6 +2,15 @@
 
 How the CLI decides a plugin download can be trusted, and how it decides a licence response and a cached licence can be trusted. Release signatures come first; licence verification starts at [What ping_api signs](#what-ping_api-signs).
 
+## Machine invocation
+
+Machine-invoked mounted plugin commands that restore secret arguments or flags
+are refused with E421 before the plugin binary starts. The CLI consumes
+`NSELF_INVOKE_SECRETS`, but the plugin protocol cannot pass those secrets onward
+without exposing them on the plugin process arguments. Calls without a secret
+hand-off continue to run. Free-form commands with no declared secret arguments
+or flags do not participate in the hand-off.
+
 ## Plugin release signatures
 
 Every plugin release is signed by the pipeline that builds it (free plugins: the `release-signing` environment of `nself-org/plugins`; licensed plugins: the owner release machine). The registry and catalog entry carries `release_signature: {key_id, alg: "ed25519", sig}`. The CLI checks it before it extracts a single file.
diff --git a/cmd/commands/plugin_mount.go b/cmd/commands/plugin_mount.go
index c1112364b..e45bcbcbb 100644
--- a/cmd/commands/plugin_mount.go
+++ b/cmd/commands/plugin_mount.go
@@ -33,6 +33,7 @@ import (
 	"strings"
 
 	"github.com/nself-org/cli/internal/compat"
+	"github.com/nself-org/cli/internal/errs"
 	"github.com/nself-org/cli/internal/plugin"
 	"github.com/nself-org/cli/internal/plugin/mount"
 	"github.com/spf13/cobra"
@@ -226,6 +227,9 @@ func installedNode(s mount.Spec) *cobra.Command {
 func proxyRunE(node *cobra.Command, s mount.Spec, segs []string) {
 	own := append([]string{}, segs...)
 	node.RunE = func(cmd *cobra.Command, args []string) error {
+		if restoredInvokeSecrets {
+			return errs.New("E421", "mounted plugins cannot receive machine-invoked secret arguments").WithWhy("the plugin protocol cannot keep restored secrets off the plugin process arguments")
+		}
 		argv := append(append([]string{}, own...), stripRootPersistentFlags(args)...)
 		return plugin.ProxyBinaryAt(s.BinaryPath, resolvePluginDir(), s.Slug, argv)
 	}
diff --git a/cmd/commands/plugin_mount_secrets_test.go b/cmd/commands/plugin_mount_secrets_test.go
new file mode 100644
index 000000000..de6114dca
--- /dev/null
+++ b/cmd/commands/plugin_mount_secrets_test.go
@@ -0,0 +1,78 @@
+package commands
+
+// Purpose: keep restored machine-invocation secrets off mounted plugin argv.
+// Inputs: synthetic secret bundles and a mounted binary that exits with 19.
+// Outputs: E421 before execution for secret hand-offs, plugin exit otherwise.
+// Constraints: shell fixture requires Unix; no real secrets or external services.
+
+import (
+	"errors"
+	"os"
+	"path/filepath"
+	"runtime"
+	"strings"
+	"testing"
+
+	"github.com/nself-org/cli/internal/invoke"
+	"github.com/nself-org/cli/internal/plugin"
+	"github.com/nself-org/cli/internal/plugin/mount"
+	"github.com/spf13/cobra"
+)
+
+func mountedSecretProxy(t *testing.T) *cobra.Command {
+	t.Helper()
+	if runtime.GOOS == "windows" {
+		t.Skip("shell fixture")
+	}
+	savedArgs, savedSecrets := os.Args, restoredInvokeSecrets
+	t.Cleanup(func() { os.Args, restoredInvokeSecrets = savedArgs, savedSecrets })
+	home := t.TempDir()
+	t.Setenv("HOME", home)
+	dir := filepath.Join(home, ".nself", "plugins")
+	if err := os.MkdirAll(dir, 0755); err != nil {
+		t.Fatal(err)
+	}
+	bin := filepath.Join(dir, "nself-demo")
+	if err := os.WriteFile(bin, []byte("#!/bin/sh\nexit 19\n"), 0755); err != nil {
+		t.Fatal(err)
+	}
+	node := &cobra.Command{Use: "demo"}
+	proxyRunE(node, mount.Spec{Slug: "demo", BinaryPath: bin}, nil)
+	return node
+}
+
+func TestMountProxyRestoredSecrets(t *testing.T) {
+	node := mountedSecretProxy(t)
+	const placeholder = "nself-invoke-secret:abcd1234abcd1234:0"
+	for _, arg := range []string{placeholder, "--token=" + placeholder} {
+		t.Run(arg, func(t *testing.T) {
+			os.Args = []string{"nself", "demo", arg}
+			t.Setenv(invoke.InvokedByEnv, "mcp")
+			t.Setenv(invoke.SecretsEnv, `{"nonce":"abcd1234abcd1234","values":["FAKE-mounted-secret"]}`)
+			if err := restoreSecretArgv(); err != nil {
+				t.Fatal(err)
+			}
+			if !strings.Contains(os.Args[2], "FAKE-mounted-secret") {
+				t.Fatal("secret was not restored")
+			}
+			err := node.RunE(node, os.Args[2:])
+			if err == nil || !strings.Contains(err.Error(), "E421") || strings.Contains(err.Error(), "FAKE-mounted-secret") {
+				t.Fatalf("expected safe E421 before executing plugin, got %v", err)
+			}
+		})
+	}
+	// A following invocation must clear the previous hand-off state. Both
+	// non-secret machine calls and direct calls must still execute the plugin.
+	for _, invokedBy := range []string{"mcp", ""} {
+		t.Setenv(invoke.InvokedByEnv, invokedBy)
+		t.Setenv(invoke.SecretsEnv, "")
+		os.Args = []string{"nself", "demo", "ordinary"}
+		if err := restoreSecretArgv(); err != nil {
+			t.Fatal(err)
+		}
+		var exit *plugin.ExitCodeError
+		if err := node.RunE(node, os.Args[2:]); !errors.As(err, &exit) || exit.Code != 19 {
+			t.Fatalf("non-secret call did not execute plugin: %v", err)
+		}
+	}
+}
diff --git a/cmd/commands/secrets_core_logmask.go b/cmd/commands/secrets_core_logmask.go
index 776dcb1a7..be6b7054f 100644
--- a/cmd/commands/secrets_core_logmask.go
+++ b/cmd/commands/secrets_core_logmask.go
@@ -25,15 +25,21 @@ import (
 	"github.com/spf13/pflag"
 )
 
+// restoredInvokeSecrets records the hand-off after its environment is consumed.
+// Mounted binaries cannot safely receive these values on their own argv.
+var restoredInvokeSecrets bool
+
 // restoreSecretArgv is the first step of Execute: a machine invoker hands the
 // secret values over the environment (D-0468); put them back into os.Args and
 // tell the command log to mask them (D-0473).
 func restoreSecretArgv() error {
+	restoredInvokeSecrets = false
 	restored, values, err := invoke.RestoreSecretArgv(os.Args)
 	if err != nil {
 		return err
 	}
 	os.Args = restored
+	restoredInvokeSecrets = len(values) > 0
 	cmdlog.MaskValues(values...)
 	return nil
 }
diff --git a/cmd/commands/secrets_core_logsite_test.go b/cmd/commands/secrets_core_logsite_test.go
index 9f856362c..4f1cee00d 100644
--- a/cmd/commands/secrets_core_logsite_test.go
+++ b/cmd/commands/secrets_core_logsite_test.go
@@ -58,7 +58,8 @@ func TestRootLogsNoSecretPositional(t *testing.T) {
 // restored value is on os.Args; a log line built from it must still hide it.
 func TestRestoreSecretArgvMasksRestoredValues(t *testing.T) {
 	saved := os.Args
-	t.Cleanup(func() { os.Args = saved })
+	savedSecrets := restoredInvokeSecrets
+	t.Cleanup(func() { os.Args, restoredInvokeSecrets = saved, savedSecrets })
 	ph := "nself-invoke-secret:abcd1234abcd1234:0"
 	os.Args = []string{"nself", "secrets", "set", "FAKEKEY", ph}
 	t.Setenv(invoke.InvokedByEnv, "mcp")
diff --git a/internal/invoke/secretenv.go b/internal/invoke/secretenv.go
index c97d99610..074818a35 100644
--- a/internal/invoke/secretenv.go
+++ b/internal/invoke/secretenv.go
@@ -14,12 +14,11 @@ package invoke
 // grandchild inherits it, and ignores it unless InvokedByEnv is set. Any
 // mismatch (bad JSON, bad index, a value unused or used twice) is E421.
 // MaxSecretEnvBytes caps the bundle per OS (secretenv_limit_*.go).
-// Known boundary: a mounted plugin binary that the nself child proxies to
-// (internal/plugin/router.go) receives the restored secret values on its own
-// argv, where ps shows them. The plugin protocol has no hand-off for the bundle
-// yet; the gap is tracked in hq debt. A free-form plugin command (no declared
-// args or flags) is also not hidden: nothing marks its words secret, so
-// Request.Argv goes on the child argv as given (argv.go).
+// Mounted plugin commands refuse a hand-off containing restored secrets with
+// E421 before executing the binary: the plugin protocol cannot carry the bundle
+// without putting those values on the plugin argv. A free-form plugin command
+// (no declared args or flags) is also not hidden: nothing marks its words
+// secret, so Request.Argv goes on the child argv as given (argv.go).
 
 import (
 	"crypto/rand"