fix: prevent completions from mutating os.Args via append side effect

completions.go

@@ -317,7 +317,10 @@ func (c *Command) getCompletions(args []string) (*Command, []Completion, ShellCo
 	// The last argument, which is not completely typed by the user,
 	// should not be part of the list of arguments
 	toComplete := args[len(args)-1]
-	trimmedArgs := args[:len(args)-1]
+	// Copy trimmedArgs to a new slice to avoid mutating the caller's
+	// backing array (which may be os.Args) when later appending "--".
+	trimmedArgs := make([]string, len(args)-1)
+	copy(trimmedArgs, args[:len(args)-1])
 
 	var finalCmd *Command
 	var finalArgs []string

completions_test.go

@@ -4072,3 +4072,58 @@ func TestCustomDefaultShellCompDirective(t *testing.T) {
 		})
 	}
 }
+
+func TestCompletionDoesNotMutateOsArgs(t *testing.T) {
+	// Test for https://github.com/spf13/cobra/issues/2257
+	// Verify that os.Args is not corrupted when shell completion runs
+	// with TraverseChildren enabled.
+	//
+	// The bug: getCompletions() calls append(finalArgs, "--") where
+	// finalArgs is a sub-slice of the original args (from os.Args[1:]).
+	// If there's spare capacity, append writes "--" into the shared
+	// backing array, mutating os.Args in place.
+
+	// Save and restore os.Args since we need to override it.
+	origArgs := os.Args
+	defer func() { os.Args = origArgs }()
+
+	// Set os.Args to simulate: root __completeNoDesc x
+	// We do NOT use SetArgs so the code falls through to os.Args[1:].
+	// The program name must not be "cobra.test" to bypass the test guard
+	// in ExecuteC().
+	os.Args = []string{"root", ShellCompNoDescRequestCmd, "x"}
+
+	rootCmd := &Command{
+		Use:              "root",
+		TraverseChildren: true,
+		ValidArgsFunction: func(cmd *Command, args []string, toComplete string) ([]string, ShellCompDirective) {
+			return []string{"mycompletion"}, ShellCompDirectiveDefault
+		},
+		Run: emptyRun,
+	}
+
+	buf := new(bytes.Buffer)
+	rootCmd.SetOut(buf)
+	rootCmd.SetErr(buf)
+
+	_, err := rootCmd.ExecuteC()
+	if err != nil {
+		t.Fatalf("Unexpected error: %v", err)
+	}
+
+	// Without the fix, os.Args[2] would be changed from "x" to "--"
+	// because append(finalArgs, "--") in getCompletions wrote into
+	// the shared backing array of os.Args.
+	if len(os.Args) != 3 {
+		t.Fatalf("os.Args length changed: expected 3, got %d", len(os.Args))
+	}
+	if os.Args[0] != "root" {
+		t.Errorf("os.Args[0] was mutated: expected %q, got %q", "root", os.Args[0])
+	}
+	if os.Args[1] != ShellCompNoDescRequestCmd {
+		t.Errorf("os.Args[1] was mutated: expected %q, got %q", ShellCompNoDescRequestCmd, os.Args[1])
+	}
+	if os.Args[2] != "x" {
+		t.Errorf("os.Args[2] was mutated: expected %q, got %q", "x", os.Args[2])
+	}
+}