diff --git a/README.md b/README.md
index a5b03d6..7dcb19f 100644
--- a/README.md
+++ b/README.md
@@ -88,17 +88,49 @@ Prints the unencrypted private key in OpenSSH format (the
 and nothing else, so it can be redirected into a file. The key's comment is the
 same as for `pub`.
 
-### `keyfunc ssh install <[user@]host> [-- ssh options...]`
+### `keyfunc ssh install <[user@]host> [-- sftp options...]`
 
-Runs the system `ssh` to the host and, on the host:
+Adds the `pub` line to `~/.ssh/authorized_keys` on the host. No command is run
+on the host: the file is fetched, changed here, and written back with the
+system `sftp` client in batch mode.
 
-- creates `~/.ssh` with mode `0700` if it is missing;
-- creates `~/.ssh/authorized_keys` with mode `0600` if it is missing;
-- appends the `pub` line only if an identical line is not already there.
+The first connection fetches `~/.ssh/authorized_keys`. The file reads as empty
+only when `sftp` reported that file as not being there — the one line naming
+that path. The same wording anywhere else in the session does not count: `ssh`
+writes `No such file or directory` about an `-i` it cannot find, on a session
+that then authenticates through the agent. When `sftp` failed for any other
+reason — the file is there and cannot be read, the connection did not come up —
+the tool prints what `sftp` said and exits with status 1 without writing
+anything, rather than put a file back holding the new key alone. What `sftp`
+cannot tell apart is a missing file and one in a directory it cannot enter, so a
+`~/.ssh` whose mode shuts the user out reads as a host with no file; the second
+connection sets that mode to `0700` and writes, as on a host that has none. If
+an identical line is already in the file, the tool prints
+`already present` and connects no further. Otherwise the line is added (after a
+newline, if the file did not end with one) and a second connection:
 
-It then prints `added` or `already present`. How this `ssh` connection
-authenticates is up to the user's normal `ssh` setup (existing keys, agent,
-password). Anything after `--` is passed to `ssh` unchanged.
+- creates `~/.ssh` and sets it to mode `0700`;
+- uploads the new file as `~/.ssh/authorized_keys.keyfunc-<random>` and sets it
+  to mode `0600`;
+- renames that file over `~/.ssh/authorized_keys`.
+
+The tool then prints `added`. So a run that adds a line connects twice. The
+rename is the step that either happens or does not: the file on the host is
+never half-written. `sftp` does it in one step against servers that offer
+OpenSSH's POSIX rename extension, as OpenSSH's own server does; a server
+without it may refuse to rename onto a file that is already there.
+
+If a step fails, the tool prints what `sftp` said, removes nothing, and exits
+with status 1. It names the uploaded file only when the step that failed was
+the upload or one after it, which is where a file of that name can be on the
+host; a failure before the upload names none. Everything `sftp`
+writes goes to standard error, so the tool's own standard output is only
+`added` or `already present`.
+
+Anything after `--` is passed to `sftp` unchanged, which is where the port goes
+(`-P 2222`, not `-p`). How the connection authenticates is up to the user's
+normal `ssh` setup, except that batch mode does not prompt: a key or an agent
+has to do it, not a typed password.
 
 ### `keyfunc ssh to <host> [ssh arguments...]`
 
diff --git a/internal/cli/ssh/install.go b/internal/cli/ssh/install.go
index 8133261..5886a69 100644
--- a/internal/cli/ssh/install.go
+++ b/internal/cli/ssh/install.go
@@ -1,57 +1,48 @@
 package ssh
 
 import (
+	"bytes"
+	"crypto/rand"
+	"encoding/hex"
 	"fmt"
+	"os"
 	"os/exec"
+	"path/filepath"
 	"slices"
 	"strings"
 
 	"github.com/spf13/cobra"
 )
 
-// script is what runs on the host. It reads the key line from its own
-// standard input, so the line never appears on a command line, where
-// anyone else on the host could read it out of the process list. It
-// contains no single quote, so the whole of it travels through ssh
-// inside one pair of them. The umask keeps anything it makes to the
-// owner from the start; the modes are then set outright, whatever the
-// umask on the host turns out to be. A file whose last line has no
-// newline at its end gets one before the key line goes on, so that the
-// two do not run into each other.
-const script = `
-set -e
-umask 077
-directory="$HOME/.ssh"
-file="$directory/authorized_keys"
-if [ ! -d "$directory" ]; then
-	mkdir -p "$directory"
-	chmod 700 "$directory"
-fi
-if [ ! -f "$file" ]; then
-	: > "$file"
-	chmod 600 "$file"
-fi
-IFS= read -r line
-if grep -q -x -F -e "$line" "$file"; then
-	echo "already present"
-else
-	if [ -s "$file" ] && [ -n "$(tail -c 1 "$file")" ]; then
-		printf "\n" >> "$file"
-	fi
-	printf "%s\n" "$line" >> "$file"
-	echo "added"
-fi
-`
+// Where the key goes on the host and what the file it arrives in is
+// called before it is renamed into place. The random end of that name
+// keeps two runs at once from writing to the same file.
+const (
+	directory     = ".ssh"
+	authorized    = ".ssh/authorized_keys"
+	sidecarPrefix = ".ssh/authorized_keys.keyfunc-"
+	sidecarBytes  = 8
+)
+
+// The modes the host is left with, as sftp's chmod spells them, and
+// the mode of the copy made here on the way.
+const (
+	directoryMode = "700"
+	fileMode      = "600"
+	localMode     = 0o600
+)
 
 // install returns the command that adds the public key to a host.
 func install() *cobra.Command {
 	cmd := &cobra.Command{
-		Use:   "install <[user@]host> [-- ssh options...]",
+		Use:   "install <[user@]host> [-- sftp options...]",
 		Short: "add the public key to a host's authorized_keys",
-		Long: "Runs the system ssh to the host, which makes ~/.ssh and " +
-			"~/.ssh/authorized_keys there if they are missing and adds " +
-			"the public key unless the same line is already in the " +
-			"file. Anything after -- is given to ssh unchanged.",
+		Long: "Downloads the host's authorized_keys with the system " +
+			"sftp, adds the public key to it here unless the same " +
+			"line is already there, and uploads the result as a file " +
+			"beside it which is then renamed over it. Nothing is run " +
+			"on the host. Anything after -- is given to sftp " +
+			"unchanged, which is where the port goes (-P).",
 		Args: cobra.MinimumNArgs(1),
 		RunE: func(cmd *cobra.Command, args []string) error {
 			key, comment, err := derived(cmd)
@@ -64,7 +55,7 @@ func install() *cobra.Command {
 				return err
 			}
 
-			return send(cmd, args[0], args[1:], line)
+			return add(cmd, args[0], args[1:], line)
 		},
 	}
 
@@ -73,24 +64,207 @@ func install() *cobra.Command {
 	return cmd
 }
 
-// send runs ssh to the host with the user's options, gives it the
-// script to run there, and writes the key line to its standard input.
-// What the host says, added or already present, is passed straight on.
-func send(cmd *cobra.Command, host string, options []string, line string) error {
-	argv := slices.Concat(options, []string{
-		host, "/bin/sh -c '" + script + "'",
-	})
-
-	//nolint:gosec // the options are the user's own, meant for ssh
-	command := exec.CommandContext(cmd.Context(), "ssh", argv...)
-	command.Stdin = strings.NewReader(line + "\n")
-	command.Stdout = cmd.OutOrStdout()
-	command.Stderr = cmd.ErrOrStderr()
-
-	err := command.Run()
+// add puts the key line in the host's authorized_keys. The file is
+// fetched in one sftp session and written back in another, so a run
+// that adds a line connects twice; a run that finds the line already
+// there connects once and stops.
+func add(cmd *cobra.Command, host string, options []string, line string) error {
+	work, err := os.MkdirTemp("", "keyfunc-install-")
 	if err != nil {
-		return fmt.Errorf("running ssh: %w", err)
+		return fmt.Errorf("making a temporary directory: %w", err)
 	}
 
-	return nil
+	defer func() { _ = os.RemoveAll(work) }()
+
+	content, err := fetch(cmd, host, options,
+		filepath.Join(work, "authorized_keys"),
+	)
+	if err != nil {
+		return err
+	}
+
+	merged, added := merge(content, line)
+	if !added {
+		return write(cmd, "already present\n")
+	}
+
+	return upload(cmd, host, options, work, merged)
+}
+
+// upload writes the new file to the host and renames it over
+// authorized_keys, which is the step that either happens or does not.
+// Nothing is removed when a step fails: the file left behind is named
+// so that it can be looked at and cleared away by hand.
+func upload(
+	cmd *cobra.Command, host string, options []string,
+	work, merged string,
+) error {
+	local := filepath.Join(work, "authorized_keys.merged")
+
+	err := os.WriteFile(local, []byte(merged), localMode)
+	if err != nil {
+		return fmt.Errorf("writing the new file: %w", err)
+	}
+
+	sidecar, err := sidecarName()
+	if err != nil {
+		return err
+	}
+
+	// The mkdir may fail: the directory is usually there already.
+	said, err := session(cmd, host, options, []string{
+		"-mkdir " + directory,
+		"chmod " + directoryMode + " " + directory,
+		"put " + quoted(local) + " " + sidecar,
+		"chmod " + fileMode + " " + sidecar,
+		"rename " + sidecar + " " + authorized,
+	})
+	if err != nil {
+		// sftp echoes each command as it runs it and stops at the
+		// first that fails, so the name is in what it said only once
+		// the put was reached, which is where a file of that name
+		// can be on the host. Before that there is none to name.
+		if strings.Contains(said, sidecar) {
+			return fmt.Errorf(
+				"%w; %s may be left on the host", err, sidecar,
+			)
+		}
+
+		return err
+	}
+
+	return write(cmd, "added\n")
+}
+
+// session runs one sftp session with the user's own options and the
+// batch of commands, which sftp reads from its standard input and
+// stops at the first of which that fails, unless it begins with a
+// dash. sftp echoes the commands as it runs them, so everything it
+// says goes to the error output and the tool's own output stays the
+// one word it prints. What it said is also given back: a session that
+// failed says there what went wrong, and the status alone does not.
+func session(
+	cmd *cobra.Command, host string, options []string, batch []string,
+) (string, error) {
+	argv := slices.Concat(
+		[]string{"-b", "-"}, options, []string{host},
+	)
+
+	var said bytes.Buffer
+
+	//nolint:gosec // the options are the user's own, meant for sftp
+	command := exec.CommandContext(cmd.Context(), "sftp", argv...)
+	command.Stdin = strings.NewReader(strings.Join(batch, "\n") + "\n")
+	command.Stdout = &said
+	command.Stderr = &said
+
+	err := command.Run()
+
+	_, _ = cmd.ErrOrStderr().Write(said.Bytes())
+
+	if err != nil {
+		return said.String(), fmt.Errorf("running sftp: %w", err)
+	}
+
+	return said.String(), nil
+}
+
+// merge returns the file with the key line on the end, and whether it
+// had to be added. A file whose last line has no newline at its end
+// gets one first, so that the two lines do not run into each other.
+func merge(content, line string) (string, bool) {
+	if slices.Contains(strings.Split(content, "\n"), line) {
+		return content, false
+	}
+
+	if content != "" && !strings.HasSuffix(content, "\n") {
+		content += "\n"
+	}
+
+	return content + line + "\n", true
+}
+
+// fetch brings the host's authorized_keys into the given path and
+// returns what is in it. A host that has no such file reads as empty,
+// but only when that is what sftp said about it: a file that is there
+// and cannot be read fails the run, because writing back over it
+// would leave the host with the new key and nothing else.
+func fetch(
+	cmd *cobra.Command, host string, options []string, into string,
+) (string, error) {
+	said, err := session(cmd, host, options, []string{
+		"get " + authorized + " " + quoted(into),
+	})
+	if err != nil {
+		if absent(said) {
+			return "", nil
+		}
+
+		return "", err
+	}
+
+	//nolint:gosec // the path is a temporary file of the tool's own
+	content, err := os.ReadFile(into)
+	if err != nil {
+		return "", fmt.Errorf("reading the fetched file: %w", err)
+	}
+
+	return string(content), nil
+}
+
+// absent says whether sftp reported the file that was asked for as
+// not being there, which is the one failure of the fetch that is read
+// as an empty authorized_keys. The reading is taken only from the
+// line in which sftp reports on that file, because ssh writes "no
+// such file" into the same output for reasons of its own — a missing
+// -i identity file draws that warning on a session that then
+// authenticates through the agent — and a real read failure on such a
+// session must not pass for an empty file.
+func absent(said string) bool {
+	for line := range strings.Lines(said) {
+		named, is := reportedNotFound(strings.TrimSpace(line))
+		if is && (named == authorized ||
+			strings.HasSuffix(named, "/"+authorized)) {
+			return true
+		}
+	}
+
+	return false
+}
+
+// reportedNotFound returns the path an sftp line reports as not being
+// there, and whether the line is such a report. The client writes one
+// wording for a remote file it cannot find, naming the path the
+// server expanded, which is the absolute one.
+func reportedNotFound(line string) (string, bool) {
+	const (
+		before = `File "`
+		after  = `" not found.`
+	)
+
+	if !strings.HasPrefix(line, before) ||
+		!strings.HasSuffix(line, after) {
+		return "", false
+	}
+
+	return strings.TrimSuffix(strings.TrimPrefix(line, before), after), true
+}
+
+// sidecarName returns the name the new file is uploaded under.
+func sidecarName() (string, error) {
+	random := make([]byte, sidecarBytes)
+
+	_, err := rand.Read(random)
+	if err != nil {
+		return "", fmt.Errorf("making a name for the new file: %w", err)
+	}
+
+	return sidecarPrefix + hex.EncodeToString(random), nil
+}
+
+// quoted puts the double quotes around a path that sftp needs when the
+// path has a space in it. Only paths of the tool's own making are
+// given to it, and they hold no quote of their own.
+func quoted(path string) string {
+	return `"` + path + `"`
 }
diff --git a/internal/cli/ssh/install_test.go b/internal/cli/ssh/install_test.go
new file mode 100644
index 0000000..051014c
--- /dev/null
+++ b/internal/cli/ssh/install_test.go
@@ -0,0 +1,78 @@
+//nolint:testpackage // absent is what these wordings are read by
+package ssh
+
+import "testing"
+
+// What a session says besides its report on the file that was asked
+// for: sftp echoes the command it is running, and ssh warns about an
+// identity file it cannot find in the words of a missing file even
+// though the session goes on to authenticate.
+const (
+	echoed = `sftp> get .ssh/authorized_keys "/tmp/keyfunc/authorized_keys"
+`
+	warning = `Warning: Identity file /gone not accessible: ` +
+		"No such file or directory.\n"
+)
+
+// TestAbsenceIsReadOnlyFromWhatSFTPSaidAboutAuthorizedKeys holds the
+// wordings the OpenSSH client was seen to use against a real server:
+// a file it cannot find is reported one way, naming the path the
+// server expanded, and everything else it says is a failure.
+func TestAbsenceIsReadOnlyFromWhatSFTPSaidAboutAuthorizedKeys(t *testing.T) {
+	t.Parallel()
+
+	sessions := map[string]struct {
+		said string
+		want bool
+	}{
+		"the file is not there": {
+			said: echoed +
+				`File "/home/someone/.ssh/authorized_keys" not found.` + "\n",
+			want: true,
+		},
+		"the file is not there, named as it was asked for": {
+			said: echoed + `File ".ssh/authorized_keys" not found.` + "\n",
+			want: true,
+		},
+		"the file is not there and an identity file is not either": {
+			said: warning + echoed +
+				`File "/home/someone/.ssh/authorized_keys" not found.` + "\n",
+			want: true,
+		},
+		"the file is there and cannot be read": {
+			said: echoed +
+				`remote open "/home/someone/.ssh/authorized_keys": ` +
+				"Permission denied\n",
+			want: false,
+		},
+		"only an identity file is not there": {
+			said: warning + echoed +
+				`remote open "/home/someone/.ssh/authorized_keys": ` +
+				"Permission denied\n",
+			want: false,
+		},
+		"some other file is not there": {
+			said: echoed + `File "/home/someone/.ssh/known_hosts" not found.` +
+				"\n",
+			want: false,
+		},
+		"the connection did not come up": {
+			said: "ssh: connect to host example.com port 22: " +
+				"Connection refused\nConnection closed\n",
+			want: false,
+		},
+	}
+
+	for name, session := range sessions {
+		t.Run(name, func(t *testing.T) {
+			t.Parallel()
+
+			if absent(session.said) != session.want {
+				t.Errorf(
+					"read as absent: %t, wanted %t, from:\n%s",
+					!session.want, session.want, session.said,
+				)
+			}
+		})
+	}
+}
diff --git a/internal/cli/ssh_test.go b/internal/cli/ssh_test.go
index 6b787fc..9aafe21 100644
--- a/internal/cli/ssh_test.go
+++ b/internal/cli/ssh_test.go
@@ -1,8 +1,10 @@
 package cli_test
 
 import (
+	"bytes"
 	"os"
 	"path/filepath"
+	"slices"
 	"strconv"
 	"strings"
 	"testing"
@@ -14,7 +16,7 @@ import (
 )
 
 // The modes the host is supposed to end up with, and the mode the
-// stand-in ssh needs so that it can be run at all.
+// stand-ins need so that they can be run at all.
 const (
 	directoryMode = 0o700
 	fileMode      = 0o600
@@ -22,8 +24,21 @@ const (
 )
 
 // failingStatus is the status the stand-in ssh ends with when a test
-// wants to see a status handed on.
-const failingStatus = 7
+// wants to see a status handed on, and failedStatus is the status the
+// tool itself ends with when something went wrong.
+const (
+	failingStatus = 7
+	failedStatus  = 1
+)
+
+// notADirectory is what a test puts where the .ssh directory belongs
+// to make a step of the write session fail.
+const notADirectory = "a file where the directory belongs\n"
+
+// missingIdentity is a path with no file at it, handed to sftp after
+// the dashes so that ssh warns about it in the words of a missing
+// file.
+const missingIdentity = "/nonexistent/keyfunc-test-identity"
 
 // The host, and where on it the key ends up.
 const (
@@ -32,20 +47,77 @@ const (
 	keptIn    = "authorized_keys"
 )
 
-// installer is a stand-in for the system ssh for the install command.
-// It writes down what it was given and then runs the command meant for
-// the host right here, with the home directory pointed at a directory
-// standing in for the host's, so that what keyfunc sends can be
-// watched doing its work.
+// The tool's own name, as it stands in the arguments a test hands to
+// Main, the ssh subcommand both commands the tests here drive live
+// under, and the one of those two these tests name most.
+const (
+	tool       = "keyfunc"
+	subcommand = "ssh"
+	installing = "install"
+)
+
+// The key line the example mnemonic gives at index 0, as it stands in
+// an authorized_keys file.
+const keyLine = vectorZero + " keyfunc/ssh/0\n"
+
+// installer is a stand-in for the system sftp for the install
+// command. It writes down the arguments and every command of the
+// batch it is given, echoes each command as sftp does, and carries
+// the commands out against a directory standing in for the host's
+// home directory, so that what keyfunc sends can be watched doing its
+// work. A command that begins with a dash may fail; any other failure
+// ends the session, as it does in sftp's own batch mode.
+//
+// The two ways a get can fail are worded as the OpenSSH client words
+// them, both naming the path the server expanded: a file that is not
+// there, which is the one failure the tool reads as an empty file, and
+// a file that is there and cannot be read, which is not. An -i naming
+// a file that is not here draws the warning ssh writes for it, which
+// carries the wording of a missing file into a session that goes on to
+// authenticate.
 const installer = `
-while [ $# -gt 1 ]; do
-	printf '%s\n' "$1" >> "$KEYFUNC_TEST_ARGUMENTS"
-	shift
+previous=
+for argument in "$@"; do
+	printf '%s\n' "$argument" >> "$KEYFUNC_TEST_ARGUMENTS"
+	if [ "$previous" = -i ] && [ ! -e "$argument" ]; then
+		printf 'Warning: Identity file %s not accessible: %s.\n' \
+			"$argument" "No such file or directory" >&2
+	fi
+	previous=$argument
+done
+home="$KEYFUNC_TEST_HOME"
+while IFS= read -r line; do
+	printf 'sftp> %s\n' "$line"
+	printf '%s\n' "$line" >> "$KEYFUNC_TEST_BATCH"
+	allowed=no
+	case "$line" in
+	-*)
+		line=${line#-}
+		allowed=yes
+		;;
+	esac
+	eval "set -- $line"
+	worked=yes
+	case "$1" in
+	get)
+		if [ ! -e "$home/$2" ]; then
+			worked=no
+			printf 'File "%s" not found.\n' "$home/$2" >&2
+		elif ! cp "$home/$2" "$3" 2>/dev/null; then
+			worked=no
+			printf 'remote open "%s": Permission denied\n' "$home/$2" >&2
+		fi
+		;;
+	put) cp "$2" "$home/$3" 2>/dev/null || worked=no ;;
+	mkdir) mkdir "$home/$2" 2>/dev/null || worked=no ;;
+	chmod) chmod "$2" "$home/$3" 2>/dev/null || worked=no ;;
+	rename) mv "$home/$2" "$home/$3" 2>/dev/null || worked=no ;;
+	esac
+	if [ "$worked" = no ] && [ "$allowed" = no ]; then
+		printf 'sftp: %s failed\n' "$1" >&2
+		exit 1
+	fi
 done
-printf '%s' "$1" > "$KEYFUNC_TEST_COMMAND"
-HOME="$KEYFUNC_TEST_HOME"
-export HOME
-eval "$1"
 `
 
 // caller is a stand-in for the system ssh for the to command. It
@@ -63,24 +135,22 @@ fi
 exit "$KEYFUNC_TEST_STATUS"
 `
 
-// pretended is where a stand-in ssh writes down what it was asked to
-// do.
+// pretended is where a stand-in writes down what it was asked to do.
 type pretended struct {
 	// home stands in for the home directory on the host.
 	home string
-	// arguments holds what ssh was given before the command, one per
-	// line.
+	// arguments holds the arguments of every session, one per line.
 	arguments string
-	// command holds what ssh was told to run on the host.
-	command string
+	// batch holds the commands of every session, one per line.
+	batch string
 }
 
-func TestTheKeyIsAddedToTheHostAndThenLeftAlone(t *testing.T) {
+func TestTheKeyIsAddedToAHostThatHasNoFileYet(t *testing.T) {
 	t.Setenv(mnemonic.Variable, example())
 
 	pretend := pretendHost(t)
 
-	require.Equal(t, "added\n", run(t, "ssh", "install", host))
+	require.Equal(t, "added\n", install(t, host))
 
 	directory, err := os.Stat(filepath.Join(pretend.home, keptUnder))
 	require.NoError(t, err)
@@ -94,11 +164,30 @@ func TestTheKeyIsAddedToTheHostAndThenLeftAlone(t *testing.T) {
 	require.NoError(t, err)
 	require.Equal(t, os.FileMode(fileMode), file.Mode().Perm())
 
-	added := read(t, path)
-	require.Equal(t, vectorZero+" keyfunc/ssh/0\n", added)
+	require.Equal(t, keyLine, read(t, path))
+}
 
-	require.Equal(t, "already present\n", run(t, "ssh", "install", host))
-	require.Equal(t, added, read(t, path))
+func TestAKeyThatIsAlreadyThereIsLeftAlone(t *testing.T) {
+	t.Setenv(mnemonic.Variable, example())
+
+	pretend := pretendHost(t)
+	path := seed(t, pretend, "somebody else\n"+keyLine)
+
+	require.Equal(t, "already present\n", install(t, host))
+	require.Equal(t, "somebody else\n"+keyLine, read(t, path))
+
+	// The fetch and nothing after it: the tool did not connect again.
+	require.Len(t, recorded(t, pretend.batch), 1)
+}
+
+func TestAnEmptyFileGetsTheKeyAndNoBlankLineBeforeIt(t *testing.T) {
+	t.Setenv(mnemonic.Variable, example())
+
+	pretend := pretendHost(t)
+	path := seed(t, pretend, "")
+
+	require.Equal(t, "added\n", install(t, host))
+	require.Equal(t, keyLine, read(t, path))
 }
 
 func TestTheKeyDoesNotRunIntoALineWithNoNewlineAtItsEnd(t *testing.T) {
@@ -106,41 +195,145 @@ func TestTheKeyDoesNotRunIntoALineWithNoNewlineAtItsEnd(t *testing.T) {
 
 	pretend := pretendHost(t)
 	already := "ssh-ed25519 AAAAsomebodyelse somebody@else"
+	path := seed(t, pretend, already)
 
-	require.NoError(t,
-		os.Mkdir(filepath.Join(pretend.home, keptUnder), directoryMode),
-	)
-
-	path := filepath.Join(pretend.home, keptUnder, keptIn)
-	require.NoError(t, os.WriteFile(path, []byte(already), fileMode))
-
-	require.Equal(t, "added\n", run(t, "ssh", "install", host))
-	require.Equal(t,
-		already+"\n"+vectorZero+" keyfunc/ssh/0\n",
-		read(t, path),
-	)
+	require.Equal(t, "added\n", install(t, host))
+	require.Equal(t, already+"\n"+keyLine, read(t, path))
 }
 
-func TestTheKeyLineIsNotOnTheCommandLine(t *testing.T) {
+func TestTheFileIsUploadedBesideTheOldOneAndThenRenamedOverIt(t *testing.T) {
 	t.Setenv(mnemonic.Variable, example())
 
 	pretend := pretendHost(t)
 
-	run(t, "ssh", "install", host)
+	require.Equal(t, "added\n", install(t, host))
+
+	sent := recorded(t, pretend.batch)
+	require.Len(t, sent, 6)
+
+	// The name of the uploaded file is random, so it is read off the
+	// put and then looked for in the two commands that follow.
+	beside := strings.Fields(sent[3])[2]
+	require.True(t,
+		strings.HasPrefix(beside, ".ssh/authorized_keys.keyfunc-"),
+	)
+
+	require.True(t, strings.HasPrefix(sent[0], "get .ssh/authorized_keys "))
+	require.Equal(t, "-mkdir .ssh", sent[1])
+	require.Equal(t, "chmod 700 .ssh", sent[2])
+	require.Equal(t, "put", strings.Fields(sent[3])[0])
+	require.Equal(t, "chmod 600 "+beside, sent[4])
+	require.Equal(t, "rename "+beside+" .ssh/authorized_keys", sent[5])
+}
+
+func TestAFileThatCannotBeReadIsNotWrittenOver(t *testing.T) {
+	t.Setenv(mnemonic.Variable, example())
+
+	pretend := pretendHost(t)
+	unreadable := unfetchable(t, pretend)
+
+	printed, said, err := attempt(t, host)
+	require.Error(t, err)
+	require.Empty(t, printed)
+	require.Contains(t, said, "Permission denied")
+
+	// The fetch and nothing after it, and what was on the host is
+	// still what is on the host.
+	require.Len(t, recorded(t, pretend.batch), 1)
+	require.DirExists(t, unreadable)
+}
+
+func TestAWarningAboutAnotherFileIsNotTakenForTheOneAskedFor(t *testing.T) {
+	t.Setenv(mnemonic.Variable, example())
+
+	pretend := pretendHost(t)
+	unreadable := unfetchable(t, pretend)
+
+	// ssh warns about an -i it cannot find in the words of a missing
+	// file, on a session that then authenticates perfectly well. That
+	// warning is not sftp reporting on authorized_keys, so the fetch
+	// failure is still a failure.
+	printed, said, err := attempt(t, host, "--", "-i", missingIdentity)
+	require.Error(t, err)
+	require.Empty(t, printed)
+	require.Contains(t, said, "No such file or directory")
+	require.Contains(t, said, "Permission denied")
+
+	require.Len(t, recorded(t, pretend.batch), 1)
+	require.DirExists(t, unreadable)
+
+	// The same run again, this way for the status it ends with.
+	given := os.Args
+
+	t.Cleanup(func() { os.Args = given })
+
+	os.Args = []string{
+		tool, subcommand, installing, host, "--", "-i", missingIdentity,
+	}
+
+	require.Equal(t, failedStatus, cli.Main())
+}
+
+func TestAFailedStepNamesTheUploadedFileAndChangesNothing(t *testing.T) {
+	t.Setenv(mnemonic.Variable, example())
+
+	pretend := pretendHost(t)
+
+	// A file where the .ssh directory belongs: nothing is there to
+	// fetch, and then the put has nowhere to put anything, so the
+	// write session ends at the put.
+	inTheWay := filepath.Join(pretend.home, keptUnder)
+	require.NoError(t,
+		os.WriteFile(inTheWay, []byte(notADirectory), fileMode),
+	)
+
+	printed, said, err := attempt(t, host)
+	require.Error(t, err)
+	require.Empty(t, printed)
+	require.Contains(t, said, "put failed")
+
+	// The put is the last command the session got to, and the file it
+	// was uploading is the one the message names.
+	sent := recorded(t, pretend.batch)
+	require.Len(t, sent, 4)
+	require.Equal(t, "put", strings.Fields(sent[3])[0])
+	require.Contains(t, err.Error(), strings.Fields(sent[3])[2])
+
+	require.Equal(t, notADirectory, read(t, inTheWay))
+
+	// The same run again, this way for the status it ends with.
+	given := os.Args
+
+	t.Cleanup(func() { os.Args = given })
+
+	os.Args = []string{tool, subcommand, installing, host}
+
+	require.Equal(t, failedStatus, cli.Main())
+}
+
+func TestTheKeyLineIsNotSentAsACommand(t *testing.T) {
+	t.Setenv(mnemonic.Variable, example())
+
+	pretend := pretendHost(t)
+
+	install(t, host)
 
 	require.NotContains(t, read(t, pretend.arguments), "ssh-ed25519")
-	require.NotContains(t, read(t, pretend.command), "ssh-ed25519")
+	require.NotContains(t, read(t, pretend.batch), "ssh-ed25519")
 }
 
-func TestWhatComesAfterTheDashesIsGivenToSSH(t *testing.T) {
+func TestWhatComesAfterTheDashesIsGivenToSFTP(t *testing.T) {
 	t.Setenv(mnemonic.Variable, example())
 
 	pretend := pretendHost(t)
 
-	run(t, "ssh", "install", host, "--", "-p", "2222")
+	install(t, host, "--", "-P", "2222")
 
+	// The same arguments twice over: adding a line takes two
+	// connections, one to fetch the file and one to write it back.
+	session := []string{"-b", "-", "-P", "2222", host}
 	require.Equal(t,
-		[]string{"-p", "2222", host},
+		slices.Concat(session, session),
 		recorded(t, pretend.arguments),
 	)
 }
@@ -150,7 +343,7 @@ func TestSSHIsPointedAtTheAgentAndItsStatusIsHandedOn(t *testing.T) {
 
 	arguments, noted := pretendCall(t)
 
-	_, err := execute(t, "ssh", "to", host, "uptime")
+	_, err := execute(t, subcommand, "to", host, "uptime")
 
 	var passed ssh.StatusError
 
@@ -177,7 +370,7 @@ func TestTheToolEndsWithTheStatusSSHEndedWith(t *testing.T) {
 
 	t.Cleanup(func() { os.Args = given })
 
-	os.Args = []string{"keyfunc", "ssh", "to", host, "uptime"}
+	os.Args = []string{tool, subcommand, "to", host, "uptime"}
 
 	require.Equal(t, failingStatus, cli.Main())
 }
@@ -190,17 +383,78 @@ func pretendHost(t *testing.T) pretended {
 	pretend := pretended{
 		home:      t.TempDir(),
 		arguments: filepath.Join(t.TempDir(), "arguments"),
-		command:   filepath.Join(t.TempDir(), "command"),
+		batch:     filepath.Join(t.TempDir(), "batch"),
 	}
 
 	t.Setenv("KEYFUNC_TEST_HOME", pretend.home)
 	t.Setenv("KEYFUNC_TEST_ARGUMENTS", pretend.arguments)
-	t.Setenv("KEYFUNC_TEST_COMMAND", pretend.command)
-	standIn(t, installer)
+	t.Setenv("KEYFUNC_TEST_BATCH", pretend.batch)
+	standIn(t, "sftp", installer)
 
 	return pretend
 }
 
+// install runs the install command, requires it to have worked, and
+// gives back what the tool itself printed.
+func install(t *testing.T, args ...string) string {
+	t.Helper()
+
+	printed, _, err := attempt(t, args...)
+	require.NoError(t, err)
+
+	return printed
+}
+
+// attempt runs the install command with the tool's own output kept
+// apart from what the stand-in said, since the stand-in echoes its
+// batch as sftp does. It gives back what the tool printed, what the
+// stand-in said, and how the run ended.
+func attempt(t *testing.T, args ...string) (string, string, error) {
+	t.Helper()
+
+	var printed, said bytes.Buffer
+
+	root := cli.Root()
+	root.SetOut(&printed)
+	root.SetErr(&said)
+	root.SetArgs(slices.Concat([]string{subcommand, installing}, args))
+
+	err := root.ExecuteContext(t.Context())
+
+	return printed.String(), said.String(), err
+}
+
+// seed puts an authorized_keys file on the stand-in host before the
+// tool runs and gives back its path.
+func seed(t *testing.T, pretend pretended, content string) string {
+	t.Helper()
+
+	directory := filepath.Join(pretend.home, keptUnder)
+	require.NoError(t, os.Mkdir(directory, directoryMode))
+
+	path := filepath.Join(directory, keptIn)
+	require.NoError(t, os.WriteFile(path, []byte(content), fileMode))
+
+	return path
+}
+
+// unfetchable puts a directory where authorized_keys belongs on the
+// stand-in host, which the stand-in can see but cannot fetch: that is
+// how a file that is there and cannot be read looks from here. It
+// gives back the path.
+func unfetchable(t *testing.T, pretend pretended) string {
+	t.Helper()
+
+	require.NoError(t,
+		os.Mkdir(filepath.Join(pretend.home, keptUnder), directoryMode),
+	)
+
+	path := filepath.Join(pretend.home, keptUnder, keptIn)
+	require.NoError(t, os.Mkdir(path, directoryMode))
+
+	return path
+}
+
 // pretendCall puts the to stand-in on the path and gives back the file
 // the arguments are written down in and the file the agent socket is
 // noted in.
@@ -213,20 +467,21 @@ func pretendCall(t *testing.T) (string, string) {
 	t.Setenv("KEYFUNC_TEST_ARGUMENTS", arguments)
 	t.Setenv("KEYFUNC_TEST_SOCKET", noted)
 	t.Setenv("KEYFUNC_TEST_STATUS", strconv.Itoa(failingStatus))
-	standIn(t, caller)
+	standIn(t, "ssh", caller)
 
 	return arguments, noted
 }
 
-// standIn writes a stand-in for the system ssh and puts it first on
-// the path, so that the tool finds it instead of the real one.
-func standIn(t *testing.T, body string) {
+// standIn writes a stand-in for one of the system programs and puts it
+// first on the path, so that the tool finds it instead of the real
+// one.
+func standIn(t *testing.T, name, body string) {
 	t.Helper()
 
 	directory := t.TempDir()
 
 	err := os.WriteFile(
-		filepath.Join(directory, "ssh"),
+		filepath.Join(directory, name),
 		[]byte("#!/bin/sh\n"+body), standInMode,
 	)
 	require.NoError(t, err)
@@ -247,7 +502,7 @@ func read(t *testing.T, path string) string {
 	return string(content)
 }
 
-// recorded returns the arguments a stand-in wrote down, one per line.
+// recorded returns the lines a stand-in wrote down.
 func recorded(t *testing.T, path string) []string {
 	t.Helper()