diff --git a/.golangci.yml b/.golangci.yml
index 26b1610..8a90995 100644
--- a/.golangci.yml
+++ b/.golangci.yml
@@ -16,6 +16,7 @@ linters:
     - depguard         # Dependency allow/block lists
     - godot            # Requires comments to end with periods
     - wsl              # Deprecated, replaced by wsl_v5
+    - gomodguard       # Deprecated, replaced by gomodguard_v2
     - wrapcheck        # Too verbose for internal packages
     - varnamelen       # Short names like db, id are idiomatic Go
   settings:
diff --git a/Makefile b/Makefile
index af82436..b977ebe 100644
--- a/Makefile
+++ b/Makefile
@@ -17,7 +17,7 @@ setup:
 	@script/setup
 
 build:
-	go build -trimpath -ldflags "$(LDFLAGS)" -o keyfunc .
+	go build -trimpath -ldflags "$(LDFLAGS)" -o keyfunc ./cmd/keyfunc
 
 test:
 	@script/test
diff --git a/README.md b/README.md
index a5b03d6..2c83234 100644
--- a/README.md
+++ b/README.md
@@ -1,16 +1,74 @@
 # keyfunc
 
-`keyfunc` turns a BIP-39 mnemonic into key pairs that can be recreated from
-that mnemonic at any time. The same mnemonic, key type and index always give the
-same key.
+`keyfunc` is a Go command-line tool by [@sneak](https://sneak.berlin) — its
+license is not yet chosen
+([#14](https://git.eeqj.de/sneak/keyfunc/issues/14)) — that turns a BIP-39
+mnemonic into SSH keys, age identities and child mnemonics, each of which can be
+recreated from that mnemonic at any time. The same mnemonic, key type and index
+always give the same key.
 
 It uses the BIP-85 entropy deriver from `git.eeqj.de/sneak/secret/pkg/bip85` and
 takes the same steps as that repository's `agehd` package.
 
-Commands are grouped by what is derived: `keyfunc ssh ...` for ed25519 SSH
-keys, `keyfunc age ...` for age identities and for encrypting and decrypting
-with them, and `keyfunc mnemonic ...` for child mnemonics derived from the
-main one.
+Commands are grouped by what is derived: `keyfunc ssh ...` for ed25519 SSH keys,
+`keyfunc age ...` for age identities and for encrypting and decrypting with
+them, and `keyfunc mnemonic ...` for child mnemonics derived from the main one.
+
+## Getting Started
+
+Build from a clone and run the binary:
+
+```
+git clone git@git.eeqj.de:sneak/keyfunc.git
+cd keyfunc
+make build
+./keyfunc --version
+```
+
+`make build` produces `./keyfunc`. Every deriving command needs a mnemonic; see
+[Giving it the mnemonic](#giving-it-the-mnemonic) for where it is read from, then
+for example:
+
+```
+./keyfunc ssh pub -n 0 --mnemonic-command 'secret get foo'
+```
+
+## Rationale
+
+A key you can derive again never has to be backed up. One mnemonic, kept safe
+once, stands behind every key this tool produces: lose a laptop and the SSH key,
+the age identity and any child mnemonic on it come back from the mnemonic alone,
+at the same index, byte for byte. Nothing else has to be written down, copied
+between machines, or stored in a secret manager, because it can always be
+derived again.
+
+## Design
+
+The entry point is a thin `cmd/keyfunc/main.go` (what `make build` builds) that
+calls into `internal/`. The packages there are:
+
+- `internal/derive` turns a mnemonic into the 32 bytes a key is made from: it
+  walks BIP-39 seed, BIP-32 master key and BIP-85 entropy, and holds the shared
+  constants (the byte count and the largest key index).
+- `internal/mnemonic` finds the mnemonic to work from — a command, an
+  environment variable, or a terminal prompt — and refuses one that fails the
+  BIP-39 checksum.
+- `internal/sshkey` turns the derived bytes into an ed25519 SSH key
+  (`sshkey.go`) and serves that key from an in-process SSH agent on a private
+  unix socket, keeping it out of any file (`agent.go`).
+- `internal/agekey` turns the derived bytes into an age identity and encrypts
+  and decrypts with it.
+- `internal/childmnemonic` derives a child mnemonic from the main one using
+  BIP-85's own mnemonic application.
+- `internal/cli` builds the cobra command tree and runs it. Under it,
+  `cli/options` holds the flags every command shares, and `cli/ssh`, `cli/age`
+  and `cli/mnemonic` are the command groups.
+
+### Adding a key type
+
+Adding a key type is one package under `internal/` that turns the 32 derived
+bytes into that type's key, plus one cobra subcommand under `internal/cli/` that
+groups its commands.
 
 ## Derivation
 
@@ -54,8 +112,14 @@ If none of these is available and standard input is not a terminal, the tool
 refuses and exits with status 1. A mnemonic that fails the BIP-39 checksum is
 refused with a message saying so.
 
+`KEYFUNC_MNEMONIC` and `KEYFUNC_MNEMONIC_COMMAND` are removed from the
+environment before the system `ssh` (`keyfunc ssh to`) and `sftp`
+(`keyfunc ssh install`) are started, so the mnemonic is never handed on to
+them.
+
 Every command takes `--index` / `-n` and `--mnemonic-command`, and has `--help`.
-`keyfunc --version` prints the version set at build time.
+`keyfunc --version` prints the version. `make build` stamps it; a binary
+installed with `go install` reports the module version instead.
 
 ## SSH keys: `keyfunc ssh`
 
@@ -88,17 +152,51 @@ 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 lists `~/.ssh` and then fetches
+`~/.ssh/authorized_keys` from it. The file reads as empty in two cases only:
+`sftp` reported `~/.ssh` itself as not being there, or the listing came up and
+the file was not in it. Any other outcome of that connection fails the run — a
+`~/.ssh` that is there but cannot be entered, an `authorized_keys` that is there
+but cannot be read, or a connection that did not come up — and 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. The listing is what tells a missing
+directory from one shut to the user, which `sftp` reports on a fetch the same
+way; the wording of a missing file elsewhere does not count either, since `ssh`
+writes `No such file or directory` about an `-i` it cannot find on a session
+that then authenticates through the agent. 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.
+- makes `~/.ssh` and sets it to mode `0700`, but only when the first connection
+  found none; a `~/.ssh` that was already there keeps the mode it had;
+- 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...]`
 
@@ -106,7 +204,9 @@ Derives the key, serves it from an SSH agent that runs inside the tool on a unix
 socket in a new private `0700` temporary directory, then runs the system `ssh`
 with `-o IdentityAgent=<that socket>` followed by the host and all remaining
 arguments unchanged. The tool exits with `ssh`'s exit status and removes the
-socket and directory on the way out. The private key is never written to disk.
+socket and directory on the way out. The private key is never written to disk. A
+SIGINT, SIGTERM or SIGHUP ends `ssh` and still removes the socket and directory,
+and the tool then exits with status 1 unless `ssh` reported one of its own.
 
 ## age identities: `keyfunc age`
 
@@ -116,6 +216,15 @@ the same steps `sneak/secret` takes in its `agehd` package. `secret` derives at
 a vendor-specific path today; for its keys to equal this tool's it moves to
 this path, which is a change in `secret`, not here.
 
+Test vectors, mnemonic
+`abandon abandon abandon abandon abandon abandon abandon abandon abandon abandon abandon about`:
+
+```
+recipient index 0: age1xwdy9y6ckyfsgjc8k02e9uhsf3fmjy0ufysewlj68kmx5n67e3nsg2mftq
+recipient index 1: age1pmm92sxaf5mazjwvjph7dx2zq9r5p8l3rarfgqm7hmakqhvgyy4q5p3w7j
+identity index 0: AGE-SECRET-KEY-19QKK2P38598XLXMQFFU3P7J9PLDD7527T70JDHGDJ7AMNF3XT44S00JFU5
+```
+
 ### `keyfunc age pub`
 
 Prints the recipient, the `age1...` public key, on one line.
@@ -148,33 +257,64 @@ not through step 4). Default 12 words. A child mnemonic is a full mnemonic in
 its own right: it can seed another `keyfunc`, another wallet, or `secret`, and
 it never has to be written down, since it can be derived again.
 
-## Adding a key type
+Test vector: the child-mnemonic step is checked against BIP-85's own published
+vectors, which derive from the specification's master key
+`xprv9s21ZrQH143K2LBWUUQRFXhucrQqBpKdRRxNVq2zBqsx8HVqFk2uYo8kmbaLLHRdqtQpUm98uKfu3vca1LqdGhUtyoFnCNkfmXRyPXLjbKb`.
+At key index 0 the 12-word English child mnemonic is:
 
-Adding a key type is one package under `internal/` that turns the 32 derived
-bytes into that type's key, plus one cobra subcommand under `internal/cli/` that
-groups its commands.
+```
+girl mad pet galaxy egg matter matrix prison refuse sense ordinary nose
+```
 
 ## Errors
 
 Errors go to standard error and the exit status is 1, except for `ssh to`,
 which passes through `ssh`'s own exit status.
 
-## Building and running
+## Entrypoints
 
-```
-make build    # produces ./keyfunc
-make check    # fmt-check, lint (golangci-lint) and tests
-```
+The repo adheres to the
+[Scripts to Rule Them All](https://github.com/github/scripts-to-rule-them-all)
+standard: most Makefile targets are thin shims over an executable in
+`script/` (`build` and `clean` are the exceptions).
 
-Examples:
+- `script/bootstrap` installs everything needed to build and develop (git, make,
+  Go), idempotently, from nix, apt, brew or apk; it does not install the linter,
+  which only runs inside Docker.
+- `script/setup` prepares a fresh clone: it runs `bootstrap`, then installs the
+  git pre-commit hook.
+- `script/projectname` prints the project name; other scripts call it so they
+  stay identical across repos.
+- `script/test` runs `go vet` and then the test suite, rerunning verbosely if a
+  test fails.
+- `script/lint` runs the linter inside the image built from `Dockerfile.lint`
+  (which pins the linter by hash), so a complaint fails the build and leaves no
+  container behind.
+- `script/fmt` formats the Go source in place.
+- `script/fmt-check` checks that formatting without writing, failing if anything
+  is unformatted.
+- `script/check` runs `test`, `lint` and `fmt-check` and changes no files.
+- `script/docker` builds the Docker image tagged with the project name.
+- `script/cibuild` is the CI build the Gitea workflow calls: it runs the linter,
+  then `docker build`.
+- `script/precommit` is what the git pre-commit hook runs: `go mod tidy` and
+  `go fmt`, failing if `go.mod` or `go.sum` changed, then `check`.
+- `script/install-precommit` installs the git pre-commit hook that runs
+  `script/precommit`.
 
-```
-keyfunc ssh pub -n 3 --mnemonic-command 'secret get foo'
-keyfunc ssh priv -n 3 > ~/.ssh/id_bip85_3
-keyfunc ssh install -n 3 user@example.com
-keyfunc ssh to -n 3 user@example.com uptime
-keyfunc age pub -n 0
-keyfunc age encrypt -n 0 --armor -o notes.age notes.txt
-keyfunc age decrypt -n 0 notes.age
-keyfunc mnemonic -n 1 --words 24
-```
+## TODO
+
+The open issues that stand between the tree and a 1.0 release:
+
+- [#14 Choose a license and add LICENSE](https://git.eeqj.de/sneak/keyfunc/issues/14)
+- [#15 Decide the Go module path before 1.0](https://git.eeqj.de/sneak/keyfunc/issues/15)
+
+## License
+
+Not yet chosen. The license is the owner's decision, still open on the tracker
+([#14](https://git.eeqj.de/sneak/keyfunc/issues/14)); the `LICENSE` file is added
+when that issue is answered.
+
+## Author
+
+[@sneak](https://sneak.berlin).
diff --git a/main.go b/cmd/keyfunc/main.go
similarity index 100%
rename from main.go
rename to cmd/keyfunc/main.go
diff --git a/go.mod b/go.mod
index 4a2f2ce..092d55c 100644
--- a/go.mod
+++ b/go.mod
@@ -1,27 +1,27 @@
 module git.eeqj.de/sneak/keyfunc
 
-go 1.26
+go 1.26.0
 
 require (
-	filippo.io/age v1.2.1
+	filippo.io/age v1.3.2
 	git.eeqj.de/sneak/secret v0.0.0-20260810132333-41cea400a7fd
-	github.com/btcsuite/btcd v0.24.2
-	github.com/btcsuite/btcd/btcutil v1.1.6
-	github.com/spf13/cobra v1.9.1
-	github.com/stretchr/testify v1.8.4
+	github.com/btcsuite/btcd v0.25.0
+	github.com/btcsuite/btcd/btcutil v1.2.0
+	github.com/spf13/cobra v1.10.2
+	github.com/stretchr/testify v1.12.1
 	github.com/tyler-smith/go-bip39 v1.1.0
-	golang.org/x/crypto v0.38.0
-	golang.org/x/term v0.32.0
+	golang.org/x/crypto v0.57.0
+	golang.org/x/term v0.46.0
 )
 
 require (
-	github.com/btcsuite/btcd/btcec/v2 v2.1.3 // indirect
-	github.com/btcsuite/btcd/chaincfg/chainhash v1.1.0 // indirect
-	github.com/davecgh/go-spew v1.1.1 // indirect
-	github.com/decred/dcrd/dcrec/secp256k1/v4 v4.0.1 // indirect
+	filippo.io/hpke v0.4.0 // indirect
+	github.com/btcsuite/btcd/btcec/v2 v2.5.0 // indirect
+	github.com/btcsuite/btcd/chaincfg/chainhash v1.2.0 // indirect
+	github.com/decred/dcrd/dcrec/secp256k1/v4 v4.4.0 // indirect
 	github.com/inconshreveable/mousetrap v1.1.0 // indirect
-	github.com/pmezard/go-difflib v1.0.0 // indirect
-	github.com/spf13/pflag v1.0.6 // indirect
-	golang.org/x/sys v0.33.0 // indirect
-	gopkg.in/yaml.v3 v3.0.1 // indirect
+	github.com/kcalvinalvin/anet v0.0.0-20251112173137-d8ddc1f6dbee // indirect
+	github.com/spf13/pflag v1.0.9 // indirect
+	go.yaml.in/yaml/v3 v3.0.5 // indirect
+	golang.org/x/sys v0.48.0 // indirect
 )
diff --git a/go.sum b/go.sum
index 4915728..57e7f4b 100644
--- a/go.sum
+++ b/go.sum
@@ -1,136 +1,50 @@
-c2sp.org/CCTV/age v0.0.0-20240306222714-3ec4d716e805 h1:u2qwJeEvnypw+OCPUHmoZE3IqwfuN5kgDfo5MLzpNM0=
-c2sp.org/CCTV/age v0.0.0-20240306222714-3ec4d716e805/go.mod h1:FomMrUJ2Lxt5jCLmZkG3FHa72zUprnhd3v/Z18Snm4w=
-filippo.io/age v1.2.1 h1:X0TZjehAZylOIj4DubWYU1vWQxv9bJpo+Uu2/LGhi1o=
-filippo.io/age v1.2.1/go.mod h1:JL9ew2lTN+Pyft4RiNGguFfOpewKwSHm5ayKD/A4004=
+c2sp.org/CCTV/age v0.0.0-20260829155415-4448f2097b2d h1:Blprhc2SbChNZtWcU+BLTM4YdoqYAS9V7cJgOwJKyAs=
+c2sp.org/CCTV/age v0.0.0-20260829155415-4448f2097b2d/go.mod h1:SrHC2C7r5GkDk8R+NFVzYy/sdj0Ypg9htaPXQq5Cqeo=
+filippo.io/age v1.3.2 h1:r6RSZLFSMm6rzKepZ7ZAYkKCu14f3/Me8c7uKYh7C8c=
+filippo.io/age v1.3.2/go.mod h1:TH/Yr2sSRhCKbaH4XPxpUV0Us8Gv6txYUpiZQWz8Evk=
+filippo.io/hpke v0.4.0 h1:p575VVQ6ted4pL+it6M00V/f2qTZITO0zgmdKCkd5+A=
+filippo.io/hpke v0.4.0/go.mod h1:EmAN849/P3qdeK+PCMkDpDm83vRHM5cDipBJ8xbQLVY=
 git.eeqj.de/sneak/secret v0.0.0-20260810132333-41cea400a7fd h1:6YFV6horz2wDFPWWhour8qx8gLGyO0qoplwEeOuQ2J4=
 git.eeqj.de/sneak/secret v0.0.0-20260810132333-41cea400a7fd/go.mod h1:gKCcMZvlBOqusn/BxR8IyFmSJQr6R4vvjJ926iNpOSI=
-github.com/aead/siphash v1.0.1/go.mod h1:Nywa3cDsYNNK3gaciGTWPwHt0wlpNV15vwmswBAUSII=
-github.com/btcsuite/btcd v0.20.1-beta/go.mod h1:wVuoA8VJLEcwgqHBwHmzLRazpKxTv13Px/pDuV7OomQ=
-github.com/btcsuite/btcd v0.22.0-beta.0.20220111032746-97732e52810c/go.mod h1:tjmYdS6MLJ5/s0Fj4DbLgSbDHbEqLJrtnHecBFkdz5M=
-github.com/btcsuite/btcd v0.23.5-0.20231215221805-96c9fd8078fd/go.mod h1:nm3Bko6zh6bWP60UxwoT5LzdGJsQJaPo6HjduXq9p6A=
-github.com/btcsuite/btcd v0.24.2 h1:aLmxPguqxza+4ag8R1I2nnJjSu2iFn/kqtHTIImswcY=
-github.com/btcsuite/btcd v0.24.2/go.mod h1:5C8ChTkl5ejr3WHj8tkQSCmydiMEPB0ZhQhehpq7Dgg=
-github.com/btcsuite/btcd/btcec/v2 v2.1.0/go.mod h1:2VzYrv4Gm4apmbVVsSq5bqf1Ec8v56E48Vt0Y/umPgA=
-github.com/btcsuite/btcd/btcec/v2 v2.1.3 h1:xM/n3yIhHAhHy04z4i43C8p4ehixJZMsnrVJkgl+MTE=
-github.com/btcsuite/btcd/btcec/v2 v2.1.3/go.mod h1:ctjw4H1kknNJmRN4iP1R7bTQ+v3GJkZBd6mui8ZsAZE=
-github.com/btcsuite/btcd/btcutil v1.0.0/go.mod h1:Uoxwv0pqYWhD//tfTiipkxNfdhG9UrLwaeswfjfdF0A=
-github.com/btcsuite/btcd/btcutil v1.1.0/go.mod h1:5OapHB7A2hBBWLm48mmw4MOHNJCcUBTwmWH/0Jn8VHE=
-github.com/btcsuite/btcd/btcutil v1.1.5/go.mod h1:PSZZ4UitpLBWzxGd5VGOrLnmOjtPP/a6HaFo12zMs00=
-github.com/btcsuite/btcd/btcutil v1.1.6 h1:zFL2+c3Lb9gEgqKNzowKUPQNb8jV7v5Oaodi/AYFd6c=
-github.com/btcsuite/btcd/btcutil v1.1.6/go.mod h1:9dFymx8HpuLqBnsPELrImQeTQfKBQqzqGbbV3jK55aE=
-github.com/btcsuite/btcd/chaincfg/chainhash v1.0.0/go.mod h1:7SFka0XMvUgj3hfZtydOrQY2mwhPclbT2snogU7SQQc=
-github.com/btcsuite/btcd/chaincfg/chainhash v1.0.1/go.mod h1:7SFka0XMvUgj3hfZtydOrQY2mwhPclbT2snogU7SQQc=
-github.com/btcsuite/btcd/chaincfg/chainhash v1.1.0 h1:59Kx4K6lzOW5w6nFlA0v5+lk/6sjybR934QNHSJZPTQ=
-github.com/btcsuite/btcd/chaincfg/chainhash v1.1.0/go.mod h1:7SFka0XMvUgj3hfZtydOrQY2mwhPclbT2snogU7SQQc=
-github.com/btcsuite/btclog v0.0.0-20170628155309-84c8d2346e9f/go.mod h1:TdznJufoqS23FtqVCzL0ZqgP5MqXbb4fg/WgDys70nA=
-github.com/btcsuite/btcutil v0.0.0-20190425235716-9e5f4b9a998d/go.mod h1:+5NJ2+qvTyV9exUAL/rxXi3DcLg2Ts+ymUAY5y4NvMg=
-github.com/btcsuite/go-socks v0.0.0-20170105172521-4720035b7bfd/go.mod h1:HHNXQzUsZCxOoE+CPiyCTO6x34Zs86zZUiwtpXoGdtg=
-github.com/btcsuite/goleveldb v0.0.0-20160330041536-7834afc9e8cd/go.mod h1:F+uVaaLLH7j4eDXPRvw78tMflu7Ie2bzYOH4Y8rRKBY=
-github.com/btcsuite/goleveldb v1.0.0/go.mod h1:QiK9vBlgftBg6rWQIj6wFzbPfRjiykIEhBH4obrXJ/I=
-github.com/btcsuite/snappy-go v0.0.0-20151229074030-0bdef8d06723/go.mod h1:8woku9dyThutzjeg+3xrA5iCpBRH8XEEg3lh6TiUghc=
-github.com/btcsuite/snappy-go v1.0.0/go.mod h1:8woku9dyThutzjeg+3xrA5iCpBRH8XEEg3lh6TiUghc=
-github.com/btcsuite/websocket v0.0.0-20150119174127-31079b680792/go.mod h1:ghJtEyQwv5/p4Mg4C0fgbePVuGr935/5ddU9Z3TmDRY=
-github.com/btcsuite/winsvc v1.0.0/go.mod h1:jsenWakMcC0zFBFurPLEAyrnc/teJEM1O46fmI40EZs=
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diff --git a/internal/cli/cli.go b/internal/cli/cli.go
index c6d63d0..2dd443e 100644
--- a/internal/cli/cli.go
+++ b/internal/cli/cli.go
@@ -2,9 +2,13 @@
 package cli
 
 import (
+	"context"
 	"errors"
 	"fmt"
 	"os"
+	"os/signal"
+	"runtime/debug"
+	"syscall"
 
 	"git.eeqj.de/sneak/keyfunc/internal/cli/age"
 	"git.eeqj.de/sneak/keyfunc/internal/cli/mnemonic"
@@ -13,20 +17,43 @@ import (
 	"github.com/spf13/cobra"
 )
 
-// Version is what --version prints. The build sets it.
+// devVersion is what Version holds until a build stamps a real one.
+const devVersion = "dev"
+
+// Version is what --version prints. make build stamps it with -ldflags.
 //
 //nolint:gochecknoglobals // set at build time with -ldflags
-var Version = "dev"
+var Version = devVersion
+
+// resolveVersion chooses what --version reports. A value stamped at
+// build time wins. Otherwise, for a binary from go install, the module
+// version recorded in the build info is used, unless that is empty or
+// the "(devel)" of a local build. When neither names a version, the
+// "dev" fallback stays.
+func resolveVersion(stamped string, info *debug.BuildInfo) string {
+	if stamped != devVersion {
+		return stamped
+	}
+
+	if info != nil && info.Main.Version != "" &&
+		info.Main.Version != "(devel)" {
+		return info.Main.Version
+	}
+
+	return devVersion
+}
 
 // Root returns the whole command tree.
 func Root() *cobra.Command {
+	info, _ := debug.ReadBuildInfo()
+
 	root := &cobra.Command{
 		Use:   "keyfunc",
 		Short: "derive key pairs from a BIP-39 mnemonic",
 		Long: "keyfunc turns a BIP-39 mnemonic into key pairs that can " +
 			"be recreated from that mnemonic at any time. The same " +
 			"mnemonic, key type and index always give the same key.",
-		Version:       Version,
+		Version:       resolveVersion(Version, info),
 		SilenceUsage:  true,
 		SilenceErrors: true,
 	}
@@ -42,8 +69,19 @@ func Root() *cobra.Command {
 // status of its own, which "ssh to" uses to hand on the status ssh
 // ended with. ssh has already said whatever it had to say in that
 // case, so nothing more is printed.
+//
+// SIGINT, SIGTERM and SIGHUP cancel the command's context instead of
+// killing the process outright, so the child ssh or sftp ends and the
+// deferred cleanup that removes the agent socket and the install
+// working directory still runs.
 func Main() int {
-	err := Root().Execute()
+	ctx, stop := signal.NotifyContext(
+		context.Background(),
+		syscall.SIGINT, syscall.SIGTERM, syscall.SIGHUP,
+	)
+	defer stop()
+
+	err := Root().ExecuteContext(ctx)
 	if err == nil {
 		return 0
 	}
diff --git a/internal/cli/ssh/install.go b/internal/cli/ssh/install.go
index 8133261..761fe3c 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 +64,271 @@ 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, present, 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, present)
+}
+
+// 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. The directory
+// is made and set to its mode only when the read found none: an .ssh
+// that was already there is left with the mode it had.
+func upload(
+	cmd *cobra.Command, host string, options []string,
+	work, merged string, present bool,
+) 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
+	}
+
+	var batch []string
+
+	if !present {
+		// The mkdir is allowed to fail in case the directory appeared
+		// between the read and now; the chmod then sets its mode.
+		batch = append(batch,
+			"-mkdir "+directory,
+			"chmod "+directoryMode+" "+directory,
+		)
+	}
+
+	batch = append(batch,
+		"put "+quoted(local)+" "+sidecar,
+		"chmod "+fileMode+" "+sidecar,
+		"rename "+sidecar+" "+authorized,
+	)
+
+	said, err := session(cmd, host, options, batch)
+	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.Env = childEnv()
+	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, and whether the .ssh directory was already
+// there. The one session lists .ssh and then gets the file, so the
+// listing settles the state of the directory before the get is read.
+//
+// The file reads as empty in just two cases: sftp reported .ssh itself
+// as not there, or the listing succeeded and the get then reported the
+// file as not there. Anything else — the listing refused, the file
+// there but unreadable, the connection down — fails the run and writes
+// nothing, because writing back over what was not read would leave the
+// host with the new key and nothing else. sftp cannot tell a missing
+// file from one in a directory it cannot enter, so the listing does:
+// a directory that is there but cannot be read is a failure, not an
+// empty file.
+func fetch(
+	cmd *cobra.Command, host string, options []string, into string,
+) (string, bool, error) {
+	said, err := session(cmd, host, options, []string{
+		"ls -1 " + directory,
+		"get " + authorized + " " + quoted(into),
+	})
+	if err != nil {
+		if directoryAbsent(said) {
+			return "", false, nil
+		}
+
+		if absent(said) {
+			return "", true, nil
+		}
+
+		return "", false, err
+	}
+
+	//nolint:gosec // the path is a temporary file of the tool's own
+	content, err := os.ReadFile(into)
+	if err != nil {
+		return "", false, fmt.Errorf("reading the fetched file: %w", err)
+	}
+
+	return string(content), true, nil
+}
+
+// directoryAbsent says whether sftp reported .ssh itself as not being
+// there, which is the one listing failure read as a host that has no
+// authorized_keys yet. The reading is taken only from the line in which
+// sftp reports on that directory: any other failure of the listing, in
+// particular a directory that is there but cannot be entered, is left
+// as a failure, so that no key is written to a host whose keys were
+// never read.
+func directoryAbsent(said string) bool {
+	for line := range strings.Lines(said) {
+		named, is := reportedCannotList(strings.TrimSpace(line))
+		if is && (named == directory ||
+			strings.HasSuffix(named, "/"+directory)) {
+			return true
+		}
+	}
+
+	return false
+}
+
+// reportedCannotList returns the path an sftp line reports it cannot
+// list for want of the directory, and whether the line is such a
+// report. The client writes this one wording when the directory a
+// listing names is not there, giving the path the server expanded.
+func reportedCannotList(line string) (string, bool) {
+	const (
+		before = `Can't ls: "`
+		after  = `" not found`
+	)
+
+	if !strings.HasPrefix(line, before) ||
+		!strings.HasSuffix(line, after) {
+		return "", false
+	}
+
+	return strings.TrimSuffix(strings.TrimPrefix(line, before), after), true
+}
+
+// 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..09d53de
--- /dev/null
+++ b/internal/cli/ssh/install_test.go
@@ -0,0 +1,133 @@
+//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"
+`
+	listed  = "sftp> ls -1 .ssh\n"
+	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,
+				)
+			}
+		})
+	}
+}
+
+// TestTheDirectoryIsReadAsAbsentOnlyFromTheListingSayingSo holds the
+// wordings the OpenSSH client was seen to use when a listing fails: a
+// directory it cannot find is reported one way, and one it cannot enter
+// another, and only the first is read as a host with no .ssh yet.
+func TestTheDirectoryIsReadAsAbsentOnlyFromTheListingSayingSo(t *testing.T) {
+	t.Parallel()
+
+	listings := map[string]struct {
+		said string
+		want bool
+	}{
+		"the directory is not there": {
+			said: listed + `Can't ls: "/home/someone/.ssh" not found` + "\n",
+			want: true,
+		},
+		"the directory is not there, named as it was asked for": {
+			said: listed + `Can't ls: ".ssh" not found` + "\n",
+			want: true,
+		},
+		"the directory is not there and an identity file is not either": {
+			said: warning + listed +
+				`Can't ls: "/home/someone/.ssh" not found` + "\n",
+			want: true,
+		},
+		"the directory is there and cannot be entered": {
+			said: listed +
+				`remote readdir("/home/someone/.ssh/"): Permission denied` + "\n",
+			want: false,
+		},
+		"some other directory is not there": {
+			said: listed + `Can't ls: "/home/someone/.config" 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, listing := range listings {
+		t.Run(name, func(t *testing.T) {
+			t.Parallel()
+
+			if directoryAbsent(listing.said) != listing.want {
+				t.Errorf(
+					"read as absent: %t, wanted %t, from:\n%s",
+					!listing.want, listing.want, listing.said,
+				)
+			}
+		})
+	}
+}
diff --git a/internal/cli/ssh/ssh.go b/internal/cli/ssh/ssh.go
index b15a08b..3df7344 100644
--- a/internal/cli/ssh/ssh.go
+++ b/internal/cli/ssh/ssh.go
@@ -3,9 +3,12 @@ package ssh
 
 import (
 	"fmt"
+	"os"
+	"strings"
 
 	"git.eeqj.de/sneak/keyfunc/internal/cli/options"
 	"git.eeqj.de/sneak/keyfunc/internal/derive"
+	"git.eeqj.de/sneak/keyfunc/internal/mnemonic"
 	"git.eeqj.de/sneak/keyfunc/internal/sshkey"
 	"github.com/spf13/cobra"
 )
@@ -84,6 +87,26 @@ func write(cmd *cobra.Command, text string) error {
 	return nil
 }
 
+// childEnv is the tool's environment with the mnemonic variables taken
+// out, for the ssh and sftp children it starts. "ssh to" exists so the
+// private key never leaves the tool; the mnemonic, from either variable,
+// must not leave it either.
+func childEnv() []string {
+	environ := os.Environ()
+	kept := make([]string, 0, len(environ))
+
+	for _, entry := range environ {
+		name, _, _ := strings.Cut(entry, "=")
+		if name == mnemonic.Variable || name == mnemonic.CommandVariable {
+			continue
+		}
+
+		kept = append(kept, entry)
+	}
+
+	return kept
+}
+
 // addComment gives a command its comment flag.
 func addComment(cmd *cobra.Command) {
 	cmd.Flags().String(
diff --git a/internal/cli/ssh/to.go b/internal/cli/ssh/to.go
index 8dc5149..6569c5c 100644
--- a/internal/cli/ssh/to.go
+++ b/internal/cli/ssh/to.go
@@ -7,6 +7,7 @@ import (
 	"os"
 	"os/exec"
 	"slices"
+	"syscall"
 
 	"github.com/spf13/cobra"
 )
@@ -70,10 +71,18 @@ func to() *cobra.Command {
 func connect(ctx context.Context, argv []string) error {
 	//nolint:gosec // the arguments are the user's own, meant for ssh
 	command := exec.CommandContext(ctx, "ssh", argv...)
+	command.Env = childEnv()
 	command.Stdin = os.Stdin
 	command.Stdout = os.Stdout
 	command.Stderr = os.Stderr
 
+	// A cancelled context means a signal ended the tool. Send ssh a
+	// SIGTERM rather than the default kill, so it puts the terminal
+	// back the way it found it before it goes.
+	command.Cancel = func() error {
+		return command.Process.Signal(syscall.SIGTERM)
+	}
+
 	err := command.Run()
 	if err == nil {
 		return nil
diff --git a/internal/cli/ssh_test.go b/internal/cli/ssh_test.go
index 6b787fc..1bf8193 100644
--- a/internal/cli/ssh_test.go
+++ b/internal/cli/ssh_test.go
@@ -1,11 +1,16 @@
 package cli_test
 
 import (
+	"bytes"
 	"os"
+	"os/exec"
 	"path/filepath"
+	"slices"
 	"strconv"
 	"strings"
+	"syscall"
 	"testing"
+	"time"
 
 	"git.eeqj.de/sneak/keyfunc/internal/cli"
 	"git.eeqj.de/sneak/keyfunc/internal/cli/ssh"
@@ -13,8 +18,25 @@ import (
 	"github.com/stretchr/testify/require"
 )
 
+// runAsTool, set in the environment of a re-executed test binary, tells
+// TestMain to run the tool through Main rather than the suite, so the
+// signal test can drive the real signal path in a process it can send a
+// signal to.
+const runAsTool = "KEYFUNC_TEST_RUN_AS_TOOL"
+
+// TestMain re-executes the test binary as the tool when runAsTool is
+// set, and otherwise runs the suite. The signal test starts the tool
+// this way, as a subprocess it can signal and watch clean up.
+func TestMain(m *testing.M) {
+	if os.Getenv(runAsTool) == "1" {
+		os.Exit(cli.Main())
+	}
+
+	os.Exit(m.Run())
+}
+
 // 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 +44,21 @@ 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 (
@@ -32,27 +67,118 @@ 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.
+// remoteCommand is the command the "to" tests hand ssh after the host.
+const remoteCommand = "uptime"
+
+// 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"
+
+// marker is a variable set beside the mnemonic ones and expected to
+// reach the stand-in, so a scrubbed environment is told apart from an
+// empty one.
+const marker = "KEYFUNC_TEST_MARKER"
+
+// 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, writes down its
+// own environment when a test asks for it, 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 listing and the two ways a get can fail are worded as the
+// OpenSSH client words them, each naming the path the server expanded.
+// A listing fails one way when .ssh is not there and another when it is
+// there but shut to the user; the first is the only failure read as a
+// host with no file. A get fails one way for a file that is not there,
+// which after a listing that came up empty is also read as no file, and
+// another for a file that is there and cannot be read, which is a
+// failure. A directory shut to the user is stood in for by mode 000,
+// which the listing reads off the mode itself so that the test does not
+// turn on the user it runs as. 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
+[ -n "$KEYFUNC_TEST_ENVIRONMENT" ] && env > "$KEYFUNC_TEST_ENVIRONMENT"
+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
+	ls)
+		dir=$2
+		[ "$dir" = -1 ] && dir=$3
+		if [ ! -e "$home/$dir" ]; then
+			worked=no
+			printf 'Can'\''t ls: "%s" not found\n' "$home/$dir" >&2
+		elif [ -d "$home/$dir" ] && [ "$(stat -c '%a' "$home/$dir")" = 0 ]; then
+			worked=no
+			printf 'remote readdir("%s/"): Permission denied\n' \
+				"$home/$dir" >&2
+		else
+			for entry in "$home/$dir"/*; do
+				[ -e "$entry" ] || continue
+				printf '%s/%s\n' "$dir" "$(basename "$entry")"
+			done
+		fi
+		;;
+	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
 // writes down the arguments it was given, notes the agent socket if
-// there really is one at the path it was handed, and ends with the
-// status the test asked for.
+// there really is one at the path it was handed, writes down its own
+// environment when a test asks for it, and ends with the status the
+// test asked for.
 const caller = `
+[ -n "$KEYFUNC_TEST_ENVIRONMENT" ] && env > "$KEYFUNC_TEST_ENVIRONMENT"
 for argument in "$@"; do
 	printf '%s\n' "$argument" >> "$KEYFUNC_TEST_ARGUMENTS"
 done
@@ -63,24 +189,34 @@ fi
 exit "$KEYFUNC_TEST_STATUS"
 `
 
-// pretended is where a stand-in ssh writes down what it was asked to
-// do.
+// sleeper is a stand-in for the system ssh that notes the agent socket
+// and then blocks, so a test can cancel the context while it is running
+// and watch the tool take the agent down. The wait ends on its own only
+// as a backstop, well after the test has cancelled and looked.
+const sleeper = `
+socket=${2#IdentityAgent=}
+if [ -S "$socket" ]; then
+	printf '%s\n' "$socket" > "$KEYFUNC_TEST_SOCKET"
+fi
+sleep 5
+`
+
+// 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 +230,30 @@ 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 read and nothing after it: the tool did not connect again.
+	require.Equal(t, 1, connections(t, pretend))
+}
+
+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 +261,192 @@ 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-"),
+	)
+
+	// The listing fails on a host with no .ssh, so the get never runs;
+	// the write session then makes the directory and puts the file.
+	require.Equal(t, "ls -1 .ssh", sent[0])
+	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 read and nothing after it, and what was on the host is
+	// still what is on the host.
+	require.Equal(t, 1, connections(t, pretend))
+	require.DirExists(t, unreadable)
+}
+
+func TestAnUnreadableDirectoryIsNotWrittenInto(t *testing.T) {
+	t.Setenv(mnemonic.Variable, example())
+
+	pretend := pretendHost(t)
+	unlistable(t, pretend)
+
+	// The listing is refused, which is not the same as no directory, so
+	// the tool writes nothing rather than treat a directory it cannot
+	// enter as a host with no file.
+	printed, said, err := attempt(t, host)
+	require.Error(t, err)
+	require.Empty(t, printed)
+	require.Contains(t, said, "Permission denied")
+
+	// The read and nothing after it: no second connection wrote a key.
+	require.Equal(t, 1, connections(t, pretend))
+}
+
+func TestAnExistingDirectoryKeepsItsModeAndIsNotRemade(t *testing.T) {
+	t.Setenv(mnemonic.Variable, example())
+
+	pretend := pretendHost(t)
+
+	// A directory that is there but holds no file yet, made with a mode
+	// of its own so that a stray chmod would show.
+	const ownMode = 0o755
+
+	directory := filepath.Join(pretend.home, keptUnder)
+	require.NoError(t, os.Mkdir(directory, ownMode))
+
+	require.Equal(t, "added\n", install(t, host))
+
+	// The key is added and the directory keeps the mode it had: the
+	// write session neither made it nor set its mode.
+	require.Equal(t, keyLine, read(t, filepath.Join(directory, keptIn)))
+
+	kept, err := os.Stat(directory)
+	require.NoError(t, err)
+	require.Equal(t, os.FileMode(ownMode), kept.Mode().Perm())
+
+	sent := recorded(t, pretend.batch)
+	require.NotContains(t, sent, "-mkdir .ssh")
+	require.NotContains(t, sent, "chmod 700 .ssh")
+}
+
+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.Equal(t, 1, connections(t, pretend))
+	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: the listing shows it and
+	// so the directory reads as already there, but 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 first and last command the write session got to,
+	// and the file it was uploading is the one the message names.
+	sent := recorded(t, pretend.batch)
+	require.Len(t, sent, 3)
+	require.Equal(t, "put", strings.Fields(sent[2])[0])
+	require.Contains(t, err.Error(), strings.Fields(sent[2])[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 +456,7 @@ func TestSSHIsPointedAtTheAgentAndItsStatusIsHandedOn(t *testing.T) {
 
 	arguments, noted := pretendCall(t)
 
-	_, err := execute(t, "ssh", "to", host, "uptime")
+	_, err := execute(t, subcommand, "to", host, remoteCommand)
 
 	var passed ssh.StatusError
 
@@ -159,7 +465,7 @@ func TestSSHIsPointedAtTheAgentAndItsStatusIsHandedOn(t *testing.T) {
 
 	given := recorded(t, arguments)
 	require.Equal(t, "-o", given[0])
-	require.Equal(t, []string{host, "uptime"}, given[2:])
+	require.Equal(t, []string{host, remoteCommand}, given[2:])
 
 	// The stand-in wrote the path down only because there really was
 	// a socket there while it ran.
@@ -177,11 +483,130 @@ 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, remoteCommand}
 
 	require.Equal(t, failingStatus, cli.Main())
 }
 
+func TestASignalTakesTheAgentDirectoryDown(t *testing.T) {
+	t.Setenv(mnemonic.Variable, example())
+
+	// The three signals the tool handles, checked one after another.
+	signals := []struct {
+		name   string
+		signal os.Signal
+	}{
+		{"SIGTERM", syscall.SIGTERM},
+		{"SIGINT", syscall.SIGINT},
+		{"SIGHUP", syscall.SIGHUP},
+	}
+
+	for _, ending := range signals {
+		signalEndsTheTool(t, ending.name, ending.signal)
+	}
+}
+
+// signalEndsTheTool runs the tool as a subprocess against a stand-in
+// ssh that blocks, waits until the agent is up and ssh is running
+// against it, sends the tool the signal, and requires the agent socket
+// and its directory to be gone once the tool has ended. The subprocess
+// goes through Main and its signal handling, so with that handling
+// removed the signal kills the tool outright, no deferred cleanup runs,
+// the directory is left behind, and the check fails.
+func signalEndsTheTool(t *testing.T, name string, signal os.Signal) {
+	t.Helper()
+
+	noted := filepath.Join(t.TempDir(), "socket")
+	t.Setenv("KEYFUNC_TEST_SOCKET", noted)
+	standIn(t, "ssh", sleeper)
+
+	//nolint:gosec // the binary is this test's own, re-run as the tool
+	command := exec.CommandContext(
+		t.Context(), os.Args[0], subcommand, "to", host, remoteCommand,
+	)
+
+	command.Env = append(os.Environ(), runAsTool+"=1")
+	require.NoError(t, command.Start())
+
+	// The stand-in notes the socket only once the agent is up and ssh
+	// is running against it, so this is where the signal lands.
+	socket := waitForSocket(t, noted)
+
+	require.NoError(t, command.Process.Signal(signal))
+	waitForTool(t, name, command)
+
+	// The signal ended the tool, and its deferred cleanup still ran:
+	// the agent socket and its directory are gone.
+	require.NoDirExists(t, filepath.Dir(socket), name)
+}
+
+// waitForTool waits for the subprocess to end, and fails the test if it
+// does not end in time.
+func waitForTool(t *testing.T, name string, command *exec.Cmd) {
+	t.Helper()
+
+	done := make(chan error, 1)
+	go func() { done <- command.Wait() }()
+
+	select {
+	case <-done:
+	case <-time.After(10 * time.Second):
+		t.Fatalf("the tool did not end after %s", name)
+	}
+}
+
+// waitForSocket waits for the stand-in to write down the agent socket
+// and gives back the path, which means the agent is up and ssh is
+// running against it.
+func waitForSocket(t *testing.T, noted string) string {
+	t.Helper()
+
+	var socket string
+
+	require.Eventually(t, func() bool {
+		content, err := os.ReadFile(noted) //nolint:gosec // test path
+		if err != nil {
+			return false
+		}
+
+		socket = strings.TrimSpace(string(content))
+
+		return socket != ""
+	}, 5*time.Second, 5*time.Millisecond)
+
+	return socket
+}
+
+func TestTheMnemonicIsNotHandedToSFTP(t *testing.T) {
+	t.Setenv(mnemonic.CommandVariable, "echo "+example())
+	t.Setenv(mnemonic.Variable, example())
+	t.Setenv(marker, "reaches the stand-in")
+
+	pretendHost(t)
+	environment := recordEnvironment(t)
+
+	install(t, host)
+
+	mnemonicWithheld(t, read(t, environment))
+}
+
+func TestTheMnemonicIsNotHandedToSSH(t *testing.T) {
+	t.Setenv(mnemonic.CommandVariable, "echo "+example())
+	t.Setenv(mnemonic.Variable, example())
+	t.Setenv(marker, "reaches the stand-in")
+
+	pretendCall(t)
+	environment := recordEnvironment(t)
+
+	_, err := execute(t, subcommand, "to", host, remoteCommand)
+
+	var passed ssh.StatusError
+
+	require.ErrorAs(t, err, &passed)
+
+	mnemonicWithheld(t, read(t, environment))
+}
+
 // pretendHost puts the install stand-in on the path and gives back the
 // places it writes to.
 func pretendHost(t *testing.T) pretended {
@@ -190,17 +615,110 @@ 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
+}
+
+// unlistable puts a .ssh on the stand-in host that is there but shut to
+// the user, a directory of mode 000, and gives back its path. Its mode
+// is put back before the temporary directory is cleared so that it can
+// be.
+func unlistable(t *testing.T, pretend pretended) string {
+	t.Helper()
+
+	directory := filepath.Join(pretend.home, keptUnder)
+	require.NoError(t, os.Mkdir(directory, directoryMode))
+	require.NoError(t, os.Chmod(directory, 0))
+
+	t.Cleanup(func() { _ = os.Chmod(directory, directoryMode) })
+
+	return directory
+}
+
+// connections returns how many times the tool ran sftp, counted from
+// the -b that opens each session's arguments.
+func connections(t *testing.T, pretend pretended) int {
+	t.Helper()
+
+	count := 0
+
+	for _, argument := range recorded(t, pretend.arguments) {
+		if argument == "-b" {
+			count++
+		}
+	}
+
+	return count
+}
+
 // 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 +731,21 @@ 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)
@@ -236,6 +755,28 @@ func standIn(t *testing.T, body string) {
 	)
 }
 
+// recordEnvironment asks the stand-in to write its environment down and
+// gives back the file it writes it to.
+func recordEnvironment(t *testing.T) string {
+	t.Helper()
+
+	path := filepath.Join(t.TempDir(), "environment")
+	t.Setenv("KEYFUNC_TEST_ENVIRONMENT", path)
+
+	return path
+}
+
+// mnemonicWithheld requires that neither mnemonic variable reached the
+// stand-in and that the marker set beside them did, so an empty
+// environment does not pass for a scrubbed one.
+func mnemonicWithheld(t *testing.T, environment string) {
+	t.Helper()
+
+	require.NotContains(t, environment, mnemonic.Variable+"=")
+	require.NotContains(t, environment, mnemonic.CommandVariable+"=")
+	require.Contains(t, environment, marker+"=")
+}
+
 // read returns what is in a file.
 func read(t *testing.T, path string) string {
 	t.Helper()
@@ -247,7 +788,7 @@ 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()
 
diff --git a/internal/cli/version_internal_test.go b/internal/cli/version_internal_test.go
new file mode 100644
index 0000000..9c01c91
--- /dev/null
+++ b/internal/cli/version_internal_test.go
@@ -0,0 +1,37 @@
+package cli
+
+import (
+	"runtime/debug"
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+func TestResolveVersion(t *testing.T) {
+	t.Parallel()
+
+	release := &debug.BuildInfo{Main: debug.Module{Version: "v1.2.3"}}
+	local := &debug.BuildInfo{Main: debug.Module{Version: "(devel)"}}
+	empty := &debug.BuildInfo{}
+
+	t.Run("stamped value wins over build info", func(t *testing.T) {
+		t.Parallel()
+		require.Equal(t, "v0.1.0", resolveVersion("v0.1.0", release))
+	})
+
+	t.Run("go install reports the module version", func(t *testing.T) {
+		t.Parallel()
+		require.Equal(t, "v1.2.3", resolveVersion(devVersion, release))
+	})
+
+	t.Run("a local build stays dev", func(t *testing.T) {
+		t.Parallel()
+		require.Equal(t, devVersion, resolveVersion(devVersion, local))
+	})
+
+	t.Run("no version anywhere stays dev", func(t *testing.T) {
+		t.Parallel()
+		require.Equal(t, devVersion, resolveVersion(devVersion, empty))
+		require.Equal(t, devVersion, resolveVersion(devVersion, nil))
+	})
+}