diff --git a/internal/probe/probe.go b/internal/probe/probe.go
new file mode 100644
index 0000000..b8c850f
--- /dev/null
+++ b/internal/probe/probe.go
@@ -0,0 +1,248 @@
+// Package probe discovers tmux sessions on localhost and configured remotes.
+package probe
+
+import (
+	"bytes"
+	"context"
+	"errors"
+	"fmt"
+	"os/exec"
+	"sync"
+	"time"
+
+	"github.com/KofTwentyTwo/limen/internal/config"
+	"github.com/KofTwentyTwo/limen/internal/ssh"
+	"github.com/KofTwentyTwo/limen/internal/tmuxinfo"
+)
+
+const (
+	// LocalhostName is the synthetic host name used for the local machine.
+	LocalhostName = "localhost"
+
+	defaultProbeTimeout = 3 * time.Second
+)
+
+// ProbeResult is one host's in-memory probe outcome.
+type ProbeResult struct {
+	HostName  string
+	Reachable bool
+	Sessions  []SessionInfo
+	Error     error
+	Latency   time.Duration
+}
+
+// SessionInfo aliases tmux session metadata for probe consumers.
+type SessionInfo = tmuxinfo.SessionInfo
+
+// CommandResult captures a completed tmux or ssh probe command.
+type CommandResult struct {
+	Stdout   string
+	Stderr   string
+	ExitCode int
+	Err      error
+}
+
+// Runner executes commands for probes.
+type Runner interface {
+	Run(ctx context.Context, name string, args ...string) CommandResult
+}
+
+// RunnerFunc adapts a function into a Runner.
+type RunnerFunc func(ctx context.Context, name string, args ...string) CommandResult
+
+// Run executes f.
+func (f RunnerFunc) Run(ctx context.Context, name string, args ...string) CommandResult {
+	return f(ctx, name, args...)
+}
+
+// Options holds probe dependencies and timing knobs.
+type Options struct {
+	LookPath func(string) (string, error)
+	Runner   Runner
+	Timeout  time.Duration
+}
+
+// Localhost probes the local tmux server.
+func Localhost(ctx context.Context, opts Options) ProbeResult {
+	deps := opts.withDefaults()
+	start := time.Now()
+	probeCtx, cancel := context.WithTimeout(ctx, deps.Timeout)
+	defer cancel()
+
+	tmuxPath, err := deps.LookPath("tmux")
+	if err != nil {
+		return ProbeResult{
+			HostName:  LocalhostName,
+			Reachable: false,
+			Error:     fmt.Errorf("tmux not found: %w", err),
+			Latency:   elapsed(start),
+		}
+	}
+
+	result := deps.Runner.Run(probeCtx, tmuxPath, "ls", "-F", tmuxinfo.LocalFormat)
+	return interpret(LocalhostName, result, tmuxinfo.ParseLocal, start)
+}
+
+// Remote probes a configured remote host through ssh.
+func Remote(ctx context.Context, host config.Host, opts Options) ProbeResult {
+	deps := opts.withDefaults()
+	start := time.Now()
+
+	probeCtx, cancel := context.WithTimeout(ctx, deps.Timeout)
+	defer cancel()
+
+	argv := ssh.ProbeArgv(host, tmuxinfo.RemoteFormat)
+	result := deps.Runner.Run(probeCtx, argv[0], argv[1:]...)
+	return interpret(host.Name, result, tmuxinfo.ParseRemote, start)
+}
+
+// Start begins one probe per host and emits results as each one completes.
+func Start(ctx context.Context, hosts []config.Host, opts Options) <-chan ProbeResult {
+	results := make(chan ProbeResult)
+
+	go func() {
+		defer close(results)
+
+		var wg sync.WaitGroup
+		send := func(result ProbeResult) {
+			select {
+			case results <- result:
+			case <-ctx.Done():
+			}
+		}
+
+		wg.Add(1)
+		go func() {
+			defer wg.Done()
+			send(Localhost(ctx, opts))
+		}()
+
+		for _, host := range hosts {
+			host := host
+			wg.Add(1)
+			go func() {
+				defer wg.Done()
+				send(Remote(ctx, host, opts))
+			}()
+		}
+
+		wg.Wait()
+	}()
+
+	return results
+}
+
+func interpret(hostName string, result CommandResult, parse func(string) ([]tmuxinfo.SessionInfo, error), start time.Time) ProbeResult {
+	combinedOutput := result.Stdout + result.Stderr
+
+	if errors.Is(result.Err, context.DeadlineExceeded) || errors.Is(result.Err, context.Canceled) {
+		return unreachable(hostName, result.Err, start)
+	}
+
+	if result.ExitCode == 1 && tmuxinfo.IsNoServerRunning(combinedOutput) {
+		return reachable(hostName, []tmuxinfo.SessionInfo{}, start)
+	}
+
+	if result.Err != nil && result.ExitCode == 0 {
+		return unreachable(hostName, commandError(result), start)
+	}
+
+	if result.ExitCode != 0 {
+		return unreachable(hostName, commandError(result), start)
+	}
+
+	sessions, err := parse(result.Stdout)
+	if err != nil {
+		if errors.Is(err, tmuxinfo.ErrNoServer) {
+			return reachable(hostName, []tmuxinfo.SessionInfo{}, start)
+		}
+		return unreachable(hostName, err, start)
+	}
+
+	return reachable(hostName, sessions, start)
+}
+
+func reachable(hostName string, sessions []tmuxinfo.SessionInfo, start time.Time) ProbeResult {
+	return ProbeResult{
+		HostName:  hostName,
+		Reachable: true,
+		Sessions:  sessions,
+		Latency:   elapsed(start),
+	}
+}
+
+func unreachable(hostName string, err error, start time.Time) ProbeResult {
+	return ProbeResult{
+		HostName:  hostName,
+		Reachable: false,
+		Sessions:  []tmuxinfo.SessionInfo{},
+		Error:     err,
+		Latency:   elapsed(start),
+	}
+}
+
+func commandError(result CommandResult) error {
+	message := result.Stderr
+	if message == "" {
+		message = result.Stdout
+	}
+	if message == "" && result.Err != nil {
+		return result.Err
+	}
+	if message == "" {
+		return fmt.Errorf("command exited with status %d", result.ExitCode)
+	}
+	return errors.New(message)
+}
+
+func elapsed(start time.Time) time.Duration {
+	duration := time.Since(start)
+	if duration <= 0 {
+		return time.Nanosecond
+	}
+	return duration
+}
+
+func (opts Options) withDefaults() Options {
+	if opts.Timeout == 0 {
+		opts.Timeout = defaultProbeTimeout
+	}
+	if opts.LookPath == nil {
+		opts.LookPath = exec.LookPath
+	}
+	if opts.Runner == nil {
+		opts.Runner = execRunner{}
+	}
+	return opts
+}
+
+type execRunner struct{}
+
+func (execRunner) Run(ctx context.Context, name string, args ...string) CommandResult {
+	cmd := exec.CommandContext(ctx, name, args...)
+
+	var stdout bytes.Buffer
+	var stderr bytes.Buffer
+	cmd.Stdout = &stdout
+	cmd.Stderr = &stderr
+
+	err := cmd.Run()
+	result := CommandResult{
+		Stdout: stdout.String(),
+		Stderr: stderr.String(),
+		Err:    err,
+	}
+	if err == nil {
+		return result
+	}
+
+	var exitErr *exec.ExitError
+	if errors.As(err, &exitErr) {
+		result.ExitCode = exitErr.ExitCode()
+	}
+	if ctxErr := ctx.Err(); ctxErr != nil {
+		result.Err = ctxErr
+	}
+
+	return result
+}
diff --git a/internal/probe/probe_test.go b/internal/probe/probe_test.go
new file mode 100644
index 0000000..0d86c3a
--- /dev/null
+++ b/internal/probe/probe_test.go
@@ -0,0 +1,469 @@
+package probe
+
+import (
+	"context"
+	"errors"
+	"os"
+	"os/exec"
+	"reflect"
+	"strings"
+	"sync"
+	"testing"
+	"time"
+
+	"github.com/KofTwentyTwo/limen/internal/config"
+	"github.com/KofTwentyTwo/limen/internal/tmuxinfo"
+)
+
+func TestLocalhostReturnsSessionsWhenTmuxSucceeds(t *testing.T) {
+	runner := &recordingRunner{
+		result: CommandResult{
+			Stdout:   "api|3|1|1779799500\n",
+			ExitCode: 0,
+		},
+	}
+
+	got := Localhost(context.Background(), Options{
+		LookPath: lookPath("/opt/homebrew/bin/tmux", nil),
+		Runner:   runner,
+	})
+
+	if got.HostName != LocalhostName {
+		t.Fatalf("HostName = %q, want %q", got.HostName, LocalhostName)
+	}
+	if !got.Reachable {
+		t.Fatalf("Reachable = false, error = %v", got.Error)
+	}
+	if len(got.Sessions) != 1 || got.Sessions[0].Name != "api" || !got.Sessions[0].Attached {
+		t.Fatalf("Sessions = %#v", got.Sessions)
+	}
+	wantArgs := []string{"ls", "-F", tmuxinfo.LocalFormat}
+	if runner.name != "/opt/homebrew/bin/tmux" || !reflect.DeepEqual(runner.args, wantArgs) {
+		t.Fatalf("runner saw %q %#v, want tmux %#v", runner.name, runner.args, wantArgs)
+	}
+	if got.Latency <= 0 {
+		t.Fatalf("Latency = %s, want positive duration", got.Latency)
+	}
+}
+
+func TestLocalhostMissingTmuxIsUnreachable(t *testing.T) {
+	got := Localhost(context.Background(), Options{
+		LookPath: lookPath("", exec.ErrNotFound),
+		Runner:   &recordingRunner{},
+	})
+
+	if got.Reachable {
+		t.Fatal("Reachable = true, want false")
+	}
+	if got.Error == nil || !strings.Contains(got.Error.Error(), "tmux not found") {
+		t.Fatalf("Error = %v, want tmux not found diagnostic", got.Error)
+	}
+}
+
+func TestLocalhostNoServerOutputIsReachableWithZeroSessions(t *testing.T) {
+	got := Localhost(context.Background(), Options{
+		LookPath: lookPath("/usr/bin/tmux", nil),
+		Runner: &recordingRunner{result: CommandResult{
+			Stderr:   "no server running on /tmp/tmux-501/default\n",
+			ExitCode: 1,
+			Err:      errors.New("exit status 1"),
+		}},
+	})
+
+	if !got.Reachable {
+		t.Fatalf("Reachable = false, error = %v", got.Error)
+	}
+	if len(got.Sessions) != 0 {
+		t.Fatalf("len(Sessions) = %d, want 0", len(got.Sessions))
+	}
+}
+
+func TestLocalhostUsesPerHostTimeout(t *testing.T) {
+	runner := RunnerFunc(func(ctx context.Context, name string, args ...string) CommandResult {
+		<-ctx.Done()
+		return CommandResult{Err: ctx.Err()}
+	})
+
+	got := Localhost(context.Background(), Options{
+		LookPath: lookPath("/usr/bin/tmux", nil),
+		Runner:   runner,
+		Timeout:  time.Nanosecond,
+	})
+
+	if got.Reachable {
+		t.Fatal("Reachable = true, want false")
+	}
+	if !errors.Is(got.Error, context.DeadlineExceeded) {
+		t.Fatalf("Error = %v, want context deadline exceeded", got.Error)
+	}
+}
+
+func TestLocalhostMalformedOutputIsUnreachable(t *testing.T) {
+	got := Localhost(context.Background(), Options{
+		LookPath: lookPath("/usr/bin/tmux", nil),
+		Runner: &recordingRunner{result: CommandResult{
+			Stdout:   "api|not-a-number|1|1779799500\n",
+			ExitCode: 0,
+		}},
+	})
+
+	if got.Reachable {
+		t.Fatal("Reachable = true, want false")
+	}
+	if got.Error == nil || !strings.Contains(got.Error.Error(), "invalid window count") {
+		t.Fatalf("Error = %v, want parse diagnostic", got.Error)
+	}
+}
+
+func TestRemoteReturnsSessionsWhenSSHSucceeds(t *testing.T) {
+	host := config.Host{Name: "prod", Hostname: "prod.example.com", User: "deploy", Port: 2222}
+	runner := &recordingRunner{result: CommandResult{
+		Stdout:   "api|3|1\n",
+		ExitCode: 0,
+	}}
+
+	got := Remote(context.Background(), host, Options{Runner: runner})
+
+	if got.HostName != "prod" {
+		t.Fatalf("HostName = %q, want prod", got.HostName)
+	}
+	if !got.Reachable {
+		t.Fatalf("Reachable = false, error = %v", got.Error)
+	}
+	if len(got.Sessions) != 1 || got.Sessions[0].Name != "api" || !got.Sessions[0].Created.IsZero() {
+		t.Fatalf("Sessions = %#v", got.Sessions)
+	}
+	wantArgs := []string{
+		"-o", "ConnectTimeout=2",
+		"-o", "BatchMode=yes",
+		"-o", "StrictHostKeyChecking=accept-new",
+		"-p", "2222",
+		"deploy@prod.example.com",
+		"tmux", "ls", "-F", tmuxinfo.RemoteFormat,
+	}
+	if runner.name != "ssh" || !reflect.DeepEqual(runner.args, wantArgs) {
+		t.Fatalf("runner saw %q %#v, want ssh %#v", runner.name, runner.args, wantArgs)
+	}
+}
+
+func TestRemoteNoOutputIsReachableWithZeroSessions(t *testing.T) {
+	host := config.Host{Name: "prod", Hostname: "prod.example.com"}
+
+	got := Remote(context.Background(), host, Options{
+		Runner: &recordingRunner{result: CommandResult{ExitCode: 0}},
+	})
+
+	if !got.Reachable {
+		t.Fatalf("Reachable = false, error = %v", got.Error)
+	}
+	if len(got.Sessions) != 0 {
+		t.Fatalf("len(Sessions) = %d, want 0", len(got.Sessions))
+	}
+}
+
+func TestRemoteNoServerOutputIsReachableWithZeroSessions(t *testing.T) {
+	host := config.Host{Name: "prod", Hostname: "prod.example.com"}
+
+	got := Remote(context.Background(), host, Options{
+		Runner: &recordingRunner{result: CommandResult{
+			Stderr:   "no server running on /tmp/tmux-501/default\n",
+			ExitCode: 1,
+			Err:      errors.New("exit status 1"),
+		}},
+	})
+
+	if !got.Reachable {
+		t.Fatalf("Reachable = false, error = %v", got.Error)
+	}
+	if len(got.Sessions) != 0 {
+		t.Fatalf("len(Sessions) = %d, want 0", len(got.Sessions))
+	}
+}
+
+func TestRemoteNoServerOnStdoutIsReachableWithZeroSessions(t *testing.T) {
+	host := config.Host{Name: "prod", Hostname: "prod.example.com"}
+
+	got := Remote(context.Background(), host, Options{
+		Runner: &recordingRunner{result: CommandResult{
+			Stdout:   "no server running on /tmp/tmux-501/default\n",
+			ExitCode: 0,
+		}},
+	})
+
+	if !got.Reachable {
+		t.Fatalf("Reachable = false, error = %v", got.Error)
+	}
+	if len(got.Sessions) != 0 {
+		t.Fatalf("len(Sessions) = %d, want 0", len(got.Sessions))
+	}
+}
+
+func TestRemoteNonZeroExitIsUnreachable(t *testing.T) {
+	host := config.Host{Name: "prod", Hostname: "prod.example.com"}
+
+	got := Remote(context.Background(), host, Options{
+		Runner: &recordingRunner{result: CommandResult{
+			Stderr:   "ssh: connect to host prod.example.com port 22: Operation timed out\n",
+			ExitCode: 255,
+			Err:      errors.New("exit status 255"),
+		}},
+	})
+
+	if got.Reachable {
+		t.Fatal("Reachable = true, want false")
+	}
+	if got.Error == nil || !strings.Contains(got.Error.Error(), "ssh: connect to host prod.example.com port 22") {
+		t.Fatalf("Error = %v, want ssh diagnostic", got.Error)
+	}
+	if len(got.Sessions) != 0 {
+		t.Fatalf("len(Sessions) = %d, want 0", len(got.Sessions))
+	}
+}
+
+func TestRemoteRunnerErrorWithoutExitCodeIsUnreachable(t *testing.T) {
+	host := config.Host{Name: "prod", Hostname: "prod.example.com"}
+
+	got := Remote(context.Background(), host, Options{
+		Runner: &recordingRunner{result: CommandResult{
+			Err: errors.New("exec: ssh: executable file not found in PATH"),
+		}},
+	})
+
+	if got.Reachable {
+		t.Fatal("Reachable = true, want false")
+	}
+	if got.Error == nil || !strings.Contains(got.Error.Error(), "executable file not found") {
+		t.Fatalf("Error = %v, want runner diagnostic", got.Error)
+	}
+}
+
+func TestRemoteUsesPerHostTimeout(t *testing.T) {
+	host := config.Host{Name: "prod", Hostname: "prod.example.com"}
+	runner := RunnerFunc(func(ctx context.Context, name string, args ...string) CommandResult {
+		<-ctx.Done()
+		return CommandResult{Err: ctx.Err()}
+	})
+
+	got := Remote(context.Background(), host, Options{
+		Runner:  runner,
+		Timeout: time.Nanosecond,
+	})
+
+	if got.Reachable {
+		t.Fatal("Reachable = true, want false")
+	}
+	if !errors.Is(got.Error, context.DeadlineExceeded) {
+		t.Fatalf("Error = %v, want context deadline exceeded", got.Error)
+	}
+}
+
+func TestStartEmitsLocalhostAndConfiguredHostsThenCloses(t *testing.T) {
+	hosts := []config.Host{
+		{Name: "prod", Hostname: "prod.example.com"},
+		{Name: "dev", Hostname: "dev.example.com"},
+	}
+	runner := RunnerFunc(func(ctx context.Context, name string, args ...string) CommandResult {
+		return CommandResult{ExitCode: 0}
+	})
+
+	results := Start(context.Background(), hosts, Options{
+		LookPath: lookPath("/usr/bin/tmux", nil),
+		Runner:   runner,
+	})
+
+	got := map[string]ProbeResult{}
+	for result := range results {
+		got[result.HostName] = result
+	}
+
+	for _, name := range []string{LocalhostName, "prod", "dev"} {
+		result, ok := got[name]
+		if !ok {
+			t.Fatalf("missing result for %s in %#v", name, got)
+		}
+		if !result.Reachable {
+			t.Fatalf("%s Reachable = false, error = %v", name, result.Error)
+		}
+	}
+}
+
+func TestStartRunsRemoteProbesConcurrently(t *testing.T) {
+	hosts := []config.Host{
+		{Name: "prod", Hostname: "prod.example.com"},
+		{Name: "dev", Hostname: "dev.example.com"},
+	}
+	started := make(chan struct{}, len(hosts))
+	release := make(chan struct{})
+	var closeRelease sync.Once
+	ctx, cancel := context.WithCancel(context.Background())
+	t.Cleanup(cancel)
+	t.Cleanup(func() {
+		closeRelease.Do(func() {
+			close(release)
+		})
+	})
+
+	runner := RunnerFunc(func(ctx context.Context, name string, args ...string) CommandResult {
+		if name != "ssh" {
+			return CommandResult{ExitCode: 0}
+		}
+
+		started <- struct{}{}
+		select {
+		case <-release:
+			return CommandResult{ExitCode: 0}
+		case <-ctx.Done():
+			return CommandResult{Err: ctx.Err()}
+		}
+	})
+
+	results := Start(ctx, hosts, Options{
+		LookPath: lookPath("/usr/bin/tmux", nil),
+		Runner:   runner,
+	})
+
+	for i := 0; i < len(hosts); i++ {
+		select {
+		case <-started:
+		case <-time.After(100 * time.Millisecond):
+			t.Fatalf("only %d remote probes started, want %d", i, len(hosts))
+		}
+	}
+
+	closeRelease.Do(func() {
+		close(release)
+	})
+
+	got := 0
+	for range results {
+		got++
+	}
+	if got != len(hosts)+1 {
+		t.Fatalf("results emitted = %d, want %d", got, len(hosts)+1)
+	}
+}
+
+func TestExecRunnerCapturesSuccessfulCommand(t *testing.T) {
+	result := execRunner{}.Run(context.Background(), os.Args[0], helperArgs("success")...)
+
+	if result.Err != nil {
+		t.Fatalf("Run returned error: %v", result.Err)
+	}
+	if result.ExitCode != 0 {
+		t.Fatalf("ExitCode = %d, want 0", result.ExitCode)
+	}
+	if result.Stdout != "probe stdout\n" {
+		t.Fatalf("Stdout = %q, want helper stdout", result.Stdout)
+	}
+	if result.Stderr != "probe stderr\n" {
+		t.Fatalf("Stderr = %q, want helper stderr", result.Stderr)
+	}
+}
+
+func TestExecRunnerCapturesExitCode(t *testing.T) {
+	result := execRunner{}.Run(context.Background(), os.Args[0], helperArgs("exit-7")...)
+
+	if result.Err == nil {
+		t.Fatal("Run returned nil error")
+	}
+	if result.ExitCode != 7 {
+		t.Fatalf("ExitCode = %d, want 7", result.ExitCode)
+	}
+	if result.Stderr != "probe failed\n" {
+		t.Fatalf("Stderr = %q, want helper stderr", result.Stderr)
+	}
+}
+
+func TestExecRunnerReportsContextTimeout(t *testing.T) {
+	ctx, cancel := context.WithTimeout(context.Background(), time.Nanosecond)
+	defer cancel()
+
+	result := execRunner{}.Run(ctx, os.Args[0], helperArgs("sleep")...)
+
+	if !errors.Is(result.Err, context.DeadlineExceeded) {
+		t.Fatalf("Err = %v, want context deadline exceeded", result.Err)
+	}
+}
+
+func TestCommandErrorUsesExitStatusWhenNoOutputExists(t *testing.T) {
+	err := commandError(CommandResult{ExitCode: 255})
+
+	if err == nil || err.Error() != "command exited with status 255" {
+		t.Fatalf("commandError() = %v, want exit status diagnostic", err)
+	}
+}
+
+func TestElapsedNeverReturnsNonPositiveDuration(t *testing.T) {
+	got := elapsed(time.Now().Add(time.Second))
+
+	if got <= 0 {
+		t.Fatalf("elapsed() = %s, want positive duration", got)
+	}
+}
+
+func TestOptionsWithDefaultsFillsDependencies(t *testing.T) {
+	got := (Options{}).withDefaults()
+
+	if got.LookPath == nil {
+		t.Fatal("LookPath is nil")
+	}
+	if got.Runner == nil {
+		t.Fatal("Runner is nil")
+	}
+}
+
+type recordingRunner struct {
+	name   string
+	args   []string
+	result CommandResult
+}
+
+func (r *recordingRunner) Run(ctx context.Context, name string, args ...string) CommandResult {
+	r.name = name
+	r.args = append([]string(nil), args...)
+	if r.result.Err == nil && ctx.Err() != nil {
+		r.result.Err = ctx.Err()
+	}
+	return r.result
+}
+
+func lookPath(path string, err error) func(string) (string, error) {
+	return func(file string) (string, error) {
+		if file != "tmux" {
+			return "", errors.New("unexpected lookpath target: " + file)
+		}
+		return path, err
+	}
+}
+
+func helperArgs(action string) []string {
+	return []string{"-test.run=TestExecRunnerHelperProcess", "--", action}
+}
+
+func TestExecRunnerHelperProcess(t *testing.T) {
+	action := ""
+	for i, arg := range os.Args {
+		if arg == "--" && i+1 < len(os.Args) {
+			action = os.Args[i+1]
+			break
+		}
+	}
+	if action == "" {
+		return
+	}
+
+	switch action {
+	case "success":
+		_, _ = os.Stdout.WriteString("probe stdout\n")
+		_, _ = os.Stderr.WriteString("probe stderr\n")
+		os.Exit(0)
+	case "exit-7":
+		_, _ = os.Stderr.WriteString("probe failed\n")
+		os.Exit(7)
+	case "sleep":
+		time.Sleep(time.Second)
+		os.Exit(0)
+	default:
+		t.Fatalf("unknown helper action %q", action)
+	}
+}
diff --git a/internal/ssh/ssh.go b/internal/ssh/ssh.go
new file mode 100644
index 0000000..7bb4c99
--- /dev/null
+++ b/internal/ssh/ssh.go
@@ -0,0 +1,52 @@
+// Package ssh builds deterministic ssh command vectors.
+package ssh
+
+import (
+	"strconv"
+
+	"github.com/KofTwentyTwo/limen/internal/config"
+)
+
+// Target returns the host target accepted by ssh.
+func Target(host config.Host) string {
+	if host.User == "" {
+		return host.Hostname
+	}
+	return host.User + "@" + host.Hostname
+}
+
+// SessionArgv builds the design-specified ssh argv for an interactive tmux command.
+func SessionArgv(host config.Host, tmuxArgs ...string) []string {
+	argv := []string{"ssh", "-t"}
+	argv = appendPort(argv, host)
+	argv = append(argv, Target(host), "tmux")
+	argv = append(argv, tmuxArgs...)
+	return argv
+}
+
+// LocalTmuxArgv builds the local tmux argv for hosts that do not need ssh.
+func LocalTmuxArgv(tmuxArgs ...string) []string {
+	argv := []string{"tmux"}
+	argv = append(argv, tmuxArgs...)
+	return argv
+}
+
+// ProbeArgv builds the non-interactive remote probe argv.
+func ProbeArgv(host config.Host, format string) []string {
+	argv := []string{
+		"ssh",
+		"-o", "ConnectTimeout=2",
+		"-o", "BatchMode=yes",
+		"-o", "StrictHostKeyChecking=accept-new",
+	}
+	argv = appendPort(argv, host)
+	argv = append(argv, Target(host), "tmux", "ls", "-F", format)
+	return argv
+}
+
+func appendPort(argv []string, host config.Host) []string {
+	if host.Port != 0 && host.Port != 22 {
+		argv = append(argv, "-p", strconv.Itoa(host.Port))
+	}
+	return argv
+}
diff --git a/internal/ssh/ssh_test.go b/internal/ssh/ssh_test.go
new file mode 100644
index 0000000..fad2742
--- /dev/null
+++ b/internal/ssh/ssh_test.go
@@ -0,0 +1,115 @@
+package ssh
+
+import (
+	"reflect"
+	"testing"
+
+	"github.com/KofTwentyTwo/limen/internal/config"
+)
+
+func TestSessionArgvBuildsDesignExamples(t *testing.T) {
+	tests := []struct {
+		name string
+		host config.Host
+		tmux []string
+		want []string
+	}{
+		{
+			name: "hostname only attach",
+			host: config.Host{Name: "prod", Hostname: "prod.example.com"},
+			tmux: []string{"attach", "-t", "api"},
+			want: []string{"ssh", "-t", "prod.example.com", "tmux", "attach", "-t", "api"},
+		},
+		{
+			name: "user new",
+			host: config.Host{Name: "dev", Hostname: "dev.lan", User: "james"},
+			tmux: []string{"new"},
+			want: []string{"ssh", "-t", "james@dev.lan", "tmux", "new"},
+		},
+		{
+			name: "user and non-default port named new",
+			host: config.Host{Name: "bldr", Hostname: "bldr.lan", User: "ci", Port: 2222},
+			tmux: []string{"new", "-s", "build"},
+			want: []string{"ssh", "-t", "-p", "2222", "ci@bldr.lan", "tmux", "new", "-s", "build"},
+		},
+		{
+			name: "non-default port without user",
+			host: config.Host{Name: "bldr", Hostname: "bldr.lan", Port: 2222},
+			tmux: []string{"new"},
+			want: []string{"ssh", "-t", "-p", "2222", "bldr.lan", "tmux", "new"},
+		},
+		{
+			name: "default port omitted",
+			host: config.Host{Name: "prod", Hostname: "prod.example.com", Port: 22},
+			tmux: []string{"new"},
+			want: []string{"ssh", "-t", "prod.example.com", "tmux", "new"},
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			if got := SessionArgv(tt.host, tt.tmux...); !reflect.DeepEqual(got, tt.want) {
+				t.Fatalf("SessionArgv() = %#v, want %#v", got, tt.want)
+			}
+		})
+	}
+}
+
+func TestLocalTmuxArgvBuildsLocalDesignExamples(t *testing.T) {
+	tests := []struct {
+		name string
+		tmux []string
+		want []string
+	}{
+		{
+			name: "attach",
+			tmux: []string{"attach", "-t", "api"},
+			want: []string{"tmux", "attach", "-t", "api"},
+		},
+		{
+			name: "new",
+			tmux: []string{"new"},
+			want: []string{"tmux", "new"},
+		},
+		{
+			name: "named new",
+			tmux: []string{"new", "-s", "build"},
+			want: []string{"tmux", "new", "-s", "build"},
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			if got := LocalTmuxArgv(tt.tmux...); !reflect.DeepEqual(got, tt.want) {
+				t.Fatalf("LocalTmuxArgv() = %#v, want %#v", got, tt.want)
+			}
+		})
+	}
+}
+
+func TestProbeArgvBuildsRemoteProbeCommand(t *testing.T) {
+	host := config.Host{Name: "prod", Hostname: "prod.example.com", User: "deploy", Port: 2222}
+
+	got := ProbeArgv(host, "#{session_name}|#{session_windows}|#{session_attached}")
+	want := []string{
+		"ssh",
+		"-o", "ConnectTimeout=2",
+		"-o", "BatchMode=yes",
+		"-o", "StrictHostKeyChecking=accept-new",
+		"-p", "2222",
+		"deploy@prod.example.com",
+		"tmux", "ls", "-F", "#{session_name}|#{session_windows}|#{session_attached}",
+	}
+
+	if !reflect.DeepEqual(got, want) {
+		t.Fatalf("ProbeArgv() = %#v, want %#v", got, want)
+	}
+}
+
+func TestTargetBuildsUserAtHostnameWhenUserIsSet(t *testing.T) {
+	host := config.Host{Name: "dev", Hostname: "dev.lan", User: "james"}
+
+	if got := Target(host); got != "james@dev.lan" {
+		t.Fatalf("Target() = %q, want james@dev.lan", got)
+	}
+}
diff --git a/internal/tmuxinfo/tmuxinfo.go b/internal/tmuxinfo/tmuxinfo.go
new file mode 100644
index 0000000..5076e22
--- /dev/null
+++ b/internal/tmuxinfo/tmuxinfo.go
@@ -0,0 +1,128 @@
+// Package tmuxinfo parses tmux session inventory output.
+package tmuxinfo
+
+import (
+	"errors"
+	"fmt"
+	"strconv"
+	"strings"
+	"time"
+)
+
+const (
+	// LocalFormat is the tmux format used when probing localhost.
+	LocalFormat = "#{session_name}|#{session_windows}|#{session_attached}|#{session_created}"
+
+	// RemoteFormat is the tmux format used when probing remote hosts.
+	RemoteFormat = "#{session_name}|#{session_windows}|#{session_attached}"
+)
+
+// ErrNoServer marks tmux's normal "no server running" response.
+var ErrNoServer = errors.New("tmuxinfo: no tmux server running")
+
+// SessionInfo is the tmux session shape used by probe results.
+type SessionInfo struct {
+	Name        string
+	Windows     int
+	WindowNames []string
+	Attached    bool
+	Created     time.Time
+}
+
+// ParseLocal parses localhost tmux ls output.
+func ParseLocal(output string) ([]SessionInfo, error) {
+	return parse(output, "local", 4)
+}
+
+// ParseRemote parses remote tmux ls output.
+func ParseRemote(output string) ([]SessionInfo, error) {
+	return parse(output, "remote", 3)
+}
+
+// IsNoServerRunning reports whether output is tmux's no-server diagnostic.
+func IsNoServerRunning(output string) bool {
+	return strings.Contains(output, "no server running on /tmp/tmux-")
+}
+
+func parse(output string, variant string, fieldCount int) ([]SessionInfo, error) {
+	if IsNoServerRunning(output) {
+		return nil, ErrNoServer
+	}
+
+	trimmed := strings.TrimSpace(output)
+	if trimmed == "" {
+		return []SessionInfo{}, nil
+	}
+
+	lines := strings.Split(trimmed, "\n")
+	sessions := make([]SessionInfo, 0, len(lines))
+	for i, line := range lines {
+		session, err := parseLine(line, variant, fieldCount, i+1)
+		if err != nil {
+			return nil, err
+		}
+		sessions = append(sessions, session)
+	}
+
+	return sessions, nil
+}
+
+func parseLine(line string, variant string, fieldCount int, row int) (SessionInfo, error) {
+	fields := strings.Split(line, "|")
+	if len(fields) != fieldCount {
+		return SessionInfo{}, fmt.Errorf("tmuxinfo: malformed %s row %d: expected %d fields, got %d", variant, row, fieldCount, len(fields))
+	}
+
+	name := fields[0]
+	if name == "" {
+		return SessionInfo{}, fmt.Errorf("tmuxinfo: %s row %d has empty session name", variant, row)
+	}
+
+	windows, err := parseNonNegativeInt(fields[1], variant, row, "window count")
+	if err != nil {
+		return SessionInfo{}, err
+	}
+
+	attachedCount, err := parseNonNegativeInt(fields[2], variant, row, "attached count")
+	if err != nil {
+		return SessionInfo{}, err
+	}
+
+	session := SessionInfo{
+		Name:     name,
+		Windows:  windows,
+		Attached: attachedCount > 0,
+	}
+
+	if fieldCount == 4 {
+		created, err := parseNonNegativeTimestamp(fields[3], variant, row)
+		if err != nil {
+			return SessionInfo{}, err
+		}
+		session.Created = time.Unix(created, 0).UTC()
+	}
+
+	return session, nil
+}
+
+func parseNonNegativeInt(value string, variant string, row int, label string) (int, error) {
+	number, err := strconv.Atoi(value)
+	if err != nil {
+		return 0, fmt.Errorf("tmuxinfo: %s row %d has invalid %s %q: %w", variant, row, label, value, err)
+	}
+	if number < 0 {
+		return 0, fmt.Errorf("tmuxinfo: %s row %d has invalid %s %q", variant, row, label, value)
+	}
+	return number, nil
+}
+
+func parseNonNegativeTimestamp(value string, variant string, row int) (int64, error) {
+	created, err := strconv.ParseInt(value, 10, 64)
+	if err != nil {
+		return 0, fmt.Errorf("tmuxinfo: %s row %d has invalid created timestamp %q: %w", variant, row, value, err)
+	}
+	if created < 0 {
+		return 0, fmt.Errorf("tmuxinfo: %s row %d has invalid created timestamp %q", variant, row, value)
+	}
+	return created, nil
+}
diff --git a/internal/tmuxinfo/tmuxinfo_test.go b/internal/tmuxinfo/tmuxinfo_test.go
new file mode 100644
index 0000000..03223e1
--- /dev/null
+++ b/internal/tmuxinfo/tmuxinfo_test.go
@@ -0,0 +1,172 @@
+package tmuxinfo
+
+import (
+	"errors"
+	"strings"
+	"testing"
+	"time"
+)
+
+func TestParseLocalReadsSessionRows(t *testing.T) {
+	got, err := ParseLocal("api|3|1|1779799500\nops|1|0|1779799600\n")
+	if err != nil {
+		t.Fatalf("ParseLocal returned error: %v", err)
+	}
+
+	if len(got) != 2 {
+		t.Fatalf("len(sessions) = %d, want 2", len(got))
+	}
+
+	api := got[0]
+	if api.Name != "api" {
+		t.Fatalf("api.Name = %q, want api", api.Name)
+	}
+	if api.Windows != 3 {
+		t.Fatalf("api.Windows = %d, want 3", api.Windows)
+	}
+	if !api.Attached {
+		t.Fatal("api.Attached = false, want true")
+	}
+	wantCreated := time.Unix(1779799500, 0).UTC()
+	if !api.Created.Equal(wantCreated) {
+		t.Fatalf("api.Created = %s, want %s", api.Created, wantCreated)
+	}
+
+	ops := got[1]
+	if ops.Name != "ops" || ops.Windows != 1 || ops.Attached || !ops.Created.Equal(time.Unix(1779799600, 0).UTC()) {
+		t.Fatalf("ops = %#v", ops)
+	}
+}
+
+func TestParseRemoteReadsSessionRowsWithoutCreatedTime(t *testing.T) {
+	got, err := ParseRemote("api|3|2\nops|1|0\n")
+	if err != nil {
+		t.Fatalf("ParseRemote returned error: %v", err)
+	}
+
+	if len(got) != 2 {
+		t.Fatalf("len(sessions) = %d, want 2", len(got))
+	}
+
+	api := got[0]
+	if api.Name != "api" || api.Windows != 3 || !api.Attached || !api.Created.IsZero() {
+		t.Fatalf("api = %#v", api)
+	}
+
+	ops := got[1]
+	if ops.Name != "ops" || ops.Windows != 1 || ops.Attached || !ops.Created.IsZero() {
+		t.Fatalf("ops = %#v", ops)
+	}
+}
+
+func TestParseEmptyOutputReturnsZeroSessions(t *testing.T) {
+	for _, parse := range []struct {
+		name string
+		fn   func(string) ([]SessionInfo, error)
+	}{
+		{name: "local", fn: ParseLocal},
+		{name: "remote", fn: ParseRemote},
+	} {
+		t.Run(parse.name, func(t *testing.T) {
+			got, err := parse.fn("\n\t ")
+			if err != nil {
+				t.Fatalf("parse returned error: %v", err)
+			}
+			if len(got) != 0 {
+				t.Fatalf("len(sessions) = %d, want 0", len(got))
+			}
+		})
+	}
+}
+
+func TestParseRejectsMalformedRowsWithUsefulErrors(t *testing.T) {
+	tests := []struct {
+		name    string
+		parse   func(string) ([]SessionInfo, error)
+		output  string
+		wantErr string
+	}{
+		{
+			name:    "local missing created field",
+			parse:   ParseLocal,
+			output:  "api|3|1\n",
+			wantErr: "tmuxinfo: malformed local row 1: expected 4 fields, got 3",
+		},
+		{
+			name:    "remote extra field",
+			parse:   ParseRemote,
+			output:  "api|3|1|1779799500\n",
+			wantErr: "tmuxinfo: malformed remote row 1: expected 3 fields, got 4",
+		},
+		{
+			name:    "invalid window count",
+			parse:   ParseRemote,
+			output:  "api|many|1\n",
+			wantErr: `tmuxinfo: remote row 1 has invalid window count "many"`,
+		},
+		{
+			name:    "invalid attached count",
+			parse:   ParseRemote,
+			output:  "api|3|yes\n",
+			wantErr: `tmuxinfo: remote row 1 has invalid attached count "yes"`,
+		},
+		{
+			name:    "invalid created timestamp",
+			parse:   ParseLocal,
+			output:  "api|3|1|not-a-time\n",
+			wantErr: `tmuxinfo: local row 1 has invalid created timestamp "not-a-time"`,
+		},
+		{
+			name:    "negative created timestamp",
+			parse:   ParseLocal,
+			output:  "api|3|1|-1\n",
+			wantErr: `tmuxinfo: local row 1 has invalid created timestamp "-1"`,
+		},
+		{
+			name:    "empty session name",
+			parse:   ParseRemote,
+			output:  "|3|1\n",
+			wantErr: "tmuxinfo: remote row 1 has empty session name",
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			_, err := tt.parse(tt.output)
+			if err == nil {
+				t.Fatal("parse returned nil error")
+			}
+			if !strings.HasPrefix(err.Error(), tt.wantErr) {
+				t.Fatalf("error = %q, want prefix %q", err.Error(), tt.wantErr)
+			}
+		})
+	}
+}
+
+func TestParseRejectsNegativeNumbersWithoutFormattingNoise(t *testing.T) {
+	_, err := ParseLocal("api|-1|0|1779799500\n")
+	if err == nil {
+		t.Fatal("ParseLocal returned nil error")
+	}
+
+	want := `tmuxinfo: local row 1 has invalid window count "-1"`
+	if err.Error() != want {
+		t.Fatalf("error = %q, want %q", err.Error(), want)
+	}
+}
+
+func TestIsNoServerRunningRecognizesTmuxMessage(t *testing.T) {
+	if !IsNoServerRunning("no server running on /tmp/tmux-501/default\n") {
+		t.Fatal("IsNoServerRunning returned false for tmux no-server message")
+	}
+	if IsNoServerRunning("ssh: connect to host prod port 22: Operation timed out") {
+		t.Fatal("IsNoServerRunning returned true for unrelated output")
+	}
+}
+
+func TestParseNoServerOutputReturnsSentinelError(t *testing.T) {
+	_, err := ParseRemote("no server running on /tmp/tmux-501/default\n")
+	if !errors.Is(err, ErrNoServer) {
+		t.Fatalf("ParseRemote error = %v, want ErrNoServer", err)
+	}
+}