diff --git a/cli/telemetry/detached.go b/cli/telemetry/detached.go
index df606ad..67bf0fc 100644
--- a/cli/telemetry/detached.go
+++ b/cli/telemetry/detached.go
@@ -1,13 +1,15 @@
 package telemetry
 
 import (
+	"crypto/rand"
+	"encoding/hex"
 	"encoding/json"
 	"os"
+	"path/filepath"
 	"runtime"
 	"strings"
 	"time"
 
-	"github.com/denisbrodbeck/machineid"
 	"github.com/posthog/posthog-go"
 	"github.com/spf13/cobra"
 	"github.com/spf13/pflag"
@@ -31,6 +33,55 @@ type EventPayload struct {
 	Timestamp  time.Time      `json:"timestamp"`
 }
 
+// userConfigDir returns the base user config directory. It is a var so
+// tests can isolate the anonymous ID file from the real user config dir.
+var userConfigDir = os.UserConfigDir
+
+// anonymousID returns a stable per-install anonymous identifier. The ID is a
+// random 128-bit value generated on first use and cached in the user config
+// dir. A random ID is used instead of a hardware-derived machine ID so
+// telemetry events cannot be correlated to a physical machine.
+func anonymousID() (string, error) {
+	dir, err := userConfigDir()
+	if err != nil {
+		return "", err
+	}
+	dir = filepath.Join(dir, "trace")
+	if err := os.MkdirAll(dir, 0o700); err != nil {
+		return "", err
+	}
+	path := filepath.Join(dir, "telemetry-id")
+	if b, err := os.ReadFile(path); err == nil {
+		if id := strings.TrimSpace(string(b)); id != "" {
+			return id, nil
+		}
+	}
+	raw := make([]byte, 16)
+	if _, err := rand.Read(raw); err != nil {
+		return "", err
+	}
+	id := hex.EncodeToString(raw)
+	if err := os.WriteFile(path, []byte(id+"\n"), 0o600); err != nil {
+		return "", err
+	}
+	return id, nil
+}
+
+// telemetryEnabled reports whether telemetry is permitted. Telemetry is
+// opt-in: nothing is sent unless TRACE_TELEMETRY_OPTIN=1 is set. The legacy
+// TRACE_TELEMETRY_OPTOUT variable continues to force-disable even when
+// opt-in is enabled.
+func telemetryEnabled() bool {
+	if os.Getenv("TRACE_TELEMETRY_OPTOUT") != "" {
+		return false
+	}
+	return os.Getenv("TRACE_TELEMETRY_OPTIN") == "1"
+}
+
+// sendDetached dispatches a payload to the detached analytics subprocess.
+// It is a var so tests can spy on whether telemetry was dispatched.
+var sendDetached = spawnDetachedAnalytics
+
 // silentLogger suppresses PostHog log output - expected for CLI best-effort telemetry
 type silentLogger struct{}
 
@@ -46,8 +97,8 @@ func BuildEventPayload(cmd *cobra.Command, agent string, isTraceEnabled bool, ve
 		return nil
 	}
 
-	// Get machine ID for distinct_id
-	machineID, err := machineid.ProtectedID("trace-cli")
+	// Get an anonymous per-install ID for distinct_id
+	machineID, err := anonymousID()
 	if err != nil {
 		return nil
 	}
@@ -87,11 +138,12 @@ func BuildEventPayload(cmd *cobra.Command, agent string, isTraceEnabled bool, ve
 // TrackCommandDetached tracks a command execution by spawning a detached subprocess.
 // This returns immediately without blocking the CLI.
 //
-// Telemetry can be disabled by setting the TRACE_TELEMETRY_OPTOUT environment variable
-// to any non-empty value (e.g., TRACE_TELEMETRY_OPTOUT=1).
+// Telemetry is opt-in: it is only sent when TRACE_TELEMETRY_OPTIN=1 is set.
+// The legacy TRACE_TELEMETRY_OPTOUT environment variable (any non-empty
+// value) force-disables telemetry regardless.
 func TrackCommandDetached(cmd *cobra.Command, agent string, isTraceEnabled bool, version string) {
-	// Check opt-out environment variables
-	if os.Getenv("TRACE_TELEMETRY_OPTOUT") != "" {
+	// Opt-in gate: nothing is sent unless explicitly enabled.
+	if !telemetryEnabled() {
 		return
 	}
 
@@ -109,14 +161,14 @@ func TrackCommandDetached(cmd *cobra.Command, agent string, isTraceEnabled bool,
 	}
 
 	if payloadJSON, err := json.Marshal(payload); err == nil {
-		spawnDetachedAnalytics(string(payloadJSON))
+		sendDetached(string(payloadJSON))
 	}
 }
 
 // TrackPluginDetached tracks a plugin invocation by spawning a detached subprocess.
 // This returns immediately without blocking the CLI.
 func TrackPluginDetached(pluginName string, isTraceEnabled bool, version string) {
-	if os.Getenv("TRACE_TELEMETRY_OPTOUT") != "" {
+	if !telemetryEnabled() {
 		return
 	}
 
@@ -126,7 +178,7 @@ func TrackPluginDetached(pluginName string, isTraceEnabled bool, version string)
 	}
 
 	if payloadJSON, err := json.Marshal(payload); err == nil {
-		spawnDetachedAnalytics(string(payloadJSON))
+		sendDetached(string(payloadJSON))
 	}
 }
 
diff --git a/cli/telemetry/detached_test.go b/cli/telemetry/detached_test.go
index 8035a2b..1199997 100644
--- a/cli/telemetry/detached_test.go
+++ b/cli/telemetry/detached_test.go
@@ -8,6 +8,24 @@ import (
 	"github.com/spf13/cobra"
 )
 
+// spySendDetached swaps the dispatch hook and returns a counter.
+func spySendDetached(t *testing.T) *int {
+	t.Helper()
+	calls := 0
+	orig := sendDetached
+	sendDetached = func(string) { calls++ }
+	t.Cleanup(func() { sendDetached = orig })
+	return &calls
+}
+
+// isolateConfigDir redirects the anonymous ID file into a temp dir.
+func isolateConfigDir(t *testing.T) {
+	t.Helper()
+	orig := userConfigDir
+	userConfigDir = func() (string, error) { return t.TempDir(), nil }
+	t.Cleanup(func() { userConfigDir = orig })
+}
+
 func TestEventPayloadSerialization(t *testing.T) {
 	payload := EventPayload{
 		Event:      "cli_command_executed",
@@ -69,14 +87,62 @@ func TestTrackCommandDetachedSkipsHiddenCommands(_ *testing.T) {
 }
 
 func TestTrackCommandDetachedRespectsOptOut(t *testing.T) {
+	t.Setenv("TRACE_TELEMETRY_OPTIN", "1")
 	t.Setenv("TRACE_TELEMETRY_OPTOUT", "1")
 
+	calls := spySendDetached(t)
 	cmd := &cobra.Command{
 		Use: "status",
 	}
 
-	// Should not panic and should respect opt-out
+	// Opt-out must win over opt-in: nothing is dispatched.
+	TrackCommandDetached(cmd, "claude-code", true, "1.0.0")
+	if *calls != 0 {
+		t.Errorf("expected 0 dispatches when opt-out is set, got %d", *calls)
+	}
+}
+
+func TestTrackCommandDetachedDisabledByDefault(t *testing.T) {
+	// Without TRACE_TELEMETRY_OPTIN, telemetry is off even with no opt-out.
+	calls := spySendDetached(t)
+	cmd := &cobra.Command{Use: "status"}
+
 	TrackCommandDetached(cmd, "claude-code", true, "1.0.0")
+	if *calls != 0 {
+		t.Errorf("expected 0 dispatches by default (opt-in), got %d", *calls)
+	}
+}
+
+func TestTrackCommandDetachedEnabledWithOptIn(t *testing.T) {
+	t.Setenv("TRACE_TELEMETRY_OPTIN", "1")
+	isolateConfigDir(t)
+
+	calls := spySendDetached(t)
+	cmd := &cobra.Command{Use: "status"}
+
+	TrackCommandDetached(cmd, "claude-code", true, "1.0.0")
+	if *calls != 1 {
+		t.Errorf("expected 1 dispatch with opt-in, got %d", *calls)
+	}
+}
+
+func TestTrackPluginDetachedDisabledByDefault(t *testing.T) {
+	calls := spySendDetached(t)
+	TrackPluginDetached("my-plugin", true, "1.0.0")
+	if *calls != 0 {
+		t.Errorf("expected 0 dispatches by default (opt-in), got %d", *calls)
+	}
+}
+
+func TestTrackPluginDetachedEnabledWithOptIn(t *testing.T) {
+	t.Setenv("TRACE_TELEMETRY_OPTIN", "1")
+	isolateConfigDir(t)
+
+	calls := spySendDetached(t)
+	TrackPluginDetached("my-plugin", true, "1.0.0")
+	if *calls != 1 {
+		t.Errorf("expected 1 dispatch with opt-in, got %d", *calls)
+	}
 }
 
 func TestBuildEventPayloadAgent(t *testing.T) {
diff --git a/go.mod b/go.mod
index 52b35e6..88e70da 100644
--- a/go.mod
+++ b/go.mod
@@ -12,7 +12,6 @@ require (
 	github.com/betterleaks/betterleaks v1.4.1
 	github.com/charmbracelet/x/ansi v0.11.7
 	github.com/creack/pty v1.1.24
-	github.com/denisbrodbeck/machineid v1.0.1
 	github.com/entireio/auth-go v0.4.0
 	github.com/go-git/go-billy/v6 v6.0.0-alpha.1.0.20260519112248-0095b064a6c6
 	github.com/go-git/go-git/v6 v6.0.0-alpha.4
diff --git a/go.sum b/go.sum
index 22111d9..879c576 100644
--- a/go.sum
+++ b/go.sum
@@ -99,8 +99,6 @@ github.com/davecgh/go-spew v1.1.0/go.mod h1:J7Y8YcW2NihsgmVo/mv3lAwl/skON4iLHjSs
 github.com/davecgh/go-spew v1.1.1/go.mod h1:J7Y8YcW2NihsgmVo/mv3lAwl/skON4iLHjSsI+c5H38=
 github.com/davecgh/go-spew v1.1.2-0.20180830191138-d8f796af33cc h1:U9qPSI2PIWSS1VwoXQT9A3Wy9MM3WgvqSxFWenqJduM=
 github.com/davecgh/go-spew v1.1.2-0.20180830191138-d8f796af33cc/go.mod h1:J7Y8YcW2NihsgmVo/mv3lAwl/skON4iLHjSsI+c5H38=
-github.com/denisbrodbeck/machineid v1.0.1 h1:geKr9qtkB876mXguW2X6TU4ZynleN6ezuMSRhl4D7AQ=
-github.com/denisbrodbeck/machineid v1.0.1/go.mod h1:dJUwb7PTidGDeYyUBmXZ2GphQBbjJCrnectwCyxcUSI=
 github.com/dlclark/regexp2 v1.12.0 h1:0j4c5qQmnC6XOWNjP3PIXURXN2gWx76rd3KvgdPkCz8=
 github.com/dlclark/regexp2 v1.12.0/go.mod h1:DHkYz0B9wPfa6wondMfaivmHpzrQ3v9q8cnmRbL6yW8=
 github.com/dlclark/regexp2/v2 v2.2.1 h1:mf4KkFUj0gJuarK8P+LgiS+Lit7m9N1yAwEfPbee7R0=