diff --git a/Cargo.lock b/Cargo.lock
index 279213138..2506235d1 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -359,12 +359,20 @@ name = "arc-util"
 version = "0.1.0"
 dependencies = [
  "aho-corasick",
+ "anyhow",
+ "base64",
+ "chrono",
  "console 0.15.11",
+ "dirs",
  "regex",
+ "reqwest 0.12.28",
  "serde",
  "serde_json",
  "termimad",
+ "tokio",
  "toml",
+ "tracing",
+ "uuid",
 ]
 
 [[package]]
diff --git a/crates/arc-util/Cargo.toml b/crates/arc-util/Cargo.toml
index b533056d8..a2c746cd0 100644
--- a/crates/arc-util/Cargo.toml
+++ b/crates/arc-util/Cargo.toml
@@ -14,6 +14,15 @@ regex.workspace = true
 termimad.workspace = true
 aho-corasick.workspace = true
 serde_json.workspace = true
+serde.workspace = true
+reqwest.workspace = true
+tokio.workspace = true
+uuid.workspace = true
+chrono.workspace = true
+base64.workspace = true
+dirs.workspace = true
+tracing.workspace = true
+anyhow.workspace = true
 
 [build-dependencies]
 toml = "0.8"
diff --git a/crates/arc-util/src/lib.rs b/crates/arc-util/src/lib.rs
index bab7da24d..330c64d81 100644
--- a/crates/arc-util/src/lib.rs
+++ b/crates/arc-util/src/lib.rs
@@ -1,3 +1,4 @@
 pub mod check_report;
 pub mod redact;
+pub mod telemetry;
 pub mod terminal;
diff --git a/crates/arc-util/src/telemetry/anonymous_id.rs b/crates/arc-util/src/telemetry/anonymous_id.rs
new file mode 100644
index 000000000..8fc4b0f4e
--- /dev/null
+++ b/crates/arc-util/src/telemetry/anonymous_id.rs
@@ -0,0 +1,47 @@
+use anyhow::{Context, Result};
+use std::fs;
+use std::path::PathBuf;
+use uuid::Uuid;
+
+fn anonymous_id_path() -> Result<PathBuf> {
+    let home = dirs::home_dir().context("could not determine home directory")?;
+    Ok(home.join(".arc").join("anonymous_id"))
+}
+
+pub fn load_or_create_anonymous_id() -> Result<String> {
+    let path = anonymous_id_path()?;
+
+    if let Ok(contents) = fs::read_to_string(&path) {
+        let id = contents.trim().to_string();
+        if !id.is_empty() {
+            return Ok(id);
+        }
+    }
+
+    let id = Uuid::new_v4().to_string();
+
+    if let Some(parent) = path.parent() {
+        fs::create_dir_all(parent)
+            .with_context(|| format!("failed to create directory {}", parent.display()))?;
+    }
+
+    fs::write(&path, &id)
+        .with_context(|| format!("failed to write anonymous id to {}", path.display()))?;
+
+    Ok(id)
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn load_or_create_returns_stable_id() {
+        // Calls the real function, which uses ~/.arc/anonymous_id
+        let id1 = load_or_create_anonymous_id().unwrap();
+        let id2 = load_or_create_anonymous_id().unwrap();
+        assert_eq!(id1, id2);
+        // Should be a valid UUID
+        Uuid::parse_str(&id1).unwrap();
+    }
+}
diff --git a/crates/arc-util/src/telemetry/context.rs b/crates/arc-util/src/telemetry/context.rs
new file mode 100644
index 000000000..f0ce2b5ab
--- /dev/null
+++ b/crates/arc-util/src/telemetry/context.rs
@@ -0,0 +1,75 @@
+use serde_json::{json, Value};
+
+pub fn build_context() -> Value {
+    json!({
+        "os": { "name": std::env::consts::OS },
+        "device": { "type": std::env::consts::ARCH },
+        "locale": current_locale(),
+        "app": { "name": "arc", "version": env!("CARGO_PKG_VERSION") }
+    })
+}
+
+fn current_locale() -> String {
+    let lang = std::env::var("LANG").unwrap_or_default();
+    parse_locale(&lang)
+}
+
+fn parse_locale(lang: &str) -> String {
+    if lang.is_empty() || lang == "C" || lang == "POSIX" {
+        return "en-US".to_string();
+    }
+
+    // Strip encoding suffix (e.g. ".UTF-8")
+    let without_encoding = lang.split('.').next().unwrap_or(lang);
+
+    // Convert underscore to hyphen (e.g. "en_US" -> "en-US")
+    without_encoding.replace('_', "-")
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn context_has_expected_keys() {
+        let ctx = build_context();
+        assert!(ctx.get("os").is_some());
+        assert!(ctx["os"].get("name").is_some());
+        assert!(ctx.get("device").is_some());
+        assert!(ctx["device"].get("type").is_some());
+        assert!(ctx.get("locale").is_some());
+        assert!(ctx.get("app").is_some());
+        assert_eq!(ctx["app"]["name"], "arc");
+        assert!(ctx["app"].get("version").is_some());
+    }
+
+    #[test]
+    fn parse_locale_en_us_utf8() {
+        assert_eq!(parse_locale("en_US.UTF-8"), "en-US");
+    }
+
+    #[test]
+    fn parse_locale_c() {
+        assert_eq!(parse_locale("C"), "en-US");
+    }
+
+    #[test]
+    fn parse_locale_posix() {
+        assert_eq!(parse_locale("POSIX"), "en-US");
+    }
+
+    #[test]
+    fn parse_locale_empty() {
+        assert_eq!(parse_locale(""), "en-US");
+    }
+
+    #[test]
+    fn parse_locale_no_encoding() {
+        assert_eq!(parse_locale("fr_FR"), "fr-FR");
+    }
+
+    #[test]
+    fn parse_locale_with_encoding() {
+        assert_eq!(parse_locale("de_DE.ISO-8859-1"), "de-DE");
+    }
+}
diff --git a/crates/arc-util/src/telemetry/event.rs b/crates/arc-util/src/telemetry/event.rs
new file mode 100644
index 000000000..a04b7662d
--- /dev/null
+++ b/crates/arc-util/src/telemetry/event.rs
@@ -0,0 +1,122 @@
+use serde::{Deserialize, Serialize};
+use serde_json::Value;
+
+#[derive(Debug, Serialize, Deserialize)]
+pub struct Track {
+    #[serde(flatten)]
+    pub user: User,
+    pub event: String,
+    pub properties: Value,
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub context: Option<Value>,
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub timestamp: Option<String>,
+    #[serde(rename = "messageId")]
+    pub message_id: String,
+}
+
+#[derive(Debug, Serialize, Deserialize)]
+#[serde(untagged)]
+pub enum User {
+    Both {
+        #[serde(rename = "userId")]
+        user_id: String,
+        #[serde(rename = "anonymousId")]
+        anonymous_id: String,
+    },
+    UserId {
+        #[serde(rename = "userId")]
+        user_id: String,
+    },
+    AnonymousId {
+        #[serde(rename = "anonymousId")]
+        anonymous_id: String,
+    },
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use serde_json::json;
+
+    #[test]
+    fn track_serialization_anonymous() {
+        let track = Track {
+            user: User::AnonymousId {
+                anonymous_id: "abc-123".to_string(),
+            },
+            event: "Command Invoked".to_string(),
+            properties: json!({"command": "run"}),
+            context: Some(json!({"os": {"name": "macos"}})),
+            timestamp: Some("2025-01-01T00:00:00Z".to_string()),
+            message_id: "msg-1".to_string(),
+        };
+
+        let value = serde_json::to_value(&track).unwrap();
+        assert_eq!(value["anonymousId"], "abc-123");
+        assert_eq!(value["event"], "Command Invoked");
+        assert_eq!(value["properties"]["command"], "run");
+        assert_eq!(value["context"]["os"]["name"], "macos");
+        assert_eq!(value["timestamp"], "2025-01-01T00:00:00Z");
+        assert_eq!(value["messageId"], "msg-1");
+        assert!(value.get("userId").is_none());
+    }
+
+    #[test]
+    fn track_serialization_user_id() {
+        let track = Track {
+            user: User::UserId {
+                user_id: "user-456".to_string(),
+            },
+            event: "test".to_string(),
+            properties: json!({}),
+            context: None,
+            timestamp: None,
+            message_id: "msg-2".to_string(),
+        };
+
+        let value = serde_json::to_value(&track).unwrap();
+        assert_eq!(value["userId"], "user-456");
+        assert!(value.get("anonymousId").is_none());
+        assert!(value.get("context").is_none());
+        assert!(value.get("timestamp").is_none());
+    }
+
+    #[test]
+    fn track_serialization_both() {
+        let track = Track {
+            user: User::Both {
+                user_id: "user-456".to_string(),
+                anonymous_id: "abc-123".to_string(),
+            },
+            event: "test".to_string(),
+            properties: json!({}),
+            context: None,
+            timestamp: None,
+            message_id: "msg-3".to_string(),
+        };
+
+        let value = serde_json::to_value(&track).unwrap();
+        assert_eq!(value["userId"], "user-456");
+        assert_eq!(value["anonymousId"], "abc-123");
+    }
+
+    #[test]
+    fn track_round_trip() {
+        let track = Track {
+            user: User::AnonymousId {
+                anonymous_id: "abc".to_string(),
+            },
+            event: "test".to_string(),
+            properties: json!({"key": "value"}),
+            context: Some(json!({"app": {"name": "arc"}})),
+            timestamp: Some("2025-06-01T12:00:00Z".to_string()),
+            message_id: "msg-rt".to_string(),
+        };
+
+        let json_str = serde_json::to_string(&track).unwrap();
+        let deserialized: Track = serde_json::from_str(&json_str).unwrap();
+        assert_eq!(deserialized.event, "test");
+        assert_eq!(deserialized.message_id, "msg-rt");
+    }
+}
diff --git a/crates/arc-util/src/telemetry/mod.rs b/crates/arc-util/src/telemetry/mod.rs
new file mode 100644
index 000000000..4b4075ad1
--- /dev/null
+++ b/crates/arc-util/src/telemetry/mod.rs
@@ -0,0 +1,154 @@
+pub mod anonymous_id;
+pub mod context;
+pub mod event;
+pub mod sender;
+
+use anyhow::Result;
+use chrono::Utc;
+use serde_json::Value;
+use uuid::Uuid;
+
+use event::{Track, User};
+
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub enum TelemetryLevel {
+    Off,
+    Errors,
+    All,
+}
+
+pub struct Telemetry {
+    level: TelemetryLevel,
+    anonymous_id: String,
+    context: Value,
+}
+
+impl Telemetry {
+    pub fn new() -> Result<Self> {
+        let level = telemetry_level();
+        let anonymous_id = anonymous_id::load_or_create_anonymous_id()?;
+        let context = context::build_context();
+
+        Ok(Self {
+            level,
+            anonymous_id,
+            context,
+        })
+    }
+
+    pub fn level(&self) -> &TelemetryLevel {
+        &self.level
+    }
+
+    pub fn should_track(&self, is_error: bool) -> bool {
+        match self.level {
+            TelemetryLevel::Off => false,
+            TelemetryLevel::Errors => is_error,
+            TelemetryLevel::All => true,
+        }
+    }
+
+    pub fn build_track(&self, event: &str, properties: Value) -> Track {
+        Track {
+            user: User::AnonymousId {
+                anonymous_id: self.anonymous_id.clone(),
+            },
+            event: event.to_string(),
+            properties,
+            context: Some(self.context.clone()),
+            timestamp: Some(Utc::now().to_rfc3339()),
+            message_id: Uuid::new_v4().to_string(),
+        }
+    }
+}
+
+pub fn telemetry_level() -> TelemetryLevel {
+    match std::env::var("ARC_TELEMETRY").as_deref() {
+        Ok("off") => TelemetryLevel::Off,
+        Ok("errors") => TelemetryLevel::Errors,
+        Ok("all") => TelemetryLevel::All,
+        _ => {
+            if cfg!(debug_assertions) {
+                TelemetryLevel::Off
+            } else {
+                TelemetryLevel::All
+            }
+        }
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use serde_json::json;
+
+    #[test]
+    fn telemetry_level_defaults_to_off_in_debug() {
+        // In test builds (debug_assertions=true), default is Off
+        std::env::remove_var("ARC_TELEMETRY");
+        assert_eq!(telemetry_level(), TelemetryLevel::Off);
+    }
+
+    #[test]
+    fn telemetry_level_parses_env_var() {
+        std::env::set_var("ARC_TELEMETRY", "all");
+        assert_eq!(telemetry_level(), TelemetryLevel::All);
+
+        std::env::set_var("ARC_TELEMETRY", "errors");
+        assert_eq!(telemetry_level(), TelemetryLevel::Errors);
+
+        std::env::set_var("ARC_TELEMETRY", "off");
+        assert_eq!(telemetry_level(), TelemetryLevel::Off);
+
+        // Clean up
+        std::env::remove_var("ARC_TELEMETRY");
+    }
+
+    #[test]
+    fn should_track_respects_level() {
+        let telemetry = Telemetry {
+            level: TelemetryLevel::Off,
+            anonymous_id: "test".to_string(),
+            context: json!({}),
+        };
+        assert!(!telemetry.should_track(false));
+        assert!(!telemetry.should_track(true));
+
+        let telemetry = Telemetry {
+            level: TelemetryLevel::Errors,
+            anonymous_id: "test".to_string(),
+            context: json!({}),
+        };
+        assert!(!telemetry.should_track(false));
+        assert!(telemetry.should_track(true));
+
+        let telemetry = Telemetry {
+            level: TelemetryLevel::All,
+            anonymous_id: "test".to_string(),
+            context: json!({}),
+        };
+        assert!(telemetry.should_track(false));
+        assert!(telemetry.should_track(true));
+    }
+
+    #[test]
+    fn build_track_populates_fields() {
+        let telemetry = Telemetry {
+            level: TelemetryLevel::All,
+            anonymous_id: "anon-123".to_string(),
+            context: json!({"app": {"name": "arc"}}),
+        };
+
+        let track = telemetry.build_track("Test Event", json!({"key": "value"}));
+        assert_eq!(track.event, "Test Event");
+        assert_eq!(track.properties["key"], "value");
+        assert!(track.context.is_some());
+        assert!(track.timestamp.is_some());
+        assert!(!track.message_id.is_empty());
+
+        match &track.user {
+            User::AnonymousId { anonymous_id } => assert_eq!(anonymous_id, "anon-123"),
+            _ => panic!("expected AnonymousId variant"),
+        }
+    }
+}
diff --git a/crates/arc-util/src/telemetry/sender.rs b/crates/arc-util/src/telemetry/sender.rs
new file mode 100644
index 000000000..a37f105e8
--- /dev/null
+++ b/crates/arc-util/src/telemetry/sender.rs
@@ -0,0 +1,71 @@
+use base64::Engine;
+use base64::engine::general_purpose::STANDARD;
+
+use super::event::Track;
+
+const SEGMENT_API_URL: &str = "https://api.segment.io/v1/track";
+const SEGMENT_WRITE_KEY: Option<&str> = option_env!("SEGMENT_WRITE_KEY");
+
+/// Spawns a fire-and-forget tokio task to send a track event to Segment.
+/// No-ops if the SEGMENT_WRITE_KEY was not set at compile time.
+pub fn send(track: Track) {
+    let Some(write_key) = SEGMENT_WRITE_KEY else {
+        tracing::debug!("telemetry: no SEGMENT_WRITE_KEY, skipping send");
+        return;
+    };
+
+    tokio::spawn(send_track(track, write_key));
+}
+
+async fn send_track(track: Track, write_key: &str) {
+    let auth = STANDARD.encode(format!("{write_key}:"));
+
+    let result = reqwest::Client::new()
+        .post(SEGMENT_API_URL)
+        .header("Authorization", format!("Basic {auth}"))
+        .json(&track)
+        .send()
+        .await;
+
+    match result {
+        Ok(resp) if !resp.status().is_success() => {
+            tracing::warn!(
+                status = %resp.status(),
+                "telemetry: segment API returned non-success status"
+            );
+        }
+        Err(err) => {
+            tracing::warn!(%err, "telemetry: failed to send event to segment");
+        }
+        Ok(_) => {
+            tracing::debug!("telemetry: event sent successfully");
+        }
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::telemetry::event::User;
+    use serde_json::json;
+
+    #[test]
+    fn send_noops_without_write_key() {
+        // SEGMENT_WRITE_KEY is not set at compile time in tests,
+        // so send() should return immediately without spawning.
+        let track = Track {
+            user: User::AnonymousId {
+                anonymous_id: "test".to_string(),
+            },
+            event: "test".to_string(),
+            properties: json!({}),
+            context: None,
+            timestamp: None,
+            message_id: "msg-test".to_string(),
+        };
+
+        // This should not panic or require a tokio runtime
+        // because it returns before reaching tokio::spawn
+        send(track);
+    }
+}