Split anonymous ID into server (UUID-on-disk) and CLI (MAC-based hash) strategies

Server is long-lived on a fixed host, so a persisted UUID at ~/.arc/.id is
appropriate. CLI runs ephemerally, so an MD5 of the MAC address avoids file
I/O and is stable per-machine. CLI falls back to ~/.arc/.id if it exists
for migration.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Cargo.lock

@@ -364,6 +364,8 @@ dependencies = [
  "chrono",
  "console 0.15.11",
  "dirs",
+ "mac_address",
+ "md5",
  "regex",
  "reqwest 0.12.28",
  "serde",
@@ -2525,6 +2527,16 @@ version = "0.1.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "c41e0c4fef86961ac6d6f8a82609f55f31b05e4fce149ac5710e439df7619ba4"
 
+[[package]]
+name = "mac_address"
+version = "1.1.8"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "c0aeb26bf5e836cc1c341c8106051b573f1766dfa05aa87f0b98be5e51b02303"
+dependencies = [
+ "nix 0.29.0",
+ "winapi",
+]
+
 [[package]]
 name = "markup5ever"
 version = "0.35.0"
@@ -2584,12 +2596,27 @@ dependencies = [
  "digest",
 ]
 
+[[package]]
+name = "md5"
+version = "0.7.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "490cc448043f947bae3cbee9c203358d62dbee0db12107a74be5c30ccfd09771"
+
 [[package]]
 name = "memchr"
 version = "2.8.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "f8ca58f447f06ed17d5fc4043ce1b10dd205e060fb3ce5b979b8ed8e59ff3f79"
 
+[[package]]
+name = "memoffset"
+version = "0.9.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "488016bfae457b036d996092f6cb448677611ce4449e970ceaf42695203f218a"
+dependencies = [
+ "autocfg",
+]
+
 [[package]]
 name = "mime"
 version = "0.3.17"
@@ -2681,6 +2708,19 @@ version = "1.0.6"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "650eef8c711430f1a879fdd01d4745a7deea475becfb90269c06775983bbf086"
 
+[[package]]
+name = "nix"
+version = "0.29.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "71e2746dc3a24dd78b3cfcb7be93368c6de9963d30f43a6a73998a9cf4b17b46"
+dependencies = [
+ "bitflags",
+ "cfg-if",
+ "cfg_aliases",
+ "libc",
+ "memoffset",
+]
+
 [[package]]
 name = "nix"
 version = "0.31.2"
@@ -3216,7 +3256,7 @@ checksum = "ccd9713fe2c91c3c85ac388b31b89de339365d2c995146e630b5e0da9d06526a"
 dependencies = [
  "futures",
  "indexmap 2.13.0",
- "nix",
+ "nix 0.31.2",
  "tokio",
  "tracing",
  "windows",

Cargo.toml

@@ -42,6 +42,8 @@ semver = "1"
 aho-corasick = "1"
 sqlx = { version = "0.8", features = ["runtime-tokio", "sqlite", "chrono"] }
 dirs = "6"
+mac_address = "1"
+md5 = "0.7"
 indicatif = "0.18"
 termimad = "0.34"
 toml = "0.8"

crates/arc-util/Cargo.toml

@@ -23,6 +23,8 @@ base64.workspace = true
 dirs.workspace = true
 tracing.workspace = true
 anyhow.workspace = true
+mac_address.workspace = true
+md5.workspace = true
 
 [build-dependencies]
 toml = "0.8"

crates/arc-util/src/telemetry/anonymous_id.rs

@@ -3,13 +3,14 @@ use std::fs;
 use std::path::PathBuf;
 use uuid::Uuid;
 
-fn anonymous_id_path() -> Result<PathBuf> {
+fn dot_id_path() -> Result<PathBuf> {
     let home = dirs::home_dir().context("could not determine home directory")?;
-    Ok(home.join(".arc").join("anonymous_id"))
+    Ok(home.join(".arc").join(".id"))
 }
 
-pub fn load_or_create_anonymous_id() -> Result<String> {
-    let path = anonymous_id_path()?;
+/// Server: UUID persisted at ~/.arc/.id
+pub fn load_or_create_server_id() -> Result<String> {
+    let path = dot_id_path()?;
 
     if let Ok(contents) = fs::read_to_string(&path) {
         let id = contents.trim().to_string();
@@ -31,17 +32,40 @@ pub fn load_or_create_anonymous_id() -> Result<String> {
     Ok(id)
 }
 
+/// CLI: prefer existing ~/.arc/.id, else MD5 of MAC address
+pub fn compute_cli_id() -> Result<String> {
+    let path = dot_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 mac = mac_address::get_mac_address()
+        .context("failed to get MAC address")?
+        .context("no MAC address found")?;
+
+    let digest = md5::compute(mac.bytes());
+    Ok(format!("{:x}", digest))
+}
+
 #[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();
+    fn compute_cli_id_returns_non_empty_string() {
+        let id = compute_cli_id().unwrap();
+        assert!(!id.is_empty());
+    }
+
+    #[test]
+    fn load_or_create_server_id_returns_stable_uuid() {
+        let id1 = load_or_create_server_id().unwrap();
+        let id2 = load_or_create_server_id().unwrap();
         assert_eq!(id1, id2);
-        // Should be a valid UUID
         Uuid::parse_str(&id1).unwrap();
     }
 }

crates/arc-util/src/telemetry/mod.rs

@@ -24,9 +24,21 @@ pub struct Telemetry {
 }
 
 impl Telemetry {
-    pub fn new() -> Result<Self> {
+    pub fn for_server() -> Result<Self> {
         let level = telemetry_level();
-        let anonymous_id = anonymous_id::load_or_create_anonymous_id()?;
+        let anonymous_id = anonymous_id::load_or_create_server_id()?;
+        let context = context::build_context();
+
+        Ok(Self {
+            level,
+            anonymous_id,
+            context,
+        })
+    }
+
+    pub fn for_cli() -> Result<Self> {
+        let level = telemetry_level();
+        let anonymous_id = anonymous_id::compute_cli_id()?;
         let context = context::build_context();
 
         Ok(Self {