diff --git a/crates/bxctl/src/main.rs b/crates/bxctl/src/main.rs
index 53f87ac..e5ed98b 100644
--- a/crates/bxctl/src/main.rs
+++ b/crates/bxctl/src/main.rs
@@ -11,7 +11,19 @@ use bxctl::escape::escape_model_text;
 use proto::{ErrorCode, SessionId, Timestamp, TurnDone};
 
 fn main() -> ExitCode {
-    let args: Vec<String> = std::env::args().skip(1).collect();
+    // `args_os`, because `args` panics on an argument that is not UTF-8. bxctl's arguments are
+    // text (ids, messages, paths it prints back), so such an argument is a usage error.
+    let args: Vec<String> = match std::env::args_os()
+        .skip(1)
+        .map(|a| a.into_string())
+        .collect()
+    {
+        Ok(args) => args,
+        Err(_) => {
+            eprintln!("bxctl: an argument is not valid UTF-8\n{USAGE}");
+            return ExitCode::from(2);
+        }
+    };
     // $BOXMAKER_HOME defaults to /var/lib/boxmaker; defaults for the sockets are read from it.
     let home = std::env::var_os("BOXMAKER_HOME")
         .map(PathBuf::from)
diff --git a/crates/bxctl/tests/args_os.rs b/crates/bxctl/tests/args_os.rs
new file mode 100644
index 0000000..cb5e09b
--- /dev/null
+++ b/crates/bxctl/tests/args_os.rs
@@ -0,0 +1,19 @@
+//! An argument that is not UTF-8 is a usage error, not a panic (M3a review finding 7).
+
+use std::ffi::OsStr;
+use std::os::unix::ffi::OsStrExt;
+use std::process::Command;
+
+#[test]
+fn an_argument_that_is_not_utf8_is_a_usage_error() {
+    let out = Command::new(env!("CARGO_BIN_EXE_bxctl"))
+        .args([OsStr::new("approve"), OsStr::from_bytes(b"4\xff")])
+        .output()
+        .unwrap();
+    assert_eq!(out.status.code(), Some(2));
+    let err = String::from_utf8_lossy(&out.stderr);
+    assert!(
+        err.starts_with("bxctl: an argument is not valid UTF-8"),
+        "{err}"
+    );
+}
diff --git a/crates/inferproxy/src/main.rs b/crates/inferproxy/src/main.rs
index b591430..2b44951 100644
--- a/crates/inferproxy/src/main.rs
+++ b/crates/inferproxy/src/main.rs
@@ -7,7 +7,11 @@ use std::process;
 use inferproxy::{Limits, serve};
 
 fn main() {
-    let args: Vec<String> = env::args().collect();
+    // `args_os`, because `args` panics on an argument that is not UTF-8.
+    let args: Vec<String> = match env::args_os().map(|a| a.into_string()).collect() {
+        Ok(args) => args,
+        Err(_) => usage(),
+    };
     let mut listen: Option<String> = None;
     let mut upstream: Option<String> = None;
     let mut i = 1;
diff --git a/crates/inferproxy/tests/args_os.rs b/crates/inferproxy/tests/args_os.rs
new file mode 100644
index 0000000..38bf3c3
--- /dev/null
+++ b/crates/inferproxy/tests/args_os.rs
@@ -0,0 +1,14 @@
+//! An argument that is not UTF-8 is a usage error, not a panic (M3a review finding 7).
+
+use std::ffi::OsStr;
+use std::os::unix::ffi::OsStrExt;
+use std::process::Command;
+
+#[test]
+fn an_argument_that_is_not_utf8_is_a_usage_error() {
+    let out = Command::new(env!("CARGO_BIN_EXE_inferproxy"))
+        .args([OsStr::new("--listen"), OsStr::from_bytes(b"/tmp/\xff.sock")])
+        .output()
+        .unwrap();
+    assert_eq!(out.status.code(), Some(2));
+}
diff --git a/crates/loopd/src/main.rs b/crates/loopd/src/main.rs
index bfb367a..f8e5c14 100644
--- a/crates/loopd/src/main.rs
+++ b/crates/loopd/src/main.rs
@@ -16,11 +16,12 @@ use loopd::selftest::{SelfTestError, run};
 use loopd::tools::{Registry, ToolPort};
 
 fn main() -> ExitCode {
-    let args: Vec<String> = std::env::args().skip(1).collect();
-    let args: Vec<&str> = args.iter().map(String::as_str).collect();
-    match args.as_slice() {
-        ["selftest", "--config", path] => run_selftest(path),
-        ["serve", "--config", path] => run_serve(path),
+    // `args_os`: the config path need not be UTF-8, and `args` would panic on one that is not.
+    let args: Vec<std::ffi::OsString> = std::env::args_os().skip(1).collect();
+    let words: Vec<Option<&str>> = args.iter().map(|a| a.to_str()).collect();
+    match (words.as_slice(), args.get(2)) {
+        ([Some("selftest"), Some("--config"), _], Some(path)) => run_selftest(Path::new(path)),
+        ([Some("serve"), Some("--config"), _], Some(path)) => run_serve(Path::new(path)),
         _ => {
             eprintln!("usage: loopd selftest --config <path>");
             eprintln!("usage: loopd serve --config <path>");
@@ -46,8 +47,8 @@ fn run_selftest_check(client: &Client) -> Result<(), SelfTestError> {
     }
 }
 
-fn run_selftest(path: &str) -> ExitCode {
-    let cfg = match Config::load(Path::new(path)) {
+fn run_selftest(path: &Path) -> ExitCode {
+    let cfg = match Config::load(path) {
         Ok(cfg) => cfg,
         Err(e) => {
             eprintln!("loopd: {e}");
@@ -62,8 +63,8 @@ fn run_selftest(path: &str) -> ExitCode {
     }
 }
 
-fn run_serve(path: &str) -> ExitCode {
-    let cfg = match Config::load(Path::new(path)) {
+fn run_serve(path: &Path) -> ExitCode {
+    let cfg = match Config::load(path) {
         Ok(cfg) => cfg,
         Err(e) => {
             eprintln!("loopd: {e}");
diff --git a/crates/loopd/tests/args_os.rs b/crates/loopd/tests/args_os.rs
new file mode 100644
index 0000000..974093c
--- /dev/null
+++ b/crates/loopd/tests/args_os.rs
@@ -0,0 +1,36 @@
+//! A config path that is not UTF-8 is read as a path, not a panic (M3a review finding 7).
+
+use std::ffi::OsStr;
+use std::os::unix::ffi::OsStrExt;
+use std::process::Command;
+
+#[test]
+fn a_config_path_that_is_not_utf8_is_read_as_a_path() {
+    let path = std::env::temp_dir().join(OsStr::from_bytes(b"loopd-missing-\xff.toml"));
+    let out = Command::new(env!("CARGO_BIN_EXE_loopd"))
+        .args([
+            OsStr::new("selftest"),
+            OsStr::new("--config"),
+            path.as_os_str(),
+        ])
+        .output()
+        .unwrap();
+    assert_eq!(
+        out.status.code(),
+        Some(1),
+        "a config error, not a panic (101)"
+    );
+}
+
+#[test]
+fn a_flag_that_is_not_utf8_is_a_usage_error() {
+    let out = Command::new(env!("CARGO_BIN_EXE_loopd"))
+        .args([
+            OsStr::from_bytes(b"serve\xff"),
+            OsStr::new("--config"),
+            OsStr::new("x"),
+        ])
+        .output()
+        .unwrap();
+    assert_eq!(out.status.code(), Some(2));
+}