diff --git a/src/config.rs b/src/config.rs
index ef9ce96..98733bc 100644
--- a/src/config.rs
+++ b/src/config.rs
@@ -21,7 +21,7 @@ pub fn build(args: Args, file_config: Option<FileConfig>) -> Result<SandboxConfi
     globals.validate_paths()?;
     profile.validate_paths()?;
 
-    let (command, command_args) = resolve_command(args.command_and_args, &profile, &globals);
+    let (command, command_args) = resolve_command(args.command_and_args, &profile, &globals)?;
     let command = resolve_binary(&command)
         .ok_or_else(|| SandboxError::CommandNotFound(PathBuf::from(&command)))?;
 
@@ -118,7 +118,7 @@ fn resolve_command(
     mut passthrough_args: Vec<OsString>,
     profile: &Options,
     globals: &Options,
-) -> (OsString, Vec<OsString>) {
+) -> Result<(OsString, Vec<OsString>), SandboxError> {
     let entrypoint = profile.entrypoint.clone().or(globals.entrypoint.clone());
     let command = profile.command.clone().or(globals.command.clone());
 
@@ -131,22 +131,16 @@ fn resolve_command(
             args.extend(default_cmd.into_vec().into_iter().map(OsString::from));
         }
 
-        return (cmd, args);
+        return Ok((cmd, args));
     }
 
     if !passthrough_args.is_empty() {
-        return (passthrough_args.remove(0), passthrough_args);
+        return Ok((passthrough_args.remove(0), passthrough_args));
     }
     if let Some(config_cmd) = command {
-        return config_cmd.into_binary_and_args();
+        return Ok(config_cmd.into_binary_and_args());
     }
-    if let Ok(cmd) = std::env::var("SANDBOX_CMD") {
-        return (OsString::from(cmd), vec![]);
-    }
-    (
-        OsString::from("claude"),
-        vec![OsString::from("--dangerously-skip-permissions")],
-    )
+    Err(SandboxError::NoCommand)
 }
 
 fn resolve_binary(name: &OsString) -> Option<PathBuf> {
@@ -712,7 +706,8 @@ mod tests {
                 ..Options::default()
             },
             &Options::default(),
-        );
+        )
+        .unwrap();
         assert_eq!(cmd, "claude");
         assert_eq!(
             args,
@@ -741,7 +736,8 @@ mod tests {
                 ..Options::default()
             },
             &Options::default(),
-        );
+        )
+        .unwrap();
         assert_eq!(cmd, "claude");
         assert_eq!(
             args,
@@ -764,7 +760,8 @@ mod tests {
                 ..Options::default()
             },
             &Options::default(),
-        );
+        )
+        .unwrap();
         assert_eq!(cmd, "claude");
         assert_eq!(args, vec![OsString::from("--dangerously-skip-permissions")]);
     }
@@ -781,7 +778,8 @@ mod tests {
                 entrypoint: Some(CommandValue::Simple("codex".into())),
                 ..Options::default()
             },
-        );
+        )
+        .unwrap();
         assert_eq!(cmd, "claude");
     }
 
@@ -800,7 +798,8 @@ mod tests {
                 entrypoint: Some(CommandValue::Simple("claude".into())),
                 ..Options::default()
             },
-        );
+        )
+        .unwrap();
         assert_eq!(cmd, "claude");
         assert_eq!(
             args,
@@ -810,6 +809,12 @@ mod tests {
                 .collect::<Vec<_>>()
         );
     }
+    #[test]
+    fn no_command_errors() {
+        let result = resolve_command(vec![], &Options::default(), &Options::default());
+        assert!(matches!(result, Err(SandboxError::NoCommand)));
+    }
+
     fn assert_paths(actual: &[PathBuf], expected: &[&str]) {
         let expected: Vec<PathBuf> = expected.iter().map(PathBuf::from).collect();
         assert_eq!(actual, &expected);
diff --git a/src/errors.rs b/src/errors.rs
index 19ed1bb..530255b 100644
--- a/src/errors.rs
+++ b/src/errors.rs
@@ -25,6 +25,7 @@ pub enum SandboxError {
     UnknownConfigKey(String),
     ConfigPathNotAbsolute(PathBuf),
     InvalidBwrapArg(String),
+    NoCommand,
 }
 
 impl std::fmt::Display for SandboxError {
@@ -69,6 +70,10 @@ impl std::fmt::Display for SandboxError {
             Self::InvalidBwrapArg(s) => {
                 write!(f, "invalid quoting in --bwrap-arg: {s}")
             }
+            Self::NoCommand => write!(
+                f,
+                "no command to run; specify a command via config, entrypoint, or pass one after --"
+            ),
         }
     }
 }