diff --git a/crates/mind-body/src/cli.rs b/crates/mind-body/src/cli.rs
index ba89647..edd59aa 100644
--- a/crates/mind-body/src/cli.rs
+++ b/crates/mind-body/src/cli.rs
@@ -7,8 +7,12 @@
 //! mind-control --instance <slug> brake hold        (also stops her running units)
 //! mind-control --instance <slug> brake release
 //! mind-control --instance <slug> dial set --heat H --base-cooldown-s S --base-run-usd U
+//! mind-control [--root <dir>] --instance <slug> check
 //! ```
 //!
+//! `check` exits 0 only when the brake is released and the dial in bounds; a
+//! systemd unit runs it as its `ExecCondition`. It writes nothing.
+//!
 //! `brake hold` writes the brake held and then, whatever the write's outcome,
 //! stops the instance's running units through the `stop_units` parameter. It
 //! fails if either step failed. No other command stops anything.
@@ -22,11 +26,11 @@ use chrono::{DateTime, Utc};
 use eureka_pipeline::{RunTurn, Slug};
 use rust_decimal::Decimal;
 
-use crate::control::{BurnRate, ControlWriter, FileSource, control_path, derive, load_state};
+use crate::control::{BurnRate, ControlWriter, Effective, FileSource, control_path, derive, load_state};
 use crate::launch::unit;
 
 const USAGE: &str =
-    "usage: mind-control [--root <dir>] --instance <slug> show | brake hold | brake release | dial set --heat H --base-cooldown-s S --base-run-usd U";
+    "usage: mind-control [--root <dir>] --instance <slug> show | check | brake hold | brake release | dial set --heat H --base-cooldown-s S --base-run-usd U";
 
 /// Who made the change: `SUDO_USER`, else `USER`, else `unknown`. An empty
 /// variable counts as unset.
@@ -59,6 +63,7 @@ pub fn run(
     let words: Vec<&str> = command.iter().map(String::as_str).collect();
     match words.as_slice() {
         ["show"] => show(root, &instance, out),
+        ["check"] => check(&path, &instance, out),
         ["brake", "hold"] => {
             let written = ControlWriter::open(&path).and_then(|mut writer| writer.set_brake(false, now, set_by));
             let stopped = stop_units(&instance);
@@ -109,6 +114,20 @@ fn need<'a>(value: Option<&'a str>, name: &str) -> Result<&'a str> {
     value.ok_or_else(|| anyhow!("{name} is required"))
 }
 
+/// The gate a unit runs before it starts: Ok only when the one derivation
+/// (`derive`, the reader behind `read_effective`) says Released. It is a pure
+/// read through `FileSource`: it opens no writer and creates no file or
+/// directory. Any other reading is an error, which `main` turns into exit 2,
+/// and systemd's `ExecCondition` skips the unit on 1-254.
+fn check(path: &Path, instance: &Slug, out: &mut impl Write) -> Result<()> {
+    let effective = derive(&FileSource::new(path), instance);
+    writeln!(out, "effective: {effective:?}")?;
+    match effective {
+        Effective::Released(_) => Ok(()),
+        Effective::Held { reason } => bail!("held: {reason:?}"),
+    }
+}
+
 fn show(root: &Path, instance: &Slug, out: &mut impl Write) -> Result<()> {
     let source = FileSource::new(control_path(root, instance)?);
     writeln!(out, "instance: {}", instance.0)?;
diff --git a/crates/mind-body/tests/cli.rs b/crates/mind-body/tests/cli.rs
index 9721e81..1be4171 100644
--- a/crates/mind-body/tests/cli.rs
+++ b/crates/mind-body/tests/cli.rs
@@ -128,6 +128,43 @@ fn the_binary_exits_nonzero_with_the_usage_when_refused() {
     assert!(output.stdout.is_empty());
 }
 
+#[test]
+fn the_binary_exits_2_for_check_while_held_and_0_after_release_with_a_dial() {
+    let root = tempfile::tempdir().unwrap();
+    let run = |args: &[&str]| {
+        std::process::Command::new(env!("CARGO_BIN_EXE_mind-control"))
+            .arg("--root")
+            .arg(root.path())
+            .args(["--instance", "eureka"])
+            .args(args)
+            .output()
+            .unwrap()
+    };
+    // Absent store: refused, and the gate wrote nothing.
+    let output = run(&["check"]);
+    assert_eq!(output.status.code(), Some(2));
+    assert!(String::from_utf8_lossy(&output.stderr).contains("held: BrakeAbsent"));
+    assert_eq!(std::fs::read_dir(root.path()).unwrap().count(), 0);
+
+    // Released with no dial is still refused; with a dial it opens.
+    assert!(run(&["brake", "release"]).status.success());
+    assert_eq!(run(&["check"]).status.code(), Some(2));
+    assert!(run(&["dial", "set", "--heat", "0.5", "--base-cooldown-s", "3600", "--base-run-usd", "5"]).status.success());
+    let open = run(&["check"]);
+    assert_eq!(open.status.code(), Some(0));
+    assert!(String::from_utf8_lossy(&open.stdout).contains("Released"));
+
+    // Holding closes it again. The brake is written through the library: the
+    // binary's `brake hold` would also ask systemd to stop units.
+    mind_body::control::ControlWriter::open(root.path().join("eureka").join("control.cc"))
+        .unwrap()
+        .set_brake(false, Utc.timestamp_opt(1_800_000_009, 0).unwrap(), "op")
+        .unwrap();
+    let closed = run(&["check"]);
+    assert_eq!(closed.status.code(), Some(2));
+    assert!(String::from_utf8_lossy(&closed.stderr).contains("held: BrakeHeld"));
+}
+
 #[test]
 fn a_leading_root_flag_replaces_the_default_root() {
     let default = tempfile::tempdir().unwrap();
@@ -152,6 +189,76 @@ fn a_root_flag_without_a_directory_or_an_instance_is_refused_and_writes_nothing(
     assert!(!std::path::Path::new("/nonexistent-root-for-test").exists());
 }
 
+fn check_out(root: &std::path::Path) -> (anyhow::Result<()>, String) {
+    go(root, &["--instance", "eureka", "check"], "op", 0)
+}
+
+#[test]
+fn check_refuses_every_reading_but_released() {
+    let root = tempfile::tempdir().unwrap();
+    let held = |root: &std::path::Path, reason: &str| {
+        let (result, out) = check_out(root);
+        let error = format!("{:#}", result.unwrap_err());
+        assert!(error.contains(reason), "{reason}: {error}");
+        assert!(out.contains(reason), "{reason}: {out}");
+    };
+    held(root.path(), "BrakeAbsent");
+    go(root.path(), &["--instance", "eureka", "brake", "hold"], "op", 1).0.unwrap();
+    held(root.path(), "BrakeHeld");
+    go(root.path(), &["--instance", "eureka", "brake", "release"], "op", 2).0.unwrap();
+    held(root.path(), "DialAbsent");
+    go(root.path(), &["--instance", "eureka", "dial", "set", "--heat", "1000", "--base-cooldown-s", "3600", "--base-run-usd", "5"], "op", 3)
+        .0
+        .unwrap();
+    held(root.path(), "DialOutOfBounds");
+    // An undecodable store is a held reading too.
+    let broken = tempfile::tempdir().unwrap();
+    std::fs::create_dir(broken.path().join("eureka")).unwrap();
+    std::fs::write(broken.path().join("eureka").join("control.cc"), b"not a store").unwrap();
+    held(broken.path(), "Undecodable");
+}
+
+#[test]
+fn check_opens_on_release_with_a_dial_and_is_per_instance() {
+    let root = tempfile::tempdir().unwrap();
+    go(root.path(), &["--instance", "eureka", "brake", "release"], "op", 0).0.unwrap();
+    go(root.path(), &["--instance", "eureka", "dial", "set", "--heat", "0.5", "--base-cooldown-s", "3600", "--base-run-usd", "5"], "op", 1)
+        .0
+        .unwrap();
+    let (result, out) = check_out(root.path());
+    result.unwrap();
+    assert!(out.contains("Released"), "{out}");
+    // Another instance's store is a different file: still absent, still refused.
+    let (other, _) = go(root.path(), &["--instance", "other", "check"], "op", 2);
+    assert!(format!("{:#}", other.unwrap_err()).contains("BrakeAbsent"));
+}
+
+#[test]
+fn check_writes_nothing_and_creates_no_directory() {
+    let root = tempfile::tempdir().unwrap();
+    assert!(check_out(root.path()).0.is_err());
+    assert_eq!(std::fs::read_dir(root.path()).unwrap().count(), 0);
+
+    go(root.path(), &["--instance", "eureka", "brake", "release"], "op", 0).0.unwrap();
+    let file = root.path().join("eureka").join("control.cc");
+    let before = (std::fs::read(&file).unwrap(), std::fs::metadata(&file).unwrap().modified().unwrap());
+    let _ = check_out(root.path());
+    let _ = check_out(root.path());
+    assert_eq!((std::fs::read(&file).unwrap(), std::fs::metadata(&file).unwrap().modified().unwrap()), before);
+    assert_eq!(std::fs::read_dir(root.path().join("eureka")).unwrap().count(), 1);
+}
+
+#[test]
+fn check_takes_no_extra_word_and_needs_an_instance() {
+    let root = tempfile::tempdir().unwrap();
+    go(root.path(), &["--instance", "eureka", "brake", "release"], "op", 0).0.unwrap();
+    for args in [&["--instance", "eureka", "check", "now"][..], &["--instance", "eureka", "check", "show"][..], &["check"][..]] {
+        let (result, out) = go(root.path(), args, "op", 1);
+        assert!(format!("{:#}", result.unwrap_err()).contains("usage: mind-control"), "{args:?}");
+        assert!(out.is_empty(), "{args:?}: {out}");
+    }
+}
+
 fn shown(root: &std::path::Path) -> String {
     go(root, &["--instance", "eureka", "show"], "op", 0).1
 }