diff --git a/core/Cargo.toml b/core/Cargo.toml
index 8111cd8..a103440 100644
--- a/core/Cargo.toml
+++ b/core/Cargo.toml
@@ -89,6 +89,7 @@ manual_let_else = "deny"
 items_after_statements = "deny"
 needless_pass_by_value = "deny"
 trivially_copy_pass_by_ref = "deny"
+verbose_file_reads = "deny"
 
 [dependencies]
 cel-interpreter = "0.10"
diff --git a/core/src/gate.rs b/core/src/gate.rs
index 3d79de5..30c13cf 100644
--- a/core/src/gate.rs
+++ b/core/src/gate.rs
@@ -250,3 +250,8 @@ fn build_block_message(
 
     msg
 }
+
+#[cfg(test)]
+#[allow(clippy::indexing_slicing, clippy::panic, clippy::unwrap_used)]
+#[path = "gate_tests.rs"]
+mod tests;
diff --git a/core/src/gate_tests.rs b/core/src/gate_tests.rs
new file mode 100644
index 0000000..b5f1bee
--- /dev/null
+++ b/core/src/gate_tests.rs
@@ -0,0 +1,238 @@
+//! Unit tests for `gate`.
+use super::*;
+use crate::state::save_state;
+use std::collections::HashMap;
+use std::path::PathBuf;
+use tempfile::TempDir;
+
+const LINEAR_FLOW: &str = r"stateDiagram-v2
+    [*] --> clean_code
+    clean_code --> tests_pass
+    tests_pass --> [*]
+    clean_code: Did you write clean code?
+    tests_pass: Do the tests pass?
+";
+
+const BRANCHING_FLOW: &str = r"stateDiagram-v2
+    [*] --> review
+    review --> ship
+    review --> abort
+    ship --> [*]
+    abort --> [*]
+    review: Reviewed the diff?
+    ship: Ship it
+    abort: Abandon the change
+";
+
+fn make_event(session: &str) -> HookEvent {
+    let mut input = HashMap::new();
+    input.insert(
+        "command".to_owned(),
+        serde_json::Value::String("git push".to_owned()),
+    );
+    HookEvent {
+        event: "tool:before".to_owned(),
+        tool: "bash".to_owned(),
+        input,
+        output: HashMap::new(),
+        session_id: session.to_owned(),
+        caller: "claude-code".to_owned(),
+    }
+}
+
+/// Writes `.steplock/checklists/<name>/` and returns `(steplock_dir, checklist_dir)`.
+fn setup(root: &Path, name: &str, reset: &str, flow: &str) -> (PathBuf, PathBuf) {
+    let steplock_dir = root.join(".steplock");
+    let checklist_dir = steplock_dir.join("checklists").join(name);
+    fs::create_dir_all(&checklist_dir).unwrap();
+    fs::write(
+        checklist_dir.join("config.toml"),
+        format!(
+            r#"on_event = "tool:before"
+on_tool = "bash"
+match_input = "input.command.contains('git push')"
+reset = "{reset}"
+"#
+        ),
+    )
+    .unwrap();
+    fs::write(checklist_dir.join("flow.mmd"), flow).unwrap();
+    (steplock_dir, checklist_dir)
+}
+
+fn state_path(steplock_dir: &Path, scope: &str, checklist: &str) -> PathBuf {
+    steplock_dir
+        .join("sessions")
+        .join(scope)
+        .join(checklist)
+        .join("state.json")
+}
+
+fn block_message(resp: Option<HookResponse>) -> String {
+    match resp {
+        Some(HookResponse::Block { message }) => message,
+        other => panic!("expected a block, got {other:?}"),
+    }
+}
+
+/// `audit::append` swallows every error, so the arguments the gate passes are
+/// only observable through the log itself.
+fn audit_lines(steplock_dir: &Path) -> Vec<serde_json::Value> {
+    fs::read_to_string(steplock_dir.join("audit.log"))
+        .unwrap()
+        .lines()
+        .map(|l| serde_json::from_str(l).unwrap())
+        .collect()
+}
+
+#[test]
+fn empty_session_id_creates_and_reuses_a_fallback_scope_key() {
+    let tmp = TempDir::new().unwrap();
+    let (steplock_dir, checklist_dir) = setup(tmp.path(), "gate", "session", LINEAR_FLOW);
+    let event = make_event("");
+
+    evaluate_checklist(&event, &steplock_dir, &checklist_dir).unwrap();
+    let fallback_path = steplock_dir.join("sessions").join("fallback-id");
+    let scope = fs::read_to_string(&fallback_path)
+        .unwrap()
+        .trim()
+        .to_owned();
+    assert!(!scope.is_empty(), "fallback id should be persisted");
+    assert!(state_path(&steplock_dir, &scope, "gate").exists());
+
+    // A second event with no session id must land in the same scope, so
+    // checklist progress survives callers that do not supply a session id.
+    evaluate_checklist(&event, &steplock_dir, &checklist_dir).unwrap();
+    let scope_again = fs::read_to_string(&fallback_path)
+        .unwrap()
+        .trim()
+        .to_owned();
+    assert_eq!(scope, scope_again);
+}
+
+#[test]
+fn single_transition_state_precomputes_next_state() {
+    let tmp = TempDir::new().unwrap();
+    let (steplock_dir, checklist_dir) = setup(tmp.path(), "gate", "session", LINEAR_FLOW);
+
+    evaluate_checklist(&make_event("sess-1"), &steplock_dir, &checklist_dir).unwrap();
+
+    let state = load_state(&state_path(&steplock_dir, "sess-1", "gate")).unwrap();
+    assert_eq!(state.current_state, "clean_code");
+    assert_eq!(state.next_state.as_deref(), Some("tests_pass"));
+    assert_eq!(state.transitions, vec!["tests_pass".to_owned()]);
+    // ack.sh needs the whole graph to advance beyond the current state.
+    assert!(state.flow_transitions.contains_key("tests_pass"));
+
+    let lines = audit_lines(&steplock_dir);
+    assert_eq!(lines.len(), 1);
+    assert_eq!(lines[0]["event"], "block");
+    assert_eq!(lines[0]["checklist"], "gate");
+    assert_eq!(lines[0]["state"], "clean_code");
+    assert_eq!(lines[0]["session"], "sess-1");
+}
+
+#[test]
+fn branching_state_leaves_next_state_unset_and_lists_every_choice() {
+    let tmp = TempDir::new().unwrap();
+    let (steplock_dir, checklist_dir) = setup(tmp.path(), "gate", "session", BRANCHING_FLOW);
+
+    let resp = evaluate_checklist(&make_event("sess-1"), &steplock_dir, &checklist_dir).unwrap();
+    let message = block_message(resp);
+
+    let state = load_state(&state_path(&steplock_dir, "sess-1", "gate")).unwrap();
+    assert_eq!(
+        state.next_state, None,
+        "an ambiguous branch must not auto-advance"
+    );
+    assert_eq!(
+        state.transitions,
+        vec!["ship".to_owned(), "abort".to_owned()]
+    );
+
+    assert!(message.contains("run one of"), "message was: {message}");
+    assert!(message.contains("ack.sh ship"), "message was: {message}");
+    assert!(message.contains("ack.sh abort"), "message was: {message}");
+}
+
+#[test]
+fn state_missing_from_the_flow_is_skipped() {
+    let tmp = TempDir::new().unwrap();
+    let (steplock_dir, checklist_dir) = setup(tmp.path(), "gate", "session", LINEAR_FLOW);
+    let path = state_path(&steplock_dir, "sess-1", "gate");
+    fs::create_dir_all(path.parent().unwrap()).unwrap();
+    // Simulates flow.mmd being edited mid-session: the stored state is gone.
+    save_state(&path, &init_state("gate", "state_that_was_removed")).unwrap();
+
+    let resp = evaluate_checklist(&make_event("sess-1"), &steplock_dir, &checklist_dir).unwrap();
+    assert!(resp.is_none(), "an unknown state must not block forever");
+}
+
+#[test]
+fn step_counter_counts_visited_states() {
+    let tmp = TempDir::new().unwrap();
+    let (steplock_dir, checklist_dir) = setup(tmp.path(), "gate", "session", LINEAR_FLOW);
+    let path = state_path(&steplock_dir, "sess-1", "gate");
+    fs::create_dir_all(path.parent().unwrap()).unwrap();
+    let mut state = init_state("gate", "tests_pass");
+    state.visited = vec!["clean_code".to_owned()];
+    save_state(&path, &state).unwrap();
+
+    let message = block_message(
+        evaluate_checklist(&make_event("sess-1"), &steplock_dir, &checklist_dir).unwrap(),
+    );
+    assert!(
+        message.starts_with("[gate: 2/2] Do the tests pass?"),
+        "message was: {message}"
+    );
+}
+
+#[test]
+fn reset_always_blocks_without_writing_session_state() {
+    let tmp = TempDir::new().unwrap();
+    let (steplock_dir, checklist_dir) = setup(tmp.path(), "gate", "always", LINEAR_FLOW);
+
+    let message = block_message(
+        evaluate_checklist(&make_event("sess-1"), &steplock_dir, &checklist_dir).unwrap(),
+    );
+    assert!(message.contains("When done, retry your original command."));
+    assert!(!message.contains("ack.sh"), "message was: {message}");
+    assert!(
+        !steplock_dir.join("sessions").exists(),
+        "reset=always keeps no session state"
+    );
+
+    // reset=always has no session scope, so the gate logs the literal "always".
+    let lines = audit_lines(&steplock_dir);
+    assert_eq!(lines.len(), 1);
+    assert_eq!(lines[0]["event"], "block");
+    assert_eq!(lines[0]["state"], "clean_code");
+    assert_eq!(lines[0]["session"], "always");
+}
+
+#[test]
+fn completed_checklist_approves_once_and_restarts() {
+    let tmp = TempDir::new().unwrap();
+    let (steplock_dir, checklist_dir) = setup(tmp.path(), "gate", "session", LINEAR_FLOW);
+    let path = state_path(&steplock_dir, "sess-1", "gate");
+    fs::create_dir_all(path.parent().unwrap()).unwrap();
+    let mut state = init_state("gate", "[*]");
+    state.visited = vec!["clean_code".to_owned(), "tests_pass".to_owned()];
+    save_state(&path, &state).unwrap();
+
+    let resp = evaluate_checklist(&make_event("sess-1"), &steplock_dir, &checklist_dir).unwrap();
+    assert!(resp.is_none(), "a complete checklist approves the attempt");
+
+    let reset = load_state(&path).unwrap();
+    assert_eq!(reset.current_state, "clean_code");
+    assert!(reset.visited.is_empty());
+
+    let lines = audit_lines(&steplock_dir);
+    assert_eq!(lines[0]["event"], "complete");
+    assert_eq!(lines[0]["state"], "[*]");
+    assert_eq!(lines[0]["session"], "sess-1");
+
+    // The next attempt blocks again from the top.
+    let next = evaluate_checklist(&make_event("sess-1"), &steplock_dir, &checklist_dir).unwrap();
+    assert!(matches!(next, Some(HookResponse::Block { .. })));
+}