diff --git a/tests/macro_bracketing.rs b/tests/macro_bracketing.rs
index 5f65010..b079930 100644
--- a/tests/macro_bracketing.rs
+++ b/tests/macro_bracketing.rs
@@ -15,6 +15,8 @@ use std::process::{Command, Output, Stdio};
 use std::thread;
 use std::time::{Duration, Instant, SystemTime, UNIX_EPOCH};
 
+const MARKERS: [&str; 4] = ["STARTED", "COMPLETED", "FAILED", "INTERRUPTED"];
+
 fn fresh_config_dir(label: &str) -> PathBuf {
     let unique = SystemTime::now()
         .duration_since(UNIX_EPOCH)
@@ -35,13 +37,27 @@ impl Drop for TempDirGuard {
     }
 }
 
-/// Builds the hook command that appends `marker` to `log` in the dialect of
-/// the shell the hook engine dispatches through: `sh -c` elsewhere, `cmd /C`
-/// on Windows. The cmd form parenthesizes the echo because `echo X >> f`
-/// under cmd writes "X " with a trailing space, which would break exact
-/// marker matching. Both forms double-quote the path, so the command also
-/// survives YAML single-quoting and spaces in temp paths.
-fn marker_command(marker: &str, log: &Path) -> String {
+/// Path of the file `marker`'s hook appends to. Every event gets its OWN
+/// file because cmd opens `>>` targets write-exclusive (no shared write
+/// access): the STARTED and terminal hook children run concurrently, and
+/// when both appended to one shared log, overlapping opens made the loser
+/// fail with a sharing violation and silently drop its line — a permanent
+/// marker loss that surfaced as a Windows-CI-only flake. Separate files
+/// make writer overlap impossible: each event fires at most once per
+/// bracket, and re-opened brackets are step-separated, so every file has
+/// one writer at a time.
+fn marker_log(dir: &Path, marker: &str) -> PathBuf {
+    dir.join(format!("agent-events-{marker}.log"))
+}
+
+/// Builds the hook command that appends `marker` to its own marker file, in
+/// the dialect of the shell the hook engine dispatches through: `sh -c`
+/// elsewhere, `cmd /C` on Windows. The cmd form parenthesizes the echo
+/// because `echo X >> f` under cmd writes "X " with a trailing space, which
+/// would break exact marker matching. Both forms double-quote the path, so
+/// the command also survives YAML single-quoting and spaces in temp paths.
+fn marker_command(marker: &str, dir: &Path) -> String {
+    let log = marker_log(dir, marker);
     let log = log.display();
     if cfg!(windows) {
         format!(r#"(echo {marker})>> "{log}""#)
@@ -51,9 +67,10 @@ fn marker_command(marker: &str, log: &Path) -> String {
 }
 
 /// Lays out a config dir with a dry_run model, global agent.* hooks that
-/// append one marker line per event to `log`, a `probe-macro` agent that
-/// whitelists everything, and a `probe` macro with the given YAML body.
-fn write_fixture(dir: &Path, log: &Path, macro_yaml: &str) {
+/// append one marker line per event to that event's marker file, a
+/// `probe-macro` agent that whitelists everything, and a `probe` macro with
+/// the given YAML body.
+fn write_fixture(dir: &Path, macro_yaml: &str) {
     // Without IS_SANDBOX (scrubbed below) a run insists on a vault password
     // file, so the fixture provides one.
     let vault_pass = dir.join("vault-pass");
@@ -92,10 +109,10 @@ fn write_fixture(dir: &Path, log: &Path, macro_yaml: &str) {
              \x20   - name: mark\n\
              \x20     command: '{interrupted}'\n",
             vault_pass = vault_pass.display(),
-            started = marker_command("STARTED", log),
-            completed = marker_command("COMPLETED", log),
-            failed = marker_command("FAILED", log),
-            interrupted = marker_command("INTERRUPTED", log),
+            started = marker_command("STARTED", dir),
+            completed = marker_command("COMPLETED", dir),
+            failed = marker_command("FAILED", dir),
+            interrupted = marker_command("INTERRUPTED", dir),
         ),
     )
     .unwrap();
@@ -130,53 +147,71 @@ fn run_coyote(dir: &Path, args: &[&str]) -> Output {
         .unwrap()
 }
 
+/// Counts `marker`'s exact-match lines in its own marker file; a missing
+/// file is zero fires. Matching stays byte-exact on both platforms because
+/// `lines()` strips the CRLF that cmd's echo emits on Windows.
+fn marker_count(dir: &Path, marker: &str) -> usize {
+    fs::read_to_string(marker_log(dir, marker))
+        .unwrap_or_default()
+        .lines()
+        .filter(|line| *line == marker)
+        .count()
+}
+
+fn dump_markers(dir: &Path) -> String {
+    MARKERS
+        .iter()
+        .map(|marker| {
+            let contents = fs::read_to_string(marker_log(dir, marker)).unwrap_or_default();
+            format!("{marker}: {contents:?}")
+        })
+        .collect::<Vec<_>>()
+        .join(", ")
+}
+
 /// Bounded poll until every expected marker is present. The binary's exit
 /// drain only acknowledges that each hook process SPAWNED — the hooks
 /// themselves keep running concurrently after `output()` returns, and
 /// nothing orders their writes (the STARTED writer can land after the
 /// terminal one). Waiting on all expected markers, not just the terminal
-/// one, keeps the exact-count assertions below race-free. Marker matching
-/// stays byte-exact on both platforms because `lines()` strips the CRLF
-/// that cmd's echo emits on Windows.
-fn wait_for_markers(log: &Path, needles: &[&str]) -> String {
+/// one, keeps the exact-count assertions below race-free.
+fn wait_for_markers(dir: &Path, needles: &[&str]) {
     let deadline = Instant::now() + Duration::from_secs(10);
     loop {
-        let contents = fs::read_to_string(log).unwrap_or_default();
-        if needles
-            .iter()
-            .all(|needle| contents.lines().any(|line| line == *needle))
-        {
-            return contents;
+        if needles.iter().all(|needle| marker_count(dir, needle) > 0) {
+            return;
         }
         assert!(
             Instant::now() < deadline,
-            "timed out waiting for {needles:?}; log so far: {contents:?}"
+            "timed out waiting for {needles:?}; markers so far: {}",
+            dump_markers(dir)
         );
         thread::sleep(Duration::from_millis(50));
     }
 }
 
-fn assert_marker_count(contents: &str, marker: &str, expected: usize) {
-    let count = contents.lines().filter(|line| *line == marker).count();
+fn assert_marker_count(dir: &Path, marker: &str, expected: usize) {
+    let count = marker_count(dir, marker);
     assert_eq!(
-        count, expected,
-        "expected {expected}x {marker}: {contents:?}"
+        count,
+        expected,
+        "expected {expected}x {marker}; markers so far: {}",
+        dump_markers(dir)
     );
 }
 
-fn assert_started_and_single_terminal(contents: &str, terminal: &str) {
-    assert_marker_count(contents, "STARTED", 1);
+fn assert_started_and_single_terminal(dir: &Path, terminal: &str) {
+    assert_marker_count(dir, "STARTED", 1);
     for marker in ["COMPLETED", "FAILED", "INTERRUPTED"] {
         let expected = usize::from(marker == terminal);
-        assert_marker_count(contents, marker, expected);
+        assert_marker_count(dir, marker, expected);
     }
 }
 
-fn probe_macro_run(label: &str, macro_yaml: &str, args: &[&str], expect_success: bool) -> String {
+fn probe_macro_run(label: &str, macro_yaml: &str, args: &[&str], expect_success: bool) {
     let dir = fresh_config_dir(label);
     let _cleanup = TempDirGuard(dir.clone());
-    let log = dir.join("agent-events.log");
-    write_fixture(&dir, &log, macro_yaml);
+    write_fixture(&dir, macro_yaml);
 
     let output = run_coyote(&dir, args);
     assert_eq!(
@@ -193,9 +228,8 @@ fn probe_macro_run(label: &str, macro_yaml: &str, args: &[&str], expect_success:
     } else {
         "FAILED"
     };
-    let contents = wait_for_markers(&log, &["STARTED", terminal]);
-    assert_started_and_single_terminal(&contents, terminal);
-    contents
+    wait_for_markers(&dir, &["STARTED", terminal]);
+    assert_started_and_single_terminal(&dir, terminal);
 }
 
 #[test]