diff --git a/apps/effectcraft/src/crash_report.rs b/apps/effectcraft/src/crash_report.rs
index 837161811..32306aa5c 100644
--- a/apps/effectcraft/src/crash_report.rs
+++ b/apps/effectcraft/src/crash_report.rs
@@ -182,7 +182,7 @@ fn clean_exit(input: &mut impl Read) -> io::Result<bool> {
 }
 
 enum SessionLog {
-    Captured(File),
+    Captured { file: File, marker: String },
     Unavailable(String),
 }
 
@@ -201,7 +201,7 @@ impl SessionLog {
             Ok(file)
         })();
         match opened {
-            Ok(file) => Self::Captured(file),
+            Ok(file) => Self::Captured { file, marker: format!("{}\n", session_marker(id)) },
             Err(e) => Self::Unavailable(e.to_string()),
         }
     }
@@ -209,7 +209,15 @@ impl SessionLog {
     fn write_tail(&mut self, report: &mut File) -> io::Result<()> {
         match self {
             Self::Unavailable(reason) => writeln!(report, "Recent log unavailable: {reason}"),
-            Self::Captured(file) => {
+            Self::Captured { file, marker } => {
+                // An older concurrent launcher may have truncated the captured file.
+                // Recheck its identity before copying any content into this report.
+                file.seek(SeekFrom::Start(0))?;
+                let mut header = [0u8; HEADER_BYTES];
+                let prefix = header.get_mut(..marker.len()).ok_or_else(|| io::Error::other("session marker too long"))?;
+                if file.read_exact(prefix).is_err() || prefix != marker.as_bytes() {
+                    return writeln!(report, "Recent log unavailable: session identity changed");
+                }
                 let size = file.metadata()?.len();
                 file.seek(SeekFrom::Start(size.saturating_sub(LOG_BYTES as u64)))?;
                 writeln!(report, "\nRecorded session log tail (last {LOG_BYTES} bytes at most; logging filters/size caps may omit later events):")?;
@@ -350,6 +358,21 @@ mod tests {
         std::fs::remove_dir_all(dir).unwrap();
     }
 
+    #[test]
+    fn captured_log_rejects_a_session_overwritten_after_open() {
+        let dir = folder();
+        let path = dir.join("log");
+        std::fs::write(&path, "crash-report session 1-2\noriginal session\n").unwrap();
+        let mut log = SessionLog::open(Some(&path), "1-2");
+        std::fs::write(&path, "crash-report session 3-4\nprivate other session\n").unwrap();
+        let report = write_report(&dir, "1-2", &mut log, None).unwrap();
+        let text = std::fs::read_to_string(report).unwrap();
+        assert!(text.contains("Recent log unavailable"));
+        assert!(!text.contains("private other session"));
+        assert!(!text.contains("session 3-4"));
+        std::fs::remove_dir_all(dir).unwrap();
+    }
+
     #[test]
     fn session_identity_requires_the_exact_first_line() {
         let dir = folder();
diff --git a/crates/engine/src/system/logging.rs b/crates/engine/src/system/logging.rs
index 198aa3b46..2bb3518a1 100644
--- a/crates/engine/src/system/logging.rs
+++ b/crates/engine/src/system/logging.rs
@@ -214,10 +214,16 @@ fn rotate(dir: &Path) -> std::io::Result<(PathBuf, File)> {
         }
     }
     let path = numbered(dir, 0);
-    let file = File::create(&path)?;
+    let file = create_session_file(&path)?;
     Ok((path, file))
 }
 
+// Another process may create the current log after our rotation. Never truncate its
+// file: its writer and crash reporter may already hold the same inode open.
+fn create_session_file(path: &Path) -> std::io::Result<File> {
+    std::fs::OpenOptions::new().write(true).create_new(true).open(path)
+}
+
 /// Rotate the session logs in `dir` (the `Logs` folder, see [`log_dir`]) and write records to
 /// a fresh `effectcraft.log` there, first the ones logged so far; returns its path.
 pub fn attach_session_log(dir: &Path) -> Result<PathBuf, String> {
@@ -428,6 +434,20 @@ mod tests {
         assert_eq!(timestamp(u64::MAX), "584556019-04-03T14:25:51.615Z");
     }
 
+    #[test]
+    fn concurrent_session_creation_does_not_truncate_the_winner() {
+        let dir = temp_dir("concurrent-create");
+        std::fs::create_dir_all(&dir).unwrap();
+        let path = dir.join(SESSION_LOG);
+        let mut winner = create_session_file(&path).unwrap();
+        winner.write_all(b"crash-report session 1-2\nwinner content\n").unwrap();
+        assert_eq!(create_session_file(&path).unwrap_err().kind(), std::io::ErrorKind::AlreadyExists);
+        winner.write_all(b"still the same session\n").unwrap();
+        assert_eq!(read(&path), "crash-report session 1-2\nwinner content\nstill the same session\n");
+        drop(winner);
+        std::fs::remove_dir_all(dir).unwrap();
+    }
+
     /// Each launch keeps the previous runs' logs, so the log of a run that crashed survives.
     #[test]
     fn rotation_keeps_the_previous_logs_and_starts_an_empty_file() {