diff --git a/crates/ghost-common/src/atomic_file.rs b/crates/ghost-common/src/atomic_file.rs
new file mode 100644
index 000000000..5721df52f
--- /dev/null
+++ b/crates/ghost-common/src/atomic_file.rs
@@ -0,0 +1,227 @@
+//! One durable, atomic, race-safe file replacement.
+//!
+//! # Why this exists in one place
+//!
+//! Several places persist a small file by rewriting it whole, and each had
+//! grown its own copy of write-temp / fsync / rename. Every copy carried the
+//! same defect: the staging file was a fixed `.tmp` sibling of the target.
+//!
+//! A fixed staging path is shared by every concurrent writer of that file. Two
+//! writers racing on it truncate each other's contents, and once the winner has
+//! renamed it away the loser's `rename` fails with `ENOENT`. Staging through a
+//! path private to each write removes both races, and doing it once means there
+//! is no next copy to get wrong.
+//!
+//! Some copies were also missing durability rather than just racing: a
+//! `fs::write` followed by `rename`, with neither the file nor the directory
+//! fsynced, reports success for a write a power loss can still take away.
+//!
+//! # What this does not do
+//!
+//! It makes a single write atomic. It does **not** make read-modify-write
+//! atomic: a caller that loads a file, edits in memory and writes the whole
+//! thing back still needs to hold a lock across all three steps, or a
+//! concurrent writer's changes are lost.
+
+use std::fs;
+use std::io::Write;
+use std::path::Path;
+use std::sync::atomic::{AtomicU64, Ordering};
+
+/// Distinguishes the staging files of two writes racing on one target path.
+/// Paired with the pid so separate processes cannot collide either.
+static TMP_SEQ: AtomicU64 = AtomicU64::new(0);
+
+/// A staging path private to one write, beside `target`.
+///
+/// For writers that stream into the staging file rather than handing over a
+/// finished body — a multi-hundred-megabyte proving key, say, which must not be
+/// buffered in memory just to reuse [`write_atomic`]. Pair it with
+/// [`sync_parent_dir`] after the rename.
+///
+/// Never a fixed `.tmp` sibling: that path is shared by every concurrent
+/// writer of the same target, so two writers truncate each other and the
+/// loser's rename fails with `ENOENT`.
+pub fn staging_path(target: &Path) -> std::path::PathBuf {
+    let seq = TMP_SEQ.fetch_add(1, Ordering::Relaxed);
+    target.with_extension(format!("tmp.{}.{seq}", std::process::id()))
+}
+
+/// fsync the directory holding `path`, so a rename into it is persisted.
+///
+/// Renaming is a metadata change. Without this the file's own fsync survives a
+/// power loss and the rename does not, which loses the write while having
+/// reported success.
+///
+/// Best effort: a filesystem that refuses to open a directory has still given
+/// us a durable staging file and an atomic rename, which is the bulk of the
+/// guarantee.
+pub fn sync_parent_dir(path: &Path) {
+    if let Some(dir) = path.parent() {
+        if let Ok(d) = fs::File::open(dir) {
+            let _ = d.sync_all();
+        }
+    }
+}
+
+/// Replace `path` with `body`, atomically and durably.
+///
+/// Creates the parent directory if absent. On unix, `mode` sets the staging
+/// file's permissions before the rename, so the file is never briefly readable
+/// at a wider mode than intended — pass `Some(0o600)` for anything secret.
+///
+/// The sequence is write-temp, fsync, rename, fsync-dir: what survives power
+/// loss on the filesystems this runs on. Skipping the directory fsync leaves
+/// the rename itself unpersisted, which loses the write while reporting
+/// success.
+pub fn write_atomic(path: &Path, body: &[u8], mode: Option<u32>) -> std::io::Result<()> {
+    if let Some(parent) = path.parent() {
+        if !parent.as_os_str().is_empty() {
+            fs::create_dir_all(parent)?;
+        }
+    }
+
+    // Staging path private to this write. See the module docs for what a
+    // shared one costs.
+    let tmp = staging_path(path);
+
+    let staged = (|| -> std::io::Result<()> {
+        let mut f = fs::File::create(&tmp)?;
+        f.write_all(body)?;
+
+        #[cfg(unix)]
+        if let Some(mode) = mode {
+            use std::os::unix::fs::PermissionsExt;
+            f.set_permissions(fs::Permissions::from_mode(mode))?;
+        }
+        #[cfg(not(unix))]
+        let _ = mode;
+
+        // Contents before the rename, or the rename can land pointing at an
+        // empty file.
+        f.sync_all()?;
+        drop(f);
+        fs::rename(&tmp, path)
+    })();
+
+    if staged.is_err() {
+        // Don't leave a staging file behind for a write that failed.
+        let _ = fs::remove_file(&tmp);
+    }
+    staged?;
+
+    sync_parent_dir(path);
+    Ok(())
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    fn scratch(tag: &str) -> std::path::PathBuf {
+        let d = std::env::temp_dir().join(format!(
+            "ghost-atomic-{tag}-{}-{}",
+            std::process::id(),
+            std::time::SystemTime::now()
+                .duration_since(std::time::UNIX_EPOCH)
+                .unwrap()
+                .as_nanos()
+        ));
+        fs::create_dir_all(&d).expect("scratch dir");
+        d
+    }
+
+    #[test]
+    fn it_creates_missing_parent_directories() {
+        let dir = scratch("parents");
+        let path = dir.join("a").join("b").join("f.json");
+        write_atomic(&path, b"hello", None).expect("write");
+        assert_eq!(fs::read(&path).unwrap(), b"hello");
+        let _ = fs::remove_dir_all(&dir);
+    }
+
+    #[test]
+    fn it_replaces_existing_content_wholesale() {
+        let dir = scratch("replace");
+        let path = dir.join("f.json");
+        write_atomic(&path, b"first-and-longer", None).expect("write 1");
+        write_atomic(&path, b"second", None).expect("write 2");
+        assert_eq!(fs::read(&path).unwrap(), b"second");
+        let _ = fs::remove_dir_all(&dir);
+    }
+
+    /// The regression this module exists for.
+    ///
+    /// With a fixed `.tmp` staging path, concurrent writers truncate each
+    /// other's staging file and the loser's rename fails with `ENOENT`.
+    #[test]
+    fn concurrent_writers_do_not_destroy_each_others_staging_file() {
+        let dir = scratch("race");
+        let path = dir.join("contended.json");
+
+        let threads: Vec<_> = (0..16u8)
+            .map(|i| {
+                let path = path.clone();
+                std::thread::spawn(move || {
+                    let body = vec![b'a' + i; 64];
+                    write_atomic(&path, &body, Some(0o600))
+                })
+            })
+            .collect();
+        for (i, t) in threads.into_iter().enumerate() {
+            t.join()
+                .expect("writer thread")
+                .unwrap_or_else(|e| panic!("writer {i} failed: {e}"));
+        }
+
+        // Every write is all-or-nothing, so whoever landed last left exactly
+        // its own body — never a mixture, never an empty file.
+        let got = fs::read(&path).expect("target exists");
+        assert_eq!(got.len(), 64, "target is a whole body, not a partial write");
+        assert!(
+            got.iter().all(|b| *b == got[0]),
+            "target interleaves two writers' bodies"
+        );
+
+        let strays: Vec<_> = fs::read_dir(&dir)
+            .unwrap()
+            .filter_map(|e| e.ok())
+            .map(|e| e.file_name().to_string_lossy().into_owned())
+            .filter(|n| n.contains(".tmp."))
+            .collect();
+        assert!(strays.is_empty(), "staging files left behind: {strays:?}");
+
+        let _ = fs::remove_dir_all(&dir);
+    }
+
+    /// Two writes to one target must never stage through the same path.
+    ///
+    /// This is the whole defect, isolated: a fixed `.tmp` sibling is shared by
+    /// every concurrent writer, so one truncates the other's staging file and
+    /// the loser's rename finds nothing there. Streaming writers (a proving key
+    /// too large to hold in memory) use this directly rather than
+    /// `write_atomic`, so it needs its own guarantee.
+    #[test]
+    fn staging_paths_are_unique_per_write() {
+        let target = std::path::Path::new("/tmp/some-target.bin");
+        let a = staging_path(target);
+        let b = staging_path(target);
+        assert_ne!(a, b, "two writes staged through the same path");
+        assert_ne!(a, target.to_path_buf());
+        // Still beside the target, so the rename stays within one filesystem —
+        // a staging file elsewhere would make `rename` cross devices and fail.
+        assert_eq!(a.parent(), target.parent());
+    }
+
+    #[cfg(unix)]
+    #[test]
+    fn a_secret_file_is_never_wider_than_requested() {
+        use std::os::unix::fs::PermissionsExt;
+        let dir = scratch("mode");
+        let path = dir.join("secret.json");
+        write_atomic(&path, b"secret", Some(0o600)).expect("write");
+        let mode = fs::metadata(&path).unwrap().permissions().mode() & 0o777;
+        assert_eq!(mode, 0o600, "secret file landed at {mode:o}");
+        let _ = fs::remove_dir_all(&dir);
+    }
+}
diff --git a/crates/ghost-common/src/lib.rs b/crates/ghost-common/src/lib.rs
index c5c99e447..15a300f45 100644
--- a/crates/ghost-common/src/lib.rs
+++ b/crates/ghost-common/src/lib.rs
@@ -31,6 +31,7 @@
 
 #![deny(unreachable_pub)]
 
+pub mod atomic_file;
 pub mod batch_two_phase;
 pub mod circuit_breaker;
 pub mod clock;
diff --git a/crates/ghost-consensus/src/mesh.rs b/crates/ghost-consensus/src/mesh.rs
index 1db8d4ad9..7117921c7 100644
--- a/crates/ghost-consensus/src/mesh.rs
+++ b/crates/ghost-consensus/src/mesh.rs
@@ -711,28 +711,9 @@ impl PeerHighWaterMarks {
         path: &std::path::Path,
         snapshot: &HashMap<String, (u64, u64)>,
     ) -> std::io::Result<()> {
-        use std::io::Write;
-
         let encoded = serde_json::to_vec(snapshot)
             .map_err(|e| std::io::Error::new(std::io::ErrorKind::InvalidData, e))?;
-        if let Some(parent) = path.parent() {
-            std::fs::create_dir_all(parent)?;
-        }
-        let tmp = path.with_extension("tmp");
-        {
-            let mut f = std::fs::File::create(&tmp)?;
-            f.write_all(&encoded)?;
-            f.sync_all()?;
-        }
-        std::fs::rename(&tmp, path)?;
-        if let Some(parent) = path.parent() {
-            // Best effort: a filesystem that refuses to open a directory still gave us a durable
-            // temp file and an atomic rename, which is the bulk of the guarantee.
-            if let Ok(dir) = std::fs::File::open(parent) {
-                let _ = dir.sync_all();
-            }
-        }
-        Ok(())
+        ghost_common::atomic_file::write_atomic(path, &encoded, None)
     }
 
     /// Move the mark to `sequence` unconditionally, including downwards. Only for a verified
@@ -1946,14 +1927,14 @@ impl MeshNetwork {
         }
     }
 
-    /// Atomically persist the sequence ceiling (write-temp-then-rename).
+    /// Atomically and durably persist the sequence ceiling.
+    ///
+    /// This used to stage with a bare `fs::write` and fsync neither the file
+    /// nor its directory, so a power loss could take back a ceiling already
+    /// reported as persisted — and the ceiling exists precisely so a restart
+    /// cannot reuse a sequence number.
     fn write_sequence_ceiling(path: &std::path::Path, ceiling: u64) -> std::io::Result<()> {
-        if let Some(parent) = path.parent() {
-            std::fs::create_dir_all(parent)?;
-        }
-        let tmp = path.with_extension("tmp");
-        std::fs::write(&tmp, ceiling.to_string())?;
-        std::fs::rename(&tmp, path)
+        ghost_common::atomic_file::write_atomic(path, ceiling.to_string().as_bytes(), None)
     }
 
     /// Reserve a fresh lease block once the counter nears the persisted ceiling.
diff --git a/crates/ghost-mpc/src/params.rs b/crates/ghost-mpc/src/params.rs
index f23922f42..a11f4a1bc 100644
--- a/crates/ghost-mpc/src/params.rs
+++ b/crates/ghost-mpc/src/params.rs
@@ -196,7 +196,7 @@ impl ParameterFiles {
 /// S-5 SECURITY: Uses atomic write (temp file + rename) to prevent corruption
 /// if the process crashes mid-write. Also uses fsync for durability.
 pub fn save_parameters(path: &Path, params: &Parameters<Bls12>) -> MpcResult<()> {
-    let temp_path = path.with_extension("tmp");
+    let temp_path = ghost_common::atomic_file::staging_path(path);
 
     // Write to temp file first
     let result = (|| -> MpcResult<()> {
@@ -220,6 +220,10 @@ pub fn save_parameters(path: &Path, params: &Parameters<Bls12>) -> MpcResult<()>
 
     // Atomic rename to target path
     fs::rename(&temp_path, path)?;
+    // The rename is a metadata change of the directory, and needs its own
+    // sync: without it the file's fsync above survives a power loss and the
+    // rename does not, losing a write already reported as saved.
+    ghost_common::atomic_file::sync_parent_dir(path);
 
     let file_size = fs::metadata(path)?.len();
     info!(
@@ -263,7 +267,7 @@ pub fn read_parameters_from_bytes(bytes: &[u8]) -> MpcResult<Parameters<Bls12>>
 ///
 /// S-5 SECURITY: Uses atomic write (temp file + rename) to prevent corruption.
 pub fn save_verifying_key(path: &Path, vk: &VerifyingKey<Bls12>) -> MpcResult<()> {
-    let temp_path = path.with_extension("tmp");
+    let temp_path = ghost_common::atomic_file::staging_path(path);
 
     let result = (|| -> MpcResult<()> {
         let file = File::create(&temp_path)?;
@@ -283,6 +287,7 @@ pub fn save_verifying_key(path: &Path, vk: &VerifyingKey<Bls12>) -> MpcResult<()
     }
 
     fs::rename(&temp_path, path)?;
+    ghost_common::atomic_file::sync_parent_dir(path);
 
     info!(path = %path.display(), "Saved verifying key (atomic write)");
 
@@ -366,7 +371,7 @@ pub fn update_current_params(files: &ParameterFiles, version: u32) -> MpcResult<
 
 /// Atomically copy a file by writing to a temp file and renaming.
 fn atomic_copy(src: &Path, dst: &Path) -> MpcResult<()> {
-    let temp_path = dst.with_extension("tmp");
+    let temp_path = ghost_common::atomic_file::staging_path(dst);
 
     let result = (|| -> MpcResult<()> {
         let mut reader = BufReader::new(File::open(src)?);
@@ -385,6 +390,7 @@ fn atomic_copy(src: &Path, dst: &Path) -> MpcResult<()> {
     }
 
     fs::rename(&temp_path, dst)?;
+    ghost_common::atomic_file::sync_parent_dir(dst);
     Ok(())
 }