diff --git a/crates/aura-registry/src/lib.rs b/crates/aura-registry/src/lib.rs
index 520ddf5..03024ce 100644
--- a/crates/aura-registry/src/lib.rs
+++ b/crates/aura-registry/src/lib.rs
@@ -42,6 +42,17 @@ pub use trace_store::{FamilyMember, NameKind, RunTraces, TraceStore, TraceStoreE
 /// `RunReport`.
 pub struct Registry {
     path: PathBuf,
+    /// Serializes the family/campaign-run append write path (#276): one
+    /// `&Registry` shared across threads must never interleave two appends'
+    /// content and newline writes, which unsynchronized `OpenOptions::append`
+    /// plus `writeln!` permits at the OS level. Held across each append's
+    /// read-the-counter-then-write critical section (`append_family`,
+    /// `append_campaign_run` in `lineage.rs`), so concurrent writers through
+    /// one handle also see a consistent per-name run counter. Store-agnostic:
+    /// it guards BOTH `families.jsonl` and the separate `campaign_runs.jsonl`
+    /// store (they never contend on the same file, but share this one lock).
+    /// In-process only; cross-process locking stays out of contract.
+    append_write_lock: std::sync::Mutex<()>,
 }
 
 impl Registry {
@@ -58,7 +69,7 @@ impl Registry {
     /// isolate runs (e.g. per-process or per-test), bind a distinct *directory*,
     /// not merely a distinct runs filename.
     pub fn open(path: impl AsRef<Path>) -> Registry {
-        Registry { path: path.as_ref().to_path_buf() }
+        Registry { path: path.as_ref().to_path_buf(), append_write_lock: std::sync::Mutex::new(()) }
     }
 
     /// Append one record as a single JSON line, creating the file (and its parent
@@ -1995,6 +2006,53 @@ mod tests {
         );
     }
 
+    /// Sibling of `concurrent_appends_keep_the_family_store_well_formed`
+    /// (#276): `append_campaign_run` shares `append_family`'s unsynchronized
+    /// `OpenOptions::append` + `writeln!` pattern at the same seam, so N
+    /// threads appending campaign runs through one shared `&Registry` must
+    /// leave `campaign_runs.jsonl` well-formed — every successful append's
+    /// record intact and parseable, none torn or lost.
+    #[test]
+    fn concurrent_campaign_run_appends_keep_the_store_well_formed() {
+        let path = temp_family_dir("concurrent_campaign_run");
+        let reg = Registry::open(&path);
+        let n_threads = 8usize;
+        let iters = 50usize;
+
+        let total_ok: usize = std::thread::scope(|scope| {
+            let handles: Vec<_> = (0..n_threads)
+                .map(|tid| {
+                    let reg = &reg;
+                    scope.spawn(move || {
+                        let mut ok = 0usize;
+                        for iter in 0..iters {
+                            let campaign = format!("t{tid}-c{iter}");
+                            if reg.append_campaign_run(&campaign_run_record(&campaign)).is_ok() {
+                                ok += 1;
+                            }
+                        }
+                        ok
+                    })
+                })
+                .collect();
+            handles.into_iter().map(|h| h.join().expect("worker thread joined")).sum()
+        });
+
+        let loaded = reg.load_campaign_runs();
+        assert!(
+            loaded.is_ok(),
+            "concurrent campaign-run appends must leave a parseable store; the writes corrupted it: {:?}",
+            loaded.err(),
+        );
+        assert_eq!(
+            loaded.expect("store parses").len(),
+            total_ok,
+            "every successful campaign-run append must survive as its own intact line",
+        );
+
+        let _ = fs::remove_dir_all(path.parent().expect("temp family dir"));
+    }
+
     #[test]
     fn load_campaign_runs_missing_file_is_empty() {
         let path = temp_family_dir("campaign_runs_missing");
diff --git a/crates/aura-registry/src/lineage.rs b/crates/aura-registry/src/lineage.rs
index ec97c24..6115a77 100644
--- a/crates/aura-registry/src/lineage.rs
+++ b/crates/aura-registry/src/lineage.rs
@@ -258,14 +258,17 @@ impl Registry {
     /// members carrying `name` + that `run`, and append them as N JSONL lines to
     /// the family store (a sibling of the flat runs store, `families.jsonl`,
     /// leaving the flat path untouched). Returns the derived id `"{name}-{run}"`.
-    /// Reads the store once to pick the run index (a read-before-write;
-    /// single-process CLI invocations do not race).
+    /// Reads the store once to pick the run index, then writes, the whole
+    /// read-then-write critical section serialized by `Registry`'s internal
+    /// `append_write_lock` (#276) — safe for concurrent writers sharing one
+    /// `&Registry`.
     pub fn append_family(
         &self,
         name: &str,
         kind: FamilyKind,
         reports: &[RunReport],
     ) -> Result<String, RegistryError> {
+        let _guard = self.append_write_lock.lock().unwrap_or_else(std::sync::PoisonError::into_inner);
         let run = next_run(name, &self.load_family_members()?);
 
         let path = self.path.with_file_name("families.jsonl");
@@ -317,12 +320,15 @@ impl Registry {
     /// run. The input record's own `run` field is ignored; a `Some`
     /// `trace_name` (the executor's claim sentinel, cycle 0109) is replaced
     /// with the derived `"{campaign8}-{run}"` on the stored line, `None`
-    /// stays `None`. Reads the store once to pick the run index (a
-    /// read-before-write; single-process CLI invocations do not race).
+    /// stays `None`. Reads the store once to pick the run index, then writes,
+    /// the whole read-then-write critical section serialized by the same
+    /// `Registry` `append_write_lock` that `append_family` uses (#276) — safe
+    /// for concurrent writers sharing one `&Registry`.
     pub fn append_campaign_run(
         &self,
         record: &CampaignRunRecord,
     ) -> Result<usize, RegistryError> {
+        let _guard = self.append_write_lock.lock().unwrap_or_else(std::sync::PoisonError::into_inner);
         let run = next_campaign_run(&record.campaign, &self.load_campaign_runs()?);
 
         let path = self.path.with_file_name("campaign_runs.jsonl");