diff --git a/crates/fparkan-resource/src/lib.rs b/crates/fparkan-resource/src/lib.rs
index 07484fb..666245f 100644
--- a/crates/fparkan-resource/src/lib.rs
+++ b/crates/fparkan-resource/src/lib.rs
@@ -449,42 +449,64 @@ impl CachedResourceRepository {
 
 impl ResourceRepository for CachedResourceRepository {
     fn open_archive(&self, path: &NormalizedPath) -> Result<ArchiveId, ResourceError> {
-        let bytes = self
-            .vfs
-            .read(path)
-            .map_err(|err| resource_error_from_vfs(path, err))?;
-        let fingerprint = sha256(&bytes);
-        let mut slot = decode_archive(path.clone(), bytes, fingerprint)?;
         let mut state = self.state.lock().map_err(|_| ResourceError::Poisoned)?;
         let key = path.identity_bytes().to_vec();
-        if let Some(id) = state.paths.get(&key).copied() {
-            let current = state.archive(id)?;
-            if current.fingerprint == fingerprint && current.document.is_some() {
-                state.touch_archive(id)?;
+        loop {
+            // Decode outside the repository lock, then verify the VFS still points
+            // at the same bytes before committing the slot under the lock.
+            drop(state);
+            let bytes = self
+                .vfs
+                .read(path)
+                .map_err(|err| resource_error_from_vfs(path, err))?;
+            let observed_fingerprint = sha256(&bytes);
+            let mut slot = decode_archive(path.clone(), bytes, observed_fingerprint)?;
+            let current_vfs_fingerprint = self
+                .vfs
+                .metadata(path)
+                .map_err(|err| resource_error_from_vfs(path, err))?
+                .fingerprint;
+            state = self.state.lock().map_err(|_| ResourceError::Poisoned)?;
+            if let Some(id) = state.paths.get(&key).copied() {
+                let current = state.archive(id)?;
+                if current.document.is_some() && current.fingerprint == current_vfs_fingerprint {
+                    state.touch_archive(id)?;
+                    return Ok(id);
+                }
+                if current_vfs_fingerprint != observed_fingerprint {
+                    continue;
+                }
+                if current.document.is_some() && current.fingerprint == observed_fingerprint {
+                    state.touch_archive(id)?;
+                    return Ok(id);
+                }
+                let current_generation = current.generation;
+                let current_fingerprint = current.fingerprint;
+                if current_fingerprint != observed_fingerprint {
+                    slot.generation = current_generation.saturating_add(1);
+                    state.payload_cache.remove_archive(id);
+                } else {
+                    slot.generation = current_generation;
+                }
+                state.unload_archive(id)?;
+                *state.archive_mut(id)? = slot;
+                state.load_archive(id)?;
+                state.evict_archives(id)?;
                 return Ok(id);
             }
-            let current_generation = current.generation;
-            let current_fingerprint = current.fingerprint;
-            if current_fingerprint != fingerprint {
-                slot.generation = current_generation.saturating_add(1);
-                state.payload_cache.remove_archive(id);
-            } else {
-                slot.generation = current_generation;
+            if current_vfs_fingerprint != observed_fingerprint {
+                continue;
             }
-            state.unload_archive(id)?;
-            *state.archive_mut(id)? = slot;
+            let id = ArchiveId(
+                u64::try_from(state.archives.len())
+                    .map_err(|_| ResourceError::HandleSpaceExhausted)?,
+            );
+            state.paths.insert(key.clone(), id);
+            state.archives.push(slot);
             state.load_archive(id)?;
             state.evict_archives(id)?;
             return Ok(id);
         }
-        let id = ArchiveId(
-            u64::try_from(state.archives.len()).map_err(|_| ResourceError::HandleSpaceExhausted)?,
-        );
-        state.paths.insert(key, id);
-        state.archives.push(slot);
-        state.load_archive(id)?;
-        state.evict_archives(id)?;
-        Ok(id)
     }
 
     fn find(
@@ -881,6 +903,8 @@ mod tests {
     use super::*;
     use fparkan_vfs::{DirectoryVfs, MemoryVfs, Vfs, VfsEntry, VfsError, VfsMetadata};
     use std::path::PathBuf;
+    use std::sync::Condvar;
+    use std::thread;
 
     enum FailingReadMode {
         Ambiguous(&'static str),
@@ -910,6 +934,74 @@ mod tests {
         }
     }
 
+    struct CoordinatedReadState {
+        current: Arc<[u8]>,
+        first_read_started: bool,
+        release_first_read: bool,
+    }
+
+    struct CoordinatedReadVfs {
+        state: Mutex<CoordinatedReadState>,
+        first_read_gate: Condvar,
+    }
+
+    impl CoordinatedReadVfs {
+        fn new(initial: Arc<[u8]>) -> Self {
+            Self {
+                state: Mutex::new(CoordinatedReadState {
+                    current: initial,
+                    first_read_started: false,
+                    release_first_read: false,
+                }),
+                first_read_gate: Condvar::new(),
+            }
+        }
+
+        fn wait_for_first_read(&self) {
+            let mut state = self.state.lock().expect("state");
+            while !state.first_read_started {
+                state = self.first_read_gate.wait(state).expect("wait");
+            }
+        }
+
+        fn replace_current(&self, bytes: Arc<[u8]>) {
+            self.state.lock().expect("state").current = bytes;
+        }
+
+        fn release_first_read(&self) {
+            let mut state = self.state.lock().expect("state");
+            state.release_first_read = true;
+            self.first_read_gate.notify_all();
+        }
+    }
+
+    impl Vfs for CoordinatedReadVfs {
+        fn metadata(&self, _path: &NormalizedPath) -> Result<VfsMetadata, VfsError> {
+            let state = self.state.lock().expect("state");
+            Ok(VfsMetadata {
+                len: state.current.len() as u64,
+                fingerprint: sha256(&state.current),
+            })
+        }
+
+        fn read(&self, _path: &NormalizedPath) -> Result<Arc<[u8]>, VfsError> {
+            let mut state = self.state.lock().expect("state");
+            let snapshot = Arc::clone(&state.current);
+            if !state.first_read_started {
+                state.first_read_started = true;
+                self.first_read_gate.notify_all();
+                while !state.release_first_read {
+                    state = self.first_read_gate.wait(state).expect("wait");
+                }
+            }
+            Ok(snapshot)
+        }
+
+        fn list(&self, _prefix: &NormalizedPath) -> Result<Vec<VfsEntry>, VfsError> {
+            unreachable!("list is not used in these tests");
+        }
+    }
+
     #[test]
     fn cached_repository_reads_synthetic_nres() {
         let path = archive_path(b"archives/test.lib").expect("path");
@@ -941,6 +1033,26 @@ mod tests {
         ));
     }
 
+    #[test]
+    fn concurrent_same_archive_open_reuses_archive_id() {
+        let path = archive_path(b"archives/test.lib").expect("path");
+        let bytes = Arc::from(build_nres(&[("Alpha.TXT", b"alpha".as_slice())]).into_boxed_slice());
+        let mut vfs = MemoryVfs::default();
+        vfs.insert(path.clone(), bytes);
+        let repo = Arc::new(CachedResourceRepository::new(Arc::new(vfs)));
+        let first_repo = Arc::clone(&repo);
+        let first_path = path.clone();
+        let first = thread::spawn(move || first_repo.open_archive(&first_path));
+        let second_repo = Arc::clone(&repo);
+        let second_path = path.clone();
+        let second = thread::spawn(move || second_repo.open_archive(&second_path));
+
+        let first = first.join().expect("first join").expect("first archive");
+        let second = second.join().expect("second join").expect("second archive");
+
+        assert_eq!(first, second);
+    }
+
     #[test]
     fn entry_handles_are_archive_qualified() {
         let first_path = archive_path(b"first.lib").expect("first path");
@@ -1125,6 +1237,36 @@ mod tests {
         let _ = std::fs::remove_dir_all(root);
     }
 
+    #[test]
+    fn concurrent_replacement_old_decode_cannot_overwrite_new() {
+        let path = archive_path(b"cache/concurrent.lib").expect("path");
+        let old_bytes =
+            Arc::from(build_nres(&[("same.bin", b"old".as_slice())]).into_boxed_slice());
+        let new_bytes =
+            Arc::from(build_nres(&[("same.bin", b"new".as_slice())]).into_boxed_slice());
+        let vfs = Arc::new(CoordinatedReadVfs::new(old_bytes));
+        let repo = Arc::new(CachedResourceRepository::new(vfs.clone()));
+        let stale_repo = Arc::clone(&repo);
+        let stale_path = path.clone();
+        let stale_open = thread::spawn(move || stale_repo.open_archive(&stale_path));
+
+        vfs.wait_for_first_read();
+        vfs.replace_current(Arc::clone(&new_bytes));
+        let current_archive = repo.open_archive(&path).expect("open current archive");
+        vfs.release_first_read();
+        let raced_archive = stale_open
+            .join()
+            .expect("join stale thread")
+            .expect("stale open");
+
+        assert_eq!(raced_archive, current_archive);
+        let handle = repo
+            .find(current_archive, &resource_name(b"same.bin"))
+            .expect("find current")
+            .expect("current handle");
+        assert_eq!(repo.read(handle).expect("read current").as_slice(), b"new");
+    }
+
     #[test]
     fn entry_read_error_carries_archive_path_and_entry_name() {
         let path = archive_path(b"bad/rsli.lib").expect("path");