diff --git a/READINESS.adoc b/READINESS.adoc
index 505a2db..cff5e6d 100644
--- a/READINESS.adoc
+++ b/READINESS.adoc
@@ -8,7 +8,7 @@ standards/testing-and-benchmarking/TESTING-TAXONOMY.adoc v1.0
 
 === CRG Grade: D (Alpha — Unstable)
 
-*Justification:* 119 tests declared (`cargo test --workspace --locked -- --list`; rust-ci runs them); the
+*Justification:* 123 tests declared (`cargo test --workspace --locked -- --list`; rust-ci runs them); the
 Idris2 ABI typechecks and 11 proofs are checked (PROVEN ledger rows: ABI, grade and transaction-model lemmas, not the
 security obligations); benchmarks exist (5 Criterion groups). But: no
 fuzz testing, no mutation testing, 225 unwrap() calls, E2E mostly skips,
@@ -65,7 +65,7 @@ handling
 (run by idris-abi.yml, agda-proofs.yml and dashboard-check)
 |===
 
-*Total declared:* 119 tests + 5 benchmark groups; 11 proofs checked (PROVEN ledger rows) *Total
+*Total declared:* 123 tests + 5 benchmark groups; 11 proofs checked (PROVEN ledger rows) *Total
 missing:* Fuzz, mutation, chaos, compatibility
 
 === Aspect Matrix
diff --git a/TOPOLOGY.adoc b/TOPOLOGY.adoc
index 469db4b..a29763a 100644
--- a/TOPOLOGY.adoc
+++ b/TOPOLOGY.adoc
@@ -79,7 +79,7 @@ INTERFACES & RESEARCH
   MPR Methodology                   ████░░░░░░  ~40%   Design documented; FORMAL PROOFS PENDING
                                                         (Idris2 ABI typechecks; 11 proofs checked: ABI, grade and transaction-model
                                                         lemmas, not the security obligations; MPR not linked to the Rust)
-  Testing (READINESS matrix)        ██████░░░░  ~60%   119 tests + 5 benches; missing fuzz, mutation,
+  Testing (READINESS matrix)        ██████░░░░  ~60%   123 tests + 5 benches; missing fuzz, mutation,
                                                         chaos, compatibility (Grade D)
 
 REPO INFRASTRUCTURE
diff --git a/crates/reversible-core/src/content_store.rs b/crates/reversible-core/src/content_store.rs
index 0233c3c..437fb7b 100644
--- a/crates/reversible-core/src/content_store.rs
+++ b/crates/reversible-core/src/content_store.rs
@@ -46,6 +46,15 @@ impl ContentHash {
         "sha256"
     }
 
+    /// True when this is exactly `sha256:` followed by 64 lowercase hex digits,
+    /// the only form `from_bytes` produces. Anything else is not a hash the
+    /// store can hold, and must never be turned into a filesystem path.
+    pub fn is_well_formed(&self) -> bool {
+        self.0.strip_prefix("sha256:").is_some_and(|hex| {
+            hex.len() == 64 && hex.bytes().all(|b| matches!(b, b'0'..=b'9' | b'a'..=b'f'))
+        })
+    }
+
     /// Verify content matches this hash
     pub fn verify(&self, content: &[u8]) -> bool {
         let computed = Self::from_bytes(content);
@@ -53,6 +62,10 @@ impl ContentHash {
     }
 }
 
+/// Name prefix of in-flight blob writes. A file with this prefix is never a
+/// stored blob; a crash can leave one behind.
+const TMP_PREFIX: &str = ".tmp-";
+
 impl std::fmt::Display for ContentHash {
     fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
         write!(f, "{}", self.0)
@@ -88,7 +101,17 @@ impl ContentStore {
     /// Get filesystem path for a content hash.
     ///
     /// Uses a 2-char prefix directory for distribution (git-style layout).
+    ///
+    /// Fails closed: a hash that is not well formed (see
+    /// [`ContentHash::is_well_formed`]) maps to a fixed path under the root that
+    /// no blob ever occupies, never to a path derived from its text. The field
+    /// is public and deserialised from metadata, so a hash string is untrusted
+    /// input; `Path::join` with an absolute component would otherwise replace
+    /// the root.
     pub fn content_path(&self, hash: &ContentHash) -> PathBuf {
+        if !hash.is_well_formed() {
+            return self.root.join("_invalid_hash");
+        }
         let raw = hash.raw_hash();
         // Use first 2 chars as directory for distribution
         let (dir, file) = raw.split_at(2.min(raw.len()));
@@ -103,33 +126,76 @@ impl ContentStore {
 
     /// Store content and return its hash.
     ///
-    /// If content with the same hash already exists, this is a no-op
-    /// (deduplication).
+    /// If an intact blob with the same hash already exists, this is a no-op
+    /// (deduplication). A blob that is present but damaged, for example
+    /// truncated by a crash or by bit rot, is replaced rather than trusted.
+    ///
+    /// The blob is written to a temporary file in its own directory, flushed to
+    /// disk, then renamed into place, so the final path only ever holds a
+    /// complete blob. Deduplication reads the existing blob back to check it;
+    /// that costs one read of a duplicate, and is what makes "store then
+    /// retrieve returns the content" hold after a crash.
     pub fn store(&self, content: &[u8]) -> Result<ContentHash> {
         let hash = ContentHash::from_bytes(content);
         let path = self.content_path(&hash);
 
-        // Skip if already stored (deduplication)
-        if path.exists() {
+        // Skip if an intact copy is already stored (deduplication)
+        if path.exists()
+            && self
+                .read_blob(&path)
+                .is_ok_and(|stored| hash.verify(&stored))
+        {
             return Ok(hash);
         }
 
-        // Create parent directory
-        if let Some(parent) = path.parent() {
-            fs::create_dir_all(parent)?;
+        let parent = path.parent().ok_or_else(|| {
+            ReversibleError::FileNotFound(format!("no parent directory for {}", hash))
+        })?;
+        fs::create_dir_all(parent)?;
+
+        let tmp = parent.join(format!("{TMP_PREFIX}{}", uuid::Uuid::new_v4()));
+        if let Err(e) = self.write_blob(&tmp, content) {
+            let _ = fs::remove_file(&tmp);
+            return Err(e);
+        }
+        if let Err(e) = fs::rename(&tmp, &path) {
+            let _ = fs::remove_file(&tmp);
+            return Err(e.into());
+        }
+        // Make the rename itself durable. Best effort: not every platform can
+        // open a directory.
+        if let Ok(dir) = File::open(parent) {
+            let _ = dir.sync_all();
         }
 
-        // Write content (optionally compressed)
+        Ok(hash)
+    }
+
+    /// Write `content` (optionally compressed) to `path` and flush it to disk.
+    fn write_blob(&self, path: &Path, content: &[u8]) -> Result<()> {
+        let mut file = File::create(path)?;
         if self.compression {
-            let file = File::create(&path)?;
-            let mut encoder = GzEncoder::new(file, Compression::default());
+            let mut encoder = GzEncoder::new(&mut file, Compression::default());
             encoder.write_all(content)?;
             encoder.finish()?;
         } else {
-            fs::write(&path, content)?;
+            file.write_all(content)?;
         }
+        file.sync_all()?;
+        Ok(())
+    }
 
-        Ok(hash)
+    /// Read a stored blob back (decompressing if needed), without verifying it.
+    fn read_blob(&self, path: &Path) -> Result<Vec<u8>> {
+        if self.compression {
+            let file = File::open(path)?;
+            let mut decoder = GzDecoder::new(file);
+            let mut content = Vec::new();
+            decoder.read_to_end(&mut content)?;
+            Ok(content)
+        } else {
+            Ok(fs::read(path)?)
+        }
     }
 
     /// Store content from a file path
@@ -146,15 +212,7 @@ impl ContentStore {
             return Err(ReversibleError::FileNotFound(hash.to_string()));
         }
 
-        let content = if self.compression {
-            let file = File::open(&path)?;
-            let mut decoder = GzDecoder::new(file);
-            let mut content = Vec::new();
-            decoder.read_to_end(&mut content)?;
-            content
-        } else {
-            fs::read(&path)?
-        };
+        let content = self.read_blob(&path)?;
 
         // Verify integrity
         if !hash.verify(&content) {
@@ -195,11 +253,13 @@ impl ContentStore {
         Ok(size)
     }
 
-    /// Count number of stored content blobs
+    /// Count number of stored content blobs (in-flight temp files are not blobs)
     pub fn count(&self) -> Result<usize> {
         let mut count = 0;
         for entry in walkdir::WalkDir::new(&self.root).into_iter().flatten() {
-            if entry.file_type().is_file() {
+            if entry.file_type().is_file()
+                && !entry.file_name().to_string_lossy().starts_with(TMP_PREFIX)
+            {
                 count += 1;
             }
         }
@@ -258,4 +318,119 @@ mod tests {
         assert_eq!(hash1, hash2);
         assert_eq!(store.count().unwrap(), 1);
     }
+
+    /// Hash strings that are not `sha256:` + 64 lowercase hex digits.
+    fn malformed_hashes(victim: &Path) -> Vec<ContentHash> {
+        let zeros = "0".repeat(64);
+        vec![
+            ContentHash(format!("sha256:ab{}", victim.display())), // absolute path after 2 chars
+            ContentHash("sha256:ab/../../escape".to_string()),
+            ContentHash("sha256:../..".to_string()),
+            ContentHash("sha256:".to_string()),
+            ContentHash(String::new()),
+            ContentHash(format!("sha256:{}", "g".repeat(64))), // not hex
+            ContentHash(format!("sha256:{}", zeros.to_uppercase().replace('0', "A"))), // uppercase
+            ContentHash(format!("sha256:{}", "0".repeat(63))), // too short
+            ContentHash(format!("sha256:{}", "0".repeat(65))), // too long
+            ContentHash(zeros),                                // no algorithm prefix
+            ContentHash(format!("sha256:{}\0", "0".repeat(63))), // NUL
+        ]
+    }
+
+    #[test]
+    fn malformed_hash_never_leaves_the_store_root() {
+        for compression in [false, true] {
+            let store_dir = TempDir::new().unwrap();
+            let victim_dir = TempDir::new().unwrap();
+            let victim = victim_dir.path().join("not-a-blob.txt");
+            fs::write(&victim, b"outside the store").unwrap();
+            let store = ContentStore::new(store_dir.path().to_path_buf(), compression).unwrap();
+
+            for hash in malformed_hashes(&victim) {
+                let path = store.content_path(&hash);
+                assert!(
+                    path.starts_with(store_dir.path()),
+                    "{:?} escaped to {}",
+                    hash.0,
+                    path.display()
+                );
+                assert!(!store.exists(&hash), "{:?} must not exist", hash.0);
+                store.delete(&hash).unwrap();
+                assert!(
+                    matches!(store.retrieve(&hash), Err(ReversibleError::FileNotFound(_))),
+                    "{:?} must read as not found",
+                    hash.0
+                );
+            }
+            assert!(victim.exists(), "a file outside the store was deleted");
+        }
+    }
+
+    #[test]
+    fn well_formed_hash_keeps_the_existing_layout() {
+        let tmp = TempDir::new().unwrap();
+        let hash = ContentHash::from_bytes(b"layout");
+        assert!(hash.is_well_formed());
+        let raw = hash.raw_hash().to_string();
+
+        let plain = ContentStore::new(tmp.path().join("plain"), false).unwrap();
+        assert_eq!(
+            plain.content_path(&hash),
+            tmp.path().join("plain").join(&raw[..2]).join(&raw[2..])
+        );
+        let gz = ContentStore::new(tmp.path().join("gz"), true).unwrap();
+        assert_eq!(
+            gz.content_path(&hash),
+            tmp.path()
+                .join("gz")
+                .join(&raw[..2])
+                .join(format!("{}.gz", &raw[2..]))
+        );
+    }
+
+    #[test]
+    fn store_repairs_a_blob_truncated_by_a_crash() {
+        for compression in [false, true] {
+            let tmp = TempDir::new().unwrap();
+            let store = ContentStore::new(tmp.path().to_path_buf(), compression).unwrap();
+            let content = b"important bytes that must survive a crash".repeat(20);
+            let hash = store.store(&content).unwrap();
+
+            // What a crash mid-write leaves at the final path.
+            let path = store.content_path(&hash);
+            let on_disk = fs::read(&path).unwrap();
+            fs::write(&path, &on_disk[..on_disk.len() / 2]).unwrap();
+            assert!(
+                store.retrieve(&hash).is_err(),
+                "setup: blob must be damaged"
+            );
+
+            assert_eq!(store.store(&content).unwrap(), hash);
+            assert_eq!(store.retrieve(&hash).unwrap(), content);
+        }
+    }
+
+    #[test]
+    fn store_leaves_no_temp_files_and_count_ignores_strays() {
+        for compression in [false, true] {
+            let tmp = TempDir::new().unwrap();
+            let store = ContentStore::new(tmp.path().to_path_buf(), compression).unwrap();
+            let hash = store.store(b"one blob").unwrap();
+            store.store(b"one blob").unwrap();
+            assert_eq!(store.count().unwrap(), 1);
+
+            let dir = store.content_path(&hash).parent().unwrap().to_path_buf();
+            let leftovers: Vec<_> = fs::read_dir(&dir)
+                .unwrap()
+                .flatten()
+                .filter(|e| e.file_name().to_string_lossy().starts_with(TMP_PREFIX))
+                .collect();
+            assert!(leftovers.is_empty(), "store left temp files behind");
+
+            // A temp file orphaned by a crash is not a stored blob.
+            fs::write(dir.join(format!("{TMP_PREFIX}orphan")), b"partial").unwrap();
+            assert_eq!(store.count().unwrap(), 1);
+            assert_eq!(store.retrieve(&hash).unwrap(), b"one blob");
+        }
+    }
 }