diff --git a/.jules/sentinel.md b/.jules/sentinel.md
new file mode 100644
index 0000000..c5299f1
--- /dev/null
+++ b/.jules/sentinel.md
@@ -0,0 +1,4 @@
+## 2025-02-14 - Predictable Temporary Filename Symlink Vulnerability
+**Vulnerability:** Predictable temporary files were created using hardcoded extensions (e.g. `path + ".tmp"`) in shared directories like `/tmp`. This allows an attacker to pre-create a symlink at the predicted location, tricking the application into overwriting an arbitrary file.
+**Learning:** Atomic file writes often involve creating a temporary file and renaming it. If the temporary filename is predictable and located in a shared directory, it is vulnerable to symlink attacks.
+**Prevention:** Always use `os.CreateTemp` to generate unpredictable temporary filenames. Explicitly set permissions using `Chmod` if matching the original file permissions is necessary, and ensure `Close()` is called before `Rename()`.
diff --git a/internal/burnrate/burnrate.go b/internal/burnrate/burnrate.go
index 8cb680c..adcd534 100644
--- a/internal/burnrate/burnrate.go
+++ b/internal/burnrate/burnrate.go
@@ -64,10 +64,29 @@ func LoadOrCreateSnapshotAt(sessionID string, currentCost float64, now time.Time
 		return nil, false, err
 	}
 
-	tmpPath := path + ".tmp"
-	if err := os.WriteFile(tmpPath, data, 0644); err != nil {
+	f, err := os.CreateTemp(filepath.Dir(path), "prism-burn-tmp-*")
+	if err != nil {
+		return nil, false, err
+	}
+	tmpPath := f.Name()
+
+	if _, err := f.Write(data); err != nil {
+		f.Close()
+		os.Remove(tmpPath)
+		return nil, false, err
+	}
+
+	if err := f.Chmod(0644); err != nil {
+		f.Close()
+		os.Remove(tmpPath)
 		return nil, false, err
 	}
+
+	if err := f.Close(); err != nil {
+		os.Remove(tmpPath)
+		return nil, false, err
+	}
+
 	if err := os.Rename(tmpPath, path); err != nil {
 		os.Remove(tmpPath)
 		return nil, false, err
diff --git a/internal/sparkline/sparkline.go b/internal/sparkline/sparkline.go
index f4c755c..110d823 100644
--- a/internal/sparkline/sparkline.go
+++ b/internal/sparkline/sparkline.go
@@ -138,10 +138,30 @@ func Save(sessionID, metric string, b *Buffer) {
 	if err != nil {
 		return
 	}
-	tmp := path + ".tmp"
-	if err := os.WriteFile(tmp, data, 0644); err != nil {
+
+	f, err := os.CreateTemp(filepath.Dir(path), "prism-spark-tmp-*")
+	if err != nil {
+		return
+	}
+	tmp := f.Name()
+
+	if _, err := f.Write(data); err != nil {
+		f.Close()
+		os.Remove(tmp)
+		return
+	}
+
+	if err := f.Chmod(0644); err != nil {
+		f.Close()
+		os.Remove(tmp)
+		return
+	}
+
+	if err := f.Close(); err != nil {
+		os.Remove(tmp)
 		return
 	}
+
 	os.Rename(tmp, path)
 }