diff --git a/.jules/sentinel.md b/.jules/sentinel.md
new file mode 100644
index 0000000..6c54982
--- /dev/null
+++ b/.jules/sentinel.md
@@ -0,0 +1,4 @@
+## 2025-05-18 - Prevent Predictable Temporary Filenames during Atomic File Writes
+**Vulnerability:** Found predictable temporary filenames in atomic file writes (e.g., `path + ".tmp"`) using `os.WriteFile` in `internal/burnrate/burnrate.go` and `internal/sparkline/sparkline.go`. This opens up symlink attacks during the predictable write, leading to potential unintended file overrides and privilege escalation.
+**Learning:** It existed because `os.WriteFile` + `os.Rename` is a common but incomplete pattern for atomic file writes. The initial write must go to a file with an unpredictable name to prevent an attacker from creating a symlink before the write happens.
+**Prevention:** Always use `os.CreateTemp` to generate an unpredictable temporary file for the initial write. Ensure it is created in the same directory as the target path (`filepath.Dir(path)`), close the file handle, and perform the `os.Rename` with explicit permission handling (`Chmod`).
diff --git a/internal/burnrate/burnrate.go b/internal/burnrate/burnrate.go
index 8cb680c..90286ca 100644
--- a/internal/burnrate/burnrate.go
+++ b/internal/burnrate/burnrate.go
@@ -64,10 +64,28 @@ 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 {
+	dir := filepath.Dir(path)
+	tmpFile, err := os.CreateTemp(dir, "prism-burn-*")
+	if err != nil {
+		return nil, false, err
+	}
+	tmpPath := tmpFile.Name()
+
+	if _, err := tmpFile.Write(data); err != nil {
+		tmpFile.Close()
+		os.Remove(tmpPath)
+		return nil, false, err
+	}
+	if err := tmpFile.Chmod(0644); err != nil {
+		tmpFile.Close()
+		os.Remove(tmpPath)
+		return nil, false, err
+	}
+	if err := tmpFile.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..ecca58a 100644
--- a/internal/sparkline/sparkline.go
+++ b/internal/sparkline/sparkline.go
@@ -138,11 +138,32 @@ func Save(sessionID, metric string, b *Buffer) {
 	if err != nil {
 		return
 	}
-	tmp := path + ".tmp"
-	if err := os.WriteFile(tmp, data, 0644); err != nil {
+
+	dir := filepath.Dir(path)
+	tmpFile, err := os.CreateTemp(dir, "prism-spark-*")
+	if err != nil {
+		return
+	}
+	tmpPath := tmpFile.Name()
+
+	if _, err := tmpFile.Write(data); err != nil {
+		tmpFile.Close()
+		os.Remove(tmpPath)
 		return
 	}
-	os.Rename(tmp, path)
+	if err := tmpFile.Chmod(0644); err != nil {
+		tmpFile.Close()
+		os.Remove(tmpPath)
+		return
+	}
+	if err := tmpFile.Close(); err != nil {
+		os.Remove(tmpPath)
+		return
+	}
+
+	if err := os.Rename(tmpPath, path); err != nil {
+		os.Remove(tmpPath)
+	}
 }
 
 // PushAndSave is a convenience that loads, pushes, saves, and returns the buffer