diff --git a/crates/core/src/config.rs b/crates/core/src/config.rs
index e0287c72..58d7d99d 100644
--- a/crates/core/src/config.rs
+++ b/crates/core/src/config.rs
@@ -351,7 +351,7 @@ pub struct RedbIndexConfig {
 }
 
 /// Configuration for the Fjall index backend.
-#[derive(Serialize, Deserialize, Clone, Debug, Default)]
+#[derive(Serialize, Deserialize, Clone, Debug)]
 pub struct FjallIndexConfig {
     /// Optional path override. If relative, resolved from storage root.
     /// If not specified, defaults to `<storage.path>/index`.
@@ -360,23 +360,37 @@ pub struct FjallIndexConfig {
     /// Size (in MB) of memory allocated for caching.
     #[serde(default)]
     pub cache: Option<usize>,
-    /// Maximum journal size in MB.
+    /// Maximum journal size in MB (default: 1024).
     #[serde(default)]
     pub max_journal_size: Option<usize>,
-    /// Flush journal after each commit.
+    /// Flush journal after each commit (default: false).
     #[serde(default)]
     pub flush_on_commit: Option<bool>,
-    /// L0 compaction threshold (default: 4, lower = more aggressive).
+    /// L0 compaction threshold (default: 8, lower = more aggressive).
     #[serde(default)]
     pub l0_threshold: Option<u8>,
-    /// Number of background compaction worker threads.
+    /// Number of background compaction worker threads (default: 8).
     #[serde(default)]
     pub worker_threads: Option<usize>,
-    /// Memtable size in MB before flush (default: 64).
+    /// Memtable size in MB before flush (default: 128).
     #[serde(default)]
     pub memtable_size_mb: Option<usize>,
 }
 
+impl Default for FjallIndexConfig {
+    fn default() -> Self {
+        Self {
+            path: None,
+            cache: None,
+            max_journal_size: Some(1024),
+            flush_on_commit: Some(false),
+            l0_threshold: Some(8),
+            worker_threads: Some(8),
+            memtable_size_mb: Some(128),
+        }
+    }
+}
+
 /// Index store configuration.
 #[derive(Serialize, Deserialize, Clone, Debug)]
 #[serde(tag = "backend", rename_all = "lowercase")]