diff --git a/furumi-agent/src/merge.rs b/furumi-agent/src/merge.rs
index eec8105..25f6e5b 100644
--- a/furumi-agent/src/merge.rs
+++ b/furumi-agent/src/merge.rs
@@ -147,6 +147,27 @@ async fn merge_db(
     proposal: &MergeProposal,
     loser_ids: &[i64],
 ) -> anyhow::Result<()> {
+    // 0. Validate proposal — ensure winner and all album IDs belong to source artists
+    let source_ids: Vec<i64> = loser_ids.iter().copied()
+        .chain(std::iter::once(proposal.winner_artist_id))
+        .collect();
+
+    // Verify winner_artist_id is one of the source artists
+    if !source_ids.contains(&proposal.winner_artist_id) {
+        anyhow::bail!(
+            "winner_artist_id {} is not among source artists {:?}",
+            proposal.winner_artist_id, source_ids
+        );
+    }
+
+    // Build set of valid album IDs (albums that actually belong to source artists)
+    let mut valid_album_ids = std::collections::HashSet::<i64>::new();
+    for &src_id in &source_ids {
+        let rows: Vec<(i64,)> = sqlx::query_as("SELECT id FROM albums WHERE artist_id = $1")
+            .bind(src_id).fetch_all(&mut **tx).await?;
+        for (id,) in rows { valid_album_ids.insert(id); }
+    }
+
     // 1. Rename winner artist to canonical name
     sqlx::query("UPDATE artists SET name = $2 WHERE id = $1")
         .bind(proposal.winner_artist_id)
@@ -155,6 +176,15 @@ async fn merge_db(
 
     // 2. Process album mappings from the proposal
     for mapping in &proposal.album_mappings {
+        // Skip albums that don't belong to any source artist (LLM hallucinated IDs)
+        if !valid_album_ids.contains(&mapping.source_album_id) {
+            tracing::warn!(
+                album_id = mapping.source_album_id,
+                "Skipping album mapping: album does not belong to source artists"
+            );
+            continue;
+        }
+
         // Skip if source was already processed (idempotent retry support)
         let src_exists: (bool,) = sqlx::query_as("SELECT EXISTS(SELECT 1 FROM albums WHERE id = $1)")
             .bind(mapping.source_album_id)