diff --git a/src/tools/memories.ts b/src/tools/memories.ts
index 84930d0..4484bed 100644
--- a/src/tools/memories.ts
+++ b/src/tools/memories.ts
@@ -43,10 +43,24 @@ interface MemoryListArgs {
 
 /**
  * Add a new memory/learning (enhanced with session_id and task_id)
+ * CF-306: Validates session_id exists before inserting to prevent foreign key violations
  */
 export async function memoryAdd(args: MemoryAddArgs): Promise<string> {
   const { category, title, content, context, project, session_id, task_id } = args;
 
+  // CF-306: Validate session_id exists if provided
+  let validSessionId = session_id || null;
+  if (session_id) {
+    const sessionExists = await queryOne<{ exists: boolean }>(
+      `SELECT EXISTS(SELECT 1 FROM sessions WHERE id = $1) as exists`,
+      [session_id]
+    );
+    if (!sessionExists?.exists) {
+      console.warn(`[CF-306] Session ${session_id} not found in database - using NULL instead`);
+      validSessionId = null;
+    }
+  }
+
   // Generate embedding for semantic search
   const embedText = `${title}. ${content}`;
   const embedding = await getEmbedding(embedText);
@@ -56,13 +70,13 @@ export async function memoryAdd(args: MemoryAddArgs): Promise<string> {
     await execute(
       `INSERT INTO memories (category, title, content, context, project, session_id, task_id, embedding)
        VALUES ($1, $2, $3, $4, $5, $6, $7, $8)`,
-      [category, title, content, context || null, project || null, session_id || null, task_id || null, embeddingValue]
+      [category, title, content, context || null, project || null, validSessionId, task_id || null, embeddingValue]
     );
   } else {
     await execute(
       `INSERT INTO memories (category, title, content, context, project, session_id, task_id)
        VALUES ($1, $2, $3, $4, $5, $6, $7)`,
-      [category, title, content, context || null, project || null, session_id || null, task_id || null]
+      [category, title, content, context || null, project || null, validSessionId, task_id || null]
     );
   }