diff --git a/packages/meta-sort-core/src/api/UnifiedAPIServer.ts b/packages/meta-sort-core/src/api/UnifiedAPIServer.ts
index ad252cd..6a64b7f 100644
--- a/packages/meta-sort-core/src/api/UnifiedAPIServer.ts
+++ b/packages/meta-sort-core/src/api/UnifiedAPIServer.ts
@@ -1198,8 +1198,8 @@ export class UnifiedAPIServer {
       }
 
       try {
-        const key = `/file/${hashId}/${property}`;
-        await this.kvClient!.setProperty(key, String(value));
+        // Write directly to Redis Hash (not flat keys)
+        await this.kvClient!.setMetadataProperty(hashId, property, String(value));
 
         return {
           status: 'ok',
@@ -1341,11 +1341,10 @@ export class UnifiedAPIServer {
               await this.kvClient!.setMetadataFlat(hashId, metadata, ['processingStatus']);
             }
 
-            // Update individual properties
+            // Update individual properties (write to Redis Hash)
             if (properties) {
               for (const [property, value] of Object.entries(properties)) {
-                const key = `/file/${hashId}/${property}`;
-                await this.kvClient!.setProperty(key, String(value));
+                await this.kvClient!.setMetadataProperty(hashId, property, String(value));
               }
             }
 
diff --git a/packages/meta-sort-core/src/api/api.test.ts b/packages/meta-sort-core/src/api/api.test.ts
index 4666f83..4b2ec27 100644
--- a/packages/meta-sort-core/src/api/api.test.ts
+++ b/packages/meta-sort-core/src/api/api.test.ts
@@ -112,6 +112,13 @@ class MockKVClient implements IKVClient {
         return entry.get(propertyPath) ?? null;
     }
 
+    async setMetadataProperty(hashId: string, property: string, value: string): Promise<void> {
+        if (!this.data.has(hashId)) {
+            this.data.set(hashId, new Map());
+        }
+        this.data.get(hashId)!.set(property, value);
+    }
+
     async deleteMetadataFlat(hashId: string): Promise<number> {
         const entry = this.data.get(hashId);
         const count = entry?.size ?? 0;
diff --git a/packages/meta-sort-core/src/kv/IKVClient.ts b/packages/meta-sort-core/src/kv/IKVClient.ts
index 6952498..e352ea5 100644
--- a/packages/meta-sort-core/src/kv/IKVClient.ts
+++ b/packages/meta-sort-core/src/kv/IKVClient.ts
@@ -96,6 +96,11 @@ export interface IKVClient {
      */
     getMetadata(hashId: string, propertyPath: string): Promise<any | null>;
 
+    /**
+     * Set a single metadata property for a file (writes to Redis Hash)
+     */
+    setMetadataProperty(hashId: string, property: string, value: string): Promise<void>;
+
     /**
      * Delete all metadata for a file
      */
diff --git a/packages/meta-sort-core/src/kv/RedisClient.ts b/packages/meta-sort-core/src/kv/RedisClient.ts
index ba9108e..cf01140 100644
--- a/packages/meta-sort-core/src/kv/RedisClient.ts
+++ b/packages/meta-sort-core/src/kv/RedisClient.ts
@@ -298,6 +298,18 @@ export class RedisKVClient implements IKVClient {
         }
     }
 
+    /**
+     * Set a single metadata property for a file
+     * Writes directly to Redis hash field
+     */
+    async setMetadataProperty(hashId: string, property: string, value: string): Promise<void> {
+        const hashKey = this.buildKey(`file:${hashId}`);
+        await this.redis.hset(hashKey, property, value);
+
+        // Also ensure the file is in the index
+        await this.redis.sadd(this.buildKey('file:__index__'), hashId);
+    }
+
     async deleteMetadataFlat(hashId: string): Promise<number> {
         const hashKey = this.buildKey(`file:${hashId}`);
         const fieldCount = await this.redis.hlen(hashKey);