chore: fix metadata test

Author: kmruiz

tests/integration/tools/mongodb/read/aggregate.test.ts

@@ -27,8 +27,19 @@ describeWithMongoDB("aggregate tool", (integration) => {
         ...databaseCollectionParameters,
         {
             name: "pipeline",
-            description:
-                "An array of aggregation stages to execute. $vectorSearch can only appear as the first stage of the aggregation pipeline or as the first stage of a $unionWith subpipeline. When using $vectorSearch, unless the user explicitly asks for the embeddings, $unset any embedding field to avoid reaching context limits.",
+            description: `An array of aggregation stages to execute.  
+\`$vectorSearch\` **MUST** be the first stage of the pipeline, or the first stage of a \`$unionWith\` subpipeline.
+### Usage Rules for \`$vectorSearch\`
+- **Unset embeddings:**  
+  Unless the user explicitly requests the embeddings, add an \`$unset\` stage **at the end of the pipeline** to remove the embedding field and avoid context limits. **The $unset stage in this situation is mandatory**.
+- **Prefiltering:**
+If the user requests additional filtering, include filters in \`$vectorSearch.filter\` only for pre-filter fields in the vector index.
+    NEVER include fields in $vectorSearch.filter that are not part of the vector index.
+- **Post-filtering:**
+    For all remaining filters, add a $match stage after $vectorSearch.
+### Note to LLM
+- If unsure which fields are filterable, use the collection-indexes tool to determine valid prefilter fields.
+- If no requested filters are valid prefilters, omit the filter key from $vectorSearch.`,
             type: "array",
             required: true,
         },