OpenSearch vs Vespa

OpenSearch

OpenSearch 3.9.0 runs vector, text and hybrid search under Apache-2.0, with an OpenAPI 3.1 description of its REST API and API keys limited by permission and index pattern. The owner runs the cluster, no status page or SLA comes with the software, and no tool in the MCP server sets a read-only or destructive annotation.

Vespa

Vespa Cloud publishes hourly unit prices, a 99.5% uptime agreement for paying customers and a trial with $300 of credits and no card. Every call needs an application package deployed first, no OpenAPI file was found in the reviewed documentation, and Vespa.ai says it has no official public MCP server.

OpenSearch

1. Create the index with index.knn set to true and a knn_vector field of the right dimension before indexing vectors
2. Put the filter clause inside the knn query for filtering during the search. A filter outside it runs afterwards and can return fewer than k hits
3. Ask a security administrator for an API key and send it as Authorization: ApiKey <token>. The token is shown once and lasts 90 days at most
4. Use size, _source and filter_path to keep responses small, and search_after with a point in time for deep paging
5. Set OPENSEARCH_SETTINGS_ALLOW_WRITE=false on the MCP server unless writes are needed. GenericOpenSearchApiTool can call any endpoint

Vespa

1. Deploy an application package with a schema before feeding or querying. Use vespa deploy and wait for the endpoint to report ready
2. Send a token as Authorization: Bearer only to the endpoint marked Token. The mTLS endpoint needs the client certificate and key
3. Set hits, offset and presentation.summary on queries. hits is capped at 400 by default and the query timeout defaults to 0.5s
4. On 429 from /document/v1 reduce the feed rate and back off. On 507 stop feeding, because the cluster is out of memory or disk
5. Use the condition parameter for test-and-set writes and expect 412 when it fails or the document is missing