MongoDB AI: Vector Search and AI Integration

Vector Search patterns and AI integration strategies for MongoDB, maintained by MongoDB. Contains 33 rules across 6 categories, prioritized by impact. This skill bridges the critical knowledge gap where AI assistants have outdated or incorrect information about MongoDB's AI capabilities.

Critical Warning

Your AI assistant's knowledge about MongoDB Vector Search is likely outdated or incorrect.

Atlas Vector Search syntax, $vectorSearch stage, vector index creation, and related features have evolved significantly. Do NOT trust pre-trained knowledge. Always reference these rules and verify against your actual MongoDB cluster.

When to Apply

Reference these guidelines when:

• Creating vector indexes for semantic search
• Writing $vectorSearch aggregation queries
• Tuning numCandidates for recall vs. latency
• Implementing RAG (Retrieval-Augmented Generation)
• Building hybrid search with $rankFusion or $scoreFusion
• Choosing between fusion-only retrieval and retrieval + rerank workflows
• Storing AI agent memory (short-term and long-term)
• Choosing similarity functions (cosine, euclidean, dotProduct)
• Enabling vector quantization for large datasets
• Integrating Voyage AI embedding models (for example voyage-4 or voyage-code-3)
• Pre-filtering vector search results
• Debugging "no results" or slow vector queries

Use mongodb-search instead when the request is primarily about lexical Search engine design ($search, analyzers, synonyms, facets, search alerts, or Community mongot operations) rather than model/provider semantics.

Rule Categories by Priority

Priority	Category	Impact	Prefix	Rules
1	Vector Index Creation	CRITICAL	index-	9
2	$vectorSearch Queries	CRITICAL	query-	7
3	Performance Tuning	HIGH	perf-	6
4	RAG Patterns	HIGH	rag-	4
5	Hybrid Search	MEDIUM	hybrid-	4
6	AI Agent Integration	MEDIUM	agent-	3

Quick Reference

1. Vector Index Creation (CRITICAL) - 9 rules

• index-vector-definition - Required fields: type, path, numDimensions, similarity
• index-similarity-function - Choosing cosine vs euclidean vs dotProduct
• index-filter-fields - Pre-filtering with filter type indexes
• index-quantization - Scalar (3.75x) vs binary (24x) RAM reduction
• index-dimensions-match - numDimensions must match embedding model
• index-multitenant - Single collection with tenant_id for SaaS apps
• index-views-partial - Partial indexing via MongoDB Views
• index-hnsw-options - maxEdges/numEdgeCandidates tuning
• index-automated-embedding - Server-side embedding with Voyage AI

2. $vectorSearch Queries (CRITICAL) - 7 rules

• query-vectorsearch-first - MUST be first stage in aggregation pipeline
• query-numcandidates-tuning - The 20x rule for recall vs latency
• query-ann-vs-enn - When to use exact: true
• query-prefiltering - Filter before vector comparison ($exists, $ne, $not)
• query-lexical-prefilter - Advanced text filters (fuzzy, phrase, geo) via $search.vectorSearch
• query-get-scores - Using $meta: "vectorSearchScore"
• query-same-embedding-model - Data/query embeddings must share space, dimensions, and correct input_type

3. Performance Tuning (HIGH) - 6 rules

• perf-quantization-scale - Enable at 100K+ vectors
• perf-index-in-memory - Vector indexes must fit in RAM
• perf-numcandidates-tradeoff - Benchmark recall/latency/cost trade-offs by model and numCandidates
• perf-prefilter-narrow - Reduce candidate set before vector comparison
• perf-explain-vectorsearch - Debug with explain() for vector queries
• perf-search-nodes - Dedicated Search Nodes for production

4. RAG Patterns (HIGH) - 4 rules

• rag-ingestion-pattern - Store documents with embeddings
• rag-retrieval-pattern - $vectorSearch for context retrieval
• rag-context-window - Managing LLM context limits
• rag-metadata-filtering - Filter by source, date, category

5. Hybrid Search (MEDIUM) - 4 rules

• hybrid-rankfusion - Combining vector + text search (MongoDB 8.0+, Preview)
• hybrid-scorefusion - Score-based hybrid search (MongoDB 8.2+, Preview)
• hybrid-weights - Per-query weight tuning
• hybrid-limitations - Stage restrictions plus decision matrix ($rankFusion vs $scoreFusion vs retrieval+rerank)

6. AI Agent Integration (MEDIUM) - 3 rules

• agent-memory-schema - Short-term vs long-term memory design
• agent-memory-retrieval - Semantic search over memories
• agent-session-context - Conversation history storage

Key Syntax Reference

Vector Index Definition

db.collection.createSearchIndex(
  "vector_index",
  "vectorSearch",
  {
    fields: [
      {
        type: "vector",
        path: "embedding",
        numDimensions: 1536,      // Must match your embedding model
        similarity: "cosine"      // or "euclidean" or "dotProduct"
      },
      {
        type: "filter",           // For pre-filtering
        path: "category"
      }
    ]
  }
)

$vectorSearch Query

db.collection.aggregate([
  {
    $vectorSearch: {
      index: "vector_index",
      path: "embedding",
      queryVector: [0.1, 0.2, ...],  // Your query embedding
      numCandidates: 200,             // 20x limit recommended
      limit: 10,
      filter: { category: "tech" }    // Optional pre-filter
    }
  },
  {
    $project: {
      title: 1,
      score: { $meta: "vectorSearchScore" }
    }
  }
])

The 20x Rule (numCandidates)

numCandidates = 20 × limit (minimum recommended)

limit	numCandidates	Max allowed
10	200	10,000
50	1,000	10,000
100	2,000	10,000

Higher numCandidates = better recall, slower queries.

How to Use

Read individual rule files for detailed explanations and code examples:

rules/index-vector-definition.md
rules/query-vectorsearch-first.md
rules/query-numcandidates-tuning.md
rules/_sections.md

Each rule file contains:

• Brief explanation of why it matters
• Incorrect code example with explanation
• Correct code example with explanation
• "When NOT to use" exceptions
• How to verify
• Performance impact

For release-sensitive behavior and fast official-doc routing, also read:

references/docs-navigation.md

Docs Quick Map (Release-Sensitive)

Need	Canonical Doc
Vector index definition (vector / autoEmbed)	Atlas Vector Search Type
Query stage syntax and operator support	Atlas $vectorSearch Stage
Hybrid overview and limitations	Atlas Hybrid Search
$vectorSearch in $rankFusion input	Vector Search with $rankFusion
Fusion stage availability	MongoDB $rankFusion and MongoDB $scoreFusion
Voyage model behavior and input_type	Voyage Quickstart and Voyage Text Embeddings
Voyage reranker model docs	Voyage Rerankers
Latest feature/release shifts	Atlas Vector Search Changelog

Production Readiness Checklist

• Confirm deployment path first (Atlas vs self-managed) and avoid mixing syntax between tracks.
• Confirm version gates for every hybrid/fusion flow before implementation.
• Confirm embedding contract: same space/family, correct input_type, and exact dimensions.
• Confirm index readiness (READY) and filter-field declarations before query tuning.
• Confirm retrieval settings with benchmark + explain (quality/latency/cost together).
• Confirm operational controls: least-privilege credentials, observability, and rollback-safe rollout plan.

---

MongoDB MCP Integration

For automatic verification, connect the MongoDB MCP Server:

{
  "mcpServers": {
    "mongodb": {
      "command": "npx",
      "args": ["-y", "mongodb-mcp-server", "--readOnly"],
      "env": {
        "MDB_MCP_CONNECTION_STRING": "mongodb+srv://user:pass@cluster.mongodb.net/mydb"
      }
    }
  }
}

When connected, I can automatically:

• Check existing vector indexes via mcp__mongodb__collection-indexes
• Analyze query performance via mcp__mongodb__explain
• Verify data patterns via mcp__mongodb__aggregate

Action Policy

I will NEVER execute write operations without your explicit approval.

Operation Type	MCP Tools	Action
Read (Safe)	find, aggregate, explain, collection-indexes	May run automatically to verify
Write (Requires Approval)	create-index, insert-many	Show command and wait for approval

---

Common Errors

"$vectorSearch is not allowed"

Cause: MongoDB version does not support $vectorSearch Fix: Upgrade cluster to MongoDB v6.0.11 or v7.0.2+

No results returned

Causes:

1. Different embedding model for data vs query
2. Index still building
3. Mismatched field path or index name

"Path 'field' needs to be indexed as token"

Cause: Filter field not indexed with type: "filter" Fix: Add filter field to index definition

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Full Compiled Document

For the complete guide with all rules expanded: AGENTS.md