virtuoso_mcp

MCP Bridge for Virtuoso (Community Edition)

A custom MCP server that lets OpenClaw (or any LLM agent) access Virtuoso Community Edition as a semantic backend without running raw SPARQL from the agent. The MCP layer exposes structured tools that orchestrate queries and later aggregate data across additional stores (PostgreSQL, CouchDB, Qdrant).

Vision

• LLMs never issue SQL/SPARQL directly—they call MCP tools.
• The MCP server handles orchestration, sanitization, rate limiting, and multi-source composition.
• Start with Virtuoso (SPARQL) and progressively add new connectors.

Architecture

LLM Agent (OpenClaw)
↓
MCP Server
├── Virtuoso (SPARQL)
├── PostgreSQL
└── Vector DBs (e.g., Qdrant)

Guardrails (current)

• sparql_query is SELECT-only and always uses a LIMIT (default SPARQL_DEFAULT_LIMIT).
• Any LIMIT above SPARQL_MAX_LIMIT is clamped.
• Example data loads are disabled unless MCP_ALLOW_EXAMPLE_LOAD=true is set.

Configuration (env)

run.sh and test.sh will source a local .env file if present. Use .env.example as a template.

• VIRTUOSO_ENDPOINT (default http://localhost:8891/sparql; can be .../sparql-auth for digest auth)
• VIRTUOSO_USER / VIRTUOSO_PASS (optional; enables HTTP Digest auth)
• GRAPH_URI (used for prefix :)
• SPARQL_TIMEOUT (seconds)
• SPARQL_UPDATE_TIMEOUT (seconds)
• SPARQL_DEFAULT_LIMIT
• SPARQL_MAX_LIMIT
• MCP_ALLOW_EXAMPLE_LOAD (true/false)
• EXAMPLE_GRAPH (graph URI for load_examples, default http://example.org/catalog#test)

Design Principles

1. Tool-based abstraction: Provide helpers such as sparql_query, get_entities_by_type, list_graphs instead of exposing raw SPARQL.
2. Gradual complexity: Ship a minimal working setup, then layer on helper tooling, schema introspection, and connectors.
3. Separation of concerns: Virtuoso stores RDF, MCP runs tool interfaces, and LLMs focus on reasoning/tool selection.
4. Guardrails: Raw queries are SELECT-only, bounded by a default LIMIT, and clamped to a maximum size.

Success Criteria

• Phase 1: MCP tool (sparql_query) returns valid SPARQL JSON results.
• Phase 2: LLM relies on helper tools instead of free-form queries (Stage 2 helpers are now present).
• Phase 3: Multiple data sources accessible through a unified MCP interface.

Example loading (test instances)

Set MCP_ALLOW_EXAMPLE_LOAD=true to enable the load_examples tool. It loads Turtle fixtures (e.g., examples/catalog_fixture.ttl) into the EXAMPLE_GRAPH (default http://example.org/catalog#test). This is meant for test instances only and uses harmless sample data.

Note: the example files are Turtle (.ttl) and the loader sends them as SPARQL Update with Turtle prefixes preserved.

Current helper tools

Core query/navigation

• sparql_query (SELECT-only, LIMIT enforced)
• list_graphs
• search_label
• get_entities_by_type
• get_predicates_for_subject
• get_labels_for_subject
• traverse_property (follow any property link, incoming or outgoing, and get labels/descriptions)

Ontology discovery (generic, reusable across domain layers)

• list_classes (list ontology classes, optional term filter)
• list_properties (list ontology properties, optional term/domain/range filters)
• describe_class (class label/comment + properties declaring it as domain)
• describe_property (property label/comment/domain/range/type + usage samples)

Relationship helpers

• describe_subject (see all predicates/objects for a subject with optional labels)
• path_traverse (walk a configured property path from a subject and return each step)
• property_usage_statistics (count property usage and sample subjects/objects)
• batch_insert (send TTL or multiple triples in a single guarded update; useful for staging domain changes)

Update/test helpers

• insert_triple (single-triple update helper)
• load_examples (optional; requires MCP_ALLOW_EXAMPLE_LOAD=true; loads fixtures such as examples/catalog_fixture.ttl)

MCP JSON-RPC compatibility (v0.1, minimal)

The /mcp endpoint supports:

1) Legacy tool router (unchanged): { "tool": "...", "input": {...} } 2) Minimal JSON-RPC 2.0 messages:

• initialize
• tools/list
• tools/call

This is intended to work with OpenClaw’s MCP bridge and allow tool discovery + calling with request ids.

Layering recommendation

Keep ontology discovery in virtuoso_mcp so any specialized layer (garden, inventory, analytics, etc.) can reuse it. Domain modules should call these generic tools instead of re-implementing ontology probing logic.

Domain plugin layers

To expose domain-specific helpers automatically, set the DOMAIN_LAYERS environment variable to a comma-separated list of Python modules (the default is garden_layer.plugin). Each module must expose a register_layer(tools) function that receives the MCP TOOLS dictionary and adds prefixed entries (e.g., garden_add_seedling). virtuoso_mcp calls those hooks at startup, so simply pip install --upgrade git+https://repo.home.world.eu.org/lucky/garden_layer.git and update DOMAIN_LAYERS to include garden_layer.plugin. The workspace already contains the garden_layer source tree (workspace/garden_layer), so during local iteration you can also install it in editable form: pip install -e /home/lucky/.openclaw/workspace/garden_layer. The new tools appear in the /mcp tool list (curl -sS http://127.0.0.1:8501/ | jq .tools) without changing the single /mcp endpoint surface.