sql-steward

Part of the Governed Agent Stack: free, on-prem building blocks for an AI agent you can point at a real database and audit.

A governed SQL gateway for AI agents, exposed over the Model Context Protocol. The agent never gets a connection string and never writes SQL. It calls typed tools; sql-steward compiles every query from a semantic layer you control, refuses blocked PII before the query runs, and returns rows. Same tools across SQL Server, Postgres and SQLite.

Most SQL MCP servers hand the model a run_sql tool and try to catch the bad queries on the way out. sql-steward removes the tool. There is no path from a prompt to raw SQL at your database, because the only thing the agent can do is name an entity or a metric and pick from allow-lists you wrote.

Three guarantees

1. Read-only by construction. There is no run_sql, query, or execute tool. The compiler can only ever build a SELECT, so a write isn't blocked, it's unrepresentable.

2. PII refused before retrieval. Every field can carry a PII tag. If a request touches a category your policy blocks, sql-steward refuses with a structured reason before any SQL is compiled or run.

3. Auditable. Every call, refusal and error can be recorded in a tamper-evident, hash-chained log via agent-blackbox, with audit-verify to prove nothing was rewritten.

Related MCP server: SLayer

See it in 10 seconds

pip install sql-steward      # or: pipx install sql-steward
sql-steward demo            # zero config, no API key, no agent, SQLite

1) get_metric('mrr_total', dimensions=['plan'])  -> safe aggregate
   compiled: SELECT subscriptions.plan, SUM(subscriptions.mrr) AS mrr_total
             FROM subscriptions GROUP BY subscriptions.plan LIMIT 1000
   {'plan': 'pro', 'mrr_total': 297.0}
   {'plan': 'team', 'mrr_total': 598.0}

2) get_metric('mrr_total', dimensions=['customers.country'])  -> auto-join
   compiled: ... INNER JOIN customers ON subscriptions.customer_id = customers.id ...

3) get_records('customers', fields=['id','email'])  -> PII refusal
   refused: {"kind": "pii_blocked", "detail": "Field 'customers.email' is tagged
             EMAIL_ADDRESS, which this policy refuses."}

The semantic layer

This YAML is the entire contract between the agent and your database. Review it like code.

dialect: postgres

entities:
  customers:
    table: customers
    fields:
      id: {type: int}
      name: {type: text, pii: PERSON}
      email: {type: text, pii: EMAIL_ADDRESS}
      country: {type: text}
  subscriptions:
    table: subscriptions
    fields:
      customer_id: {type: int}
      plan: {type: text}
      mrr: {type: numeric}

joins:                              # nothing reachable that isn't listed here
  - left: subscriptions
    right: customers
    on: subscriptions.customer_id = customers.id

metrics:
  mrr_total:                        # the aggregation is fixed; the agent only
    entity: subscriptions           # chooses dimensions/filters from the lists
    aggregate: sum
    field: mrr
    dimensions_allowed: [plan, status, customers.country]
    filters_allowed: [status, customers.country]

policy:
  block_pii: [EMAIL_ADDRESS, CREDIT_CARD]
  max_rows: 1000

Ask for a join that isn't defined and you get unreachable_entity, not an invented relationship. Ask to group a metric by a dimension that isn't listed and you get dimension_not_allowed.

Tools exposed to the agent

Filters are {field, op, value}; operators are =, !=, <, <=, >, >=, like, in, not in, is null, is not null. Values are always bound parameters, never inlined.

Wire it into an MCP client

servers.yaml lives wherever you point SQL_STEWARD_LAYER. Claude Desktop (claude_desktop_config.json):

{
  "mcpServers": {
    "sql-steward": {
      "command": "sql-steward",
      "env": {
        "SQL_STEWARD_LAYER": "/full/path/to/semantic.yaml",
        "SQL_STEWARD_DB_URL": "postgresql+psycopg://readonly@db.internal/warehouse"
      }
    }
  }
}

SQL_STEWARD_DB_URL is a SQLAlchemy URL, so the same server reads SQL Server (mssql+pyodbc://...), Postgres (postgresql+psycopg://...) or SQLite (sqlite:///path.db). Install the matching driver with the extras: pip install "sql-steward[postgres]" or "[mssql]".

Optional: the rest of the stack

The semantic layer is the primary control. These are extra layers, all opt-in, and no-ops if the library isn't installed:

pip install "sql-steward[rbac,mask,audit]"

export SQL_STEWARD_POLICY=/path/to/policy.yaml   # query-warden second-pass role check
export SQL_STEWARD_ROLE=analyst
export SQL_STEWARD_MASK=1                         # pii-veil masks anything left in results
export SQL_STEWARD_AUDIT_DB=logs/steward.db       # agent-blackbox audit chain (on if installed)
export SQL_STEWARD_QUERY_BUDGET=200               # hard cap on queries per role per session
export SQL_STEWARD_EMBED_URL=http://localhost:11434/api/embeddings  # local embeddings for semantic_search
export SQL_STEWARD_EMBED_MODEL=nomic-embed-text

Semantic search (pgvector)

Give an entity a search block pointing at a pgvector column and the agent gets a semantic_search tool, governed exactly like everything else (PII refused, results masked, calls audited):

entities:
  documents:
    table: documents
    fields:
      id: {type: int}
      title: {type: text}
      embedding: {type: vector}
    search:
      vector_column: embedding
      dim: 768
      returns: [id, title]

The query text is embedded locally (set SQL_STEWARD_EMBED_URL to a local Ollama endpoint, so nothing leaves the building), and matched with pgvector's <=> operator. PostgreSQL only. The embedding column is never returned.

• query-warden re-checks the compiled SQL against a role policy.

• pii-veil masks any PII that survives into result rows.

• agent-blackbox records every call in a hash-chained ledger; sql-steward audit-verify checks it.

How this is different

A typical SQL MCP validates arbitrary SQL the model wrote (a blocklist: catch what's bad). sql-steward compiles SQL from definitions you wrote (an allow-list: only what's described exists). The read-only and PII guarantees hold by construction rather than by inspection, and the query surface is the same across three engines.

Develop

git clone https://github.com/Pawansingh3889/sql-steward
cd sql-steward
pip install -e ".[dev]"
pytest -q

License

MIT