QueryShield

Secure SQL proxy between AI agents and enterprise databases.

Agents call a single endpoint in plain English (or structured SQL). QueryShield:

1. Translates natural language → SQL via Claude with prompt caching.

2. Validates every query at the AST level — only SELECT is allowed, no stacked statements, no forbidden functions, LIMIT required.

3. Applies per-agent row-level security: schema/table whitelists and WHERE clause injection.

4. Executes against the customer DB and returns rows.

5. Logs every attempt to an append-only audit table — metadata only, never row contents.

Agents never see connection strings.

Quickstart

pip install -r requirements.txt
cp .env.example .env
# Set ANTHROPIC_API_KEY, DATABASE_URL, VAULT_KEY (see below)

python -m queryshield.start

Generate a Fernet key for VAULT_KEY once and never lose it:

python -c "from cryptography.fernet import Fernet; print(Fernet.generate_key().decode())"

Related MCP server: mcp-postgres

End-to-end flow (curl)

# 1) Boot a tenant. Returns the admin API key — copy it.
curl -X POST localhost:8000/v1/tenants?name=Acme

# 2) Register the customer DB. Connection string is encrypted at rest.
curl -X POST localhost:8000/v1/databases \
  -H 'X-Admin-Key: qs_...' \
  -H 'Content-Type: application/json' \
  -d '{
    "alias": "prod",
    "db_type": "postgresql",
    "connection_string": "postgresql://reader:secret@db.acme.internal:5432/app",
    "allowed_tables": ["users", "orders"]
  }'

# 3) Provision a scoped agent (different from admin) for your AI app.
curl -X POST localhost:8000/v1/agents \
  -H 'X-Admin-Key: qs_...' \
  -H 'Content-Type: application/json' \
  -d '{ "name": "reporting", "tenant_id": "<tenant>" }'

# 4) Set the agent's RLS policy.
curl -X POST localhost:8000/v1/policies \
  -H 'X-Admin-Key: qs_...' \
  -H 'Content-Type: application/json' \
  -d '{
    "agent_id": "<agent>",
    "database_alias": "prod",
    "allowed_tables": ["users", "orders"],
    "row_filters": { "users": "tenant_id = 42" }
  }'

# 5) The agent queries.
curl -X POST localhost:8000/v1/query \
  -H 'X-API-Key: qs_...' \
  -H 'Content-Type: application/json' \
  -d '{
    "database_alias": "prod",
    "query": "how many active users do we have?",
    "mode": "nl",
    "max_rows": 10
  }'

MCP integration

Listed in the official MCP Registry as io.github.bch1212/queryshield.

Install the client:

pip install queryshield-mcp

Then drop this into your Claude Desktop / Cursor / agent config:

{
  "queryshield": {
    "command": "queryshield-mcp",
    "env": { "QUERYSHIELD_API_KEY": "qs_..." }
  }
}

Source for the standalone PyPI package lives in packages/queryshield-mcp/.

MCP integration (legacy)

Drop this into any MCP-aware client (Claude Desktop, Cursor, custom agents):

{
  "queryshield": {
    "command": "python",
    "args": ["-m", "queryshield.mcp_server"],
    "env": {
      "QUERYSHIELD_API_KEY": "qs_...",
      "QUERYSHIELD_BASE_URL": "https://api.queryshield.io"
    }
  }
}

Tools exposed:

• query_database(database_alias, question, max_rows) — natural-language

• query_database_sql(database_alias, sql, max_rows) — pre-built SELECT

• get_audit_log(limit) — recent attempts for the calling agent

Security model

safety.py is the single most important module. Every additional check that lands there should ship with a test in tests/test_safety.py.

Pricing

Targets $32.5K MRR @ 15 customers (10 Pro + 5 Enterprise).

Deploy

The repo is Railway-ready. python -m queryshield.start is the entrypoint (reads PORT via os.getenv, since Railway exec's the start command without a shell). Provision Postgres + (optionally) Redis from Railway's marketplace and the rest is env vars.

railway up

/health is the liveness check. /ready returns 503 if the control-plane DB is unreachable.

Tests

pip install pytest
python -m pytest tests/

42 tests cover:

• AST safety (24 cases — direct DDL, comments, encoded keywords, multiple statements, forbidden functions, missing LIMIT)

• RLS engine (6 cases — whitelist enforcement, WHERE injection, conjunction with existing predicates)

• Proxy end-to-end against a SQLite "customer DB" (5 cases — happy path, blocked DML, RLS row filtering, table whitelist, cache hit)

• HTTP integration via FastAPI TestClient (7 cases — full provisioning → query flow, scoped agent with RLS, auth failures)