a2db MCPサーバー

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Agent: "Show me active users and their recent orders"
  ↓
a2db execute → 2 queries, 1 call, structured results
  ↓
Agent: "Got it — 847 active users, avg order $42.50"

Why a2db?

Most database MCP servers make you run one query at a time, repeat connection details on every call, and return results double-encoded inside JSON strings. a2db fixes all of that:

• Pre-configured connections — define databases in .mcp.json with --register, agent queries immediately
• Batch queries — run multiple named queries in a single tool call
• Default connection — set connection once, use it across all queries in a batch
• Clean output — structured JSON envelope with compact TSV data and per-query timing (see why TSV?)
• Read-only enforced — SQLGlot AST parsing blocks all write operations
• All drivers bundled — pip install a2db and you're done
• Secrets stay in env — ${DB_PASSWORD} in DSNs, expanded only at connection time

Supported Databases

Database	Driver	Async
PostgreSQL	asyncpg	native
SQLite	aiosqlite	native
MySQL / MariaDB	mysql-connector-python	wrapped
Oracle	oracledb	wrapped
SQL Server	pymssql	wrapped

Quick Start

pip install a2db

As an MCP Server (recommended)

Claude Code (with pre-configured connection):

claude mcp add -s user a2db -- a2db-mcp \
  --register myapp/prod/main 'postgresql://user:${DB_PASSWORD}@host/mydb'

Claude Code (minimal — agent calls login on demand):

claude mcp add -s user a2db -- a2db-mcp

Claude Desktop / Cursor / any MCP client (.mcp.json):

{
  "mcpServers": {
    "a2db": {
      "command": "uvx",
      "args": [
        "a2db-mcp",
        "--register", "myapp/prod/main", "postgresql://user:${DB_PASSWORD}@host/mydb"
      ],
      "env": {
        "DB_PASSWORD": "your-password-here"
      }
    }
  }
}

Multiple databases:

{
  "args": [
    "a2db-mcp",
    "--register", "myapp/prod/main", "postgresql://user:${DB_PASSWORD}@host/maindb",
    "--register", "myapp/prod/analytics", "postgresql://user:${DB_PASSWORD}@host/analytics"
  ]
}

--register pre-registers connections at server startup — the agent can query immediately. Passwords use ${ENV_VAR} syntax and are expanded at connection time, never stored in plaintext.

As a CLI

# Save a connection (validates immediately)
a2db login -p myapp -e prod -d main 'postgresql://user:${DB_PASSWORD}@localhost/mydb'

# Query
a2db query -p myapp -e prod -d main "SELECT * FROM users LIMIT 10"

# JSON output
a2db query -p myapp -e prod -d main -f json "SELECT * FROM users LIMIT 10"

# Explore schema
a2db schema -p myapp -e prod -d main tables
a2db schema -p myapp -e prod -d main columns -t users

# List / remove connections
a2db connections
a2db logout -p myapp -e prod -d main

MCP Tools

Tool	Description
login	Save a connection — validates by connecting first
logout	Remove a saved connection
list_connections	List connections (no secrets exposed)
execute	Run named batch queries with pagination
search_objects	Explore schema — tables, columns, with detail levels

execute — the core tool

Named dict with default connection (preferred):

{
  "connection": {"project": "myapp", "env": "prod", "db": "main"},
  "queries": {
    "active_users": {"sql": "SELECT id, name FROM users WHERE active = true"},
    "recent_orders": {"sql": "SELECT id, total FROM orders ORDER BY created_at DESC LIMIT 5"}
  }
}

List format (auto-named q1, q2, ...):

{
  "connection": {"project": "myapp", "env": "prod", "db": "main"},
  "queries": [
    {"sql": "SELECT COUNT(*) AS cnt FROM users"},
    {"sql": "SELECT AVG(total) AS avg_order FROM orders"}
  ]
}

Response (TSV format — default):

{
  "active_users": {
    "data": "id\tname\n1\tAlice\n2\tBob\n3\tCharlie",
    "rows": 3,
    "truncated": false,
    "time_ms": 12
  },
  "recent_orders": {
    "data": "id\ttotal\n501\t129.00\n500\t49.99",
    "rows": 2,
    "truncated": false,
    "time_ms": 8
  }
}

No ::text casts needed — integers, floats, timestamps, arrays, NULLs all work natively.

Error context

When a query fails with a column error, a2db enriches the message:

column "nme" does not exist
Did you mean: name?
Available columns: id (integer), name (text), email (text), active (integer)

Why TSV?

LLM context windows are expensive. JSON row data is verbose — every row repeats every column name, adds braces, commas, and quotes. TSV is a flat grid: one header row, then just values separated by tabs.

For a 100-row, 5-column result set, TSV typically uses 40-60% fewer tokens than JSON row format. The structured JSON envelope still gives you metadata (row count, truncation status) — only the row payload is TSV.

Set format="json" if you need full structured output with column names on every row.

Security

Read-Only Enforcement

Every query is parsed by SQLGlot before execution:

• Blocked: INSERT, UPDATE, DELETE, DROP, TRUNCATE, ALTER, CREATE, GRANT, REVOKE
• Bypass-resistant: multi-statement attacks and comment-wrapped writes are caught at the AST level, not just keyword matching
• Allowed: SELECT, UNION, EXPLAIN, SHOW, DESCRIBE, PRAGMA

This is defense-in-depth — you should also use a read-only database user, but a2db won't let writes through even if the user has write permissions.

Write support is implemented in the core but not yet exposed via MCP. Planned: per-connection write permissions, explicitly enabled by the human operator — not the agent. See TODO.md.

Credential Storage

Connections are saved in ~/.config/a2db/connections/ as TOML files.

• ${DB_PASSWORD} syntax — environment variable references are stored literally and expanded only at connection time. Secrets stay in your environment, not on disk.
• No secrets in list output — list_connections shows project/env/db and database type, never DSNs or passwords
• Connection files are local to your machine and outside any repository

Deployment Scope

a2db currently runs as a local stdio MCP server. It inherits environment variables from the process that launches it (your shell, Claude Code, Docker). This is the standard model for local MCP servers — the same approach used by DBHub, Google Toolbox, and others.

Planned: remote HTTP transport with OAuth 2.1 per the MCP spec. For now, if running in Docker, inject secrets via environment variables at container runtime.

Comparison

Feature	a2db	DBHub	Google Toolbox	PGMCP	Supabase MCP
Databases	5 (PG, SQLite, MySQL, Oracle, MSSQL)	5 (PG, MySQL, MSSQL, MariaDB, SQLite)	40+ (cloud + OSS)	PG only	PG (Supabase)
Batch queries	Named dict + list	Semicolon-separated	No	No	No
Default connection	Set once, use for all	Per-query	N/A	Single DB	Single project
Read-only	SQLGlot AST (enforced)	Keyword check (config)	Hint/annotation	Read-only tx + regex	Config flag
Write support	Planned (per-connection)	Config flag	Via tool definition	No	Config flag
Output	JSON + TSV data	Structured text	MCP protocol	Table / JSON / CSV	JSON
Schema discovery	3 detail levels	Dedicated tool	Prebuilt tools	Via NL-to-SQL	Dedicated tools
Pre-configured	--register in MCP config	Config file	YAML config	Env var	Cloud-managed
Credentials	${ENV_VAR} in DSN	DSN strings	Env vars + GCP IAM	Env var	OAuth 2.1
Drivers bundled	All included	All included	Varies	Built-in	Managed
CLI	Yes	No	Yes	Yes	No
Error context	Column suggestions + types	No	No	No	No
License	Apache 2.0	MIT	Apache 2.0	Apache 2.0	Apache 2.0

When to use what:

• a2db — multi-DB batch queries with clean output, agent-first design, fast setup
• DBHub — custom tools via TOML config, web workbench UI
• Google Toolbox — GCP ecosystem, IAM integration, 40+ sources
• PGMCP — natural-language-to-SQL for PostgreSQL (requires OpenAI key)
• Supabase MCP — full Supabase platform management (edge functions, branching, storage)

Setup by Environment

Local (macOS / Linux)

pip install a2db

# CLI
a2db login -p myapp -e dev -d main 'postgresql://user:pass@localhost/mydb'

# Or add as MCP server (see Quick Start)

Docker

FROM python:3.12-slim
RUN pip install a2db
CMD ["a2db-mcp", "--register", "myapp/prod/main", "postgresql://user:${DB_PASSWORD}@host/mydb"]

docker run -e DB_PASSWORD=secret -i my-a2db-image

Secrets are injected as environment variables at runtime — never baked into the image.

CI / Automation

pip install a2db

# Pre-configured — no login needed
a2db-mcp --register myapp/ci/main "postgresql://ci_user:${CI_DB_PASSWORD}@db-host/mydb"

# Or use CLI directly
a2db login -p myapp -e ci -d main "postgresql://ci_user:${CI_DB_PASSWORD}@db-host/mydb"
a2db query -p myapp -e ci -d main "SELECT COUNT(*) FROM migrations"

Development

make bootstrap   # Install deps + hooks
make check       # Lint + test + security (full gate)
make test        # Tests with coverage (90% minimum)
make lint        # Lint only (never modifies files)
make fix         # Auto-fix + lint

License

Apache 2.0

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