pglens gives agents the context to get it right the first time: column_values shows real distinct values with counts, find_join_path does BFS over the FK graph and returns join conditions through intermediate tables, describe_table gives columns/PKs/FKs/indexes in one call. Plus production health tools like bloat_stats, blocking_locks, and sequence_health.
Everything runs in readonly transactions, identifiers escaped via Postgres's quote_ident(), no extensions required. Works on any Postgres 12+ (self-hosted, RDS, Aurora, etc.). Two dependencies: asyncpg and mcp.
https://github.com/janbjorge/pglens
pip install pglen
On query guardrails: every query runs in a readonly transaction and results are capped at 500 rows via a wrapping SELECT * FROM (...) sub LIMIT 500. There's also explain_query which returns the plan without executing, so agents can check before running something expensive. That said, there's no cost-based gate that blocks a bad plan automatically; that's an interesting idea worth exploring.
