introspect (Tier-1 — live read-only)

Tier 0 reads files. Many checks need runtime truth a file cannot give: real index usage, FK-without-index joins, table/row sizes, RLS state, extensions, engine version, autovacuum/wraparound state. introspect is the only path to that, and it is read-only by contract. See references/data-tiers.md.

Connection path (prefer MCP)

1. DB MCP server (preferred) — cleaner permission boundary. Use only read-class tools whose names match *query* / *read* / *list* / *describe* / *schema*. A generic write-capable query tool is routed through the same read-only validator below; a PreToolUse hook on mcp__.* backs this up. List available tools first; never call a tool that mutates.
2. Else a least-privilege read-only connection string read from $DATABASE_URL in the environment — never echoed, never written into a finding. redactSecrets() scrubs any credential that reaches a finding, report, or log.
3. Neither available → emit status: needs_api on every dependent check with the reason and the minimal grant needed. Never silently pass and never fabricate row counts, sizes, or plans.

Read-only contract (enforced before any query)

• SET default_transaction_read_only = on;
• a bounded statement_timeout (e.g. SET statement_timeout = '15s';).
• only SELECT / EXPLAIN (no ANALYZE write side effects beyond plan) / catalog reads. Reject anything else.

Catalogs read (no row data)

• Postgres: version(), information_schema, pg_catalog, pg_class, pg_index, pg_indexes, pg_constraint, pg_policies, pg_extension. Tier-2 adds pg_stat_user_indexes / pg_stat_user_tables, pg_stat_statements, age(datfrozenxid).
• MySQL/MariaDB: information_schema (TABLES, STATISTICS, KEY_COLUMN_USAGE), SHOW ENGINE INNODB STATUS (read).
• Mongo: db.stats(), $indexStats, $collStats (Tier-2).
• Cassandra: system_schema, nodetool tablehistograms (Tier-2).