X_MCP_QUERY_REJECTED
Means
db.query was not given one read-only statement
Typical cause
empty input, a batch, a non-read leading keyword, a mutating keyword anywhere at statement level (including a data-modifying CTE), a locking clause (FOR UPDATE/FOR SHARE), EXPLAIN ANALYZE, or a call from a banned function family matched by prefix (pg_read_*, pg_ls_*, lo_*, dblink, pg_advisory_*, pg_sleep*, set_config, …)
Fix
send exactly one read-only SELECT/WITH/EXPLAIN/SHOW/TABLE/VALUES statement. db.query has no write path at all: change data by calling an action exposed with mcp: { expose: true }, and change schema with db.migrate after x db branch create <name>