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Describe the enhancement you want to request

Problem

The agent can run an analytical query that scans terabytes. e.g. SELECT * FROM events and incur a large warehouse bill before anyone realizes. Today there is no pre-execution cost estimate or budget guardrail in sql_execute. The FinOps tooling that exists is all historical (analyzing spend after the fact), not preventive.

Proposed enhancement

An opt-in cost firewall that estimates a query's scan cost before it runs and asks for confirmation when it exceeds a configured budget:

• Estimate via warehouse-native dry-run where available (BigQuery dryRun returns exact bytes processed for free; no execution, no cost).
• New governance config: max_query_cost_usd, max_bytes_scanned, cost_per_tib_usd.
• When a query is over budget, prompt the user (with a hint to try sql_optimize first).
• Disabled by default; warehouses without estimation support are unaffected (the guard fails open).

Benefits

Cost control is core to the data-engineering workflow. A pre-flight guardrail turns a surprise 40ドル query into approve, or optimize first without changing behavior for anyone who doesn't set a budget.

Notes

I have an implementation ready (BigQuery estimator + the default-off guard framework) and can open a PR referencing this issue. The estimator is designed to extend to other warehouses (Snowflake via EXPLAIN, etc.).