Cost Guard MCP Server

cost-guard-mcp

Pre-flight query cost & result-size guardrails for AI agents, across BigQuery, Snowflake, and Databricks — before the query ever runs.

Why

An AI agent using a warehouse MCP can silently trigger a full-table scan that costs hundreds of dollars, or return millions of rows that flood its own context window. No existing warehouse MCP tells the agent "how much will this cost" or "how much data will this return" before running the query.

What makes this different

• Every cost estimate discloses its accuracy tier — PRECISE (BigQuery dryRun), UPPER_BOUND (Snowflake EXPLAIN), or HEURISTIC (Databricks EXPLAIN COST) — so your agent never over-trusts a heuristic number.
• Per-call bounds — run_query_bounded takes max_bytes_billed / max_rows / max_estimated_cost_usd on each call; no shared session state required.
• Zero infrastructure — a single local stdio process. No database, no gateway, no Docker Compose.

Tools

• check_credentials(engine, warehouse?) — verifies credentials/connectivity without running any real query; call this first after configuring a new engine. Supports BigQuery, Snowflake, and Databricks.
• describe_engine_capabilities(engine) — what's exact vs. approximate for this engine.
• estimate_query_cost(engine, sql, warehouse?, warehouse_size?, edition?) — pre-flight cost estimate, tagged with its accuracy tier. Supports BigQuery, Snowflake, and Databricks. warehouse_size (Snowflake/Databricks) and edition (Snowflake) default to the smallest/standard tier if omitted — set them to match the warehouse you actually run on, or the dollar figure understates cost on a larger one.
• run_query_bounded(engine, sql, max_bytes_billed?, max_rows?, max_estimated_cost_usd?, warehouse?, warehouse_size?, edition?) — refuses to run if the estimate exceeds your bound. Supports BigQuery, Snowflake, and Databricks.

Setup

BigQuery

Set GOOGLE_APPLICATION_CREDENTIALS to a service-account key file path (or run gcloud auth application-default login).

Snowflake

Set SNOWFLAKE_ACCOUNT, SNOWFLAKE_USER, SNOWFLAKE_ROLE (required — no default, never ACCOUNTADMIN), and either SNOWFLAKE_PRIVATE_KEY_PATH (preferred) or SNOWFLAKE_PASSWORD (discouraged).

Databricks

Set DATABRICKS_SERVER_HOSTNAME and DATABRICKS_HTTP_PATH (from the SQL warehouse's Connection Details tab), and either DATABRICKS_TOKEN (a personal access token, simplest) or DATABRICKS_CLIENT_ID + DATABRICKS_CLIENT_SECRET (OAuth machine-to-machine via a service principal, preferred for automated use). Only Serverless SQL warehouses are priced accurately — see Known Limitations.

A note on credentials with MCP hosts

Whatever MCP client/host you use (Claude Desktop, etc.) spawns this server as its own subprocess — it does not automatically inherit your shell's environment variables, even if they're set in your .zshrc/.bashrc. Put them directly in the host's server config instead — see .mcp.json.example for the exact block, and the "Use with other AI coding tools" section below for where each specific tool wants it.

Install

uvx cost-guard-mcp

Also published on the official MCP Registry as io.github.mcpsmiths/cost-guard-mcp.

For local development instead:

git clone https://github.com/mcpsmiths/cost-guard-mcp.git
cd cost-guard-mcp
uv sync
uv run cost-guard-mcp

Or via Docker:

docker build -t cost-guard-mcp .
docker run -i --rm -e GOOGLE_APPLICATION_CREDENTIALS=/creds.json -v /path/to/service-account.json:/creds.json cost-guard-mcp

Quickstart (~5 minutes to your first estimate)

This walks through the fastest path to a real tool call — no data of your own required (it uses a public BigQuery dataset), no Snowflake trial signup needed.

1. Get a GCP project with the BigQuery API enabled. Any project works, including the free-tier Sandbox mode (no billing card required to run dryRun, which is all estimate_query_cost does). Create one at console.cloud.google.com if you don't have one.

2. Get Application Default Credentials: run gcloud auth application-default login locally, or create a service-account key and point GOOGLE_APPLICATION_CREDENTIALS at its JSON file.

3. Add the server to your MCP client — see .mcp.json.example, filling in only GOOGLE_APPLICATION_CREDENTIALS (leave the Snowflake vars out entirely for this quickstart).

4. Restart your MCP client so it picks up the new server config, then ask your agent to call check_credentials on bigquery. This confirms your setup without running any real query — you should get back "ok": true and a detail line naming your project. If you get "ok": false instead, the detail field explains exactly what's missing (usually GOOGLE_APPLICATION_CREDENTIALS not making it through to the server process — see the credentials note above, and double check the value is set inside the client's own server config block, not just your shell).

5. Ask your agent to call estimate_query_cost against a public dataset — for example:

   Use cost-guard-mcp's estimate_query_cost tool on bigquery for this query: SELECT name, SUM(number) AS total FROM bigquery-public-data.usa_names.usa_1910_2013 GROUP BY name ORDER BY total DESC LIMIT 10

6. You'll know it worked when the response looks like this — the exact numbers will differ, but accuracy_tier should read PRECISE:

   {
     "engine": "bigquery",
     "accuracy_tier": "PRECISE",
     "estimated_bytes": 320866545,
     "estimated_cost_usd": 0.001842,
     "currency": "USD",
     "caveats": []
   }
   

Use with other AI coding tools

cost-guard-mcp is a standard stdio MCP server — any MCP-compatible client works, not just Claude Desktop. Every client ultimately runs the same command/args/env; only the wrapping file format differs, so there's one canonical definition — .mcp.json.example — instead of a separately maintained copy per tool below.

There is no single file every tool reads automatically (each looks in its own location), but three of the four use the exact same mcpServers wrapper .mcp.json.example already has, so those need nothing more than copying it into place. Fill in your real credential values, then:

Codex is the one genuine exception (TOML, not JSON), so it still needs its own block:

[mcp_servers.cost-guard-mcp]
command = "uvx"
args = ["cost-guard-mcp"]

[mcp_servers.cost-guard-mcp.env]
GOOGLE_APPLICATION_CREDENTIALS = "/path/to/service-account.json"
BIGQUERY_PROJECT = "your-project-id"
SNOWFLAKE_ACCOUNT = "your-account"
SNOWFLAKE_USER = "your-user"
SNOWFLAKE_ROLE = "your-role"
SNOWFLAKE_PRIVATE_KEY_PATH = "/path/to/rsa_key.p8"
DATABRICKS_SERVER_HOSTNAME = "your-workspace.cloud.databricks.com"
DATABRICKS_HTTP_PATH = "/sql/1.0/warehouses/your-warehouse-id"
DATABRICKS_TOKEN = "your-personal-access-token"

Observability

• Structured logging (always on, no configuration needed) — every tool call and warehouse-client failure is logged via Python's standard logging module. Since stdout is the MCP transport channel in stdio mode, logging's default (stderr) is what this server relies on — never redirect these loggers to stdout. What gets logged:

  • Every tool call (check_credentials, describe_engine_capabilities, estimate_query_cost, run_query_bounded) logs one INFO record on completion: tool=<name> outcome=<success|error> elapsed_ms=<n>.
  • Every warehouse-client failure (BigQuery/Snowflake/Databricks) logs one WARNING record: engine=<engine> warehouse_client_call_failed message=<redacted> — message is always the same secret-redacted text the caller gets back, never the raw exception.
  • Every run_query_bounded refusal logs one INFO record naming the engine and the specific refusal reason (cost_cap_exceeded, byte_cap_exceeded, or row_cap_exceeded).
  • Each engine's 120-second execution watchdog logs one WARNING record before cancelling a still-running query.
  • None of the above ever logs a credential, connection string, or raw (unredacted) warehouse-client exception message — the same redact_secrets helper that sanitizes what a tool caller sees is applied before anything is logged.

• OpenTelemetry tracing (opt-in, off by default) — the underlying mcp SDK ships an OpenTelemetryMiddleware on by default for every server, wrapping each inbound message in a SERVER span, but that middleware is a documented no-op until a real exporter is registered — this project registers none unless you ask for it. Set OTEL_EXPORTER_OTLP_ENDPOINT to your OTel Collector's endpoint (e.g. http://localhost:4317) to turn it on: at that point cost-guard-mcp constructs a TracerProvider with a gRPC OTLP exporter pointed at that endpoint and registers it as the global tracer provider before the server starts running. Leave the env var unset and nothing changes — no exporter is constructed, and the two extra dependencies below never need to be installed. Requires the otel extra:

  uv sync --extra otel
  # or: pip install "cost-guard-mcp[otel]"
  

Known limitations

• Snowflake cost estimates are calibrated from the caller's own recent query history (INFORMATION_SCHEMA.QUERY_HISTORY, no elevated privilege required) when an exact repeat of the same SQL text has run before - falling back to a coarse byte-size-tier heuristic otherwise. This only fires on an exact repeated query; a genuinely novel query always uses the heuristic. Result-cache hits are deliberately excluded from the average (a cached, near-instant repeat would otherwise corrupt calibration toward underestimating future runtime). Query history ingestion has its own latency - a query run moments ago may not yet be visible to the lookup, in which case it safely falls back to the heuristic rather than erroring.
• Databricks calibration was investigated and found blocked: its Query History REST API returns the query text as "<REDACTED>" unconditionally on the account tested, even for the caller's own queries and even with include_metrics=True - confirmed server-side via a direct SDK source read, not something a client-side parameter can bypass. Not implemented for Databricks as a result; may be revisited if a future paid workspace confirms this is a toggleable setting there.
• Databricks cost estimates are always HEURISTIC (the least precise tier) - Databricks has no dry-run, and EXPLAIN COST's byte estimates are frequently unavailable.
• Databricks pricing only models Serverless SQL warehouses - Classic/Pro warehouses use different (lower) DBU rates plus a separate cloud VM cost not modeled here.
• Databricks has no per-query warehouse override - the SQL warehouse is fixed by DATABRICKS_HTTP_PATH at connect time.
• Snowflake's UPPER_BOUND estimate excludes Cortex AI Function ("AI Credits") cost.
• Snowflake warehouse generation (Gen1 vs. the newer, pricier Gen2) is detected on a best-effort basis via SHOW WAREHOUSES and CURRENT_REGION() (both ordinary, non-privileged SQL) to pick the correct credit rate — Gen2 bills ~1.35x Gen1 on AWS/GCP and ~1.25x on Azure. Detection needs a warehouse to be specified; if it isn't, or the lookup fails for any reason (permission, timeout, unrecognized response shape), the estimate safely falls back to Gen1 rates with an explicit caveat rather than erroring — since Gen2 is now the default for new standard warehouses in most regions, an undetectable generation means the real cost may be higher than this estimate. Live- verified 2026-09-18 against a real trial account: its default warehouse (COMPUTE_WH) is genuinely Gen2 on AWS, and this detection correctly identified it and applied the Gen2 rate.
• BigQuery Editions/capacity-billed projects cannot get a dollar estimate — only a byte count (capacity billing has no fixed $/byte rate).
• BigQuery dry runs always report 0 bytes processed for tables protected by row-level security, by design, to prevent a side-channel — dry_run adds a caveat when it sees 0 bytes against a non-empty referenced_tables list, but a $0.00 estimate on such a query must never be treated as proof the query is free to run.
• BigQuery remote functions and BigQuery ML remote-model inference (e.g. ML.GENERATE_TEXT) incur separate Cloud Run/Vertex AI billing that this byte-based dollar estimate does not include — dry_run flags this with a conservative text-based heuristic (ML.GENERATE_TEXT or CREATE FUNCTION + REMOTE in the query text) rather than the dry-run response's referencedRoutines field. Attempted to resolve this live (2026-09-23) once BigQuery credentials existed in CI: the deliberately least-privilege service account correctly lacks bigquery.datasets.create, so a throwaway routine could not be created to test dry-run population of referencedRoutines — and granting broader access just to answer this research question would contradict the least-privilege posture this project maintains everywhere else. Still an open follow-up for whoever next has a project where they can test this safely.
• run_query_bounded gives up on a still-running query after 120 seconds and cancels it (BigQuery: QueryJob.cancel(); Snowflake: SYSTEM$CANCEL_QUERY; Databricks: Cursor.cancel() from a watchdog thread) rather than waiting indefinitely — a query stuck behind slot contention or a cold/suspended warehouse would otherwise block the tool call, and keep burning warehouse-seconds the whole time, defeating the point of a "bounded" tool.
• The underlying mcp SDK can drop an in-flight tool-call response if the client closes stdin before the tool finishes (upstream issue modelcontextprotocol/python-sdk#2678, open since 2026-05, unresolved after several attempted fixes) — no known real-world exposure for well-behaved clients that keep stdin open for the session, but worth knowing about given this server's tool calls can run up to 120 seconds.
• A client-sent MCP cancellation notification against an in-flight run_query_bounded call now detaches promptly at the MCP bookkeeping level, but the warehouse-side query itself keeps running in the abandoned background thread until the existing per-engine watchdog (~120s, see above) fires on its own — this fix does not by itself stop the warehouse from billing for that abandoned query any sooner.

More docs

• ARCHITECTURE.md — component/data-flow map
• DECISIONS.md — why the design looks the way it does
• CONTEXT.md — terminology glossary (BigQuery/Snowflake concepts that sound alike but aren't)
• CHANGELOG.md — release history
• CONTRIBUTING.md / AGENTS.md — contributing and build/test/lint commands
• SECURITY.md — vulnerability reporting

License

MIT