Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?

Annotations already declare readOnlyHint/destructiveHint/openWorld, but the description adds substantial value beyond them: DB-enforced read-only role, tier-based statement timeout, hard 300-row cap, per-5-second Query Unit pricing, and the exact auth failure mode ('a trial key belongs to no organization and is refused'). The only blemish is describing a `tier` selector and `explain_only` flag that the schema does not expose.

Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.

Is the description appropriately sized, front-loaded, and free of redundancy?

The core constraint is correctly front-loaded in the first sentence, but the body is long and repeats itself — the per-5-second pricing and the hold-then-settle mechanic are explained in the pricing paragraph, again in the Access paragraph, and a third time under `usage.units_charged`. Roughly a third of the text could be cut without losing information.

Shorter descriptions cost fewer tokens and are easier for agents to parse. Every sentence should earn its place.

Given the tool's complexity, does the description cover enough for an agent to succeed on first attempt?

With no output schema, documenting the return fields (rows, row_count, duration_ms, quote, usage) is genuinely necessary and done well, including the warning that planner_cost is advisory and uncalibrated. Auth requirements, row cap and timeouts are all covered; only the phantom `tier`/`explain_only` references keep it from being fully trustworthy.

Complex tools with many parameters or behaviors need more documentation. Simple tools need less. This dimension scales expectations accordingly.

Does the description clarify parameter syntax, constraints, interactions, or defaults beyond what the schema provides?

Schema coverage is 100% for the two real parameters, so the baseline would be 3, but the description actively introduces semantics for parameters that do not exist ('`tier` buys the ceiling (standard 15s, extended 60s)' and 'send `explain_only`'). An agent reading this would construct calls that fail validation, which is worse than adding nothing.

Input schemas describe structure but not intent. Descriptions should explain non-obvious parameter relationships and valid value ranges.

Does the description clearly state what the tool does and how it differs from similar tools?

States a specific verb and resource ('One SELECT (or WITH) over the public data-layer tables') plus the enforcement model, which immediately separates it from the list_*/get_* siblings that return canned collections. An agent can tell this is the ad-hoc query escape hatch without opening the schema.

Agents choose between tools based on descriptions. A clear purpose with a specific verb and resource helps agents select the right tool.

Does the description explain when to use this tool, when not to, or what alternatives exist?

It gives real routing advice — 'Call get_account for the tier table' — and frames the cost model so an agent can judge whether a query is worth running. But it also directs the agent to 'send `explain_only`' and says '`tier` buys the ceiling', and neither is a parameter in the schema (additionalProperties: false), so the guidance points at options that will be rejected. That undercuts otherwise strong when-to-use context.

Agents often have multiple tools that could apply. Explicit usage guidance like "use X instead of Y when Z" prevents misuse.