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

Annotations only cover readOnly/openWorld, but the description adds substantive behaviour: source is the Bank's official CSV, values are as published with no interpretation, and the pricing model is spelled out ($0.002 USDC per successful call via x402 on Base/Solana/Arc, failed calls not charged). Cost and failure-charging semantics are exactly the kind of context an agent needs before invoking a paid tool.

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?

Purpose and defaults are front-loaded in the first clause, and every element (source, no-interpretation caveat, sibling routing, price) carries information. It reads as one long run-on sentence, and the pricing/alternative clauses could be separated for scanability, but there is no filler.

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?

For a two-parameter read-only tool with 100% schema coverage and an output schema, this covers everything relevant: what it returns, its provenance, its safety profile via annotations, cost, and sibling differentiation. No critical gap remains for correct invocation.

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 high, so the baseline is within the range of a 5; the description adds the default series code and the rationale for the days parameter (calendar days back), which matches the schema's own descriptions and confirms default 30 days. Little new meaning beyond the schema, but nothing misleading and it reinforces the defaults an agent must know.

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: official Bank of England daily statistics (IADB) for a single series code, from the Bank's CSV download, over the last N days. It also names the default series (XUDLUSS) and explicitly disclaims interpretation, so an agent can tell exactly what data comes back and how it differs from the ADB and BCB siblings.

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?

Explicitly routes to alternatives with conditions ('finance_asian_dev_bank_data when you need ADB financing...', 'finance_bcb_brazil_central_bank when you need SGS time series'), which is real when-to-use guidance. It stops short of stating when this tool is insufficient (e.g., discovering valid series codes or non-daily BoE data), so selection is clear but not fully bounded.

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