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

Annotations already mark this as read-only, idempotent, and non-destructive; the description adds useful behavioral context: it is monthly, returns a trailing window of N months, requires no auth, and carries a UK Open Government Licence. No contradiction with annotations.

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?

Three sentences: definition, parameter behavior, and data/licensing context. Front-loaded with the key definition; 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 single-optional-parameter read-only tool with an output schema, the description covers data identity, source, frequency, default window, auth, and licensing. Nothing needed to select and invoke it is missing.

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?

Input schema coverage is 100% and already documents last_n's minimum, maximum, default, and monthly nature. The description repeats the default/max and adds no new parameter semantics beyond that.

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?

The description opens with a precise definition: monthly average quoted UK household interest rate for a 2-year fixed-rate mortgage at 75% LTV, and adds the canonical series identifier IUMBV34. It also states the operation ('Returns the most recent N months'), making the tool's purpose unambiguous and distinct from sibling BoE rate tools.

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?

Clear context: anyone needing the BoE 2-year fixed mortgage rate series can infer this is the tool. It notes monthly cadence, default/max range, and no auth requirement. It does not explicitly name alternatives such as bank_rate or sonia, but those are clearly different series.

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