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

Annotations already declare readOnlyHint=true, idempotentHint=true, openWorldHint=true and destructiveHint=false, so safety is covered. The description adds useful non-annotation context by disclosing a cost of 1 credit. However, it does not explain defaults for dataset/frequency, pagination behavior via limit, or what happens when a CDID is not found.

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?

One front-loaded sentence states the operation and key parameter, followed by four compact examples and a parenthetical cost. No sentence is wasted and the most important scoping detail (by CDID) comes first.

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 four-parameter retrieval tool with no output schema, the definition covers the core lookup concept and cost but leaves gaps: it does not describe the return shape, default dataset or frequency, or limit semantics. The schema also marks cdid as 'required' while the required-parameter signal says zero required, and the description does not resolve that ambiguity.

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 description coverage is 100%, so the schema already documents all four parameters, making 3 the baseline. The description reinforces that cdid is the key input and gives concrete CDID examples, but adds no meaning about dataset, frequency, or limit beyond what the schema provides.

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 ('Observations') and resource ('ONS time series') and clarifies retrieval is keyed by CDID. The four CDID examples (D7G7, L55O, MGSX, ABMI) concretely illustrate the resource. It does not explicitly differentiate from the sibling ons_search, so it stops short of a 5.

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?

No when-to-use guidance is given: it never says to use this when the CDID is already known and to use ons_search to discover CDIDs. No prerequisites or exclusions are stated. The examples imply a lookup workflow but do not route the agent between alternatives.

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