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

Annotations already declare readOnlyHint, openWorldHint, idempotentHint, and destructiveHint=false, covering safety. The description adds valuable context beyond that: 'Keyless' indicates no API key is required, and 'Returns the most recent value per site × parameter' clarifies the granularity of results. This extra transparency around auth and output structure justifies a score above baseline.

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 description is three concise sentences, each earning its place: purpose, output semantics, and parameter reference. It is front-loaded with the primary action and does not repeat schema information. The 'Keyless' note is a compact but useful piece of behavioral info.

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 simple 2-param tool with 100% schema coverage and rich annotations, the description is complete. It explains the core return concept (most recent value per site × parameter), common parameter codes, and keyless access. No output schema exists, but the description provides enough conceptual structure for an agent to interpret results.

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 fully documents both parameters. The description adds extra value by listing common parameter codes with their meanings and units (e.g., 00060 = discharge in ft³/s), which is not in the schema. This helps an agent select appropriate parameter codes without external lookup.

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 clearly states the tool's function: 'Latest instantaneous (real-time) readings for one or more USGS gauge sites.' This is a specific verb+resource with a clear scope that distinguishes it from historical or alert-oriented siblings. The phrase 'real-time' and the focus on current readings differentiate it from tools like daily_values without needing to inspect schemas.

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?

The description provides clear context for when to use this tool: when you need the most recent instantaneous readings for USGS sites. It does not explicitly name alternatives or give exclusion criteria, but the 'Latest instantaneous' framing strongly implies it is not for historical or aggregated data. Absence of explicit when-not guidance prevents a 5.

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