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

Annotations already declare readOnlyHint=true, so safety is covered. The description adds one genuinely behavioral fact beyond them — that the call is free — but says nothing about what is returned (two counts? a diff?), latency, or whether upstream must be reachable.

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?

It is a single short fragment with no filler, which is good, but it is compressed to the point of ambiguity rather than true conciseness — the key noun (what is measured or returned) is missing.

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 and no explanation of the result, the description leaves the agent guessing what it will receive from a comparison tool. For a simple read-only utility the bar is low, but the return shape is exactly what a measurement tool must communicate.

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?

There is a single optional parameter, and the schema already documents it fully (100% coverage: "Upstream MCP URL, if not in ?upstream="). The description adds no meaning beyond that, so the baseline of 3 applies when the schema does all the work.

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 phrase "tokens of full tools/list vs slim index" implies a measurement comparing the token cost of a full tool listing against the slim index, which is a recognizable purpose. But it is telegraphic and never states a verb or what exactly is produced, so the agent must infer the operation from the tool name rather than the description.

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?

"Free:" hints that calling it carries no cost, which mildly encourages use, but there is no statement of when to call this versus siblings such as slim_index or slim_report, and no preconditions. The agent gets no routing guidance at all.

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