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

Annotations already declare readOnlyHint, idempotentHint, and destructiveHint=false, so safety is covered. The description adds meaningful provenance beyond that: the numbers are an aggregation of 'daily-collected observatory data,' which tells the agent the figures are derived and refresh daily rather than being live. "No side effects" is redundant with the annotations, and no staleness or caching caveat is given.

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?

Two sentences, zero filler: the first front-loads the deliverable and its components, the second supplies provenance and the invocation trigger. Every clause carries information an agent would otherwise have to guess.

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?

An output schema exists, so return shape need not be described, and the description still previews the metric categories the caller will receive. For a zero-parameter, read-only summary tool this covers everything needed to select and call it.

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?

The tool takes zero parameters and the schema is an empty object, so there is nothing for the description to disambiguate. Baseline for a parameterless tool applies, with no gaps to compensate for.

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 names a specific verb (summarize) and resource (the MCP server ecosystem) and then enumerates the exact metrics returned: unique servers, 7-day arrivals vs the prior 7 days, per-registry counts, and new arrivals per category. That enumeration cleanly separates it from siblings like search_mcp_servers (lookup) and get_ecosystem_events (event stream) without needing to name them.

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?

"Use when the user asks how big or how fast-growing the MCP ecosystem is" gives a concrete trigger condition tied to the two headline metrics. It stops short of naming an alternative tool or stating when not to use it, so it sits below the explicit routing seen in the top-tier example.

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