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

Annotations already declare read-only, idempotent, non-destructive, closed-world behavior, so the bar is lower. The description adds useful context the annotations do not carry: the default section returns a few KB, what summary contains, and that the full file remains accessible at the url.

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?

Four compact sentences, front-loaded with the action and artifact, then usage, then default behavior, then the section override. Every sentence carries distinct information with no redundancy.

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-parameter fetch with an output schema already describing the return, the description covers what the agent needs: what it retrieves, when to use it, the default payload contents, and how to narrow to a section. The summary advertising a list of sections also enables self-discovery of valid values.

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 coverage is 0%, so the description must carry the section parameter, and it does well: default value, the meaning of section=<name> (one section), concrete examples (rankings, assets, arena_verified_registry), and the note that the full file stays at url. It omits the 64-char limit and does not enumerate every valid section, but the summary's self-describing section list mitigates 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?

States a specific verb (Fetch) and resource (the citable World Arena snapshot at /facts/world-arena.json), and clarifies it is the source for numbers rather than invented scores. It does not, however, explicitly differentiate itself from near-neighbors like arena_snapshot or arena_leaderboard, leaving the agent to infer the distinction.

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 this for numbers; do not invent market scores" gives clear context on when to reach for this tool, and the default-vs-section guidance tells the agent how to size the call. It stops short of naming an alternative tool or stating exclusions, so it is clear context without explicit when-not routing.

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