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

No annotations exist, so the description carries the full burden, and it does substantial work: it discloses the audit envelope (provenance, licence and redistribution status, attribution requirement, verification of per-gas sums, GWP set, publisher omissions) and the graceful failure mode of returning candidate keys. It stops short of stating read-only semantics, permissions, or any rate limits, but the disclosure is well above typical.

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?

Three sentences, front-loaded with the core action before the return-envelope detail. The nested parentheticals are dense but every clause conveys distinct information; nothing is padding, though the licence/provenance list is longer than strictly needed.

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?

There is no output schema and no annotations, so the description must describe what comes back, and it does so in detail for both success (value plus audit envelope) and failure (candidate keys). An agent has everything needed to call this correctly and interpret the result.

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?

Only one parameter, and schema coverage is 100% with an example key format already present in the schema. The description restates the 'canonical key' concept and adds the meaningful failure behavior for a bad key, but adds no syntax or format detail beyond the schema, so the baseline 3 applies.

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 and resource with scope: 'Look up a single greenhouse-gas emission factor by its canonical key.' The word 'single' and 'by its canonical key' implicitly separate it from lookup_factors (plural) and search_factors, and the description explicitly routes to search_factors when the key is unknown.

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?

Gives an explicit condition for the alternative: 'Use search_factors first if you do not know the key.' That is clear when-to-use guidance. It does not, however, explain when to prefer this over the closely-named siblings lookup_factors or resolve_factor, so the agent must infer that from 'single'.

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