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

Annotations already declare idempotentHint=true and destructiveHint=false, so the description's job is narrower. It still adds value annotations can't: marking is monotonic ('never unmarks'), episodes are numbered within that one entry, and a film is through_episode 1. It stops short of stating permission/error behavior or what happens if the episode count exceeds the show's run.

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 tight sentences, front-loaded with the action and scope, then the safety note, then the edge case, then the input hint. No sentence is filler.

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 3-parameter mutation with no output schema and annotations covering the safety profile, this covers scope, monotonicity, indexing semantics, and inputs. Only failure modes and what the response returns are left implicit, which is minor here.

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 100%, so baseline is 3, and the description genuinely adds meaning beyond the schema: episode numbers reset per entry (season 2 starts at 1) and a film is through_episode 1, plus the title-or-catalog-ID choice. It doesn't clarify what happens when through_episode exceeds the actual episode count, which is the main remaining ambiguity.

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 ('Mark episodes ... as watched') plus a precise scope: one anime season or film, from episode 1 through the number given, on the signed-in person's list. The 'never unmarks' and per-entry episode-counting notes make it distinguishable from add_to_list even though no sibling is named.

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?

It gives real usage context ('Give a title or a catalog ID', episode 1 through N, films use through_episode 1) but never states when to use this instead of add_to_list or what to do when the entry doesn't exist yet. Usage is implied rather than routed.

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